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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?

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    \$\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 '18 at 13:16
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    \$\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 '18 at 13:19
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    \$\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 '18 at 15:55
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    \$\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\$ – Spehro Pefhany Oct 27 '18 at 16:07
  • \$\begingroup\$ Thanks a Lot guys! I shall rebuild and add the zener and report back! \$\endgroup\$ – Paul Ferreira Oct 27 '18 at 22:19
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  • 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).

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  • \$\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\$ – Paul Ferreira Oct 27 '18 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\$ – Paul Ferreira Oct 27 '18 at 22:44

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