0
\$\begingroup\$

I'm making a simple design where a power N-channel MOSFET is driven from a microcontroller pin. In all sources I can find they say it can be done using a single resistor. However, my friend says that due to large gate-to drain capacity of the MOSFET (I' using IRF3205, which has 3.2 nF gate capacity) the microcontroller pin circuit can be destroyed by a higher drain voltage (24V). He says that he experienced it and recommends placing a Zener diode from gate to 0.

I never read about this possibility before, so the question is: can this happen? Can a 24 volt drain voltage destroy a 6 volt CMOS transistor pair via 3nF capacitor or it is just an imagination?

| improve this question | |
\$\endgroup\$
  • 4
    \$\begingroup\$ You need a gate driver. Everything else is bullshit. \$\endgroup\$ – Gregory Kornblum Dec 2 '17 at 7:37
  • \$\begingroup\$ @GregoryKornblum, please explain further. is the gate driver an IC? \$\endgroup\$ – jsotola Dec 2 '17 at 8:00
  • 2
    \$\begingroup\$ It can be. But you should better google it and get all available information. \$\endgroup\$ – Gregory Kornblum Dec 2 '17 at 8:10
  • \$\begingroup\$ What switching frequency? Nature of load? \$\endgroup\$ – bobflux Dec 2 '17 at 10:00
  • \$\begingroup\$ Switching frequency is very low, it's just on-off switch. Load is inductive. Low R. \$\endgroup\$ – AlexN Dec 2 '17 at 10:30
-1
\$\begingroup\$

Yes! I speak here of experience. I blew up two micro controller ports because I did not do that. One switched twice before it went. I was driving a relay and after the ports blew I checked about ten times if my protection diode was the right way around. It was correct and still my ports died.
A zener diode is expensive. Much cheaper is an RC circuit. I have been 'naughty' as I put the capacitor direct over the output pin. Thus the controller has to drive a rather high capacitive load. The pull-down resistor is because at start-up the I/O pin is floating and I want the FET to be guaranteed off.

Cut out of FET driver

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ I also use 2 resistors for the same reason. Isn't it better to place a capacitor at the other side of R42 and make its capacity not so large? This will increase the switching time, but it is not always needed. \$\endgroup\$ – AlexN Dec 2 '17 at 12:29
  • \$\begingroup\$ So it blew due to a spike going through Cgd? \$\endgroup\$ – bobflux Dec 2 '17 at 12:31
  • \$\begingroup\$ So with zero frequency the MOSFET will get hot from a single switching. \$\endgroup\$ – Gregory Kornblum Dec 2 '17 at 12:59
  • \$\begingroup\$ @AlexN I wanted to have an RC filter. The way I see it, this way the inductance of the capacitor is not an issue. Which may be an issue if the spike frequency is very high. The OD0 switch time is very, very slow (seconds). The 'naughty' cap was because I did not have PCB space left for another resistor and it is another cost. (This is in a commercial product)<br>@peufeu I don't know why they both blew up but it was my best guess and with the new circuit there are thousands of boards out there still alive. \$\endgroup\$ – Oldfart Dec 2 '17 at 13:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.