Please help me understand this.

enter image description here

I built this circuit, exactly, with the exception of the Zener.

When I test the output of the Channel mosfet, it shows approx 2V below whatever voltage I used to trigger the gate on the N channel side.

For example, if I place 12V on the N channels gate, the P channel Drain, measured at the drain is 10.5V, and if I put 24V on the N Channel's gate I get 21.3 on the P channel's Drain.

I have searched and read and all seems to point that I should be getting approx 24V (VDD)-IlRDS at the drain.

What on earth am I not understanding here? Surely, irrespective of whether I apply 12 or 24 to the N channels gate, should I not be getting 24V-IlRds on the P channel's drain? Why am I getting 10.5 or 21.3V on the drain?

  • 4
    \$\begingroup\$ If you're putting 24V on the gate of the IRF3205, you've destroyed your IRF3205. It's rated for a maximum \$V_{GS}\$ of 20V. \$\endgroup\$
    – Hearth
    Oct 27, 2018 at 13:16
  • 3
    \$\begingroup\$ On top of that, if that zener diode is not populated as stated in your question, you've probably also destroyed your IRF4905, because when the n-channel fet is on, \$V_{GS}\$ on the p-channel one goes to -21.8V according to a quick simulation, which is also exceeding the maximum of ±20V. \$\endgroup\$
    – Hearth
    Oct 27, 2018 at 13:19
  • 3
    \$\begingroup\$ Also are you sure you got the pin assignments of the FETs correct? A photo of your physical circuit could help us check that. \$\endgroup\$
    – The Photon
    Oct 27, 2018 at 15:55
  • 2
    \$\begingroup\$ Tend to agree with @ThePhoton here, and think there is a connection issue. While you are exceeding the abs max ratings, they virtually never fail that close (24V vs. 20V) to the rating. \$\endgroup\$ Oct 27, 2018 at 16:07
  • \$\begingroup\$ Thanks a Lot guys! I shall rebuild and add the zener and report back! \$\endgroup\$ Oct 27, 2018 at 22:19

1 Answer 1

  • I built this circuit, exactly, with the exception of the Zener.
  • I put 24V on the N Channel's gate

Both MOSFETs are irreparably compromised due to excessive voltages between gate and source due to: -

  1. Using a 24 volt supply and removing the zener (damaging the P-channel FET)
  2. Feeding 24 volts to the gate on the N-channel FET

Once damaged, both MOSFETs will conduct in "strange ways" because the gate-source-drain insulation has broken down. Respect the maximum gate-source breakdown voltage of MOSFETs (usually a maximum of 20 volts but can be less).

  • \$\begingroup\$ Thanks @andy-aka, FYI I learnt alot from your other answers on other posts. I shall retry with new mosfets and report back. \$\endgroup\$ Oct 27, 2018 at 22:20
  • \$\begingroup\$ Hi GUys, yes indeed. I added the 15v Zener and used 12v at the gate and it worked fine! Thanks again! \$\endgroup\$ Oct 27, 2018 at 22:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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