Power Transformers - Switch Mode What differentiates a power transformer and a switch mode power transformer from other transformers? Power transformers (and inductors) are essentially A.C. (alternating current) devices. They cannot sustain transformer operation from a fixed D.C. (direct current) voltage source. However they can sustain transformer operation in a transient condition(s) that allows resetting or reversal of the transformer’s magnetic flux levels. An A.C. voltage source keeps reversing the polarity of the voltage being applied across the transformer. Consequently the magnetic fields keeps reversing. Voltage reversal can also be accomplished with a D.C. source such as a battery. The connections between the D.C. source and the transformers are repeatedly switched, thereby reversing the voltage polarity across the transformer, hence reversing the magnetic field. The transformer can also be switched off from the D.C. source. In this case the magnetic field simply collapses until it reaches its residual value (ideally equal to zero). This collapse “resets” the transformer’s magnetic field. Switch mode power transformers (and supplies) get their name from the switching action needed to sustain transformer operation. By controlling the amount of “on time” and “off time” of the switches, one can also control the amount of power delivered to the transformer’s load (or load circuit). The voltage can be fed to the switch mode power transformer in voltage pulses. The pulse duration is a portion of an overall cycle time. The cycle time is equal to the inverse of the operating frequency. The terms “duty cycle” and “pulse width modulation” arise from the control of the switching “on time” and “off time”. Switch mode power transformers are used extensively in electronic applications, usually within a switch mode power supply. A switch mode
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Power Transformers - Switch Mode
What differentiates a power transformer and a switch mode power
transformer from other transformers? Power transformers (and inductors) are
essentially A.C. (alternating current) devices. They cannot sustain
transformer operation from a fixed D.C. (direct current) voltage source.
However they can sustain transformer operation in a transient condition(s)
that allows resetting or reversal of the transformer’s magnetic flux levels. An
A.C. voltage source keeps reversing the polarity of the voltage being applied
across the transformer. Consequently the magnetic fields keeps reversing.
Voltage reversal can also be accomplished with a D.C. source such as a
battery. The connections between the D.C. source and the transformers are
repeatedly switched, thereby reversing the voltage polarity across the
transformer, hence reversing the magnetic field. The transformer can also be
switched off from the D.C. source. In this case the magnetic field simply
collapses until it reaches its residual value (ideally equal to zero). This
collapse “resets” the transformer’s magnetic field. Switch mode power
transformers (and supplies) get their name from the switching action needed
to sustain transformer operation.
By controlling the amount of “on time” and “off time” of the switches, one
can also control the amount of power delivered to the transformer’s load (or
load circuit). The voltage can be fed to the switch mode power transformer
in voltage pulses. The pulse duration is a portion of an overall cycle time.
The cycle time is equal to the inverse of the operating frequency. The terms
“duty cycle” and “pulse width modulation” arise from the control of the
switching “on time” and “off time”.
Switch mode power transformers are used extensively in electronic
applications, usually within a switch mode power supply. A switch mode
power supply is usually powered from a D.C. source, such as a battery. The
switching mode power supply converts the input D.C. source to one or more
output D.C. sources. The power supplies are often referred to as “DC to DC”
converters. In similar fashion, the switch mode power transformers are often
referred to as “DC to DC” transformers (or “DC-DC” transformers). A
switch mode power transformer can have several secondary windings.
Consequently, the switch mode transformers permits multiple outputs which
can be electrically isolated from one another. Transformer action permits
one to “step up” or “step down” the voltage as needed via an appropriate
turns ratio. Pulse width modulation is used to provide voltage regulation.
Many electronic applications require some sort of power supply which
converts power from the conventional low frequency sinusoidal A.C. wall
socket (for example, 115V 60 Hz) to the necessary voltage, current, and/or
waveform required by the circuit. Typically the circuits need a well-
regulated D.C. voltage. Designers often choose either a rectifier type circuit
(to convert A.C. voltage to D.C. voltage), a switch mode power supply, or
both. For the “both” case, the A.C. voltage is first rectified to provide a D.C.
voltage. The D.C. voltage varies as the A.C. voltage varies, hence good
voltage regulation cannot be assured. One or more switching mode power
supplies follow the rectifying circuitry. The switching mode power supplies
provide a more tightly regulated output voltage. A.C. rectification is not a
necessity. Although tricky, it is possible, through switching actions, to
divide (“chop”) the A.C. waveform into a series of pulses, which are directly
fed into the switching mode power transformer. Pulse width modulation is
used to control the regulation.
Butler Winding can make (and has made) switching mode power
transformers (and /or inductors) for Buck, Flyback, and Boost applications
(discussed below) in a wide variety of shapes and sizes. This includes;
various standard types of “core with bobbin” structures (E, EP, EFD, PQ,
POT, U and others), toroids, and some custom designs. Our upper limits are
40 pounds of weight and 2 kilowatts of power. We have experience with foil
windings, litz wire windings, and perfect layering. For toroids, we can (and
have done) sector winding, progressive winding, bank winding, and
progressive bank winding.
Butler winding has a variety of winding machines, bobbin/tube and toroid.
That includes two programmable automated machines and a taping machine
for toroids. Butler Winding has vacuum chamber(s) for vacuum
impregnation and can also encapsulate. To ensure quality, Butler Winding
purchased two programmable automated testing machines. Most of our
production is 100% tested on these machines. For more information on
Butler Winding’s capabilities, click on our “capabilities” link.
Switching Mode Power Transformers, Basic Application Circuits
The design of a switch mode power transformer will differ depending upon
the type of circuit used. There are many variations of switching mode power
supplies, but they can be narrowed down to three basic circuit configurations
(each also has a mirrored configuration); “Buck”, “Boost”, and “Flyback”.
Be aware that the name for the “Buck” circuit varies from industry to
industry and from person to person. It may also be referred to as an
“inverter”, “D.C. converter”, “forward converter”, “feed forward”, and
others. There are also unipolar and bipolar (push-pull) versions.
The basic “Buck” circuit is illustrated in Figure 1A with an inductor and in
Figure 1B with both a switch mode power transformer and an inductor. A
push-pull version is shown in Figure 4. The basic “Flyback” circuit is
illustrated in Figure 2A with an inductor and in Figure 2B with a switch
mdoe power transformer. The basic boost circuit is illustrated in Figure 3A
with an inductor, Figure 3B and 3C with a transformer and in Figure 5 with
a push-pull forward converter type of switch mode power transformer. The
circuits shown in Figures 1A, 2A, and 3A, which have no switch mode
power transformers, are the simplest circuits. They are useful for explaining
the operating theory.
The Forward Converter (Buck) Circuit
The inductors in all of the buck circuits act as filtering elements to smooth
out the ripple and reduce peak currents. Since they must store energy for part
of a cycle they usually have a discrete air gap(s) or a distributed air gap in
the magnetic core path.
The switch mode power transformer in the Buck Circuit of Figure 1B
couples energy from the input side (primary) to the output side (secondary).
An ideal transformer does not store any energy and consequently does not
provide any ripple filtering. The inductor does the ripple filtering. Ideally, a
Buck circuit transformer couples energy without storing it (hence it meets
the true definition of a transformer). The transformer does not need to do
any ripple filtering.
The transformer should have minimal air gap. The “on time” on the
transistor (switch) controls how much energy is delivered to the capacitor
hence it regulates the output voltage. Note that for the inductor circuit of
Figure 1, the average capacitor voltage can never be more than the source
voltage even for ideal circuit components. Real life voltage drops (diode,
transistor, winding resistance) ensure that the average output voltage will be
less than the source voltage. The transformer in Figures 1B remove this
voltage limit and can also provide electrical isolation between input and
output.
The circuits of Figures 1A and 1B are unipolar applications of forward
converters. Push-pull versions, such as that shown in Figure 4, are bipolar
applications. Unipolar and bipolar applications are explained further below.
Click on the available link for more information about push-pull switching
mode power transformers.
Inductive Flyback (Kickback) in Switch Mode Power Transformers
Unlike the Buck transformer; the flyback inductor, flyback transformer,
boost inductor, and boost transformer intentionally store energy during the
“on time” (charging portion) of a cycle and then discharge energy during the
“off time” portion. (Technically, since they intentionally store energy, the
switch mode flyback and boost power transformers are not true
transformers.) They usually have a discrete air gap(s) or a distributed gap in
their core’s magnetic path. The transistor is turned on and current flows into
the inductor or transformer (which has inductance). When the transistor is
turned off, the input current that formed and maintained the core’s magnetic
field become zero. The magnetic field collapses causing a voltage reversal to
occur in the inductor or transformer. The collapsing magnetic field induces
sufficiently high voltage (known as inductive kickback voltage) to discharge
energy into the capacitor connected to the inductor or to the switch mode
power transformer secondary.
Inductive discharge into the capacitor continues until the magnetic field
completely dissipates or power is restored to the input. Restoring the power
starts the inductive charging cycle again. The use of inductive kickback
permit the output voltages of the inductor circuits of Figures 2A and 3A to
be either lower, equal, or greater than the input source voltage. A
transformer “step up” is not needed to achieve voltages higher than the
source voltage. Flyback transformers are usually preferred over flyback
inductors. The appropriate turns ratio can optimize current levels. The
transformer can provide voltage isolation between input and output, and
removes a polarity restriction that comes with a flyback inductor design.
Boost Inductor Circuits
You might ask what distinguishes the boost inductor application from the
flyback inductor application. One characteristic is the polarity reversal of the
output capacitor due to the placement of the circuit components. Compare
the circuits of Figures 2A and 3A. The diode in the flyback circuit, Figure
2A, completely blocks direct flow of current from the input source to the
capacitor regardless of the capacitor’s voltage value. The capacitor can only
be charged by the inductive kickback. The diode in the boost circuit, Figure
3A, permits current flow from the input source to the capacitor without the
use of inductive kickback if the capacitor voltage is sufficiently low.
