AN2528 datasheet(8/21 Pages) STMICROELECTRONICS

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Parasitic capacitances and related issues

AN2528

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Parasitic capacitances and related issues

In a flyback converter stage it is important to take into account the parasitic capacitances

since their influence may affect the correct operation of the converter itself. Figure 2 shows

the small signal equivalent model of a main switch, transformer and main parasitic effects.

The parasitic capacitances between the ESBT collector and ground are mainly due to three

components (see Figure 2):

C1, the primary inter-winding capacitance;

C2, the intrinsic capacitance of ESBT between its collector and source;

C3, the parasitic capacitance between the collector of the ESBT and the heat-sink.

Usually transistors are mounted on a heat-sink by interposing an insulation layer. The heat-

sink has to be grounded either for safety reasons, or to minimize the RFI. The resulting total

parasitic capacitance C is equal to C1 + C2 +C3. C may be large enough to produce

additional and non-negligible switch-on power dissipation. Large parasitic capacitances may

cause ringing and produce noise problems. The effect of parasitic capacitances is worse at

higher input voltages, like those observed in a 3-phase power supply.

Figure 3.

The small signal equivalent circuit

Considering that the power managed by the system is low, another goal to achieve is to

keep the power dissipation very low on every part on the system. Our target is to get less

than 1.5 W power loss on the switch. Achieving this target leads to two benefits: high

efficiency and no need of heat-sink (cost reduction).

Therefore the effect of C3 must not be considered in this case. C2 is related only to

additional power dissipation during switch-on and does not affect system stability. C1 has

the most important effect on flyback converter design. We have only two ways to reduce C1:

Parasitic transformer inter-winding capacitance

Layout parasitic capacitance

Care is needed when designing the layout and building the transformer.

ESBT

Heatsink

Cbus

ESBT

Heatsink

ESBT

Insulaion Pad

Ic2

Ic3

Ic1

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