AN2708 datasheet(15/42 Pages) STMICROELECTRONICS

15 / 42 page

AN2708

High-frequency ballast

15/42

Once the lamp and its parameters have been chosen, the ballast design will be optimized by

selecting the resonant components L and C as follows:

●

Set Tpre

●

Select frunmin (> 20 kHz)

●

Choose

∆f = fmax-frunmin

●

Select L & C such that fph > frun

●

Select half-bridge switches

●

Select L6574 biasing circuitry

The magnitude of the transfer function (lamp voltage divided by input voltage) for the two

circuit configurations (preheating-ignition and running) illustrates the operating frequencies

and where they lie with respect to one another.

Figure 6.

Ballast transfer functions (magnitude)

The currents and voltages corresponding to the resulting operating frequencies determine

the maximum current and voltage ratings for the inductor, capacitor, and the switches,

which, in turn, directly determine the size and cost of the ballast.

Moreover the zero-voltage switching is ensured as shown by the curves above in Figure 6.

STP8NM50 (8 A, 550 V) has been selected as power switch according to the current stress

and the input DC voltage.

The half-bridge inverter driving circuitry is based on the high performance L6574 which is an

OFF-LINE half-bridge driver designed in 600 V BCD technology, including all the features

needed to drive and properly control the tubes. A dedicated timing section in the L6574

allows setting the necessary parameters for proper preheat and ignition of the lamps. Also,

an op-amp is available to implement closed-loop control of the lamp current during normal

lamp burning. To avoid cross conduction of the power MOSFETs the internal logic ensures a

minimum deadtime. Moreover the L6574 is provided with two lamp status control functions

to protect the application against lamp failure as well as lamp disconnection. Finally it is

AN2708 Datasheet(PDF) 15 Page - STMicroelectronics