Consequently it both “stores” energy and “passes through” energy during the
charging portion of a cycle. “Pass through” current flow stops whenever the
capacitor voltage approaches the value of the source voltage minus the diode
voltage drop. (Further increase requires the inductive kickback voltage.)
This may be a desirable feature for rapid power supply startup
Few designers are aware of the boost transformer circuit shown in Figure 3B
because the circuit is not very practical. With only half-wave rectification it
is either a forward (Buck) converter transformer application or a flyback
transformer application depending on choice of polarity. Full wave
rectification, as shown, permits it to duplicate the boost inductor actions
discussed in the prior paragraph; both storing energy and “passing through”
energy (by transformer coupling like a Buck transformer) during the
charging portion of a cycle if the secondary capacitor voltage is sufficiently
low. It acts likes a flyback transformer during the discharging portion of the
cycle. It is rarely used with the full wave rectification as shown. It has seen
some limited use as modified in the circuit shown in Figure 3C. The
transformer has two secondary windings. One is used as a
Forward (Buck) converter. The other is used as a flyback. It effectively
divides the full-wave rectification into two half-wave applications. A more
common boost inductor application is shown in Figure 5. A boost inductor is
used with a push-pull (Buck) transformer. “High power” power supplies
might use this type of circuit. In this application both switches are not open
at the same time. Both switches are closed to charge the inductor, otherwise
the switches are alternated on and off with “one closed and one open”.
Unipolar versus Bipolar
What is the difference? When a current flows through an inductor or a
transformer a magnetic field is created in its core. The value of the magnetic
field will be greater than zero and it will have a direction associated with it.
This direction is also referred to as the polarity of the field. If the value of
the current varies, then the value of the magnetic field will vary accordingly,
but the field polarity (direction) will remain the same as long as the current
direction does not reverse. When an inductor or transformer continually
operates with the same magnetic polarity it is a unipolar application. The
circuits shown in Figures 1 through 3, including A thru C versions, are all
unipolar applications.
Applications were the magnetic field polarity is continually reversing are
bipolar applications. A.C. applications are bipolar applications. Push-Pull
types of forward converters (Buck) are bipolar applications. “Push-pull”
transformers are often used in “inverter circuits” to create A.C. voltage from
a D.C. source. A “push-pull” center-tap application is shown in Figure 4.
There are several types of “push-pull’ applications. More information about
push-pull transformer applications is available on this website. Click on the
available link.
Need More Power TransformerTechnical Information?
More information about the theory of operation for flyback transformers is
available. Click on the available link for flyback transformer. Much of its
theory of operation also applies to the boost inductor. There is also a link for
forward (Buck) converters and links for other types of power transformers,
inductors, chokes, etc.
Also, feel free to contact Butler Winding and ask for technical or
engineering assistance.
Toroidal Transformer
Toroidal transformers are the high performers among transformers. They
offer the smallest size (by volume and weight), less leakage inductance, and
lower electromagnetic interference (EMI). Their windings cool better
because of the proportionally larger surface area. A 360 degree wound
toroidal transformer has a high degree of symmetry. Its geometry leads to
near complete magnetic field cancellation outside of its coil, hence the
toroidal transformer has less leakage inductance and less EMI when
compared against other transformers of equal power rating. Toroidal
transformers with a round core cross section are better performers than
toroidal transformers with a rectangular cross section.
The cancellation is more complete for the round cross section. The round
cross section also gives a shorter turn length per unit of cross sectional area,
hence lower winding resistances. The toroidal transformer also has better
winding to winding magnetic coupling because of its toroidal shape. The
coupling is dependent on the winding being wound a full 360 degrees
around the core and wound directly over the prior winding, hence sector
wound windings do not couple as well and have higher leakage inductance.
As winding turns are positioned further away from the core less complete
coupling will occur; hence toroidal transformers with multi-layered
windings will exhibit more leakage inductance.
Toroidal transformers can be used in any electronic transformer application
that can accommodate its shape. Although usable, toroidal transformers are
not always practical for some applications. Gapped toroidal transformers
usually require that the gap be filled with some type of insulating material to
facilitate the winding process. This is an extra expense. Split core current
transformers can be assembled directly on a conductor while toroids must be
passed over a disconnected end of the conductor. A toroid can be split in
two, but a suitable clamping mechanism (difficult and costly) is required.
Some printed circuit boards are space critical. Mounting a toroidal
transformer flat on the board may take up too much precious board area.
Some applications also have restricted height so the toroid cannot be
mounted vertically.
Generally speaking toroidal transformers are more expensive than bobbin or
tube wound transformers. Sufficient winding wire must first be wound
(loaded) onto the winding shuttle, then wound onto the toroidal
transformer’s core. After that, the best situation, from a cost perspective, is
no insulation required over the winding and the next winding uses the same
wire size. If the wire is different, then the leftover wire must be removed and
the wire for the next winding must be loaded. However, if the winding must
be insulated, then if must either be insulated (taped) by hand or the toroidal
transformer must be removed and taken to a separate taping machine, then
placed back on the toroid winding machine after taping. The shuttle must
then be loaded with the wire size and type for the toroidal transformer’s next
winding. A toroidal transformer with a single winding (auto-transformer,
current transformer) wound on a coated core will probably be cost
competitive with an equivalent bobbin or tube wound transformer since the
toroidal transformer will not require a bobbin or tube. The cost differential
will then depend on the method and cost of mounting the transformers.
Toroidal transformer cores are available in many materials: silicon steel,
nickel iron, moly-permalloy powder, iron powdered, amorphous, ferrites,
and others. Silicon steel and nickel iron are available as tape wound cores or
laminated pieces. Non-magnetic toroids are also available to make air core
toroidal transformers.
Butler Winding manufactures toroidal transformers in a wide variety of
materials and sizes. To ensure quality, Butler Winding purchased two
programmable automated testing machines. Most of our production is 100%
tested on these machines. For more information on Butler Winding’s
capabilities, click on our “capabilities” link.
Need More Technical Information about Electronic Transformers in
general ?
More information is available on other web pages included in this web site.
Saturation and the volt-second product are discussed in the “pulse
transformer” web page. An equivalent circuit for a transformer is included in
the “power transformers” web page. The “inverter transformer” and “push
pull” web pages include some discussion about magnetic “bipolar” and
“unipolar” operating modes. There are web pages for various types
(applications) of electronic transformers (and inductors). Click on one of the
available links.
Also, feel free to contact Butler Winding and ask for technical or
engineering assistance.
Flyback Transformers - Kickback Transformers
A simple and low cost power supply is bound to be quite popular. The single
ended flyback circuit topology fits this description. The flyback transformer
utilizes the "flyback" action ( also known as "kickback" ) of an inductor or
flyback transformer to convert the input voltage and current to the desired
output voltage and current. Figures 1A and 1B show simple flyback
transformer schematics for an inductor and a flyback transformer. These
schematics do not show any parasitic effects ( such as leakage inductance
and winding capacitance ). Modern flyback transformer and circuit design
now permit use in excess of 300 watts of power, but most applications are
less than 50 watts.
By definition a transformer directly couples energy from one winding to
another winding. A flyback transformer does not act as a true transformer. A
flyback transformer first stores energy received from the input power supply
(charging portion of a cycle) and then transfers energy (discharge portion of
a cycle) to the output, usually a storage capacitor with a load connected
across its terminals. An application in which a complete discharge is
followed by a short period of inactivity (known as idle time) is defined to be
operating in a discontinuous mode. An application in which a partial
discharge is followed by charging is defined to be operating in the
continuous mode. See figures 2A and 2B for illustration.
Gapped core structures increase the magnetizing force needed to reach
saturation and lower the inductance of the flyback transformer (or inductor).
Consequently, a gapped flyback transformer (or inductor) can handle higher
peak current values, and thereby storing more energy, most of which is
stored in the magnetic field of the gap. For these reasons almost all flyback
transformers (or inductors) are gapped. The gap may be a discrete physical
gap, several smaller discrete physical gaps or a distributed gap. Distributed
gaps are inherently present in low permeability powdered cores. The bulk of
the stored energy is stored in the magnetic field of the gap(s). Most modern
flyback transformers are operated at high frequency hence gapped ferrite
core materials are typically used.
Butler winding can make (and has made) flyback transformers in a wide
variety of shapes and sizes. This includes; various standard types of “core
with bobbin” structures (E, EP, EFD, EC, ETD, PQ, POT, U and others),
toroids, and some custom designs. We have experience with foil windings,
litz wire windings, and perfect layering. For toroids, we can (and have done)
sector winding, progressive winding, bank winding, and progressive bank
winding. Butler winding has a variety of winding machines, bobbin/tube and
toroid. That includes two programmable automated machines and a taping
machine for toroids. To ensure quality, Butler Winding purchased two
programmable automated testing machines. Most of our production is 100%
tested on
these machines. For more information on our capabilities, click on our
"capabilities" link.
How does a flyback transformer ( or inductor ) work?
Flyback circuits repeat a cycle of two or three stages; a charging stage, a
discharging stage, and in some applications idle time following a complete
discharge. Charging creates a magnetic field. Discharging action results
from the collapse of the magnetic field. The typical flyback transformer
application is a “unipolar” application. The magnetic field flux density
varies up in down in value ( 0 or larger ) but keeps the same ( hence unipolar
) direction.
Charging Stage
The flyback transformer ( or inductor ) draws current from the power source.
