AN2794 datasheet(23/39 Pages) STMICROELECTRONICS

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AN2794

Schematic description

Doc ID 14827 Rev 2

23/39

Schematic description

The schematic of the converter is shown in Figure 11. Three MOSFETs are paralleled in

order to transfer power to each primary winding of the transformer. Both RC and RCD

networks can be connected between the drain and source of the MOSFETs to reduce the

overvoltages and voltage ringing caused by unclamped leakage inductance. The output of

the transformer is rectified by a full bridge of ultrafast soft-recovery diodes. An RCD network

is connected across the rectifier output to clamp the diode voltage to its steady state value

and recover the reverse recovery energy stored in the leakage inductance. This energy is

first transferred to the clamp capacitor and then partially diverted to the output through a

resistor.

The IGBT full bridge is connected to the output of the push-pull stage. Their control signals

are generated by an SG3525 voltage mode PWM modulator. Its internal clock, necessary to

generate the 100 kHz modulation, is set by an external RC network. The PWM output stage

is capable of sourcing or sinking up to 100 mA which can be enough to directly drive the

gate of the MOSFETs devices. The PWM controller power dissipation, given by the sum of

its own power consumption and the power needed to drive six STP160N75F3s at 100 kHz,

can be evaluated with the following equation:

Equation 67

where Vs and Is are the supply voltage and current.

Since this power dissipation would result in a high operating temperature of the IC, a totem

pole driving circuit has been used to handle the power losses and peak currents, achieving

a more favorable operating condition. This circuit was implemented by means of an NPN-

PNP complementary pair of BJT transistors. The control and driver stage schematic is

shown in Figure 11.

drive

tot

Contoller

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