Half bridge converter for electronic lamp ballast

AN993

4/27

Doc ID 5656 Rev 10

1

Half bridge converter for electronic lamp ballast

Voltage-fed, series-resonant half-bridge inverters are currently used for fluorescent lamps

(Figure 1). This topology facilitates operation in zero voltage switching (ZVS) resonant

mode, dramatically reducing the transistor switching losses and the electromagnetic

interference.

To design a cost-effective, compact and smart electronic lamp ballast, a dedicated IC could

be used to drive directly the power MOSFETs of the half bridge. Such controllers require a

high voltage capability for the high-side floating transistor driver.

1.1

Lamp requirements

To prolong lamp life and to ensure efficient ignition of the lamp, the cathodes must be

preheated. In fact, the preheating of the filaments allows an easy strike of the lamp, reducing

the ignition voltage. During the preheating time, the lamp is characterized by a high

impedance and the current flows only in the filaments. The resistance value of the filaments

strictly depends on the type of lamp. Typically, these filaments present an initial low value (a

few Ohms) that will increase by four to five times during the preheating phase.

After the preheating phase, the lamp must be ignited by increasing the voltage across it. The

ignition voltage value also depends on the type of lamp, and it increases with the aging of

the lamp. For a typical TL 58 W, the ignition voltage value is not much less than 1000 V.

When a simple inverter with a constant switching frequency is used, external circuitry is also

necessary (for example, a PTC or discrete timer). However, with ST’s L6574 smart

controller, both the preheating and ignition functions are achieved by using simple resistors

and a capacitor, which set all the start-up procedures.

Figure 1.

Half bridge topology

AM01309v1

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