The current increases over time. The current flow creates a magnetic field
flux that also increases over time. Energy is stored within the magnetic field.
The associated positive flux change over time induces a voltage in the
flyback transformer ( or inductor ) which opposes the source voltage.
Typically, a diode and a capacitor are series connected across a flyback
transformer winding ( or inductor ). A load resistor is then connected across
the capacitor.
The diode is oriented to block current flow from the flyback transformer ( or
source ) to the capacitor and the load resistor during the charging stage.
Controlling the charging time duration (known as duty cycle) in a cycle can
control the amount of energy stored during each cycle. Stored energy value,
E = ( I x I x L ) / 2, where E is in joules, I = current in amps, L = inductance
in Henries. Current is defined by the differential equation V(t) = L x di/dt.
Applying this equation to applications with constant source voltage and
constant inductance value one obtains the following equation; I = Io + V x t /
L , where I = currents in amps, Io = starting current in amps, V = voltage in
volts across the flyback transformer winding ( or inductor ), L = inductance
in Henries, and t = elapsed time in seconds.
Note that increasing L will decrease the current. Stored energy will
consequently decrease because effects of the “current squared decrease” will
more than offset the effects of the inductance increase. Also be aware that
the flyback transformer ( or inductor ) input voltage is less than the source
voltage due to switching and resistive voltage drops in the circuit.
Discharge Stage
The current ( which creates the magnetic field ) from the source is then
interrupted by opening a switch, thereby causing the magnetic field to
collapse or decrease, hence a reversal in the direction of the magnetic field
flux change ( negative flux change over time ). The negative flux change
induces a voltage in the opposite direction from that induced during the
charging stage. The terms “flyback” or “kickback” originate from the
induced voltage reversal that occurs when the supply current is interrupted.
The reversed induced voltage(s) tries to create ( induce ) a current flow. The
open switch prevents current from flowing through the power supply. With
the voltage reversed, the diode now permits current flow through it, hence
current flows into the capacitor and the load across the capacitor. If current
can flow, then the resulting flow of current is in the direction, which tries to
maintain the existing magnetic field. The induced current cannot maintain
this field but does slow down the decline of the magnetic field.
A slower decline translates to a lower induced flyback voltage. If current
cannot flow, the magnetic field will decline very rapidly and consequently
create a much higher induced voltage. In effect, the flyback action will
create the necessary voltage needed to discharge the energy stored in the
flyback transformer or inductor. This principle, along with controlling the
duration of the charging stage, allows a flyback inductor to increase or
decrease the voltage without the use of a step-up or step-down turns ratio. In
the typical flyback circuit, the output capacitor clamps the flyback voltage to
the capacitor voltage plus the diode and resistive voltage drops.
For a sufficiently large & fully charged capacitor, the clamping capacitor
voltage can be treated as a constant value. The equations V(t) = L x di/dt,
and I = Io + V x t / L can also be applied to the discharge stage. Use the
inductance value of the discharging winding and the time duration of the
discharging stage. The time will either be the cycle time minus the charging
time ( no idle time ), or the time it takes to fully discharge the magnetic field
thereby reaching zero current. The cycle time equals the period which equals
1 / frequency.
Idle Stage: This stage occurs whenever the flyback transformer ( or inductor
) has completely discharged its stored energy. Input and output current ( of
the transformer or inductor ) is at zero value.
Other Principles of Operation
Equal Ampere-Turns Condition: A magnetic field is created by the current
flow through the winding(s). The current creates a magnetizing force, H, and
a magnetic field flux density B. A core dependent correlation will exist
between B and H. B is not usually linear with H. By definition H is
proportional to the product of the winding turns and the current flowing
through the winding, hence ampere-turns. In classical physics, the magnetic
field flux cannot instantaneously change value if the source of the field ( the
current flow ) is removed. When the source current is removed from the
flyback transformer ( or inductor ) the charging stage ends and the discharge
stage begins.
The value of the magnetic field will be the same for both stages at that point
in time ( cannot instantaneously change to another value ). The same
magnetic core is used for both stages, hence if the magnetic field is the
same, then the magnetizing force, H, must be the same. Consequently the
ampere-turns at the end of the charging stage must equal the ampere-turns at
the start of the discharge stage. If there are multiple outputs then the total
amperes turns of all outputs at the start of the discharge stage must equal the
ampere-turns at the end of the charging stage. The same condition applies at
the start of the charging stage. The total ampere-turns of all outputs at the
start of the charging stage must equal the ampere-turns at the end of the
discharge stage. Note that there are zero ampere-turns at both the start and
end of an idle stage when an idle stage exists.
Zero Average Voltage
During steady state operation, the average voltage across the charging
winding must equal the average voltage across the discharge winding, or
equivalently, the volt-seconds of the charging stage must equal the volt-
seconds of the discharge stage. If not, flux density increases over time and
the core saturates. Assuming a 1:1 turns ratio, then from V1 x t1 = V2 x t2
one can obtain t1 / t2 = V2 / V1 for both continuous and discontinuous
modes of operation. For continuous mode operation, t1 + t2 = 1 / operating
frequency.
Conservation of Energy
Power out cannot exceed power in. Sum up output power ( V x I ) of each
output at maximum steady state load plus allowances for parasitic output
power losses ( diode and resistive losses ). Divide power in watts by
operating frequency. The result is the energy in Joules that must be
discharged each cycle into the output storage capacitor during steady state
operation. It is also the amount of energy that must be added to the flyback
transformer ( or inductor ) during the charging stage. The energy being
transferred equals ( Ipeak x Ipeak – Imin. x Imin. ) x L /2.
If operating in the continuous mode, the stored energy will exceed the
energy being transferred because the starting level of stored energy is above
zero ( Imin. > 0 ). The flyback transformer ( or inductor ) must be designed
to handle the peak stored energy, Ipeak x Ipeak x L / 2. The power source
will have to supply the transferred energy plus the parasitic switching and
resistive losses of the charging circuit, plus some power allowance for
transient conditions. Take this value and divide by the power supply voltage.
The result will be the average input current.
Need additional information about Flyback Transformers?
Contact Butler Winding. Ask for engineering assistance.
Electronic Transformer - Inverter Transformer
The term "inverter" is associated with several different electronic
applications.
In logic circuits "inverter" may be a logic inverter, the equivalent of a "Not"
gate. In analogue signal processing an inverter can be a circuit which inverts
the phase of the signal being transmitted. In power conversion applications
an inverter is an electronic transformer which converts power from a Direct
Current (D.C.) source into Alternating Current (A.C.) power. Power
conversion inverters can be divided into two sub-categories, voltage-fed
inverters and current-fed inverters.
Voltage-fed inverters are more common than the current-fed inverters. The
electronic transformers used in inverter circuits are often called inverter
transformers. Inverters produce A.C. power by switching the polarity of the
D.C. power source across the D.C. power source’s load. The early inverters
used mechanical switches to do the switching. Vacuum tubes replaced
mechanical switches in low power applications. Eventually semiconductor
based switches (diodes, transistors, F.E.T.s, S.C.R.s, etc.) replaced both
mechanical and vacuum tube switches.
The schematic in Figure 1A illustrates a very simple inverter circuit. The
circuit does not have an inverter electronic transformer. The switches are
alternated on and off (“cycled”), but are not on at the same time. The load
will see alternating square wave pulses of voltage equal to the source voltage
minus the circuit’s resistive voltage drops. The pulse voltage cannot be
adjusted, but the average load voltage can be made less than the source
voltage by holding both switches “open” (“off”) at the same time.
The portion (ratio < 1) of time during a cycle that a switch is “on” is called
the “duty cycle”. The inverter schematic in Figure 1B utilizes a capacitor
and another switch to provide a lower load voltage. One switch controls the
amount of charge delivered to the capacitor hence it also controls the
capacitor voltage. The set of two switches alternately switches the polarity
for the connection between the capacitor and the load. The load voltage
cannot exceed the input source voltage.
The inverter schematic of Figure 1C adds an electronic transformer inverter
with two secondary windings. The switching action sends alternating current
through the inverter transformer’s primary winding. This is referred to as
“push-pull” action. The core has “bipolar” utilization. Bipolar utilization is
discussed further below. The inverter transformer’s turns ratio can permit
either higher or lower load voltage. The inverter transformer’s output is an
A.C. square wave. Output filter networks can be used to obtain sine wave
output. The inverter transformer can also provide electrical isolation
between the inverter transformer’s input and output sides. Full wave
rectification can be applied to the inverter transformer’s outputs to obtain a
D.C. voltage of different value than that of the input source. This is shown in
the schematic of Figure 1D.
Compare the schematic of Figure 2A to the one in Figure 1D. Note in figure
2A the center-tap connections on the electronic transformer windings, a set
of two switches instead of a set of four switches on the input side, the two
diodes on the secondary instead of four, and the output filter inductor
between the capacitor and load. The inverter transformer center-taps allow
use of fewer switches and diodes. The inductors smooth out the current
surges from the rectification thereby maintaining tighter output voltage
regulation (less ripple voltage). The circuit in Figures 2A depicts a typical
“Push-Pull” “Forward Converter” circuit. Be aware that the name for a
“Forward Converter” circuit (and transformer) varies from industry to
industry and from person to person. It may also be referred to as “Buck”,
“inverter”, “D.C. converter”, “feed forward”, and others. There are also
unipolar versions and there are bipolar versions that utilize saturable
transformers to trigger transistor switching.
