APPLICATION NOTE STMicroelectronics
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 110 V a.c. or 220 V 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, 50 Hz 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 selfoscillating circuit where the two transistors are driven in opposing phase by feedback from the output circuit.
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Figure 1. Electronic transformer for 12 V Halogen Lamp
Figure 2. Transformer with short-circuit protection added (shown shaded grey)
