# SMPS Simulation Help

Finally bit the bullet and considered getting help with a simulation I'm looking to get right. Just a heads up, I have limited experience with SMPS simulation in both LTSpice and Multisim. The circuit I'm attempting to simulate is an interesting ZVS and ZCS flyback I found in an IEEE report. It utilizes a resonant clamp along with a half bridge voltage multiplier to achieve the zero voltage and current switching.

Here it is in multisim:

I first attempted to follow along with the report's calculations and use my own parameters for fun, but the waveforms looked nothing like the expected or tested waveforms they presented. I then went back and plugged in the component values listed in the report to try to get the correct waveforms with no success. Here's the theoretical waveforms for the primary side:

And here's what the simulation gave me:

The voltage waveform looks clipped because the IRF250 has a breakdown of 200V. Not intending to operate at this level. The characteristics of the transformer are 9:30 turns ratio, 22 uH magnetizing inductance, and a 2.4 uH leakage inductance if anyone's interested. The gates of S1 and S2 are driven by 15V signals opposite each other and at 60V a duty cycle of 40% gets it close to a 340V output (370W).

My question is, is there anything glaringly wrong with the simulation? If not, what could I be doing wrong?

• Check that the transformer is not a "DC trafo". Better use coupled inductors. It looks like 24.4u and 244.4u will give you those values. I don't knwo how Multisim does it, but also make sure that the drivers are correctly driving the switches relative to their source. And is V1 a pulsed source? Shouldn't that be a DC level for the PWM block afterwards? – a concerned citizen Apr 15 at 22:41
• Beware how you are driving Q2. You have the source flapping around at a high voltage node. In an actual circuit, you will blow your driver circuit and probably destroy Q2. Since this is for educational purposes, replace the MOSFETs with a switch component (LTspice has a SW part which has floating inputs) and add in the parasitic diodes and capacitances. – qrk Apr 15 at 23:12
• @qrk Since I would like to learn as much as possible, how would you suggest I drive Q2? – justaguy Apr 15 at 23:54

• Nicely done! Two things: 1) there is no need for L3 since those values are meant to be used with a coupling of 0.9 (2.4u for a 22u magnetizing inductance means about 0.9), but it won't hurt much this way, either, and 2) instead of the default diode, D, use MUR460, or something similar. And if you want a complementary pulse generator with more flexibility, see this (td sets the dead-time, but it can be used to set the duty cycle, too). If you just want to see "how it works", using VCSW as in the link will be much faster. +1 – a concerned citizen Apr 16 at 7:11