Self Oscillating Dimmable Electronic Ballast Azlan Kamil … electronic ballast/12V... · Self Oscillating Dimmable Electronic Ballast ... broadly employed in recent lighting systems

Self Oscillating Dimmable Electronic Ballast

Azlan Kamil B Moharnrnad Fauzy

This Report Is Submitted In Partial Fulfillment Of Requirements For The Bachelor

Degree of Electronic Engineering (Industrial Electronic)

Fakulti Kejuruteraan Elektronik dan Kejuruteraan Komputer

Kolej Universiti Teknikal Kebangsaan Malaysia

March 2006

ABSTRACT

The lighting system provides many opportunities for cost-effective energy saving

without any sacrifice. The system is now part of the Energy Conservation program over

the world and reduction of energy consumption by implementing energy conservation

schemes is needed. Incandescent lamps convert just five per cent of energy into light and

the remainder into heat where as fluorescent lamps turn 25 per cent of energy into light.

As the rapid development of power electronics technology, inverters are now used

for energizjng the lamp. These inverter systems are now referred as the electronic ballast

and can eliminate the conventional magnetic ballast's disadvantages. A simple

explanation for the system is the main input is rectified to a DC voltage which is then

inverted into a high frequency AC voltage to drive the fluorescent lamp. Electronic

ballasts can also be provided with a dimming capability. Dimming controls have been

broadly employed in recent lighting systems to provide energy savings and improved

economic.

The self oscillating dimmable electronic ballast that will construct in this project

is hope will be useful not only in energy saving but also low cost and high reliability.

ABSTRAK

Sistem pencahayaan memberi banyak peluang bagi penjimatan tenaga secara kos

efektif tmpa sebarang pengorbanan. Sistem ini sekarang merupakan sebahagian daripada

program pemeliharaan tenaga (Energy Conservation) di seluruh dunia dan pengurangan

penggunaan tenaga dengan rnelaksanakan skim pengekalan yang diperlukan.

Dengan pembangunan yang pesat di dalam teknologi elektronik kuasa, pengubah

(inverter) dapat digunakan untuk penyalaan lampu. Sistem pengubah ini di kenali sebagai

ballast elektronik dan ia boleh menghilangkan kelemahan ballast magnetic yang masih di

guna pakai sekarang. Penerangan yang ringkas menegnai system ini ialah masukan utama

di tukar kepada voltan arus terus yang mana kemudiannya di ubah kepada voltan arus

ulang alik yang berfrekuensi tinggi untuk memacu larnpu fluorescent. Ballast elektronik

juga dapat disertakan dengan fungsi pemalap kawalan malapan telah banyak digunakan

di dalam system pencahayaan yang ada sekarang bertujuan untuk menjimatkan tenaga

dan meningkatkan ekonomi.

Litar yang akan dibina didalm projek ini di harap agar dapat digunakan bukan

sahaja bagi menjimatkan tenaga tetapi juga murah dan berkeupayaan tinggi.

CHAPTER I

INTRODUCTION

Ballasts are electrical devices that convert line current into the proper voltage,

amperage, and waveform to operate fluorescent lamps. Over the past 10 years, ballast

have been develops toward more efficient equipment. Electronic ballasts are the best

choice in most applications today, either as replacements for magnetic ballasts in

existing fixtures or in new installations.

To drive a fluorescent lamp, the electronic ballast, with respect to the magnetic

ballast, presents the following features: reduced ballast loss (higher efficiency) and

weight, facility on lamp power control, more eficient tube ignition, no flickering and

operating conditions improving lamp life.

This project involves voltage fed series half bridge converters. This topology is

operating in Zero Voltage Switching (ZVS) resonant mode which is reducing the

switching losses. The project are based on L6569 device that are able to directly

control a symmetric half bridge inverter of a fluorescent lamp ballast.

