|26th May 2010, 05:11 PM||#31|
Join Date: Jan 2008
I think I solved my problem.
I had a wrong estimation of the primary mosfets Coss so that the shim inductance was too high.
Now I have reduced my shim inductance to 2.5uH (it was 10uH) and everything is fine.
With some work on RC and RCD snubbers my peak voltage on the output
mosfetss is clamped at around 90V with acceptable ringing. The power
dissipation in every RCD clamp resistor is around 2.5W which is something I can manage without problem with 2 parallel 3W resistors.
With the reduction of the shim inductance the A-B leg of the H-bridge will enter in ZVS at an higher output current: at around 40A out the A-B leg enters ZVS and the C-D leg enters ZVS at around 10A.
Even when it works in hard switching the efficiency does not suffer too much: it remains above 92.5% from 20A output and above.
"Do you have pics of the layout? "
Attached you will find the secondary side layout. It is a 4 layers pcb.
All the snubber components are not mounted on the pcb but mounted flying on the back side triying to keep the connections as short as possible
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