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Thanks, though the Vout is 400V, and as you know, the diode off state voltage ends up being far more with a forward converter, whose Duty is limited to 0.45, than in a Full Bridge, where the Duty can go up to 0.8Using a full bridge in DCM means you'll never be utilizing the true power capacity of the converter, at 100W you might as well use a forward converter.
The type of shoot through that I speak of in the top post certainly isn't a problem for the LLC. As you know, in the LLC, the FET turn-on happens when the FET Cds capacitances have charged such that the voltage across the fet that’s turning on is almost zero, so there’s none of this spurious turn-on with an LLC.Shoot through isn't a problem in adequate designs, and it isn't especially challenging in full bridge converters (otherwise LLC converters would be a nightmare).
Even when hard switching, shoot through should not be a problem. If it is then its a shortcoming of the drive circuitry, and maybe the FETs. Hard switched bridge converters are used successfully in the range of many kW.The type of shoot through that I speak of in the top post certainly isn't a problem for the LLC. As you know, in the LLC, the FET turn-on happens when the FET Cds capacitances have charged such that the voltage across the fet that’s turning on is almost zero, so there’s none of this spurious turn-on with an LLC.
The attached pdf shows the shoot-through that I speak of (in a CCM Full Bridge). As you know, those current spikes that you see are due to the fact that when either the upper or lower fet of a leg turn on, the other FET gets temporarily turned on by the sudden flow of current through its junction capacitances, which result in a temporary Vgs voltage which momentarily turns on that fet..resulting in shoot-through which is visible here.Even when hard switching, shoot through should not be a problem. If it is then its a shortcoming of the drive circuitry
..Well yes, though as we know, a very highly experienced engineer has recently been on this forum to warn about that. But as we know, the only reason those multi kw full bridges work is because the heatsinking is just increased to take care of it.Hard switched bridge converters are used successfully in the range of many kW.
Why? Changing di/dt or dv/dt won't change the amount of energy dissipated associated with Coss. That energy is only a function of the total charge and voltage on Coss, which is independent of load current/duty cycle. So in order to maximize efficiency, you should try to maximize the energy delivered to the load per cycle.The thing is, whether its shoot through or charging and discharging of the Coss of the upper and lower fets of a leg, we still want to minimise the di/dt of it and the magnitude of it.
Hard switching of Coss should not be a substantial contributor to EMC at 100W.
As discussed, its only the fet turn-on transient that we will slug, the turn-off transient will be very fast.kneecapping the entire converter by running it in DCM with sluggish gate drive isn't going to give good results.
And due to the higher peak current in DCM, the turn-off losses will be increased vs a CCM design...As discussed, its only the fet turn-on transient that we will slug, the turn-off transient will be very fast.
The reason we didn’t do an LLC was because it can't manage the full Vf and dimming range without having too high switching frequency at lower load (very dimmed).
he reason for DCM is because CCM Full bridge’s suffer the problem where the bottom/top FET in a ‘leg’ gets spuriously turned ON when the top/bottom FET in the same leg turns ON..resulting in momentary ‘shoot-through’ current.
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