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PNP turn off circuit not needed for hi-side LLC converter (bipolar) fet drive?

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treez

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Hello,
Do you agree that a PNP turn off circuit is not needed in a bipolar hi side drive for
LLC converter (7kw, vin=380vdc, vout = 276vdc).

Ltspice and schem attached
 

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i'm taking it that pnp not necessary in such case because the greater voltage across the pull down resistance means you get fast enough switch off, as long as your series R's aren't too big.
Another point is the ringing seen as the pulsing of the hi side pulse transformers begins (as seen in the first few 100's of microseconds of the above ltspice simulation).....I admit this can be mitigated by damping resistance, but that also slows switching time, so I prefer low resistance, but that ringing is not wanted. Its something not likeable about these simple high side drives. Reduced by making the magnetising inductance of the pulse transformer low henries, {or rather making sqrt(L/C) low}but still not liked.
There have been posters on this forum saying they've done pulse transformer high side drive for LLC with no damping resistance whatsoever, other than the gate resistance and the little bit of driver R(out). How did you manage the ringing?
 

It's almost a certainty that your turn off losses will be excessive without the solid low-Z turn off provided by pnp xtors

Many have tried to design simple xfmr gate drive for high power levels, almost all have failed - pnp's etc are so cheap - why not ensure decent product life?

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p.s. ringing is a function of all the L in series to the gate and back, including the drive Tx, if you can keep this to < 300nH then the ringing might be a non problem - gate Zeners will clip and dissipate some of the energy and give you a square wave at the gate, turning off large FET die quickly requires ~ 1-2 amp in <50nS, assuming a 12V drive the max L can be (v/L = di/dt) 240nH, to get to 1A in 20nS, even a good pnp struggles to get to 1A in 50nS...
 
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It should be noted that gate transformers with coupling factors in the supposed 0.9995 range have unconveniently high coupling capacitance, e.g. 100 pF, which is even worse for a fast switcher than a certain amount of leakage inductance.

"There's no thing like a free lunch".
 
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even a good pnp struggles to get to 1A in 50nS...

It should be noted that gate transformers with coupling factors in the supposed 0.9995 range have unconveniently high coupling capacitance, e.g. 100 pF, which is even worse for a fast switcher than a certain amount of leakage inductance.

...Thanks, these make me think that only fet drives that heavily damp the pulse transformer, and then have a fet driver in the hi side, with an isolated supply for it, are really favourable at high power with big fets...as attached
 

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