Electronic Transformer For A 12V Halogen Lamp CD00003902

User Manual: Halogen Lamp

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APPLICATION NOTE

1/4AN528/0999

ELECTRONIC TRANSFORMER FOR A 12V HALOGEN LAMP

by P. Fichera, R. Scollo

1. INTRODUCTION

Lighting that uses halogen lamps is commonly found

in residential environments, where it offers a warm

light in comparison with that generated by fluorescent

tubes, and greater efficiency than conventional

filament bulbs.

These lamps are available with voltage ratings of 6,

12 or 24 Volts, and so a transformer is needed in

order to provide the lamp with a low voltage supply

from either 110V a.c. or 220V a.c. mains. They are

generally available with power ratings of 50, 75, 100

and 250 Watts.

The “electronic” transformer represents an alternative

means of power conversion to the more standard

iron core, 50Hz transformer technique.

The advantages of the electronic transformer

compared with the classical solution are:

· The output power from the electronic transformer

to the lamp can be varied, thus dimming control

can be added;

· It is possible to include protection against short

circuit of the lamp filament;

· Weight can be reduced and the construction made

more compact; and

· Acoustic noise (mains hum) is eliminated.

2. THE ELECTRONIC SOLUTION

The topology of the circuit is the classic half-bridge.

The control circuit could be realised using an IC (so

fixing the operating frequency), but there is a more

economical solution which consists of a self-

oscillating circuit where the two transistors are driven

in opposing phase by feedback from the output circuit.

3. CIRCUIT DESCRIPTION

The line voltage is rectified by the full-bridge rectifier,

generating a semi-sinusoidal voltage at double the

line frequency. The diac starts to conduct during

each cycle, starting the half-bridge oscillation. The

turn-on of the diac can be varied by modifying the

time constant of the RC network formed by R and

T'1

T''1

TR1

TR2

Figure 1. Electronic transformer for 12V Halogen Lamp

APPLICATION NOTE

2/4

C1. This can be varied to provide a lamp dimming

feature.

Once the cycle has started, the diode D maintains

the voltage across C1 at a value less than the diac

conduction threshold voltage, so the transistor can

switch off.

The frequency of oscillation depends mainly upon

the size and maximum flux density of the core used

in the feedback transformer, and the storage time of

the transistors. When the cycle has started, the

current in the feedback transformer increases until

the core saturates. At this point the feedback drive

of the active transistors is therefore removed, and,

once its storage time has passed, it turns off. In this

application the oscillation frequency would be around

35kHz, generally around ten times the natural

frequency of the circuit.

The dependence upon the storage time is minimised

by the RC network at the base of the transistor,

which increases the rate of charge extraction from

the base at turn-off. The network also serves to

decouple the base from the oscillation caused by

the base transformer at turn-off, preventing spurious

turn-on of the device.

The anti-parallel diodes allow current to flow when

devices are off.

3.1 Short Circuit Protection

Figure 2 shows a circuit with circuitry to protect

against a short circuit of the load added (note - some

components have been removed for clarity). The

circuit operates as follows.

A short circuit of the lamp will cause an excessive

current to flow through the transistors, which will

eventually cause them to overheat and be destroyed.

However, this current will also cause the voltage

across RE to increase. This causes transistor TRS to

turn on, which in turn prevents the diac from triggering

the circuit at the start of each cycle. RS and CS serve

to delay the turn-on of the transistor, preventing the

protection being triggered during the inrush phase at

the turn on of the lamp (when the lamp filament is

cold, it has a very low resistance, causing a large

but brief current to flow through the transistors at

turn-on. This resistance increases when the lamp

heats up, and the current through the transistors

falls to its steady-state value). Diode DS prevents

the small-value resistor RE disrupting the operation

of the filter.

After a short time (a few operating cycles), the

capacitor CS will become discharged and so will be

unable to keep TRS turned on, and the circuit will

attempt to restart itself. If the fault condition still

Figure 2. Transformer with short-circuit protection added (shown shaded grey)

3/4

APPLICATION NOTE

exists, the protection will be re-triggered after a

short time. In this way the circuit limits the energy

dissipated by the transistors.