1.1 Project Objective

The objective of this final project is to gain knowledge in developing

electronic ballast. By study the prospect and ability of electronic ballast, a simple and

low cost electronic ballast circuit will be constructed. The circuit will be able to

operate in selected frequency range for an 18 watt lamp and possibility for dimming

are been study.

1.2 Project Scope

The scope for this project is to studies the electronic ballast function and then

planning for the required specification of the electronic ballast. By using software, a

development of the circuit by simulation is used to convince the theoretical of

electronic ballast. Beside in scope of work for this project followed by:-

1. Studies and development of the PCB design.

2. Studiesllearn the appropriate testing and troubleshooting technique.

CHAPTER I1

LITERATURE REVIEW

The first fluorescent lamp was patented over 100 years ago by American

inventor, Peter Cooper Hewitt. Cooper Hewitt's low pressure mercury arc lamp is the

direct parent for the generation of today modem fluorescent lamp. Fluorescent lamps

are far more efficient than incandescent lamps, fluorescent lamp use electricity to

excite molecules of Argon or krypton together with mercury vapor to create

luminescence. Unlike incandescent lamps, a fluorescent lamp cannot be connected

directly to electric lines. Fluorescent lamp required ballasts to stabilize the flow of

current or else they quickly become inoperable. Ballast provides the starting voltage

for a fluorescent lamp and limits the current passing through it. The ballast also

provides proper electrode or filament heating.

2.2 FLUORESCENT LAMP

A fluorescent tube is a low pressure mercury vapors discharge lamp containing

an inert gas that consisting of argon or krypton at low pressure (below 1 atmosphere)

plus a small measured dose of mercury. There is a filament at each end which when

hot, emits electrons to sustain the discharge when the lamp is operating. The mercury

vapors discharge produces ultraviolet light which is converted to visible light by the

phosphors coating the inside of the glass tube. The glass blocks the exit of the

ultraviolet radiation but allows the visible radiation through.

A non-operating fluorescent tube will appear as an open circuit, since there is

no electrical connection from one end to the other. To "strike the arc", a high voltage

must be applied across the lamp which will ionizes the gas and this will instantly "cold

start" the lamp and shorten its life by sputtering electronemitting material from its

cathodes. However, if the cathodes are first preheated to generate a space charge of

electrons at each end of the lamp, the strike voltage is considerably reduced and lamp

life will not be unduly compromised by the start-up. As soon as the discharge current

flows, the lamp's electrical impedance will drop. It now becomes as negative

impedance, where an increase in current is accompanied by a reduction in lamp

voltage. Therefore they will be a current-limiting device in series with the lamp which

compensates with a positive impedance characteristic to prevent current runaway and

rapid destruction of the lamp.

low pressure argon or crypton fjllmg

chatode

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Figure 2.1: Fluorescent Lamp

2.3 BALLAST

Ballasts are electrical devices that convert line current into the proper voltage,

amperage, and waveform to operate fluorescent lamps. Electrical distribution systems

deliver fixed AC voltage (50 or 60 Hz) and expect connected electrical loads to limit

the current drawn from the source. Ballasts provide system stability by limiting the

curre. .L that can be drawn. Ballasts use inductive and capacitive components because

they impede alternating current with little power consumption. Resistive components

generate high loss and are usually avoided. The mix of ballasts has been shifting

steadily toward more efficient equipment over the past 15 years. Magnetic ballast and

electronic ballast are the common ballast that is being use now days.

23.1 MAGNETIC BALLAST

The requirements of fluorescent lamp ballast are to:

(a) Preheat the cathodes to induce electron emission,

(b) Provide the starting voltage to initiate the discharge,

(c) Limit the running current to the correct value.

There are several types of mains frequency "magnetic" ballast available. By far

the most common circuit for 230V mains supplies has traditionally been the switch

start ballast, where lamp ballasting is provided by the choke. Other circuits include, in

order of popularity, the semi-resonant circuit and the quick start circuit.

