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Pfet as a power switch

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Mosaic

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Hi,
I want to control a < 40VDC line @ around 10A max with a PFET as a switch, high side.

Would the parasitic diode cause leakage when the PFET is switched off by a 0 Vgs?
Could that diode 'blow' shorted and disable the PFET as a high side switch?
 

alec_t

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Most Helpful Member
If Vgs=0 there should be negligible leakage.
If the body diode (or the mains source-drain path) fails shorted then yes, the PFET fails to shut off the supply.
 

spec

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Considering this one as an option with a basic bolt on heat sink.
http://www.farnell.com/datasheets/136391.pdf?_ga=1.165570815.1386623266.1453766078
Hi Mosaic,

You would be sailing a bit close to the wind with an IRF5305 switching 10 Amps.

The IRF5305 RDss is 60mR at 25 Deg C junction temperature. Add a factor of 1.5 for the actual junction temperature and you get, 1.5 * 60mR= 90mR.

At 10A ID that would give a voltage drop of 900mV and a power dissipation of 9W.

The other thing about the IRF5305 is that its thermal resistance, junction to case, is 1.5 Deg/C/W which means that you would need a sizeable heat sink.

Also the IRF5305 needs about 10V gate drive.

The IRF5305 is a nice PMOSFET and is widely used but, for your application, I would suggest one of the newer PMOSFETs with the following characteristics:

RDss at 25 Deg C: 6mR Max

Thermal resistance Junction to case 0.5 Deg/C/W Max

That would give an operating voltage drop at 10A of, 10A * 1.5 * 6mR= 90mV and a dissipation of 900mW so, depending on the temperature in your equipment, a heatsink would probably not be required.

If you would like to change MOSFET type but do not want to search for a suitable PMOSFET I can give you a suggestion.

spec

PS: If you are using pulse width modulation a few other issues arise
 
Last edited:

spec

Well-Known Member
Most Helpful Member
Hi,
I want to control a < 40VDC line @ around 10A max with a PFET as a switch, high side.

Would the parasitic diode cause leakage when the PFET is switched off by a 0 Vgs?
Could that diode 'blow' shorted and disable the PFET as a high side switch?
When using a MOSFET as a switch the drain/source parasitic diode only becomes a problem when the load retains a voltage (if the load were a battery for example) and the source goes to a low resistance when the source is turned off. In this situation the load will feed current back into the source through the parasitic diode. Otherwise you can forget about the parasitic diode as it will be reverse biased.

If you ever need to negate the effect of the parasitic diode you can use two MOSFETs in series in a 'reverse blocking' configuration. http://www.electronic-products-desi...ics/mosfets/using-mosfets-as-general-switches

MOSFETs can have relatively high leakage currents though. For the IRF5305 the leakage current can be 250uA, but with bigger MOSFETs the leakage current can be a couple of mA.

spec
 
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MikeMl

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Another caution: Most Power FETs (both N and P) have a limited Vgs rating, usually far short of 40V, so you will have to go to some effort to limit the gate voltage. In other words, you cannot simply connect the source to +40V, and ground the gate to turn on the PFET. That would put the full 40V from source to gate...
 

Mosaic

Well-Known Member
I worked out the LTspice sim limiting the Vgs etc driving the PFET with an NPN from a uC pin. Split the 36V with 2 10Ks for 18Vgs, all's fine. But I'm still looking for an alternative as the ultimate purpose is to add a layer of redundancy in a battery charger. This way adds components in a tight PCB and can reverse bias the PFET body diode being a battery load.

My purpose in all this => Turns out I got a concatenated failure over 2 months....first the NFET temp sensing current limiter went, which then stressed the NFET drivers which eventually failed shorted and caused battery overcharge.
I had an inline 8A glass fuse, but depending on the battery draw it may not cause the fuse to blow. Thus either a P-FET secondary e-fuse or perhaps a crowbar for the glass fuse.

I just patched the firmware to 'notice' current flow when the NFEts are supposed to be off and display an A/V alert. But this doesn't stop current flowing in an unattended situation with shorted drivers.

I'm looking at triggering a nichrome loading circuit segment which 'exercises' batteries (when not under charge) as a means to slam a half Ω load across the output for a couple seconds as a crowbar for the glass fuse.
 

Mosaic

Well-Known Member
Ended up using an IRFP3206 n channel low side as the switch as it won't need heat sinking etc.
Left the glass fuse in place , in case of a hard short due to wiring error or solder balls.
 
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