Two-switch forward converter general concepts

AN4638

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DocID027309 Rev 1

Other distinctive features include:

•

The clamp diodes recover the magnetizing energy stored in the core during on-time,

which is fed back to the supply; therefore, any voltage spikes generated by leakage

inductance are clamped to the DC input bulk voltage level.

•

No IGBT antiparallel co-packaged diode conduction is required under any operating

condition.

•

The maximum duty cycle is limited to 50% to guarantee full demagnetization of the

•

The core flux only operates in one quadrant of the BH loop.

•

The configuration is unable to operate in zero-voltage switching (ZVS) mode because

this limits its operation frequency.

Even if the IGBT voltage stress should, in theory, be clamped to the maximum DC input bulk

voltage level and recycled back to the input capacitor via the activation of the fast recovery

diodes, the IGBTs may actually see an additional spike during turn-off switching which is

affected by the leakage inductance energy, the switching speed, and the circuit layout.

Leakage inductance is difficult to control and often varies in production[4].

Refer to “Topology equations and device selection” in the Appendix, which details some

useful equations for the selection of the IGBTs used in testing.