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For 3 phase 6kW there is no PFC front end as no standards require it...!
..at turn on its hardly any, run the simulation of post#1 and see for yourself, the leakage inductor cuts most of it out.actually there is some overlap in a std full bridge
As you know, we don't run off 650v in commercial domestic vehicle market, and we do see all the RFI that you speak of due to our very hard switching pfc stages, and its nothing of great concern. We had a consultancy approach us and tell us we should pay them 100's of thousands for a special resonant converter that only they could do properly.....they told us that hard switching was not possible at 7kw...then we asked them how does the pfc stage work then, and they went quiet and left our offices soon afterwards.if you ever do you will see the losses and the RFI generated by the hard switch full bridge..!
I understand, though as you know we aren't running full bridges off the output of a three phase rectifier.I have never seen a hard switched FB at 6kW (or anywhere near that power) running off rectified 3 mains any where near 100kHz,
Sorry but surely you agree that the boost converter PFC is one of the most hard switching topologies out there.....during the switching transistion there is rapid discharge of the Cds capacitor, also horrendous reverse recovery, as well as overlap of v and I during the switching transition.boost converters are far different as they process only a fraction of the thru power...
thanks but sorry I though it was the other way round, PSFB isn't good for high ratios because a PSFB suffers 'duty cycle loss' due to its (designed) higher leakage term in the transformer.?So more suitable for extremely
high boost ratios than a fixed phase bridge perhaps
the simulation in the top post shows that the normal leakage inductance in the full bridge transformer prevents the reverse recovery current (of the secondary diodes) from flowing in the full bridge FETs...I deliberately implemented ridiculously bad diodes in that simulation to show this point.I see your line of argument, but actually the turn on losses at 100kHz at 400 - 600V DC bus are higher than you might expect for a std H bridge, there is a contribution from the recovering o/p diodes (and their snubbers) that you can't ignore
..turning the diodes on quickly causes power losses?because you cannot turn the fets on really fast and hard in a std H bridge as you will hit the o/p diodes really hard in doing so, upping losses and RFI
Yes, look at a datasheet for a power rectifier and you will see that its reverse recovery charge depends on the di/dt. At higher di/dt, Qrr becomes much larger, and therefore so will the dissipation. The extra leakage in the PSFB causes slower recovery with less losses. The ringing is no more severe, since the leakage inductance is not the cause of ringing in a full bridge (the parasitic inductances in series with your switching devices are)...turning the diodes on quickly causes power losses?
if you run the simulation in the top post with an ideal transformer, (no leakage), you will see that you get no ringing across the secondary diodes, because the ringing across the secondary diodes was caused by the leakage inductance in the transformer.the leakage inductance is not the cause of ringing in a full bridge
Oh I thought you were talking about snubbing of the bridge, my mistake.if you run the simulation in the top post with an ideal transformer, (no leakage), you will see that you get no ringing across the secondary diodes, because the ringing across the secondary diodes was caused by the leakage inductance in the transformer.
Sure.PSFB ringing across secondary diodes needs snubbing, that we agree on...and psfb ring across secondary diodes is a lower frequency ring than in a plain full bridge (generally)
I don't agree. This would be in some topologies where the leakage energy is not recovered, but in a PSFB it is not the case. Just to see, I ran a couple simulations on one of your PSFB models while changing the leakage, and the loss in the snubber resistors does not change substantially (assuming the snubber values have been re-optimized for the new leakage values).so it takes more dissipation to snub it
Hello Treez, respectfully, you put too much faith in simulation and then reason from incorrect results, had you build a 3kW converter you would see the high levels of RFI resulting from diode turn off even at modest di/dt (and dv/dt) resulting from modest turn pn speeds of the fets, wind up the turn on of the fets and everything gets much worse, affecting control.the simulation in the top post shows that the normal leakage inductance in the full bridge transformer prevents the reverse recovery current
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