The switch start circuit has been widely adopted because of its simplicity, low

cost and improved efficiency when compared with the alternative options mentioned

above. Another reason is that the 230V mains voltage is sufficiently higher than the

tube running voltage to allow the use of the simple series impedance ballast in almost

all cases. . CHOKE

STARTER

Fignre 23: A fluorescent tube.

When the voltage is applied to the circuit, the lamp does not operate at first, so

the full mains voltage appears across the starter via the choke and lamp cathodes. The

starter consists of bi metallic contacts sealed within a small discharge bulb with an

inert gas filling such as argon or neon. The mains voltage causes a glow discharge

within the starter which heats up the bi metallic contacts, causing them to close. This

completes the circuit and allows preheat current to flow through the choke and both

cathodes. Since the glow discharge within the starter has now ceased, the bi metallic

contacts cool down and open. Because the inductance of the choke tries to maintain

current flow, the voltage across the lamp rises rapidly and strikes the lamp. If it does

n ~ t , the starter's contacts close again and the cycle repeats. Once the lamp has started,

the choke controls its current and voltage to the correct levels. The lamp running

current is enough to keep the cathodes (heaters) hot and emitting electrons without the

need for separate heater supplies, which would otherwise be wastehl of energy. Since

the lamp's running voltage is much lower than the &air& voltage, there is now not

enough voltage to cause a glow discharge in the starter, so it remains open circuit.

2.3.2 ELECTRONIC BALLAST

Electronic ballasts have been available since at least the beginning of the

1980's. Improvements in ballast performance and ever-increasing energy costs have

resulted in an upsurge in electronic ballast use since the beginning of the 1990's.

Replacing the most efficient low loss mains frequency switch start ballast with

electronic ballast leads to reduced energy consumption and improved perfo..nance.

The advantages of electronic ballast are:

1. Increased light output

2. Flicker eliminated

3. Audible noise eliminated

4. Lower ballast power

5. Extended lamp life

6. Versatile lamp control

7. Compact and light weight

23.2.1 INCREASED LIGHT OUTPUT

If the cperating frequency is increased from 50Hz to above the audible limit of

20 kHz, fluorescent lamps can produce around 10% more light for the same input

power. Alternatively, the input power can be reduced for the same light output.

Figure 23: Typical fluorescent lamp eflicacy

2.3.2.2 FLICKER ELIMINATED

A fluorescent lamp operating at 50160Hz will extinguish twice every cycle as

the mains sine wave passes through zero. This produces 1001120Hz flicker which is

noticeable or irritating to some people. If the lamp is operated at high frequency,

however, its light output is continuous. This is because the time constant, hence the

response time of the discharge is too slow for the lamp to have a chance to extinguish

during each cycle.

The output waveform of magnetic ballast will usually be slightly modulated by

1 001 120Hz "ripple". Figure 2.4 shows the measured voltage and current waveforms of

a lamp operating at 60 Hz. After every line zero crossing, the lamp voltage waveform

has a restrike voltage peak, during the rest of the cycle, the voltage does not vary

much. This causes two big ~roblems: The lamp electrode wearing is significant, and

the lamp's output light is highly susceptible to the line voltage, which results in an

annoying visible flickering [I].

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mechanical vibrations in its laminated steel core and, possibly, its coil as well. This

can excite vibrations in the steel body of the lighting fixture and the surface to which . it is fixed, which amplifies the original noise even further.

23.2.4 LOWER BALLAST iOWER

Electronic ballast will consume less power and therefore dissipate less heat

than mains frequency magnetic ballast. These power =ductions are because:

a) At high frequency, the' lamp can be run at a lower power for the same light output.

b) The power loss in electronic ballast is much lower than the power loss in mains

frequency magnetic ballasts.

