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P Channel Power MOSFET - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement



lna
Post subject: P channel Power MOSFET Posted: Wed Jun 10, 2009 12:32 am

Captain

Joined: Mon May 04, 2009 6:47 pm
Posts: 22
Hi, there. Can someone suggest a p channel power mosfet I need such one to switch +28VDC rail max current of 2A. The MOSFET is controlled by a open drain output comparator, which is 5VDC powered. As I can see, the max IGS of the MOSFET needs to be larger than +/- 25VDC. I have checked onsemi and found nothing meet my requirement.

Thanks!


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Mike_From_Plano
Post subject: Re: P channel Power MOSFETPosted: Fri Jun 12, 2009 2:20 pm

Captain

Joined: Mon Apr 20, 2009 2:04 pm
Posts: 14
Hello,

I don't think you'll find a MOSFET with such a high gate punch through, but that's not the end of the world, you just need to modify the circuit slightly.
I'd send you a picture, but I don't know how to get it across... SO, here goes a way to describe what you need:

BOM:
1. (1) Q1, MMBT3906 (or other NPN)
2. (1) Q2, FDD8685 (or similar P-MOSFET)
3. (1) R1, 2.2K, 1/10W, 5%
4. (1) R2, 2.7K, 1/10W, 5%

Netlist:
R1, Comparitor_out, NPN_Emitter
Q1, (E)NPN_Emitter, (B)+5V, (C)MOSFET_Gate
R2, MOSFET_Gate, +28V
Q2, (S) +28V, (G) MOSFET_Gate, (D) Switched_28V

I hope this isn't too much a mess, communicating it by net list. Also, this circuit is only valid for slow switching. If you need something in the kHz, this won't get it because the MOSFET turns on & off very slowly.

If you need something fast, post it, and I'll give you something a little more complicated + faster.

- Mike


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Mike_From_Plano
Post subject: Re: P channel Power MOSFETPosted: Fri Jun 12, 2009 6:27 pm

Captain

Joined: Mon Apr 20, 2009 2:04 pm
Posts: 14
Oops,
Q1 should be MMBT 3904 (the NPN version!)
and
R2 should be 4.7K

I plead old age....


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lna
Post subject: Re: P channel Power MOSFETPosted: Tue Jun 16, 2009 9:39 pm

Captain

Joined: Mon May 04, 2009 6:47 pm
Posts: 22
Thanks a lot, Mike. This is a good solution but the logic is reversed to my application so a NOT gate is needed to fix the logic, or maybe connect comparator output to base and emitter to GND.

Is it possible to control P-channel MOSFET gate by a N-channel FET? I found some N-channel FEt with 30VDC VDS however max VDG is not specified . I wonder why VDG is not important.







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