Butler Winding makes electronic transformers and inverter transformers in a
wide variety of shapes and sizes. This includes; various standard types of
“core with bobbin” structures (E, EP, EFD, PQ, POT, U and others), toroids,
and some custom designs. Our upper limits are 40 pounds of weight and 2
kilowatts of power. We have experience with foil windings, litz wire
windings, and perfect layering. For toroids, we can (and have done) sector
winding, progressive winding, bank winding, and progressive bank winding.
Most of our production is 100% tested on these machines. For more
information on Butler Winding’s capabilities, click on our “capabilities”
link.
The Difference between Bipolar and Unipolar Applications
Since the connections of the electronic transformer "inverter" are alternated,
the current direction through the electronic transformer will also alternate.
Consequently the magnetic field polarity of the inverter transformer’s core
will alternate between positive and negative flux directions. This is known
as “bipolar” utilization of the inverter transformer’s core. This is graphically
illustrated in Figure 2B. The “B-H” curve shown is also known as a
hysteresis loop. The area inside the loop is related to the core loss. A thinner
loop means less core loss. Also note the residual flux density point. In a
Unipolar application the flux density, B, would never return to zero value.
It would stop at Br when the current (hence also the magnetizing force, H)
returns to zero. The applied voltage reversal (by switching action) ensures
that the flux density returns to zero. Bipolar utilization permits use of a
smaller core than unipolar utilization because it permits a larger change in
the core’s flux density. Fewer turns are needed to handle the same amount of
power. Compare Figure 2B to Figures 3C, 4C, and 5C.
Unipolar utilization occurs if the magnetic flux remains in one direction. The
value may vary up and down but does not cross zero value. A unipolar
application is illustrated in Figures 3A, 3B, and 3C. Some designers may
refer to the transformer in Figure 3A as an inverter transformer, but it is not.
It is serving as a pulse transformer with a resistive load. If we assume it to be
an ideal transformer, then there is no core loss, no leakage inductance, does
not store any energy, and the residual flux density is zero. Figure 3B shows
the expected output if a rectangular voltage pulse is placed across the
transformer (turn switch on, then off). The output will also be a rectangular
pulse without any distortion. There will be a change in amplitude because of
the transformer’s turns ratio. The ideal transformer’s lack of stored energy
eliminates the possibility of an inductive kickback voltage spike. This circuit
does not produce an A.C. output, hence no true inverter action.
A non-ideal electronic transformer has finite inductance hence it stores some
inductive energy in its magnetic field. A lower inductance results in more
stored energy. Consider the non-ideal gapped transformer in the circuit
shown in Figure 4A. The gap lowers the inductance of the transformer;
consequently more current can flow when the switch is closed (compared to
no gap). When the switch is closed the transformer directly couples power to
the load plus it stores energy in its magnetic field. The field is created by the
magnetizing current. The current flow due to the load does not contribute to
the stored energy. When the switch is opened the magnetic field collapses.
The collapse creates an inductive kickback voltage of reversed polarity. The
induced secondary voltage causes current to flow through the load resistor in
the reversed direction. (This is how a flyback transformer functions.) The
load sees alternating current although it usually has an asymmetrical
waveform. One could claim that the circuits and transformer have inverter
action.
The energy stored in the electronic transformer’s magnetic field is dissipated
as heat produced by current flowing through the load resistor. Current of
declining value will continue to flow until either all of the stored energy is
dissipated or the switch is closed again. If completely dissipated, then the
output shown in Figure 4B and the generalized hysteresis loop of Figure 4C
apply. The transformer is said to be operating in discontinuous mode. The
load voltage and load current reach zero value, and the core’s flux density
reaches its residual value. Note that the flux density averaged over time is
greater than zero. This holds for all unipolar applications. If the switch is
closed again before all the energy is dissipated, then the output shown in
Figure 5B and the generalized hysteresis curve of Figure 5C applies. The
transformer is said to be operating in the continuous mode. The load voltage
and load current remain above zero value, and the flux density does not
reach its residual value. The output waveform in Figure 5B is more
rectangular than that of Figure 4B.
The circuits in Figures 4A and 5A are not very practical inverter transformer
circuits. To be useful the transformer must store as much energy as it
directly couples to its load. Consequently, the transformer will tend to be
lightly loaded and designed to have appreciable magnetizing current. Output
filters would be required to produce a more symmetrical output waveform.
These circuits find little use as shown here. There are D.C. biased unipolar
applications, which function as inverters. They are not discussed here.
Saturable Transformers as Inverter Transformers
Figure 6A shows a “Royer Inverter Circuit” schematic that uses saturable
transformers. The saturable transformer also functions as the inverter
transformer. Figure 6B shows a “Jensen Circuit” which uses a saturable
transformer and a power transformer. The power transformer functions as
the inverter transformer. Both of these circuits make use of “push-pull”
switching to achieve the inverter action. The feature of these two circuits is
the transistor switching action that is activated by a voltage spike created
when the saturable transformer enters saturation.
An oscillation develops which maintains the necessary switching action.
The theory of operation is not discussed here. It may be available on this
website at some future date from the issue date of this website page. Check
the available links.
Additional Technical Information
This website covers a variety of transformer and inductor applications.
Check the available website links to see if your topic of interest is available.
If a link is not available or if a link does not provide enough information,
please feel free to contact Butler Winding and ask for engineering or
technical assistance. Select the “Contact” link for contact information.
Buck Boost Transformer - Push Pull Transformer
When it comes to power conversion, the buck boost or "push pull"
transformer application is well known. The buck boost transformer
configuration is widely used in converting direct current (D.C.) voltage into
another value of D.C. voltage, and in inverters. Inverters convert direct
current into alternating current (A.C.). The push pull transformer is usually
the preferred choice in high power switching transformer applications
exceeding one kilowatt. It is usually used in a circuit known as a "forward
converter" circuit. Be aware that the name for the "forward converter" circuit
varies from industry to industry and from person to person. It may also be
referred to as an "inverter", "D.C. converter", "buck", "feed forward", and
others. A basic "forward converter" transformer circuit is illustrated in
Figure 1A. It is not a push pull transformer application. The output inductor
reduces ripple voltage. Pulse width modulation is used to control the value
of the output voltage
A center-tapped buck boost transformer application circuit is illustrated in
Figure 2A. Figure 2A only shows one output. Multiple voltage outputs are
possible by using either a tapped secondary winding or using multiple
secondaries. Some other buck boost transformer versions are discussed
further below. They are illustrated in Figures 3, 4, 5, and 6. (These include
some push pull transformers without the center-taps.)
The core of the transformer in Figure 1A is operated in a unipolar fashion.
Unipolar operation is depicted graphically in Figure 1B. The core's magnetic
"B-H" loop remains in one quadrant of the "B-H" grid. A loop occurs once
every cycle. The flux density "B" and the magnetizing force "H" never cross
zero hence always retain the same (or one) polarity. "H" does not have to
return to zero value. The core in a push pull transformer has bipolar
operation. Both "B" and "H" cross zero value and reverse polarity. Bipolar
operation is depicted graphically in Figure 2B. Note that the "dB" value
(change in B) in Figure 2B for the bipolar push pull transformer can be more
than twice the "dB" value shown in Figure 1B for the unipolar forward
converter (assuming the same core material). Push pull transformer (bipolar)
operation permits one to handle the same amount of power in a smaller
package than for that of a unipolar operation. There are tradeoffs. The buck
boost transformer operation requires more switching elements and its control
circuitry is more complicated. Consequently a push pull transformer
application is more expensive. The voltage pulses must be adequately
controlled to avoid phenomena known as saturation walk. Center tapped
push pull transformers have winding capacitance issues at higher
frequencies. Winding imbalances can contribute to saturation walk.
Power ratings for push pull or buck boost transformer can vary from a
fraction of a Watt to Kilowatts. Megawatts is possible, but definitely beyond
Butler Winding's capabilities. Size correlates with power hence size (and
weight) can vary from a fraction of a cubic centimeter (several grams) to
multiple cubic meters (thousands of kilograms). Buck boost transformers
can be wound on toroids, bobbins, and tubes. Core materials vary depending
on the application. Laminated or tape wound grain oriented silicon steel is
common for low frequency inverter buck boost transformers. Ferrite core
materials are common for high frequency switching push pull transformers.
If minimal size is a requirement, nickel-iron alloys may be chosen for the 1
to 20 kilohertz range. Minimal energy storage is desired so cores have
minimal air gaps in their structure.
Butler Winding manufactures buck boost transformers in a wide variety of
shapes and sizes. This includes; various standard types of “core with
bobbin” structures (E, EP, EFD, PQ, POT, U and others), toroids, and some
custom designs. Our upper limits are 40 pounds of weight and 2 kilowatts of
power. We have experience with foil windings, litz wire windings, and
perfect layering. For toroids, we can (and have done) sector winding,
progressive winding, bank winding, and progressive bank winding. Butler
winding has a variety of winding machines, bobbin/tube and toroid. That
includes two programmable automated machines and a taping machine for
toroids. Butler Winding has vacuum chamber(s) for vacuum impregnation
and can also encapsulate. To ensure quality, Butler Winding purchased two
programmable automated testing machines. Most of our production is 100%
tested on these machines. For more information on Butler Winding's
capabilities, click on our "capabilities" link.
Push Pull - Buck Boost Transformer Rectification
The push pull / buck boost transformer in Figure 3 is the same as the push
pull transformer in Figure 2A except for secondary rectification. Figure 2A
achieves full wave rectification using a center-tap. It requires two diodes.
Figure 3 achieves full wave rectification with a full wave bridge. It requires
four diodes. Four diodes result in more power loss, but elimination of the
center-tap simplifies transformer construction and reduces winding
capacitance. The primary and secondary winding halves as shown in Figure
2A conduct current on alternate half cycles. Their maximum duty cycle is a
0.5 ratio (or 50%). Figure 3 requires approximately half of the secondary
turns of Figure 2A, but its secondary winding may see a maximum duty
cycle near 1 (or 100%), hence its wire must handle twice the r.m.s. current
value. Both transformers are about the same size.
