Dead time in Full Bridge Converter

Hi guys,
I made a full bridge converter for Vin=400V, Vout = 400V, 100kHz, two diodes at the secondary, which I'm testing right now. For testing purposes I'm using only 40V and I reduced the frequency to 50kHz.
The problem I'm having is the dead-time occuring at the secondary side. When the converter is operating at 100% duty cycle (left and right leg 180°out of phase) and there is no load, I get the following waveform at the secondary:


The waveform seems fine (to me at least). However, when there's a 4A load, I get the following:


There's an aproximately 2us dead time occuring, which I have no idea where it comes from. The mosfet dead time is set to only 100ns, and the bridge duty-cycle is at 100%. Also, during that dead-time, there is 40V voltage at the primary, but no voltage at the secondard. 8-O
The transformer I'm using is an ETD-54 core and has 8mH primary inductance and 10uH leakage inductance.

Does anyone have an idea, why that dead time is occuring?

George

It takes 1uS for 4 amp to ramp down to zero with 40 volt drive on 10uH leakage, and a further 1uS to ramp up again to 4 amp with 40 volt of drive, you are seeing the net effect of leakage inductance impedance opposing the AC current you are trying to push through the transformer.

While the current thru the Tx is less than the current in the o/p choke and diodes - the diodes (all) will be on - giving an effective short on the Tx - which you see as a zero volt dead time, until the full o/p current is established in the Tx secondary again and one set of diode(s) turn off - hope this makes sense when you have digested the explanation.
Regards, Orson Cart