Two switch forward converter reset

[Slinzer]

Designing a 50W two switch forward converter. The input voltage is 600V. On reset the transformer primary should clamp to the input bus voltage. On my prototype during reset the primary voltage reverses but only hits about 500V and never forward bias the clamp diodes. What could be going on? Is the primary doing a type of resonant reset with parasitic capacitance? Any thoughts.

 

[Comcokid]

There are several configurations of the two-switch forward when it comes to the circuit and reset. I'm assuming you are using a single winding with two diodes for the reset? Are you diodes fast enough - both on the primary (and secondary)? Are you sure you've probing it correctly - you might need a HV differential probe because if you're using a regular scope probe without scope isolation your ground can shunt the lower MOSFET leaving you with a pseudo single switch. A lot of SMPS questions are answered on the Ridley group on linkdin.