I have PWM of very low frequency and very low duty cycle. And max voltage comes out are 3.3. But thats not enough to open the MOSFET's gate(IRFP250N). Now I want to amplify the voltage. I do not have op-amp right now. But I tried to use LM393P comparator but its seems too slow. So if I put LED direct to PWM output I can see it blinking but if I put it on output from comparator it stays on. So I assume comparator is not switching fast enough to make it blink.

Now, I'm thinking of using PNP transistor to switch on the MOSFET gate, but I'm not sure which one to buy ? or should I need to try something else ?

PS: I'm learning through internet and still very beginner. So I might have missed something.

Edit :

  • PWM Frequency : 4Hz
  • PWM DutyCycle : 2ms
  • PWM Max Voltage : 3.3V

Edit 2 : For testing I used 12V instead of 200V.

And the test with LM393, the LED stays on, it doesn't blink. But if I turn off PWM, LED gets off too.

  • \$\begingroup\$ What is the largest voltage supply currently available in your system? Is it 3.3V ? \$\endgroup\$ – Nick Alexeev Sep 12 '14 at 5:56
  • \$\begingroup\$ 3.3V is PWM, however I need to control 80VDC through MOSFET. \$\endgroup\$ – xmen Sep 12 '14 at 6:06
  • \$\begingroup\$ How much current will pass through the transistor? \$\endgroup\$ – Dan Laks Sep 12 '14 at 6:25
  • \$\begingroup\$ 5-10V I assume, I think 10V is enough to open MOSFET gate. So yea 10V through transistor. \$\endgroup\$ – xmen Sep 12 '14 at 6:30
  • \$\begingroup\$ Please re-read my question. I asked how much current will pass through the transistor, not how much voltage you will apply to the gate. Better yet, post a schematic of your circuit. \$\endgroup\$ – Dan Laks Sep 12 '14 at 6:32

Based on the information you've provided so far, the schematic posted below will allow you to build something that works and that you can play around with using the active stuff you have on hand.

Since the LM393 is a dual comparator, you should spare out the unused one by connecting its + and - inputs, and its output, to ground.

If you want to, you can also connect your LED in series with R5 to watch it blink, but make sure you don't connect it backwards or you'll destroy it.

Also, watch out for that 80 volts; it can give you a nasty little sting.

enter image description here


The LM393 has an open collector output.
You MUST use a pullup resistor to use it to drive a load high.

There are many N Channel MOSFETs available which can be well driven at 3V. The IRFP250N is not one of them.

Low frequency PWM can be driven by a uC pin OK..
As frequency is increased a driver with more current drive capacity will be needed.

  • \$\begingroup\$ Yes I did use pullup resistor. Can you please tell me the names of N Channel MOSFETs that can hold upto 200VDC that can be open with 3VDC PWM. IRFP250N is just for testing till 80VDC. \$\endgroup\$ – xmen Sep 12 '14 at 12:52

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