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Old 08-02-2014, 12:54 AM #1
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Default Incredibly fast high current P-Channel MOSFET?

I've been playing around with some very high current buck driver designs, and I've run into a bottle-neck that I think is related to the MOSFET on/off time.

I need a P-Channel MOSFET that can deal with 2Mhz switching, which in theory means that the MOSFET has 500 ns to do it's business. I was using this:
http://www.vishay.com/docs/63262/sis413dn.pdf

But I think it might not be fast enough (or there may be other issues with the design - but I'm starting with the MOSFET as the weak link).

Does anyone have any suggestions for locating a fast switching optimized MOSFET? The normal sources don't seem to let you filter by such specs. I've already identified this as slightly better:
http://www.vishay.com/docs/67042/sq7415aen.pdf

Plus, it's pin compatible, which is nice. Anyone have any other thoughts?


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Old 08-02-2014, 01:58 AM #2
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Default Re: Incredibly fast high current P-Channel MOSFET?

What are the switching requiremts of your application? Need to know the Vds, Vgs, max RdsON, if reverse recovery speed will be important, Ids, and form factor.
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Old 08-02-2014, 02:14 AM #3
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Default Re: Incredibly fast high current P-Channel MOSFET?

That's not fast at all for a mosfet. But the gate current will be quite high. Be sure you're driving it appropriately.
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Old 08-02-2014, 02:40 AM #4
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Default Re: Incredibly fast high current P-Channel MOSFET?

Quote:
Originally Posted by Sigurthr View Post
What are the switching requiremts of your application? Need to know the Vds, Vgs, max RdsON, if reverse recovery speed will be important, Ids, and form factor.
I'll open the datasheet and grab some specs when I'm at a PC, but the IC in this case is the LM3475.
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Old 08-02-2014, 02:51 AM #5
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Default Re: Incredibly fast high current P-Channel MOSFET?

There's a reason high efficiency designs almost always use N channel fets even on the high side switch as there are a lot more choices and are usually better performing than the P ch counterparts.

Last edited by djQUAN; 08-02-2014 at 02:52 AM.
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Old 08-02-2014, 08:53 PM #6
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Default Re: Incredibly fast high current P-Channel MOSFET?

I decided to go oldschool on this and do some DIY boards for faster iteration. That was a throwback - I don't think I've etched boards in quite honestly 10 years. Vinegar, peroxide, and salt work remarkably well.

At any rate, a few boards later, and I've identified that the MOSFET wasn't the bottleneck. It was the current sense monitor I had been using, as well as the power source (a bench supply). I may need to order a new bench supply, because mine doesn't seem to like driving really high current switching regulators.

At any rate, I have this buck design with the LM3475 up to 10A - although with a lazy layout that is just dumping FB voltage directly over the set resistor(s).

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