AN1756

High-frequency tube lamp ballast applications

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High-frequency tube lamp ballast applications

Basically, a HF-TL ballast converts the 50-60 Hz AC Line input or a DC voltage to a high

frequency output, usually in the range of 25-125 kHz. A rectifier block and a DC to high

frequency inverter usually make up a ballast (please refer to 1 in Section 4: References).

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

This topology allows to easily operate in Zero Voltage Switching (ZVS) resonant mode,

reducing the transistor switching losses and the electromagnetic interference (please refer

to 2 in Section 4: References).

The half bridge of the inverter can be controlled in different ways with different ICs:

●

Half-bridge controller like, for example, the L6574.

Figure 1.

Typical topology with a half-bridge controller

●

Half-bridge driver like, for example, the L6384 + some kind of control.

Figure 2.

Typical topology with a half-bridge driver

What is the difference between a half-bridge driver and a half-bridge controller?

A driver is “just” able to drive the floating (high side) transistor of a symmetric half bridge

inverter, from a Pulse Width Modulation (PWM) signal. The controller also includes some

form of oscillator, usually tunable by using a few external components (resistor, capacitor).

Varying the switching frequency is the mainly used control principle. This allows the current

in the lamp, and therefore the output power to be modulated (please refer to 2 in Section 4:

References). So the lamp can be dimmed and input voltage variations can be compensated.

To know more about high frequency tube lamp ballast applications, please refer to 2 in

Section 4: References.