AN4213 datasheet(22/42 Pages) STMICROELECTRONICS

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DC/AC converter and lamp

AN4213

22/42

DocID023984 Rev 2

DC/AC converter and lamp

The DC/AC converter uses a half-bridge voltage-fed topology with two Power MOSFETs

driven by the L6569 driver. It is a high-voltage half-bridge driver with an oscillator inside.

Here below is the complete electrical schematic of DC/AC inverter in half-bridge topology:

Figure 12. DC/AC converter

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

This topology makes easy to operate in zero-voltage switching (ZVS) resonant mode,

dramatically reducing transistor switching losses and the electromagnetic interference. This

type of inverter can operate up to 150 kHz in ZVS mode. For this project, the frequency has

been selected between 50 and 100 kHz, specifically

run

≈

57 kHz.

The frequency of the internal oscillator can be programmed using an external circuitry

composed of a resistor R

and a capacitor C

. The nominal oscillator frequency can be

calculated using the following

Equation 39:

Equation 39

From the above equation, imposing C

=560 pF, R

will be equal to 22 k

Ω.

4.1

Bootstrap circuit

The bootstrap block is needed to supply the high-voltage section. This function is normally

accomplished by a high-voltage fast-recovery diode. The L6569 has an internal bootstrap

circuit that replaces the external diode. The bootstrap capacitor (C

BOOT

) is charged every

time the low-side driver is on and the output pin is below the supply voltage (V

) of the

gate driver. The bootstrap capacitor is discharged only when the high-side switch is turned

on. This bootstrap capacitor is the supply voltage (V

) for the high circuit section.

AM17326v1

GND

BOOT

HVG

OUT

LVG

L6569

LAMP

400Vdc

Cboot

Csn

PTC

kHz

OSC

3863

∗

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