I am having a problem with the circuit below.

I am trying to use the IRLR8726TRPBF to switch a 24V pump by turning the Gate HIGH and LOW using the Arduino. However, there when I connect power to the board, there is a short between the Gate and the Source of the MOSFET.

enter image description here

Here is my power supply circuit. The GND common everywhere. enter image description here

Before I noticed the short, I would turn the Gate HIGH and the MOSFET would get really hot quite fast. And it would switch an LED but for long, then it would go off.

What am I doing wrong? Any suggestions on how to solve the problem? Thank you.

  • 1
    \$\begingroup\$ Gate-source short indicates device damage - possibly ESD. Are you using proper ESD procedures? Getting hot fast may be related, or suggests either poor gate drive (though 5V should be OK in the absence of a short) or inadequate heatsinking for the power dissipation (that's easy math) \$\endgroup\$ – Brian Drummond Mar 8 '18 at 11:32
  • \$\begingroup\$ Are you sure you have it pinned out correctly? The schematic pin numbers match the datasheet, but we can't see your actual PCB design. Do you have a non-isolated heatsink pad connected to the tab? Have you tried a second MOSFET (to see if this one is damaged)? \$\endgroup\$ – DerStrom8 Mar 8 '18 at 11:32
  • \$\begingroup\$ Also, 24V is pretty close to Vds breakdown; any spikes over 30V, say, from brushes on the pump, will kill it. Your flyback diode dumps any spike into the supply ... but how stiff is the supply, do these take it over 30V? \$\endgroup\$ – Brian Drummond Mar 8 '18 at 11:35
  • \$\begingroup\$ Also beware of the miller capacitance. I have had two blown up CPU ports on my prototype. I can't prove it was that but after adding a gate resistor plus capacitor non of the 5000 produced have blown-up since (Funny: it was down-voted by somebody who claimed the RC was not needed). \$\endgroup\$ – Oldfart Mar 8 '18 at 11:44
  • \$\begingroup\$ Unrelated, but I noticed it: 120R seems pretty low to a LED at 24V. Unless it is a high power led. I am thinking of a small status led. \$\endgroup\$ – ricardomenzer Mar 8 '18 at 11:50

Check that the flyback diode is installed with the correct polarity.

That is the only thing I see that would explain all three of your observations.

The MOSFET getting hot would be due to excessive current. You have enough gate drive voltage.

Gate-source short could be due to overheating of the die.

The LED going out could be due to the diode melting inside into a short or due to MOSFET damage. It's only a 1A diode and that MOSFET is quite capable of driving enough current to get sufficient forward voltage to light a red LED (some tens of A or more).

Incidentally, it's important to check the actual PCB and part and not just the silk screen vs. component, because it's possible to get the mapping from schematic symbol to footprint wrong, especially the first time you've used a newly minted symbol-footprint combo.

  • 2
    \$\begingroup\$ You are right. The polarity of the diode was inverted during assembly. Thank you so much. I hope that is the only problem. \$\endgroup\$ – B. Cleo Mar 8 '18 at 12:19
  • \$\begingroup\$ would you be so kind as to explain the effect of reversing the polarity of the flyback diode? How would it lead to a short? \$\endgroup\$ – B. Cleo Mar 8 '18 at 12:23
  • \$\begingroup\$ R52 looks low as commented above. Also it's often a good idea to add a series gate resistor which might save the Arduino in case of a gate-drain short. \$\endgroup\$ – Spehro Pefhany Mar 8 '18 at 12:23
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    \$\begingroup\$ If you reverse the diode it conducts when the MOSFET turns on, so it will conduct a great deal of current. Diodes are not perfect so it can have a few volts across it when that way. \$\endgroup\$ – Spehro Pefhany Mar 8 '18 at 12:26
  • 2
    \$\begingroup\$ That is exactly what happened. I going through the design right now and I just noticed that the pinout of the diode match the footprint. But it is inverted. \$\endgroup\$ – B. Cleo Mar 8 '18 at 12:50

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