Half Bridge Push-Pull Transformers
Compare figure 4 to figure 2A. Figure 4 is a “half bridge” push pull / buck
boost transformer application. This configuration eliminates the primary
center-tap and reduces primary winding capacitance. The two series
connected capacitors shown in Figure 4 effectively cut the input voltage to
the push pull transformer in half. Consequently, for the same power rating,
the push pull / buck boost transformer requires one quarter of the total
primary turns to support the halved voltage, but it must handle twice the
amount of input current.
The primary winding may see a maximum current duty cycle near 1, hence
its wire may see 4 times the r.m.s current value as wire used in the primary
winding halves of Figure 2A. Both transformers are about the same size. To
achieve the same output voltage, the number of secondary turns is about the
same as that of figure 2A, but the secondary over primary turns ratio is
quadrupled because the primary turns of figure 4 are one quarter of that of
figure 2A. The output of figure 4 is a full wave center-tap configuration.
Alternately, it could be a full wave bridge configuration with approximately
half the number of secondary turns.
Full Bridge Push Pull Transformers
Compare figure 5 to figure 4. Figure 5 is a “full bridge” push pull / buck
boost transformer application. Like the half bridge configuration of figure 4,
this configuration eliminates the primary center-tap, reduces primary
winding capac-itance, & is about the same size. The two series connected
capacitors are replaced by two additional transistors as shown in Figure 4.
The voltage supplied to the input of the push pull transformer of figure 5 is
the same as that for figure 2A. For the same power rating and source
voltage, the push pull transformer of figure 5 requires half the primary turns
as that of figure 2A and it must handle the same amount of input current.
The primary winding of figure 5 may see a max current duty cycle near 1,
hence its wire may see 2 times the r.m.s current value as wire used in the
primary winding halves of Figure 2A. For the same output voltage, the
number of secondary turns is about the same as that of figure 2A, but the
secondary over primary turns ratio is doubled because the primary turns (fig.
5) are halved. The output of figure 5 is a full wave center-tap configuration.
Alternately, it could be a full wave bridge configuration with approximately
half the number of secondary turns.
The Boost Push Pull Transformer Application
The prior push pull transformer applications utilize an inductor in the output
circuit to reduce output voltage ripple. If there were more than one output,
an inductor would be used with each output. An alternate would be to place
one inductor in series with the primary center-tap of a push-pull center-tap
transformer. This circuit is illustrated in Figure 6. To charge the inductor the
two transistors are made to conduct at the same time. Charging current flow
through both halves of the primary winding but in opposite directions
resulting in magnetic cancellation of each other hence the transformer
windings act as a short to ground. Opening one of the transistor switches
results in current flow in only one of the primary winding halves. Alternate
opening of the transistor switches results in a push-pull transformer action.
Control circuitry is more complex.
Need More Technical Information?
A push pull transformers is a type of forward converter transformer. More
information about the theory of operation for forward converter transformers
is available under the category of switch mode (switching) transformers.
Click on the available link for switch mode power transformers. Much of the
theory for flyback transformers also applies to boost inductors Click on the
available link for flyback transformers. There are also links for other types
of transformers, inductors, chokes, etc.
Also, feel free to contact Butler Winding and ask for technical or
engineering assistance.
Pulse Transformers
The magnetic flux in a typical A.C. transformer core alternates between
positive and negative values. The magnetic flux in the typical pulse
transformer does not. The typical pulse transformer operates in an “unipolar”
mode ( flux density may meet but does not cross zero ).
A fixed D.C. current could be used to create a biasing D.C. magnetic field in
the transformer core, thereby forcing the field to cross over the zero line.
Pulse transformers usually (not always) operate at high frequency
necessitating use of low loss cores (usually ferrites). Figure 1A shows the
electrical schematic for a pulse transformer. Figure 1B shows an equivalent
high frequency circuit representation for a transformer which is applicable to
pulse transformers. The circuit treats parasitic elements, leakage inductances
and winding capacitance, as lumped circuit elements, but they are actually
distributed elements. Pulse transformers can be divided into two major
types, power and signal.
An example of a power pulse transformer application would be precise
control of a heating element from a fixed D.C. voltage source. The voltage
may be stepped up or down as needed by the pulse transformer’s turns ratio.
The power to the pulse transformer is turned on and off using a switch (or
switching device) at an operating frequency and a pulse duration that
delivers the required amount of power. Consequently, the temperature is also
controlled. The transformer provides electrical isolation between the input
and output. The transformers used in forward converter power supplies are
essentially power type pulse transformers. There exists high-power pulse
transformer designs that have exceeded 500 kilowatts of power capacity.
The design of “signal” type of pulse transformer focuses on the delivery of a
signal at the output. The transformer delivers a “pulse-like” signal or a series
of pulses. The turns ratio of the pulse transformer can be used to adjust
signal amplitude and provide impedance matching between the source and
load. Pulse transformers are often used in the transmittal of digital data and
in the gate drive circuitry of transistors, F.E.T.s, S.C.R.s, and etc. In the
latter application, the pulse transformers may be referred to as “gate
transformers” or “gate drive transformers”. Signal type of pulse transformers
handle relatively low levels of power. For digital data transmission,
transformers are designed to minimized signal distortion. The transformers
might be operated with a D.C. bias current. Many signal type pulse
transformers are also categorized as wideband transformers. Signal type
pulse transformers are frequently used in communication systems and digital
networks.
Pulse transformer designs vary widely in terms of power rating, inductance,
voltage level (low to high), operating frequency, size, impedance, bandwidth
(frequency response), packaging, winding capacitance, and other parameters.
Designers try to minimize parasitic elements such as leakage inductance and
winding capacitance by using winding configurations which optimize the
coupling between the windings.
Butler Winding can make (and has made) pulse transformers in a wide
variety of shapes and sizes. This includes; various standard types of “core
with bobbin” structures ( E, EP, EFD, PQ, POT, U and others ), toroids, and
some custom designs. Our upper limits are 40 pounds of weight and 2
kilowatts of power. We have experience with foil windings, litz wire
windings, and perfect layering. For toroids, we can ( and have done ) sector
winding, progressive winding, bank winding, and progressive bank winding.
Butler winding has a variety of winding machines, bobbin/tube and toroid.
That includes two programmable automated machines and a taping machine
for toroids. Butler winding has vacuum chamber(s) for vacuum
impregnation and can also encapsulate. To ensure quality, Butler Winding
purchased two programmable automated testing machines. Most of our
production is 100% tested on these machines. For more information on
Butler Winding’s capabilities, click on our “capabilities” link.
PULSE TRANSFORMER OPERATING PRINCIPLES
Pulse transformer designers usually seek to minimize voltage droop, rise
time, and pulse distortion. Droop is the decline of the output pulse voltage
over the duration of one pulse. It is cause by the magnetizing current
increasing during the time duration of the pulse. To understand how voltage
droop and pulse distortion occurs, one needs to understand the magnetizing (
exciting, or no-load ) current effects, load current effects, and the effects of
leakage inductance and winding capacitance. The designer also needs to
avoid core saturation and therefore needs to understand the voltage-time
product.
Magnetizing ( No-Load ) Current, its Effects, and Its Relation to
Saturation
Consider the simple pulse transformer circuit of Figure 2A and its equivalent
circuit of Figure 2B. There is no source impedance, winding capacitances, or
secondary leakage inductance to worry about. With both switches open,
there cannot be any primary or secondary currents flowing. Now close the
primary switch. Since the secondary load is not connected, the pulse
transformer’s primary winding acts like an inductor placed across a voltage
source. Primary current begins to flow. This is the magnetizing current ( no
secondary current ) and is governed by the differential equation V(t) = L x
d(I)/dt + Rp x I(t), with units of volts, henries, amps, and seconds. If the
power supply has constant voltage, Rp = zero, & L = Lkp+Lm is constant,
the differential equation can be solved for I(t), I(t) = Io + V x t / ( Lkp+Lm ),
where Io = the initial current which equals zero.
Notice that the current increases at a linear rate over time and that the rate in
inversely proportional to the inductance. The current flows through Np turns
creating Np x I(t) amount of magnetizing force ( amp-turns ) which in turns
creates a magnetic flux density in the pulse transformer core. Eventually the
increasing primary magnetizing current would exceed the magnetic flux
capacity of the pulse transformer core and will saturate the core. Once
saturation occurs the primary current rapidly increases towards infinity ( in
theory ). In a real circuit the primary winding resistance ( and source
impedance ) would limit the current. See Figure 3A for graphical illustration.
For non-zero Rp, I(t) = Io + ( V/Rp ) x ( 1 – e to the ( -Rp x t / ( Lkp + Lm ))
power ). The effect of Rp is graphically illustrated in Figures 3B and 3C. Rp
extends the time it takes for the unloaded transformer ( or an inductor ) to
saturate. If Rp is sufficiently large, it prevents the transformer ( or inductor )
from saturating altogether. Regardless of saturation, Rp places an upper limit
on the primary current value.
Voltage Droop
For Rp = 0 the source voltage divides proportional across Lkp and Lm hence
the voltage across Lm = V x Lm / ( Lm+Lkp ) = Vm. The induced secondary
voltage becomes equal to Ns x Vm / Np. For Rp > zero a voltage drop
occurs across Rp. The value of this drop increases in value as the primary
current increases with time, hence Vm decrease over time and consequently
the secondary voltage declines over time. Thus Rp and magnetizing current
contribute to secondary voltage droop. Lkp does not contribute to the droop
in the “no-load” case but does contribute to a lower secondary starting
voltage for both the “no load” and “under load” cases. Droop is graphically
illustrated in Figure 4B. Compare it against the ideal pulse shown in Figure
4A.
