AN993 Datasheet, PDF (4/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

Half bridge converter for electronic lamp ballast
AN993
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.
Figure 1. Half bridge topology
HV
DRIVER
L_RES
C_RES
AM01309v1
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.
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Doc ID 5656 Rev 10