AN2747 datasheet(10/44 Pages) STMICROELECTRONICS

10 / 44 page

General circuit description

AN2747

10/44

Equation 1

where

●

VL= lamp voltage

●

Vdc = DC bus voltage

●

Vlamp = lamp voltage

When the high side device H1 is switched OFF, the current flows in the low side

devices. The freewheeling diode integrated in the low side device IGBT L1

operates at high frequency with duty cycle 1-D (see Figure 10).

Figure 10.

Inductor current during discharge phase

The voltage across the inductor is:

Equation 2

The current through L decreases linearly. This circuit is actually a synchronous rectifier buck

converter. To minimize the current ripple through the lamp, the circuit operates in continuous

conduction mode (CCM) with fixed frequency.

The circuit operates in mode A and B complementarily in low frequency supplying the lamp

with square wave alternate current.

2.2

Lamp power management

The lamp power is obtained by multiplying the lamp voltage signal and the lamp current.

The lamp voltage is sensed directly across the lamp while the lamp current is obtained using

the relation given below. In continuous mode the lamp current is coincident with average

buck coil current. Starting from peak inductor current the average value is:

Equation 3

lamp

–

Lamp

DC Bus

OFF

lamp

–

lamp

peak

∆

-----

–

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