Semiconductor Power Switching Devices-1 (Lecture-4) R S Ananda Murthy Associate Professor and Head Department of Electrical & Electronics Engineering, Sri Jayachamarajendra College of Engineering, Mysore 570 006 R S Ananda Murthy Semiconductor Power Switching Devices-1
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Semiconductor Power Switching Devices-1(Lecture-4)
R S Ananda Murthy
Associate Professor and HeadDepartment of Electrical & Electronics Engineering,
Sri Jayachamarajendra College of Engineering,Mysore 570 006
R S Ananda Murthy Semiconductor Power Switching Devices-1
Static i-v Characteristics of Ideal Switch
+ -
(a)
+ -(b)
ON
OFF
FullyControlled
ON
OFF
FullyControlled (c)
We use power semiconductor devices as switches inconverters.
R S Ananda Murthy Semiconductor Power Switching Devices-1
Properties of Ideal Switch
When switch is OFF, i = 0 and −∞≤ v ≤+∞ which impliesthat PON = 0When switch is ON, v = 0 and −∞≤ i ≤+∞ which impliesthat POFF = 0It should be possible to easily turn the switch ON and OFF
by applying an appropriate control signal.The power terminals of the switch should be electricallyisolated from the control terminals.
R S Ananda Murthy Semiconductor Power Switching Devices-1
Properties of Ideal Switch
The power required to keep the switch in a particular state,or to switch it ON/OFF should be infinitesimally small.Should be able to change state instantaneously whichimplies that tON = 0, tOFF = 0 and PSW = 0.Should be able to withstand infinite temperature whichmeans that its power handling capability is infinite, i.e.,PHmax → ∞.Should be able to withstand infinite value of di/dt duringturn ON and infinite value of dv/dt during turn OFF.
R S Ananda Murthy Semiconductor Power Switching Devices-1
Static i-v Characteristics of Practical Switch
−V1
+V2
Limit
Power
Limit
Power
III
II I
IV
v
i
+ Ito
toOFF State close
v − axisi − axis
ON State close
− I
The switch is assumed to be bilateral while drawing this.
R S Ananda Murthy Semiconductor Power Switching Devices-1
Classification of Power Semiconductor Devices
Bipolar voltage blocking
Pulse triggered (latched)
Continuously triggered
Unilateral
Bilateral
Semi−controlled Fully−controlledUncontrolled
Unidirectional
Power Semicondutor Switching Devices
Unipolar voltage blocking
R S Ananda Murthy Semiconductor Power Switching Devices-1
Classification of Power Semiconductor Devices
Device Control Input Controllability Conduction Blocking
SITH – Static Induction Thyristor; MCT – MOS Controlled Thyristor.
R S Ananda Murthy Semiconductor Power Switching Devices-1
Power Losses in a Switch
t
p = vi
t
p = vi
i
Turn−on time Turn−off time
v
i
v
The total average power loss over a time interval T is
PD = PON +POFF +PG +PSW ≈ PON +PSW =1T
ˆ T
0pdt
as POFF +PG is negligible as compared to PON +PSW .
R S Ananda Murthy Semiconductor Power Switching Devices-1
Static Characteristics Power Diodes
Reverse
leakage
current
VBR
VF( )
Forward
conduction
drop
A K
Circuit symbol
+ −
i
v
A K
Structure
p i n
i
v
i
v
Linear Approximation
i
v
Ideal Characteristics
Avalanche
breakdown
VF = Vγ + IF RF where RF is the ON-state bulk resistance.
R S Ananda Murthy Semiconductor Power Switching Devices-1
Reverse Recovery Characteristics of Power Diodes
IR
t1 t2 t3
VF
IF
QRR
IRM
t 3
t 2SF =
VR
IRMdtdiQ
RR
QRR2
di/dttrr =
dtdi
< 0
Diode with snubber
Turn OFFstarts here
turn OFFends here
v
t
t
Area =
RMV - E
0.25 IRM
= 2
0
Reverse Recovery Charge
Reverse Recovery Timeis the time taken by theminority carriers in the diodeto recombine when the diodeis reverse biased
R S Ananda Murthy Semiconductor Power Switching Devices-1
Important Specifications of Power Diodes
Reverse Blocking Voltage (VRRM).Forward Average Current (IF (AVE)).Forward RMS Current (IF (RMS)).Surge Current Rating (IFSM).Maximum On-state Voltage Drop (VFM).I2t Rating.Reverse Recovery Time (trr ).The maximum allowable junction and case temperatures(θJM and θCM ).Junction-to-case and case-to-sink thermal resistances(RθJC and RθCS).
R S Ananda Murthy Semiconductor Power Switching Devices-1
Types of Power Diodes
1 Standard Recovery Diodes2 Fast Recovery Diodes3 Schottky Diodes4 Silicon Carbide Diodes.
R S Ananda Murthy Semiconductor Power Switching Devices-1
Standard Recovery Diodes
trr ≈ 25 µs.Leakage current in the OFF-state is of the order of few µA.Have lower ON-state voltage drop.Available with ratings of several kV and kA.Typically used in rectifiers at power frequencies i.e., at 50Hz or 60 Hz.
R S Ananda Murthy Semiconductor Power Switching Devices-1
Fast Recovery Diodes
trr ≤ 5 µsHave relatively higher ON-state voltage drop.Typically used in D.C.-to-D.C. converters and invertersoperating at higher frequencies as freewheeling diodes.
R S Ananda Murthy Semiconductor Power Switching Devices-1
Schottky Diodes
A K
Barrier Metal
n−type silicon layer
Heavily dopedsilicon substrate
A K
Barrier Metal
n−type silicon layer
Heavily dopedsilicon substrate
n+
trr is typically around few ns.Have very low ON-state voltage drop of the order of0.15-0.45 V and consequently very low PON .Typically available in voltage ratings in the range 50-200 V.Used in circuits having very low output voltages likeswitched mode power supplies.
R S Ananda Murthy Semiconductor Power Switching Devices-1
Silicon Carbide Diodes
These are Schottky diodes constructed from siliconcarbide.Have very low power loss.Have extremely fast switching behavior with ultra-low trr .Can operate at junction temperatures > 225◦ C.Switching behavior is independent of ON-state forwardcurrent and temperature.Have much lower reverse leakage current than siliconSchottky diodes, and higher reverse voltage rating.Very expensive as compared to Si Schottky diodes.
R S Ananda Murthy Semiconductor Power Switching Devices-1
Some Diode Packages
Axial Pack Plastic Pack Plastic Pack
Stud Type Stud Type Disc Type
Source: www.irf.com
R S Ananda Murthy Semiconductor Power Switching Devices-1