The problem I have is with the oscillation at the top of the output waveform when a load is attached. Applying a load to the transformer only makes ringing worse. I have tested the gates of all the mosfets and the waveforms are very clean with no spikes even when the transformer is loaded. The only problem is with the bridge output waveform. The board has a 1uf film capacitor in the center of the board. I have tried adding a 2200uf capacitor right at the main voltage rail next to the mosfet as shown in the image below. I also have a current transformer to measure the capacitor current.
The output waveform improves with transformer still connected when electrolytic cap is added. CH1 : Full bridge output voltage CH2: Electrolytic capacitor current.
The problem with this is: the electrolytic cap gets warm under very light loading of the full bridge. At high loads the current through the capacitor was about 30 amps at the peak. The capacitor was very hot. If adding more capacitance to the supply rail would improve the ringing, what kind of capacitor should I use? Would a larger film capacitor help the ringing? Is the ringing a layout problem? If so, should the pcb power traces be shorter?