Trying to develop MOSFET intuition

I'm trying to rebuild parts of the Microchip PICDEM FS USB board schematic partly as an experiment but mostly as a learning experience. There's a part of the circuit that appears to allow the device to auto-select between USB bus power and wall-brick power, that looks sort of like this:

NDS352P 1N5817 [VBUS>---+--+--[S G D]---->|--+-[+5V>

| | | | | | | | [VREG>---|--|-----|------->|--+ \ | \ 1N5817 100k / =.1uF / 33k \ | \ | | | +--+-----+--| GND

From what I read, NDS352P is a P-channel MOSFET. I also can't seem to locate this part on Allied, and didn't have any better luck with a couple of cross-references.

I'm thinking that this is an opportunity for me to learn about MOSFETs since I've never used one before. I only recently really learned about BJTs, and can only really keep it straight because, in general, only a small selection of the figures are relevant: hFE (the current gain Ib to Ic), which isn't crucial when used as a switch instead of an analog amplifier, Vce (voltage drop collector to emitter, i.e., how much less than Vcc is dropped across the output load), and max Ic (how much current can be passed through the load).

What are the important (analogous?) figures for a MOSFET? What features should I be looking for in a replacement for NDS352P?

Thanks PSM

Reply to
Peter S. May
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On a sunny day (Thu, 11 Oct 2007 10:19:04 -0400) it happened "Peter S. May" wrote in :

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I could write about MOSFETS for hours, but somebody already did a good job in the above link.

Also: Buy some of different type and experiment.

Reply to
Jan Panteltje

I've actually read quite a bit about how they work, but it is information overload--much like I don't have to think about PN junctions, electrons, and holes to know the high-level functionality of BJTs, surely I don't have to have every word of the Wikipedia article on MOSFETs down to be able to use the things.

In other words, I humbly beg for a summary of, say, the three to five most relevant stats to look at, because I lack the talent to pick out the important parts of a wordy article. :-)

Thanks PSM

Reply to
Peter S. May

On a sunny day (Thu, 11 Oct 2007 10:56:23 -0400) it happened "Peter S. May" wrote in :

Yes, sure, but if you just bought some, and / or looked up the datasheet, you would find out!! What is important depends on what you want to do. Speed (Ton / Toff) may not be important, input capacitance may not be important, gain (A/V) may not be important, cut off point (Vgs for Id zero) may not be important, drain source voltage (Vds max) may not be important, Hell in the time I typed this you could look up XXXFET.pdf and study it.

I cannot drink water for you, and I cannot give you hands on experience.

Reply to
Jan Panteltje

Fair enough--let's get some hands-on experience.

In reading so far, I get the impression that the 2N3904 and the 2N2222 are generally universal representations of the NPN transistor (and

2N3906 for PNP), and I've found this out for myself by using a few. If there is a handful of similarly ubiquitous MOSFETs, which do you think they'd be? I ask this primarily because there is a wide variety and can't tell the difference between the common and the extreme.

Also, here's a question I have about another specific application that just came to mind: I have an arbitrary power-hungry 5VDC circuit (let's say 500mA) that I want to be able to switch on and off using HC logic. The Vce on a bipolar transistor for that kind of load is too steep, but MOSFETs seem to act a bit more like normal switches. Would one be appropriate here?

Thanks, and I apologize if I'm trying your patience... PSM

Reply to
Peter S. May

On a sunny day (Thu, 11 Oct 2007 12:09:54 -0400) it happened "Peter S. May" wrote in :

Yes, but those are very old. My TUN and TUP (Transistor Universal NPN, Transistor Universal PNP) are in Europe more like BC548, BC558. Those are small signal Si. Universal is perhaps also 2N3055....., the BD series.

Well, I have a lot of IRFZ44A laying about, high current low voltage switch. Also some dual gate MOSFETS BF191 BF192, very nice for RF circuits. Dual gates are magic! Used a lot of BUZ44A for higher voltage (no longer made I think).

I have some IRLZ34N, those will switch on from 5V logic, max treshold voltage is 2V, it carries max 30A, and allows 50V.

OK It is easier than making an animation in Blender.

Reply to
Jan Panteltje

A mosfet is like a driving a complicated capacitor and it conducts like a complicated resistor.

D from BC

Reply to
D from BC

Go to any of the parts house that have stock numbers listed. Search for mosfets. Now sort by "Number in stock" (Newark does this easily, I don't recall if there others typically allow you to sort by it in results or not). Buy some from the "several thousand in stock" class, those are probably pretty ubiquitous.

Heavy duty, old-school, probably terrible numbers per more up to date parts, still work, cheapish to buy - IRF540 (IRF9540 for P-channel)

Slightly more modern, find some on a junk hard drive board near you for free (not every one, of course, but they seem common there), 2 per package - IRF7101 (not all that common per the above test, however - probably obsoleting faster, while the 540 has decades of staying power)

--
Cats, coffee, chocolate...vices to live by
Reply to
Ecnerwal

For fun.. The most expense P mosfet I can find on Digikey is $12.07Can each

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The most expensive N mosfet I found is $94.23Can each.

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?name=IXFN36N100-ND D from BC

Reply to
D from BC

The Schottky diodes in the circuit allow whichever voltage is higher to supply power to the circuit. It's not clear why anyone thought they needed the PFET in this case. At most, it might reduce the magnitude of a power up surge, since the 33K slows turn on slightly. The C is simply for bypass. The 100K discharges C when VBUS goes away.

Simplest mental model of MOSFET: VOLTAGE Gate to source reduces RESISTANCE between Drain and Source (Rdson) when Vgs is of correct polarity for type of MOSFET. For Vgs > threshold, Rdson doesn't decrease much further. A BODY DIODE shunts Drain and Source, such that Drain and Source act like a diode for reverse polarity Vds. Dynamic capacitances, gate spreading resistance, gate oxide breakdown, and thermal characteristics complicate the model. Paul Mathews

Reply to
Paul Mathews

Posted originally in 2004....

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...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Similar concept:

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It's a bit expensive though.

Allan

Reply to
Allan Herriman

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