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P channel Power MOSFET - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.



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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