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I've designed in reverse current protection on a board that I'm designing. The input voltage is 24V with a potential max current draw of 1A (depends on what is plugged in, parts of the PCB are modular). enter image description here

Here is the P-FET https://docs.rs-online.com/7785/0900766b80f35049.pdf

According to the datasheet the Max continuous current drain is 2.3A at ambient vgs@10V . The package seems too small to support currents in the ampere range. Would this be an appropriate choice for this circuit or do I need to go for a bigger package with higher current drain? thanks

ChrisD91
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  • If you doubt the manufacturers specification, order one and test it out ? – Sorenp Jun 18 '21 at 12:22
  • Reverse current or reverse polarity? There is a difference between connecting input voltage backwards and having that reverse polarity push current into the circuit versus the load side dumping power and pushing a reverse current back to the source. A simple diode protects against both. A simple PMOS circuit like yours only protects against the former. https://electronics.stackexchange.com/questions/525248/is-this-mosfet-upside-down/525249#525249 Also think about what voltages appear between PMOS pins if you connect 24V in reverse. Vds isn't the only max voltage rating. – DKNguyen Jun 18 '21 at 13:36
  • @DKNguyen while a diode does both, it will also produce a lot of heat at currents above 1A. This circuit is really only intended to be a simple way to protect the board if upon installation the power is put in backwards .Thanks, I'll check the link out. – ChrisD91 Jun 18 '21 at 13:45
  • I know. I wasn't suggesting you replace it with a diode. Just suggesting some things are taken for granted with a diode. – DKNguyen Jun 18 '21 at 13:47
  • Interesting, looking at the link I hadn't considered the possibly of the load pushing power to the source. Not an issue in my application, but something I'll bear in mind. – ChrisD91 Jun 18 '21 at 13:49
  • Usually current limits are specified with a *huge* thermal pad (typically 1sq/in or 6sq/cm, read note 3 on the datasheet). The rdson at 0.165 is not really great and 100°C/W is typical for a SOT23. At 2.3A it's 800mW which is about +90°C of heating. I agree with the datasheet. It all depends on your Ta and current needs – Lorenzo Marcantonio Jun 29 '21 at 12:34

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Absolute maximum rated G-S voltage is +/-20V and you're apply 24V (plus any transients). That could have reliability consequences. It can be fixed with a zener.

The 100K is rather high- you'll have quite slow switching so the MOSFET SOA limits may be violated.

Also, keep in mind that there are probably static brown-out conditions (input voltage) that could cause the MOSFET to neatly unsolder itself and fall off the board.

Spehro Pefhany
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  • I added a 6V Zener as it seemed quite difficult to get an appropriate FET with a G-S of more than +/-20V .Most of what I was looking at are rated at +/-15V – ChrisD91 Jun 18 '21 at 13:53
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    @ChrisD91 You are forgetting zeners act as a short and will clamp the voltage to the load 6V as it fries. Reposition the zener so it only clamps Vgs and not Vout. – DKNguyen Jun 18 '21 at 13:56
  • @ChrisD91 Why wouldn't you use a 12V zener when the MOSFET is already a bit on the high side in Rds(on)? More voltage = lower Rds(on). – Spehro Pefhany Jun 18 '21 at 13:56
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    @DKNguyen Zener goes from gate to source. – Spehro Pefhany Jun 18 '21 at 13:57
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    My comment was meant for OP, sorry. – DKNguyen Jun 18 '21 at 13:58
  • I just realised the layout mistake, thanks for pointing it out! Also, I'm using a 6V Zener for another part of the circuit so I was looking to keep the BOM smaller. I can always swap for 12V if it would make a difference. – ChrisD91 Jun 18 '21 at 13:59
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    Surface tension should be enough to retain a desoldered SOT23 :) it depends on the vibration I guess, – Jasen Слава Україні Jun 19 '21 at 00:24