Two switch flyback converter MOSFETs voltage stress

Question

I am exploring and experimenting the two switch flyback converter. I have a doubt, in this two switch flyback converter configuration, two MOSFETs are connected in series with sandwiched primary coil between them.

For example, voltage source of 1000 V is applied to the primary side, does the high side and low side MOSFETs share the voltage (500 V +500 V) between them, like LEDs connected in series. From my hunch, can I select the high side and low side MOSFETs rated for 500 V, thus forming the total 1000 volts.

Correct me if I am wrong and appreciate for the help.

Link for the application report: Improving the Performance of Traditional Flyback-Topology With Two-Switch –Approach

Answer 1

voltage source of 1000 V is applied to the primary side, does the high side and low side MOSFETs share the voltage (500 V +500 V) between them, like LEDs connected in series.

No, they don't.

Look at the clamping diodes on the primary side. During de-magnetisation of the core the diodes will be in conduction to reset the core and clamp the MOSFETs' drain-source voltages to rails i.e. the low-side MOSFET has its drain clamped to positive rail and the one on high-side has its source clamped to negative rail. That makes the maximum voltage stress across the MOSFETs equal to the input voltage plus a diode drop.

So,

From my hunch, can I select the high side and low side MOSFETs rated for 500 V, thus forming the total 1000 volts.

No. The MOSFETs you select should have a drain-source breakdown voltage rating higher than the maximum possible DC input voltage.

One more thing: Since the MOSFETs can never turn on at the exact same time due to the imperfections, one MOSFET will turn on before so the other MOSFET will see full bus voltage (if we assume the voltage across the primary winding is zero before the turn-on pulses are sent) through the R_ON of that MOSFET and the R_DC (DC winding resistance) of the primary winding. Although this will happen momentarily, still can be destructive if you use 500V-rated MOSFETs.