Turn-off loss in H-Bridge converter MOSFET resulting from ZVS turn-on of complimentary device

[akh_power]

Generally ZVS turn-on of MOSFET is considered as a good feature in power converters. In H-bridge converters, the MOSFETs in a switching leg operate in complimentary manner. So, when one device is undergoing ZVS , it is certain that the complimentary device turn-off happened when it was carrying a non-zero current. Because, only after the device is completely off, the diode in the turning on device can conduct. This would result in turn-off loss in the device. Why ZVS turn-on is still considered as favorable feature even though it results increased turn-off loss in complimentary device.

 

[dick_freebird]

Tuning the "handoff" timing is key interest in power stage design. Can be done "ballistic" or some folks will servo to a minimal SW undershoot to get "just enough" current to finish the job (Near Zero Current switching).

Cross conduction is bad,. Momentary open is bad, there is a happy place in between.

 

[cupoftea]

Do you mean like the ZVS turn ON in a Phase shift full bridge converter?
I agree there is turn off loss in the devices....but its not increased because of the zvs turn on.
Some people put caps across the fets (vds), and smash them off really quickly to reduce turn off loss in PSFB...
....somewhere in this long thread they speak of this....

Phase Shift Full Bridge SMPS is massively over-hyped?

Hello, The Phase Shift Full Bridge (PSFB) converter is a hoax. –The LTspice simulation attached of a “plain” Full Bridge SMPS bears this out. PSFB claims to be able to reduce switching losses compared to the “plain” Full Bridge SMPS. However the PSFB does have Turn-OFF switching losses, just...

www.edaboard.com

 

[Easy peasy]

turn off losses in a ZVS designed H bridge are largely governed by the speed of the gate drive at turn off - unless the gate can go from 10V to 0v in <= 30nS you will not get low loss turn off, additional caps across mosfets are sometimes used to reduce dv/dt at turn off, having enough fwd current in the mosfet at turn off is essential to get a full voltage transition so that the other mosfet can turn on under ZVS - for just enough energy - the timing of the turn on is critical ... for excess energy you get a larger time window for turn on ...