2.3.2.5 EXTENDED LAMP LIFE

An electronic ballasts which "soft starts" the lamp will not sputter away the

electronemitting material from the cathodes during starting. This will give longer

lamp life when compared to the uncontrolled impulses to which the lamp is subjected

in a switch start circuit.

23.2.6 VERSETILE LAMP CONTROL

Electronic ballasts are available which permit lamp dimming. This gives

substantial energy savings in situations where the lights are linked to an automatic

control system which detects ambient light levels and adjusts lamp output to maintain

a constant level of illumination. Lights may also be programmed to dim during

intervals when ,eas are not in use, for example during lunch breaks.

Electronic ballasts can incorporate feedback to detect the operating conditions

of the lamp(s) so that failed lamps can be switched off to avoid annoying flicker and

possible ballast damage. They can also incorporate regulation, whereby a constant

light output is maintained over a range of input voltages.

23.2.7 COMPACT AND LIGHT WEIGHT

Owing to the high frequency of operation, the magnetic components in

electronic ballast are compact and lightweight with cores of ferrite material, whereas

at mains frequency the ballast choke must be larger and heavier with bulkier copper

windings and a core of laminated steel. The shape and geometry of a mains tiequency

choke is determined by magnetic efficiency requirements, whereas the circuitry within

electronic ballast can be arranged to produce a very slim final pacpcge. This permits

new levels of slimness and compactness for the final ballast and the lighting fixture.

few cycles, the voltage on capacitor Cres reaches enough value to strike the tube.

After that, the current will flow between the cathodes and the lamp can be considered

like a resistor (see figure 2.7) 151.

Figure 2.7: Simplified Schematic after lamp ignition

Using cool ignition, high current values (3, 4 times the nominal value) and

voltage values are present in the lamp for a short time, consequently the tube life is

reduced [5].

2.4.2 WARM IGNITION

To provide long life and to insure an efficient ignition of the lamps the

cathodes must be preheated. The preheating of the filaments allows an easy strike of

the lamp thanks to the ignition voltage reduction. During preheating time the tube

presents high impedance so the current flows through the filaments. Its resistance

value is strictly dependent on the lamp model. A simple rule to determine the right

preheating currentltime value is the following: the ratio between the cathode

resistance before and after the preheating has to be in the range of 3-5.Thei-e are two 5

methods to obtain the cathodes preheating:

Voltage mode heating;

Current mode heating.

2.4.2.1 VOLTAGE MODE HEAITVG

This function is achieved by heating the lamp cathodes by means of two

auxiliary windings. These ones are magnetically coupled with the main inductance as

shown in figure 2.8.

I , . I.\-5 1_ ti. 1 '"<'

Figure 2.8: Voltage Mode Heating lamp connection

During the preheating phase, the lamp can be considered an open circuit, the

current flows through L'res and Cres. The voltage across L'res is transferred to the

secondary windings L"res, generating a current heating the cathodes. The primary

current value is related to the half bridge working frequency, so the preheating

fkquency is chosen according to the tube specs (rms currentltime) [ 5 ] .

4

The capacitor Cres must be chosen considering that, during the preheating, the

voltage across it must be lower than the ignition voltage. At the end of the preheating

During the preheating phase the lamp is an open circuit and the current flows

through Lres and Cres heating the lamp cathodes. The current in this phase depends

on the half bridge working fkquency. At the end of the preheating time the ICs

perform the lamp ignition sequence reducing the frequency towards the resonant

frequency fixed by Cres and Lres. In this way the c m n t and the voltage on Cres

increase causing the strike of the lamp. Once the lamp is successfblly ignited, the ICs

determine the steady state frequency for a given power lamp [5] .

2.43.2.2 CURRENT MODE HEATING I N SELF OSCILLATING

BALLAST

When discrete components (i.e. BJT MOSFET IGBT etc.) are used to make

the DC/AC converter (half bridge), external circuits are necessary to perform all

control hnctions of the power switches. One method to obtain the preheating cathodes

is to adopt a PTC resistor. One method to obtain the preheating cathodes is to adopt a

PTC resistor. Figure 2.10 shows its typical connection [5].

