Big problem with SMPS transformer gate drive

[Flyback]

Hello,
The attached shows a disastrous problem with a DC restored transformer FET gate drive. (shown here with a cascaded buck SMPS)
…when the converter is suddenly no-loaded from full load, the duty cycle obviously goes fairly quickly to zero. Ater the duty cycle has finally gone to zero, the FET gate voltage of the high side FET lingers high for many switching periods…….BOOM!

As far as we can see, this can only be stopped by very heavy resistive damping…damping which is far too heavy for a transformer gate drive. So what’s the answer?

The attached shows the overall schematic, also a Ltspice simulation which demonstrates the problem, and the high side gate drive waveform which shows the FET gate-source voltage lingering high for far too long after D has gone to zero.

 

[zahwi]

The 47nF between the pulse transformer and the gate drive should have never been there because, as you have already noticed, when the pulses are narrow there is no energy to discharge the gate. Bad circuit.
The reason they put 47nF and 1N4148 to ground is for clipping the pulse to -0.7V so you get 12V positive which needed to switch the FET on properly.
One way is to short the 47nF and to remove the 1N4148 and also to double v13,v14 so you get enough gate drive.
The other way that might work is to put 220 ohm across the 47nF so when the pulse is narrow or 0 the low resistance of the pulse transformer will discharge the gate.

 

[Flyback]

Thanks, if we double v13 and v14 and get rid of the DC restore circuit then negative gate drive will go below -6v, and our sic fet has a max negative vgs of just -6v. The fet is SCT2160

SCT2160 SIC FET
http://www.wakamatsu-net.com/biz/sct2160ke.pdf

This is for a MWD application, ambient temp is 170degc

 

[zahwi]

Using pulse transformer for driving a mosfet gate is ok only for cases that the waveform doesn't change. It is possible that the circuit was intended for constant load and only small change in input voltage.
You can add 2 zeners to each gate to protect it from over voltage.
R77 may cause the oscillator to stop when the mosfet stays partly on, these are the conditions for the mosfet to burn. Try removing R77.

 

[Flyback]

Thanks but I don't see r77 stopping the oscillator as it doesn't stop the CT cap from charging /discharging