AN2747 datasheet(9/44 Pages) STMICROELECTRONICS

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AN2747

General circuit description

9/44

General circuit description

2.1

Block diagram

The block diagram of the ballast is shown in Figure 8. The complete circuit is composed of

two stages:

●

The boost converter which regulates the output voltage and performs the power factor

correction.

●

The inverter stage composed of a full bridge that converts the DC current coming from

the PFC stage into an AC current for the lamp.

The operation mode of the full bridge realizes a synchronous buck converter. The full bridge

moreover drives the igniter block to generate the high-voltage pulses.

Figure 8.

250 W HID lamp ballast - block diagram

To generate a square wave current in the lamp, the circuit is driven in the following way:

When low side device L2 is switched ON, the high side Power MOSFET H1

operates with a high-frequency pulse width modulation (PWM). The duty cycle D

is established by a constant-current control circuit.

Figure 9.

Inductor current during charge phase

In this condition the current increases linearly and the voltage across the inductor L is:

VIPER12

L6385

STGF10NB60SD

Main

STF12NM50N

STP20NM50FD

ST7FLITE39

74AC00

LM119

TS272I

LE50AB

Lamp

Filter

Bridge

L6562

IGNITER

STTH8R06DIRG

Lamp

DC Bus

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