12V Full bridge waveform doubt

[mrinalmani]

Hi
I am testing a 12V full bridge converter connected to a step up transformer. The output of the transformer is rectified using a SiC bridge diode. There's no inductor between the SiC output and the bulk capacitor.
The transformer is being driven by simple PWM. Alternate high and low FETs are on for a brief interval followed by a large deadband where all switches are off. During deadband, the current through the transformer leakage should reset to zero. But instead of stopping at 0A, it shoots negative and then snaps back to zero. The negative overshoot is significant. What could be the reason for the negative overshoot? Could this cause potential heating? Please share your opinion.
Here is an attached oscilloscope snapshot.

 

[KlausST]

Hi,

I guess the problem is caused by D1.

Klaus

 

[FvM]

Diode reverse recovery.

 

[Easy peasy]

your current measuring device may be in error ... esp if it is a CT that needs resetting ....

 

[BradtheRad]

Your waveform looks as though it goes with a modified sinewave. It has a similar dead zone between the positive and negative pulses. The dead zone involves abrupt shut-off of current.

However it's difficult to make an inductive load (or transformer) compatible with this. The transistor shuts off current but the inductor wants to continue yanking current through the wires.

I've seen a similar overshoot in my simulations of a transformer driven by pulses via an H-bridge.

It could help if you turn off current less abruptly. Or, by providing a path for transformer current during dead time. It might be a snubber network, or diode clamping, etc.