It should be noted that the transistor must be robust

enough to withstand the fault condition while the

short circuit protection reacts - that is to say devices

with a large RBSOA must be used.

4. TRANSISTOR SELECTION

Because of the tight cost constraints and the voltage

range of this application, the bipolar is the usual

choice of transistor. The voltage and current ratings

of the power devices used must be selected based

on the circuit topology, the required output power

and the frequency of operation.

4.1 Voltage rating

The required voltage rating of the devices is defined

by the half-bridge topology. Supplying the circuit

with 220V RMS A.C. mains, calculating peak value,

and adding a safety margin, gives a maximum supply

voltage VCC of:

VCC(max) = 220V x √2 + 10%

= 310V + 10%.

≈350V.

To this figure must also be added the overvoltage

generated by the input filter at turn-off. In practice,

devices are used with a rating of:

VCE(max) = 450 - 500V

4.2 Current rating

The nature of the half-bridge topology is such that in

normal operation, half the supply voltage is dropped

across each device, so from the above figures VCE

in the steady state is 310V /2, 155V. Hence the

collector current in the steady state can be calculated

using:

POUT =I

C(RMS) . VCE(RMS)

VCE(RMS) =1/2 . Vmains

IC(RMS) =2. POUT

Vmains

IC(RMS) =IC(peak)

√2

IC(peak) =2 . √2 . POUT

Vmains

=2 . √2 . 50W

220V

IC(peak) = 0.64A

As stated above, when the circuit is first turned on,

the low initial resistance the lamp filament causes a

large current to flow through the transistors. This

current can be up to ten times the current in the

steady state, and the devices must be selected to

withstand this.

In this example then it is recommended that the

device used is bipolar transistor, rated at 450V and

around 7A. An example is the BUL38-D, which also

incorporates an integrated anti-parallel diode.

4.3 Storage and fall times

The fall time, tfall, of the transistors influences the

losses of the circuit, while the storage time, ts, is

important as it affects the switching frequency of the

converter. The nature of the processes used to

produce bipolar transistors means that the storage

time between batches of transistors may vary

considerably. The transistors used must be

manufactured, tested and selected to have storage

times within certain limits.

Transistors with too large a storage time may cause

the circuit to oscillate below the operating limits of

the output transformer, causing saturation of the

core towards the end of each cycle. This will cause

a spike in the collector current of the transistors

every cycle, which will eventually cause them to

overheat and be destroyed.

5. POWER FACTOR CORRECTION

As the capacitor at the input of the circuit is relatively

small, there is little deformation of the input current

waveform. However, this type of circuit generates a

certain amount of electro-magnetic interference, due

to the high frequency source that feeds the resonant

network, and so a suitable filter must be inserted in

the circuit before the rectifier bridge to prevent this

interference being fed back to the mains. This filter

must satisfy VDS and UL norms concerning

maximum RFI levels, clearance and creepage

distances and the frequency response of the filter.

CONCLUSION

Electronic transformers can offer an economic,

simple and compact alternative to the traditional

50Hz transformer. This technique is particularly

useful in halogen lamp applications because of the

short circuit protection, reduced weight, absence of

acoustic noise and the possibility of including a

dimming facility.

APPLICATION NOTE

4/4

The simple general circuit shown and the guidance

given in choosing a suitable transistor should provide

a useful starting point for developing more specific

applications.

Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences

of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is

granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specification mentioned in this publication are

subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products

are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.

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Electronic Transformer Design-in Guidelines

Type Package Typical

Application

BUL39D*

BUL38D*

BUL49D*

BUL58D*

BUL59

BUL67

BUL510

BUL89

BUL810

400

450

400

450

400

450

850

800

850

800

700

1000

850

1000

TO-220

TO-218

100

110

125

up to 50W

up to 75W

up to 105W

up to 150W

up to 250W

up to 300W

cew

(V) V

ces

cev

(V) I

(A) P

tot

(W)

*Integrated freewheeling diode

Electronic Transformer For A 12V Halogen Lamp CD00003902

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