Voltage-time product
Pulse transformers, being typically unipolar (D.C.) applications, require the
primary switch to be opened ( thereby removing the voltage source ) before
saturation occurs, whereas A.C. applications reversed the applied voltage
before saturation occurs. Unipolar applications require that sufficient time be
allowed to pass to re-set the core before starting the next pulse. This time
permits the magnetic field to collapse ( reset ).
The field does not completely collapse to zero value ( unless forced to zero,
or lower ) because of core material remanence. A slight air gap may be used
to bring remanence closer to zero value. The gap lowers the pulse
transformer inductance. The flux range between remanence and the
maximum flux is referred to as dB, the maximum change in flux density
during the pulse duration, dt.
The dB of the typical pulse transformer is less than half for that of an A.C.
application because flux in A.C. applications can go from positive Bmax to
negative Bmax. Operating frequency and maximum expected temperature
affect the choice of maximum usable flux density value, Bmax. Saturation
can be avoided by applying the following equation; dB x Np x Ac x Sf = V x
dt x 100000000, where dt is the maximum time duration of the pulse, Ac is
the core’s cross-sectional area and Sf is the core stacking factor ratio. Units
are gausses, turns, square centimeters, volts and seconds. Be aware that dt
does not include reset time, tr. Maximum operating frequency equals 1 / ( dt
+ tr ). The voltage-time product, V x dt is quite useful. The size and cost of a
pulse transformer is roughly proportional to this product.
Kickback Voltage
In the foregoing discussion the primary switch was opened thereby
interrupting the current flowing through the transformer primary. The
resulting collapse in the magnetic field will induce a voltage reversal in the
transformer windings. The more rapid the field collapse is, the higher the
induced voltage. The transformer will try to dissipate the energy stored in its
collapsing magnetic field. If the transformer was under load, the induced
voltage would cause current to flow into the load. In the “no-load” case of
this example, the transformer does not have any readily available place to
dissipate the energy. The transformer will generate the voltage necessary to
dissipate the stored energy, hence a high voltage “kickback” ( or flyback or
backswing ) voltage will occur in the windings. In a real circuit the
transformer will induce eddy currents in its core thereby dissipating the
energy as core loss. In a real circuit the high voltages can damage the
switching elements ( transistors, F.E.T.s, S.C.R.s, etc. ). Many designs
include protective circuitry across the primary winding.
Secondary Load Current Effects and Rise Time
Consider again the simple pulse transformer circuit of Figure 2A and its
equivalent circuit of Figure 2B. Initally, with both switches open, there
cannot be any primary or secondary currents flowing. Close the secondary
load switch and then close the primary switch. Current flows through the
primary winding. The L x dI(t)/dt action induces a voltage in the primary
winding which opposes the source voltage. A voltage, Vsi, is also induced in
the secondary winding causing secondary current to flow. The ampere-turns
created by the secondary current work against the induced voltage that
opposes the source voltage.
Consequently, the source voltage supplies more current flow through the
primary. Currents rapidly increase until either the secondary current or
primary current encounters a current limitation. Examples of such limits are
the secondary load and winding resistances limiting the secondary current or
the source impedance and primary winding resistance and primary leakage
inductance limiting the primary current. Once a limit is encountered, an
equilibrium is quickly established except for the magnetizing current. The
primary current has two components; Irs, the load current transformed (
reflected ) to the primary winding and Im, the magnetizing current. As in the
“no-load” case, the magnetizing current starts at zero and increases over
time. The pulse transformer must be “switched off” before saturation occurs.
In this example the load is resistive, there is no secondary leakage
inductance, and there is no secondary winding capacitance; hence a purely
resistive load current is reflected to the primary winding. The primary
current is larger than it was in the “no-load” case, hence more voltage drop
is expected across the primary winding resistance. Consequently less
voltage, Vm, is available across Lm which results in less induced voltage in
the secondary winding. Secondary current flow through the secondary
winding resistance causes another voltage drop hence lower transformer
output voltage. Under load, both the primary and secondary winding
resistance contribute to a lower secondary voltage. The secondary winding
resistance does not contribute to pulse droop.
The reflected load current, Irs, does not flow thorughthe mutual inductance,
Lm, but doe flow through the primary leakage inductance, Lkp. Lkp restricts
the flow of the primary current ( hence reflected load current also ).
Consequently the reflected load current cannot immediately reach its full
value ( nor can the secondary current ). It is effectively delayed. Until the
reflected load current reaches its full value, a larger voltage drop will occur
across Lkp then there was in the “no-load” case. This larger voltage
diminishes in value over time. Consequently Vm exhibits a time delay in
reaching peak voltage value. This delay is also seen in the secondary output
voltage. This delay is known as rise time. Rise time is graphically illustrated
in Figure 4B.
Effects of Winding Capacitance, Secondary Leakage Inductance, and
Core loss
Now consider the equivalent pulse transformer circuit of Figure 5. The
circuit has all the components of the circuit in Figure 2B, but also has
primary winding capacitance, secondary winding capacitance, core loss, and
secondary leakage inductance. Start with both switches open and no
capacitive energy and no inductive energy. All currents are initially zero.
Close the secondary switch then close the primary switch. The primary
leakage inductance, Lkp, restricts the flow of primary current by opposing
the source voltage. The opposing voltage is generated by Lkp x d(I)/dt
action. Current flow ( from the source ) finds the uncharged winding
capacitance, Cp to be a much easier path, hence a relatively large amount of
current flows into the winding capacitance. This large amount of current
could be called a surge current because it will diminish over time as the
capacitance is charged. The surge causes a relatively large voltage drop
across the primary winding resistance, Rp, thereby initially lowering the
voltage available to Lkp and Lm. Over time, as the surge current diminishes,
the voltage drop across Rp diminishes, and the voltage across Lkp and Lm
reaches full ( peak ) value. The surge effectively delays the peak voltage
across Lm. This in turn delays peak secondary voltage. The delay
contributes to rise time, hence Cp contributes to rise time. As discussed
earlier, Lpk restricts flow of the reflected load current and consequently also
contributes to rise TIME
A similar consequence occurs with the secondary winding capacitance, Cs.
Any current supplied by induced secondary voltage must charge Cs as the
secondary voltage tries to rise to peak value. This delays the secondary in
reaching peak voltage, hence Cs also contributes to rise time.
Secondary leakage inductance, Lks, restricts secondary current flow just like
Lkp restricted primary current flow. Lks also delays the secondary peak
output voltage, hence it also contributes to rise time.
Core loss resistance, Rc, provides a relatively small current shunt path across
Lm just like the reflected secondary load current does. It has the same effect
but the effect is much smaller.
To summarize, Winding capacitances and leakage inductances act to
increase rise time. ( They also generate trailing edges which is discussed
later. ) They may also contribute to spurious oscillations. In a typical pulse
transformer design, core loss does not have much effect.
The Trailing Edge
For an ideal pulse transformer, once the primary switch is opened the
secondary pulse should immediately end. This does not happen. The pulse
transformer tries to dissipate the energy stored in Lm and in the parasitic
components Cp, Cs, Lkp, and Lks. The inductance will induce voltages as
their magnetic fields collapse. The capacitor charge will drain, but will not
drain instantaneously. The capacitances may temporarily supply current to
the inductances. As a result, there is a sloped decline of the secondary output
voltage after the primary switch is opened. This sloped decline is referred to
as the “trailing edge”. Some combinations of capactiance and inductance
could produce spurious oscillations ( known as ringing ). A trailing edge is
graphically illustrated in Figure 3B.
Pulse Distortion
Ideally the output pulse waveform should be identical in shape to the input
pulse waveform except for a desired amplitude change due to the “step-up”
or “step-down” turns ratio. Any other deviation is considered to be
distortion. Rise time, droop, trailing edges, and spurious oscillations are all
considered to be signal distortions.
Figure 3B illustrates all of these distortions.
Electronic Transformer - Trigger Transformers
There are many types of eletronic transformers. What distinguishes a trigger
transformer from other types of electronic transformers? Basically, it is
application! As the word “trigger” implies, a trigger transformer is used in a
circuit that initiates some sort of action or event. Once initiated, some
applications may no longer require continued presence of a voltage to
complete the action or event. Other applications may need the voltage but
for a limited amount of time. Regardless, the application provides a voltage
pulse to the trigger transformer’s primary.
The trigger transformer’s turns ratio steps up or steps down the secondary
voltage as needed. The trigger transformer’s secondary then supplies voltage
or current to its load. The load is usually the gate of a semiconductor switch
such as a transistor, F.E.T., S.C.R., etc.. The trigger transformer also
provides voltage isolation between the primary side circuit and the
secondary side circuit. Most circuit designers would refer to the trigger
transformer as a type of pulse transformer. This website provides some
explanation on pulse transformer operation. Click on the “Electronic
Transformers” button and then select “Pulse Transformer”.
One example of a trigger transformer application is the electronic flash in
modern cameras. A basic circuit is shown in Figure 1. A charging circuit
takes energy from a battery and charges two electrolytic capacitors ( approx.
300V ). The negative sides are both connected to ground. One capacitor is
much larger than the other is. It is connected to the electrodes of a glass tube
filled with xenon gas. This capacitor provides the energy needed to produce
the flash, but lacks sufficient voltage to initiate the flash. The primary of the
trigger transformer is attached to the positive side of the smaller capacitor
through a switch.
