I am trying to design a simple buck converter. But it doesn't work as expected. Output voltage should be 19V but it is very close to input with value 48.5V. Also there is a problem with pwm generator. If anyone could help,I will appreciate for it.Thanks.

enter image description here

With this configuration pwm generator (comparator LT1001) doesn't work well, but seperately it works. enter image description here

And the Output: enter image description here

feedback goes through differential opamp: enter image description here

  • \$\begingroup\$ For a start, the MOSFET is connected backward, no flyback diode? \$\endgroup\$
    – G36
    Commented Jun 19, 2020 at 20:14
  • \$\begingroup\$ You never turn the FET off for a start, plus it's reversed, so the body diode conducts. Plus yu have almost certainly exceeded its Vgs(max) and destroyed it. \$\endgroup\$
    – user16324
    Commented Jun 19, 2020 at 20:21
  • \$\begingroup\$ where's your diode ?1 (between the common tap of mosfet & inductor and Gnd) \$\endgroup\$ Commented Jun 19, 2020 at 20:22
  • \$\begingroup\$ sorry, I should have removed diode accidentally. I edited the results \$\endgroup\$
    – T B.
    Commented Jun 19, 2020 at 21:00

1 Answer 1


enter image description here

  • The MOSFET has an internal parasitic diode that will always cause conduction from the input voltage to the output voltage.

  • The MOSFET's gate will be destroyed by ineffective gate voltage control

I'm stopping counting the faults now!

  • \$\begingroup\$ More things: 1. Why R7 and R8? Your fb node is set for a ~5/31 ratio to your output, so 19V you'd expect ~3V at the fb node. With R7 and R8 your reference is 1.5V so you'd regulate to only ~9.3V. 2. Please add some compensation to U3, for pure voltage mode control you need a Type 3 compensator. 3. Your ramp looks too triangley, you want it more saw-toothy (these are the technical terms). 4. You need V+ to be higher than V1 or else the NMOS FET you chose won't turn on. Idk if LTSpice has a high side gate driver, but use that also. \$\endgroup\$
    – Stiddily
    Commented Jun 19, 2020 at 21:27

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