I'm working on a project to power a 56 volt BMS with a buck converter that bucks 120V DC (rectified from 120AC) to 56V DC, but I've gotten to the point where I'm hard stuck.

Here is what I have so far.

I'm most likely driving the N-Channel MOSFET wrong. I've tried to make higher voltage versions of other buck converters online but adapted it to the materials I have.

Ignore the voltage reading, also instead of an IRF840 I have a C2M0280120, but Multisim doesn't have it listed. Also, instead of that PWM generator in the bottom left, I have a TL494 at 250kHz in real life, but that isn't too relevant right now. I'd appreciate any insight.

The TL494 has a feedback function, but I'm not using it right now. Right now I'm trying to get a proper buck converter in its most basic form. The TL494 gives me duty cycle control with a potentiometer, so I'm using the duty cycle to check how well it accurately bucks at different duty cycles, but at the moment the circuit is simply not wired correctly at all and I'm stuck on finding how to solve that issue in particular.

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  • 7
    \$\begingroup\$ There are dedicated chips for buck converters. 120 VAC has a peak value of 170 V, so your DC peaks to that value. You'll need to have feedback from the output to control the buck converted. Dedicated ICs have the feedback loop built-in. \$\endgroup\$
    – Ralph
    Commented Mar 1, 2022 at 8:55
  • 4
    \$\begingroup\$ Head over to Texas Instruments' site and have a look at Webench. Input a few parameters and it will give options based on available components, and you can even do some basic simulations, such as transient response webench.ti.com/power-designer/switching-regulator?powerSupply=0 \$\endgroup\$
    – raaymaan
    Commented Mar 1, 2022 at 9:09

1 Answer 1


There is indeed a problem with your circuit wiring. The supply to U1 needs to be relative to the source of Q2, not the circuit ground. i.e. the ground of V3 should be connected to the cathode of D1, otherwise your MOSFET will not turn on correctly.

  • \$\begingroup\$ I just tried it, thank you it worked instantly, I just have to make other adjustments to my inductor and capacitor to get the type of output I'm looking for but overall you have been unbelievably helpful. \$\endgroup\$ Commented Mar 1, 2022 at 9:36
  • \$\begingroup\$ You're welcome. Good luck. Since you're new here, don't forget to upvote and accept the response that answers your question. \$\endgroup\$
    – mr_js
    Commented Mar 1, 2022 at 9:40

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