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):

1.

2.

3.

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

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).

1.

Parasitic transformer inter-winding capacitance

2.

Layout parasitic capacitance

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

ESBT

T

Heatsink

+

Cbus

ESBT

T

C2

C1

C3

Heatsink

ESBT

T

C2

C1

C3

Insulaion Pad

Ic2

Ic3

Ic1

Ic