Shindengen Electric MFG.CO.,LTD

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6.1.2 Ringing voltage at turn-off of main switching device

A significant voltage surge component is generated in the main switch if the transformer leakage inductance is too

large or if a relatively high current is output. The most effective way to reduce the voltage surge component is to

reduce the leakage inductance. The voltage surge component can be also reduced by a clamp circuit. Reducing

the voltage surge component protects the main switch. In the case of a multi-output power supply, it also improves

cross regulation in the outputs.

(1) Transformer leakage inductance

When the main switching device is turned off, a ringing

voltage is added to the voltage, as shown in Fig. 6.4, due to

the leakage inductance of the transformer primary coil.

The voltage applied to the main switching device must be

designed to accommodate the ringing voltage. The leakage

inductance of the primary coil is measured as shown in Fig.

6.4.

(2) Clamp circuit

A clamp circuit may be required if the withstand voltage limit of the main switching device is exceeded due to load or

other conditions or if the design margin is insufficient due to a ringing voltage caused by the leakage inductance.

We recommend a DCR snubber circuit as a clamp circuit. See the next

page for DCR snubber circuit design procedures.

Fig. 6.4 Leakage inductance

MR4000

3

5

9

Fig. 6.5 DCR snubber circuit

6. Supplementary design information