Will the diode current waveform throughout the reverse recovery time look the same as it did in parts a and b? Or do I need to use Qr? How does Qr help me?
I think the intent of the circuit is to turn on Ms to store the energy from the diode D reverse recovery in the inductor Ls. Then at the end of the reverse recovery time, turn on M (zero voltage switching) and shortly after turn off Ms. The energy stored in Ls will then be dumped back to the load via diode Ds. I think we need to equate the energy stored in Ls to the energy that would be lost due to diode reverse recovery. However I don’t know where to start. Any hints?
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Turning on Ms causes Ls to see V, so current iLs rises a at constant rate V/Ls. Given that L is large, current iL is constant, so the diode current will decrease as iLs rises. If you size Ls so that the rate of decrease of the diode current is equal to the 10A/us given ealier in the problem, you can eliminte Qr. If you also time the turn-on of Ms correctly, the diode current will be 0 when M turns on, so there will be no reverse recovery loss. This reasoning allows you to find the turn-on time of Ms and the inductance Ls.
What I'm trying to figure out is what's happening in the time when g and gs overlap. Whatever's happening, it's related to the turn-on on M and maybe the resetting of iLs to 0, but I'm still working on details. Any ideas?
My guess is that the overlap of the gate signals is to ensure a smooth handoff between M and Ms. Once Ms is done discharging Qr thru Ls it remains ON so when M turns ON Qr=0. If Ms were to turn off before M turned on, the diode would build up some more Qr between the time Ms turned off and M turned ON. The overlap ensures that there is no Qr to deal with when M turns ON thus eliminately the switching loss in M.
Ms wouldn't have to be ON very long after M turns ON. I'm not sure what would determine how long the overlap should be.
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