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Why hasnt the pushpull SMPS been banned yet?

cupoftea

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Hi,
We have a 300W synch pushpull, 18-36vin, 32vout, 300wout, 300khz, 1:2
Cust wants us to fix it.

It has no snubbers on the fets. The pushpull is particularly victim to leakage inductance problems, as well as having to have snubbers to burn up the magnetising current energy, so as to reset that too. Unlike the full bridge and 2tranForwardwhich just lets magnetising current and leakage inductor current "ring out"....no related dissipation to speak of.
So why has the pushpull nto been banned yet?
People "Like" it due to low side drive......but its "fool's gold".

The sec FETs will see 144V off state voltage at 36vin....but then we must add the leakage inductance spike to that....so its more......the FETs are 200V rated , so we have to have TVS's which act within 145V-195V....not an easy pick.

Having to burn up the primary magnetising current is also a massive downer...dont know why pushpull even exists?
 

Easy peasy

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One can wind a PP Tx with low leakage, one can also design catch ckts for the turn off energy and send it to the output, one can design a resonant push pull with less voltage stress on the output diodes, one can design a boost stage so that the PP is essentially buck only, PP are good - but less so at higher powers with higher currents ...
 

cupoftea

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Thanks, yes, though the thing about the PP that seems very poor, is the fact that in the standard PP, the magnetising current has to be burned up in order to reset it...unlike the full bridge where it just rings out in the primary.....in the PP, it is trapped when the FET turns OFF...and has to get burned up in the primary snubber.
 

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..or rather should i say.....the bit of the magnetising inductor which isnt fully coupled to the core...the energy associated with the currnt in that bit needs to be dissipated.
 

Easy peasy

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hardly a helpful comment - transformer winding skills and layout skills are paramount for a good working push pull up to say 200Vin and about 10A per fet.
 

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