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Boost converter push pull topology

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ShayanZulfiqar

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Hello everyone,
I am currently designing a boost converter based on push pull topology. I am attaching the link to the schematic. The problem is that when i check the wave at the drain of either of the mosfet then the positve peak of the square wave has alot of noise. The peak of square wave is like a exponentially decaying sine wave. Kindly tell me why this is happening and how to solve it.

Link to schematic: https://tahmidmc.blogspot.com/2013/01/using-sg3525-pwm-controller-explanation.html?m=1
 

CataM

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Do you mean like this?
Ring.png

RC snubber across each FET could help.
 

dick_freebird

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In Tahmid's blog there is mention / question re the need
for snubbers. The reply focused on the FET stress and
neglected the ringing / EMI concern. Abrupt dI/dt or dV/dt
(and you have both) will kick the tank (Coss of FET, and
some inductance term(s) of the xfmr primary) and nothing
to scrub off that energy.

Another push-pull concern is "flux walk" which comes from
leg-leg circuit imbalances such as on-time, component
mismatches etc. More sophisticated controllers may have
features to sense and correct that, otherwise be sure to
leave yourself plenty of core saturation margin and maybe
a soft saturation characteristic (i.e. no "cliff").
 

ShayanZulfiqar

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CataM

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One thing i dont understand is that how the output DC voltage is increased by varying the duty cycle. I get that the RMS will vary but the rectifier will just convert the -ve portion into +ve of same amplitude
When Q1 ON --> secondary of xformer =Vin/N
When Q1 OFF --> secondary of xformer=0
When Q2 ON --> secondary of xformer= -Vin/N
When Q2 OFF --> secondary of xformer =0
---Cycle repeats--

One thing to notice is that the output filter inductor sees 2 times the frequency of the signal applied to the xformer, then, the filter inductor sees a square wave between Vin/N and 0. (N=N1/N2)

- - - Updated - - -

When duty cycle=50% (in practice one should go a bit less depending of switching times of transistors... e.g. 49%, 48% ...), you get what you are saying i.e. the output voltage rectified "Vin/N".
 

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