The trigger transformer secondary is connected to a metal plate(s) or grid(s)
that partially surrounds the glass tube. The trigger transformer is designed to
step up the voltage to high voltage levels. When the switch is closed the
trigger transformer places high voltage across the plates. The high voltage
ionizes the gas inside the tube. The gas becomes conductive. The large
capacitor discharges through the gas thereby producing a bright white flash.
The capacitor rapidly discharges its energy and must be recharged to
produce another flash.
The switch between the trigger transformer and the smaller capacitor is
opened. A small drain resistor is placed across the high voltage plates to
discharge the voltage on the plates. In this example the trigger transformer
aided the initiation ( or triggering ) of the flash by delivering a stepped up
voltage pulse. Figure 1 shows the trigger transformer windings grounded
together. With proper circuit design the trigger transformer could also
provide voltage isolation.
In the preceding example, the trigger transformer ( which is a pulse
electronic transformer ) design does not saturate the core and usually
employs unipolar core utilization. There are trigger transformer applications
that use bipolar core utilization and/or intentionally saturates the core.
Bipolar core utilization mean the magnetic flux alternates between positive
and negative directions. Unipolar means the flux direction remains either
positive or negative. Two examples of this are found in the “Royer Inverter
Circuit” and the closely related “Jensen Circuit”.
These are shown in Figure 2A and 2B. Operating theory will not be
discussed in detail here but is briefly summarized; transformer saturation
repeatedly occurs in alternating directions which in turn triggers ( switches )
the transistors on and off in alternating fashion, thereby creating an A.C.
output voltage. The switching of the transistors forces the current direction
to alternate which then forces the alternating direction of core saturation. For
more information about saturable transformers click on the “Electronic
Transformers” button, then select “Saturable Transformers”.
Figure 3 is a unipolar application which shows how a trigger transformer can
use core saturation can to shorten the time duration of a pulse. The trigger
transformer usually has a high impedance load ( lightly loaded ) hence it acts
much like a saturated inductor but with voltage step up or step down
capability and voltage isolation. The primary winding of the trigger
transformer has much higher impedance than the series resistor until
saturation occurs. Before saturation most of the circuit’s voltage drop is
across the trigger transformer’s primary. The trigger transformer’s turns
ratio can adjust the secondary output voltage. There will be voltage droop.
After saturation, most of the voltage drop is across the resistor, the
secondary output voltage is substantially reduced, and the time duration of
the output pulse has been reduced. The pulse’s time duration can be
calculated from the transformer’s volt-second product. This website provides
some explanation of the volt-second product. Click on the “Electronic
Transformers” button and then select “Pulse Transformer”.
Butler Winding can make ( and has made ) pulse and trigger transformers.
There are a wide variety of shapes and sizes available. This includes; various
standard types of “core with bobbin” structures ( E, EP, EFD, PQ, POT, U
and others ), toroids, and some custom designs. Our upper limits are 40
pounds of weight and 2 kilowatts of power. We have experience with foil
windings, litz wire windings, and perfect layering. For toroids, we can ( and
have done ) sector winding, progressive winding, bank winding, and
progressive bank winding. Butler winding has a variety of winding
machines, bobbin/tube and toroid. That includes two programmable
automated machines and a taping machine for toroids. Butler winding has
vacuum chamber(s) for vacuum impregnation and can also encapsulate. To
ensure quality, Butler Winding purchased two programmable automated
testing machines. Most of our production is 100% tested on these machines.
For more information on Butler Winding’s capabilities, click on our
“capabilities” link.
Gate Drive Transformers - Electronic Transformer
There are many types of transformers. What distinguishes a gate drive
transformer from other types of transformers? Basically, it is application!
Modern day electronic circuits utilize many gated semiconductor devices
such as ordinary transistors, field effect transistors, and S.C.R.s and others.
Gate drive transformers are used in some of these circuits. A signal must be
supplied to ( or removed from ) the device’s gate node to activate ( or
deactivate ) the device. When used, gate drive transformers are located
within the circuitry driving the gate. Gate drive transformers are used to
modify the voltage level to the gate, provide impedance matching, and to
provide voltage isolation. Gate drive transformer may be used to deliver
voltage to the grids or plates of a vacuum tube or flash tube.
Some gate drive transformers simply deliver a voltage pulse or a series of
voltage pulses to a semiconductor gate. A gate drive transformer functioning
in this manner could also be called a pulse transformer. Most circuit
designers would consider these gate drive transformers to be a type of pulse
transformer. If the gate drive transformer’s pulse initiates some action or
event, the gate drive transformer could be called a trigger transformer. Some
applications require a close reproduction of the pulse. The gate transformer
designer will seek to minimize winding capacitance and leakage inductance
because these parasitic components distort the signal. This website includes
information about trigger transformers and pulse transformers. The latter
includes information on the theory of operation. Click on the available links
if you want to view them.
Some amplifying circuits use a gate drive transformer to deliver a signal to a
semiconductor gate. Here the objective is to reproduce the signal, but with
increased power and increased voltage or current. The gate transformer
designer will seek to minimize winding capacitance and leakage inductance
because these parasitic components distort the signal. In most amplifying
circuits the signal is injected into a direct current biased transistor circuit,
hence the gate transformer may have to tolerate a D.C. current bias. Even
though these gate drive transformers drive a gate, circuit designers will
usually refer to them as signal transformers.
Gate drive transformers exist in a variety of shapes and sizes. There is also a
wide variety of core materials available for use with different applications. If
you need more information please contact Butler Winding and ask for
Engineering.
Butler Winding can make ( and has made ) gate drive transformers. There
are a wide variety of shapes and sizes available. This includes; various
standard types of “core with bobbin” structures ( E, EP, EFD, PQ, POT, U
and others ), toroids, and some custom designs. Our upper limits are 40
pounds of weight and 2 kilowatts of power. We have experience with foil
windings, litz wire windings, and perfect layering. For toroids, we can ( and
have done ) sector winding, progressive winding, bank winding, and
progressive bank winding. Butler winding has a variety of winding
machines, bobbin/tube and toroid. That includes two programmable
automated machines and a taping machine for toroids. Butler winding has
vacuum chamber(s) for vacuum impregnation and can also encapsulate. To
ensure quality, Butler Winding purchased two programmable automated
testing machines. Most of our production is 100% tested on these machines.
For more information on Butler Winding’s capabilities, click on our
“capabilities” link.
Current Transformers
What is the purpose of a current transformer? It measures alternating current
flowing through a conductor. Since it is used to measure current, a current
transformer is often classified as a type of instrument transformer. One could
measure the voltage drop across a known resistor. This is okay for low
current applications but is often impractical for high current applications.
The resistor consumes a lot of power (lowering efficiency) unless the
resistor is very low in value, in which case there may be very little voltage to
measure. The resistor could be excessively large.
The resistor’s heat may affect the resistor value, thereby reducing the
accuracy of the measurement. A current transformer can accurately measure
the alternating current and put out a reasonable voltage, which is
proportional to the current, but without as much heat and size that an
appropriate resistor would require. The current transformer can perform its
function with very little insertion loss into the conductor current being
measured. The current transformer also provides voltage isolation between
the conductor and the measuring circuitry. Proper function of a current
transformer requires use of a load resistor. The load resistor is often referred
to as a “burden resistor”.
The best core structure for a current transformer in terms of electrical
performance is a toroidal coil. Many toroidal current transformers have only
one winding. This winding is usually a “high turns” winding which
functions as the secondary winding. In application, the toroidal current
transformer is slipped over an end of a high current wire or buss bar, which
conducts the primary current. Said wire or buss bar constitutes a one turn
primary winding. Split core current transformers are designed so that they
can be assembled around a buss bar without disconnecting the buss bar. "C"-
cores and "U" core structures are commonly used for split-core current
transformers because they are relatively easy to take apart and put back
together around the buss bar. Historically, this has not been practical for
toroidal coils, but there are now some flexible toroids, which permit the
“split-core” feature of installing it around a buss bar. They have limited
application. Some printed circuit board applications will utilize bobbin
wound current transformers with two or more windings. One winding is an
integral part of the circuitry, while the other winding acts the secondary.
Butler Winding can make (and has made) current transformers in a wide
variety of shapes and sizes. This includes toroids, “U” and “C” cores for
split-core applications; various standard types of "core with bobbin"
structures (E, EP, EFD, PQ, POT, and others), and some custom designs.
Our upper limits are 40 pounds of weight and 2 kilowatts of power. We have
experience with foil windings, litz wire windings, and perfect layering. For
toroids, we can ( and have done ) sector winding, progressive winding, bank
winding, and progressive bank winding. Butler winding has a variety of
winding machines, bobbin/tube and toroid. That includes two programmable
automated machines and a taping machine for toroids. Butler winding has
vacuum chamber(s) for vacuum impregnation and can also encapsulate. To
ensure quality, Butler Winding purchased two programmable automated
testing machines. Most of our production is 100% tested on these machines.
For more information on Butler Winding’s capabilities, click on our
“capabilities” link.
Current Transformer Theory of Operation.
In the typical current transformer application, the primary winding consists
of one to a few turns of wire. The primary wire size is much larger than the
secondary wire size. The number of secondary winding turns is a selected
multiple of the primary turns. Figure 1 gives a circuit schematic of a current
transformer application. The current transformer shown represents an ideal
transformer. The ideal transformer has infinite no-load input impedance,
100% magnetic coupling between transformer windings ( hence no leakage
inductance), zero winding resistance, zero core losses, and no capacitance. (
Capacitance, leakage inductance, winding resistance, and core losses are
considered to be parasitic components. ) The output voltage is exactly
proportional to the primary voltage times the turns' ratio. There is no
regulation drop. There are no losses. Since there are no parasitic components
the ideal current transformer is 100% accurate. The conservation of energy
requires that the output power equals the input power, hence Vp x Ip must
equal Vs x Is. Since Vs = Vp x Ns / Np, it can be shown that Is = Ip x Np /
Ns. Is = Vs / RL, hence Ip = Ns x Vs / ( RL x Np ). With an ideal current
transformer there is no phase shift ( except 180 degrees depending on the
choice of output connections ).