Figure 2.10: Preheating circuit using PTC

Referring to Figure 2.10 the following relationship between the capacitors has to be

respected: . a

Cres>C'res>C"res (1)

In this case the current flows through the series formed by Cures and C'res.

According to (I) the equivalent capacitance (C9'res series C'res) across the lamp . becomes lower than the initial value (C'res) increasing the capacitive reactance and

allowing the tube ignition. After the ignition, C"res C'res and WC can be considered

a high impedance in parallel to the tube, thus its contribution can be neglected [S ] .

2.5 POWER FACTOR (PF)

Power Factor (PF) is the measurement of how effectively ballast converts the

voltage and current supplied by the power source into watts of usable power

delivered to the ballast and lamps. Perfect power utilization would result in a power

factor of one.

PF = Inwt Watts

Input Current x Input Voltage

Ballast's power factor may be classified under any one of the following

categories:

High Power Factor (HPF) - 0.90 or greater

Power Factor Corrected (PFC) - 0.80 to 0.89

Normal (Low) Power Factor (NPF) - 0.79 or less

Power factor measurements pertain only to the effective use of power

supplied to the ballast. They are not an indication of ';he +allasts ability to supply

light through the lamps. Because low power factor ballasts required about twice the

current needed by high power factor ballasts, they allowed fewer fixtures per circuit

and added wiring costs. High power factor ballast is generally specified for all

commercial lighting applications.

I I Main Power

Factor Correction

Figure 2.13: System Block Diagram

2.6 MAJOR ELECTRONIC BALLAST TOPOLOGIES @C/AC

INVERTER STAGE)

Electronic ballasts are expected to perform the following functions: supply

proper starting and operating voltage for the lamp; maintain a running current at the

designed value with a low CF; regulate the lamp current output against supply voltage

variations; and have a high overall efficiency. To obtain extra energy savings andlor

make intelligent lighting, the controllable light output or dimming feature is expected.

In addition, low cost and high reliability are very important considerations [I].

From a historical perspective, electronic ballasts originated from the solid-state

radio frequency (RF) power amplifiers (PAS). RF PAS are usually identified by their

classes of operat&n, that is, Classes A, B, C, D, E, F, G, H and S. Based on how the

transistor is biased and driven, all these classes of PAS are placed in the following

three categories:

I. linear-mode PAS

2. switching-mode PAS

3. mixed-mode PAS

2.6.1 LINEAR-MODE PAS

In the linear-mode PAS, the power transistors act as a high-resistance current

source to produce a magnified replica of the input signal voltage or current wave.

Because of the high voltage and current product (power dissipated) inherent in power

transistors, this category of Pas usually has low efficiency [I].

2.6.2 SWITCHING-MODE PAS

In switching-mode PAS, the power transistors operate as a switch, alternately

opencircuited and shortcircuited. Ideally, a switch has either zero voltage across it or

zero current through it at all times (i.e., zero resistance when on, infinite resistance

when off, no associated parasitic capacitance or inductance, and zero transition times).

Therefore, this category of PAS can theoretically achieve eficiencies of 100% [I.]. fi

2.63 MIXED-MODE PAS

In mixed-mode PAS, the power transistors basically act as a current source, but

partially also as a low-resistance "on" switch. Compared with linear-mode PAS, this

category of PAS can improve efficiency due to operating the power switch into

saturation. Obviously, the biggest achievements of switching-mode PAS are their high

efficiency, low power dissipation, high reliability, small ,ize and low cost [I].

Classes D, E and S usually comprise switching-mode PAS in RF engineering.