The ideal transformer’s secondary resistive load consumes power equal to Is
x Is x RL. This same amount of power must be consumed at the primary
terminals. The secondary load RL can be replaced ( commonly referred to as
“reflected” ) with a resistor across the primary terminals, RLr. By applying
the conservation of energy, one can show that RLr equals Np x Np x RL /
(Ns x Ns), OR RLr equals RL times the turns ratio squared (where turns
ratio = Np / Ns). If Np / Ns is small, then the RLr is very small. The primary
voltage drop is Ip x RLr. A very small value for RLr means that the current
transformer presents a low insertion loss to the primary current and a low
primary voltage drop.
The reflected load impedance acts in parallel to the transformers own input
impedance. The ideal current transformer has infinite input impedance. This
infinite impedance would correlate to an infinite inductance inserted in
series into the path of the primary conductor. Without the load (or burden)
the current transformer acts like an inductor and would completely block the
primary current flow. Any constant value of alternating current would, in
theory, produce an infinite primary voltage drop. In reality the current
transformer’s input inductance (hence also impedance) cannot be infinity.
The current transformer has an inductance value which acts in parallel to the
reflected load. The core has losses that can be represented as a resistor in
parallel with the reflected load and the transformer’s self-inductance (no
load inductance). Without the load resistor the inductance and core loss will
place an upper limit on the primary voltage, but this voltage could still be
substantial. Core saturation is also a possibility. A turns ratio step-up would
result in even higher secondary voltage. Any circuitry beyond the secondary
load resistor could be subjected to high voltage, possibly resulting in circuit
damage. Because of this potential high voltage, the load resistor should
never be removed from the secondary when the current transformer is being
powered. Figure 2A shows an equivalent circuit schematic for a current
transformer with load RL. The ideal (induced) secondary voltage is now
denoted as Vsi and Vs now denotes the voltage at the secondary terminals.
Notice that the schematic contains the ideal current transformer and load as
before plus transformer mutual inductance Lm, secondary winding
resistance Rs, core loss resistor Rc, secondary leakage inductance Lks, and
primary leakage inductance Lkp. Just like for the load resistor, the other
secondary circuit components can be reflected to the primary side of the
transformer. This is illustrated in Figure 2C.
The parasitic components, Rs, Lkp, and Lks, all act to lower the output
voltage across RL, hence the output voltage, Vout, will not equal the
induced secondary voltage Vsi. Rs and Lks act in series with RL and are
reflected to the primary side along with Rs. Their presence presents added
impedance to the primary current hence an increase in primary voltage in
proportion to the impedance. Consequently, RL still has the same voltage
drop and current flow as it did without Lks and Rs even though Vs does not
equal Vout. The phase shift associated with Lks will cause some slight
deviation from the ideal current ratio (which equals the turns ratio).
The current transformer’s self (no-load) inductance Lm and the core loss Rc
shunt current away from the reflected load and reflected parasitic
components. Their impedances act in parallel to the reflected impedances,
consequently lowering the impedance seen by the primary current and the
resulting primary voltage. Less primary voltage means less output voltage
and less secondary current. Consequently Lm and Rc also cause deviation
from the ideal current ratio.
As long as Rc, Lm, Lkp, Lks, and Rs are constant in value, The actual
current ratio will be some fixed ratio times the ideal (or desired) current
ratio. One can compensate for the deviation from the desired current ratio by
appropriate choice of secondary turns. The number of turns will be a little
lower than that for the associated ideal turns ratio. For constant values
accuracy could be 100% except for any turn resolution limitations (full turns
versus fractional turns).
Accuracy concerns arise from non-constant values for Rc, Lm, and to a
lesser degree from Lkp and Lks. These values usually vary with core
induction levels; hence they vary over the range of primary current being
measured. (Air core transformers are stable but magnetic coupling is
relatively poor hence relatively large leakage inductances.) Since Rc and Lm
impedances act in parallel to the reflected load, higher Rc and Lm values
have a smaller effect and consequently increase accuracy. Cores materials
with high permeability and low core loss are preferred for high accuracy
applications.
At higher frequencies winding capacitance becomes a concern. Figure 3
gives an equivalent circuit schematic, which includes winding capacitance.
Leakage inductance and winding capacitance are actually distributed
components, but are shown as lumped approximate equivalent components.
Like Lm, winding capacitances shunt current around the reflected load. The
inductances and capacitances can interact and consequently may produce
spurious oscillations. It is also possible to develop “parallel resonance”.
High frequency coil designs seek to minimize winding capacitances.
If you need assistance with your current transformer design, please contact
Butler Winding and ask for Engineering.
Toroidal Current Transformers
Like other types of current transformers, the toroidal current transformer
measures alternating current flowing through a conductor. Since they are
used to measure current, current transformers are often classified as a type of
instrument transformer. One way of distinguishing types of current
transformers is by the type of cores used in their construction. The term
“toroidal” refers to the shape of the core that the winding of a toroidal
current transformer is wound on. The core is circular. Its cross-section may
be rectangular or round. The round cross-section gives better electrical
performance. The cores are often called “ring” cores. In contrast, the term
“split-core” in split-core current transformers is used because the
transformer core is split into two pieces which allow it to be assembled and
disassembled around a buss bar without disconnecting either end of the buss
bar. It is possible to make a split-core toroidal current transformer.
Historically, it has been impractical to do so, but there are now some flexible
toroids, which permit the “split-core” feature of installing it around a buss
bar. They have limited application.
Toroidal current transformers give better electrical performance than other
types of current transformers. Their shape minimizes the magnetic path
length, minimizes the winding turn length, produces less stray magnetic flux,
and optimizes magnetic coupling, and minimizes leakage inductance.
The toroidal current transformer is the most common way to measuring large
amounts of alternating (or even pulsing) current. It is preferred over the
measurement of the voltage drop across a known resistor and over split-core
transformers. The resistor is usually impractical for high current
applications. The toroidal current transformer can accurately measure the
alternating current and put out a reasonable voltage, which is proportional to
the current. The toroidal current transformer does so with very little insertion
loss, while an appropriate resistor would produce lots of heat and
consequently produce considerable insertion loss.
Like other current transformers, the toroidal current transformer also
provides voltage isolation between the conductor and the measuring
circuitry. Measurement over a resistor does not.
Proper function of the toroidal current transformer requires use of a load
resistor. The load resistor is often referred to as a “burden resistor”. Presence
of the load resistor enables a current transformer to perform its function with
little insertion loss. Without the load resistor the core could saturate and no
longer have the desired current ratio, or the no-load inductance could limit
primary current flow. Core materials with high permeability and low core
losses give better electrical performance. Further explanation and theory
about the operation of current transformers is given further below.
Current transformers, including the toroidal current transformer, may have
multiple windings. The typical toroidal current transformers have only one
winding. This winding is usually a “high turns” winding which functions as
the secondary winding. In application, the toroidal current transformer is
slipped over an end of a high current wire or buss bar, which conducts the
primary current. Said wire or buss bar constitutes a one turn primary
winding.
Butler Winding can make (and has made) toroidal current transformers in a
wide variety of sizes and in a variety of core materials. Our upper limits are
40 pounds of weight and 2 kilowatts of power. We have experience with foil
windings, litz wire windings, and some limited perfect layering. Butler
Winding can (and has done) sector winding, progressive winding, bank
winding, and progressive bank winding. Butler winding has a variety of
toroid winding machines. That includes toroid-taping machines. Butler
winding has vacuum chamber(s) for vacuum impregnation and can also
encapsulate. To ensure quality, Butler Winding purchased two
programmable automated testing machines. Most of our production is 100%
tested on these machines. For more information on Butler Winding’s
capabilities, click on our “capabilities” link.
Current Transformer Design Specifications
The designer must either determine or be supplied with the information
needed to design the current transformer. The needed information is listed
below along with a brief description if needed. Add any additional items
required by your particular application.
Describe Primary Current – State maximum current value and type of
measurement (r.m.s. average, peak, etc.), Give type of waveform (sine wave,
square wave, triangular, etc.). State either continuous current or describe the
applicable duty cycle.
Give Number of Primary Turns – This is the number of times the primary
conductor (buss bar) passes through the core window.
The Desired Current Ratio – This is simply the desired secondary current
value (at a specified value of primary current) divided by the primary current
value that generates said value of secondary current. Alternatively, a turns
ratio could be specified. but don’t expect the current ratio to exactly equal
the turns ratio.
Define the Output Burden ( Load Resistor ) – Specify the value and type
of the intended secondary load. The type of load is usually resistive ( a
resistor ), but could be inductive or capacitive ( which complicates things ).
Alternatively, the desired output voltage per unit of primary current can be
specified. The value of the load resistor can then be calculated.
Required Accuracy – This is usually expressed as either a maximum
percentage or maximum absolute change over the entire primary current
range. It includes both measurement tolerances and variations over the
operating range(s). It may be expressed over a portion of the operating range
or at specific operating points.
Minimum Inside Window Dimensions – This is the primary conductor (
buss bar ) dimensions plus any additional distance needed to clear any
obstacles encountered during installation of the current transformer..