Essentially, Class S PAS are wideband PWM DCIDC converters with low-pass filters

to allow only a slowly varying DC or average voltage component to appear on the

load. The desired output signal is obtained by controlling the pulse width of the input

signal, and for this reason, it requires PWM control. In power electronics, Class S PAS

are appropriated for applications in variable-speed AC motor drives and

unintermptible power supplies (UPSs), which use batteries to provide standby AC

power. Classes D and E PAs, on the contrary, are essentially resonant power

converters with high-fkquency AC output voltage and current. This is most favored in

electronic ballast applications in which a sinusoidal current source is needed.

Actually, Class D and Class E electronic ballasts represent two major categories in

today's electronic ballast market [I] .

Class D PAS, employ a pair of active switches and a tuned network. The

switches are driven to act as a two-pole switch that defines either a rectangular voltage

or rectangular current waveforms. The output network is tuned to the switching

frequency and removes its harmonics, resulting in a sinusoidal output. This

characteristic of Class D PAS easily finds its application in electronic ballasts. With its

sinusoidal current drive, the lamp efficacy is highest and the EM1 is smallest.

Additionally, the tuned network also functions as an impedance match to the lamp.

The tuned network is probably the most cost-effective way to generate AC curreni as

well as allowing impedance matching.

4

Figure 2.17: voltage-fed balf-bridge series-resonant parallel-loaded ballast.

In the Class D voltage-fed push-pull (VFPP) electronic ballasts (Figure 2.14),

a square-wave voltage is fed into a series-resonant or series-resonantderived network,

thereby producing sinusoidal resonant current, as opposed to the CFPP electronic

ballasts in which a square-wave current is fed into a parallel-resonant or parallel-

resonantderived network, thereby producing sinusoidal resonant voltage such that the

voltage and current waveforms are interchanged.

Class D current-fed half-bridge (CFHB) and voltage-fed half-bridge (VFHB)

electronic ballasts are shown in Figures 2.15 and 2.16, respectively. Although the

circuit topologies are different, the circuit operations are essentially the same as those

of their push-pull counterparts. In contrast, Class D CFPP electronic ballasts have the

best performances. Due to the parallel resonant configuration with current feed, they

can operate indefinitely under virtually any load condition, including short, open, and

the most severe situation in which a lamp is socketed into a powered fixture. Another

important advantage is that they can achieve parallel lamp operation simply by adding

a single ballast capacitor per additional lamp, and part of the lamps' replacements

might occur while the rest of the lamps are in normal operation. But the downside is

that they have the largest component count. The required large inductor and step-up

transformer make the circuit bulky and lossy. High voltage stress on the two switches

also makes them less attractive, especially when operated after a PFC pre-regulator

[I]. . n

The Class D VFHB electronic ballasts, on the contrary, are simple circuits

requiring fcw components. When they operate at the undamped natural frequency,

they show curreni source characteristics such that no step up transformer is necessary.

If isolation is needed, a smaller transformer can be used because the secondary

winding voltage is equal to the lamp voltage, unlike the Class D CFPP electronic

ballasts in which separate ballast impedance is necessary to absorb the voltage

difference between the winding voltage (striking voltage) and the lamp's normal

operating . Atage. Another attractive feature is that the voltage stress of the two

switches is equal to the bus voltage, which allows low-voltage-rating MOSFETs to be

used [I].

2.7 PFC TECHNIQUES IN ELECTRONIC BALLAST

There are generally two methods for correcting the PF and suppressing

harmonic distortion: the passive PFC approach and the active PFC approach. Passive

PFC refers to using only 1 ine-frequency reactive components plus uncontrolled

rectifiers, while the active PFC uses active devices and high- requency reactive

components as well as passive switches such as diodes. The advantages of the passive

PFC approach are its simplicity, reliability, robustness, lack of EM1 generated, and

low cost. However, the physical size and weight of the line frequency components

renders the passive approach very unattractive. The active approach, on the other

hand, not only easily meets the specifications, but also significantly reduces the circuit

size and weight. The major issues are circuit complexity, reliability and cost [I].