Trying to develop MOSFET intuition

[Peter S. May]

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

 

[Jan Panteltje]

I'm thinking that this is an opportunity for me to learn about MOSFETs
since I've never used one before.

http://en.wikipedia.org/wiki/MOSFET

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.

 

[Peter S. May]

http://en.wikipedia.org/wiki/MOSFET

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.

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

 

[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

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.

 

[Jan Panteltje]

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.

And I forgot Ron, case type, thermal resistance, max junction temp,
price, second source, Pmax, Id max, and a few more.

All of these may or may not be important.

 

[Peter S. May]

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.

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

 

[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.

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.

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.

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).

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?

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

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

OK
It is easier than making an animation in Blender.

 

[D from BC]

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

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


D from BC

 

[Ecnerwal]

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.

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)

 

[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)

For fun..
The most expense P mosfet I can find on Digikey is $12.07Can each
http://ca.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=IXTH10P50-ND


The most expensive N mosfet I found is $94.23Can each.
http://ca.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=IXFN36N100-ND
D from BC

 

[Fred Bloggs]

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

That doesn't look like an auto-select circuit to me.

 

[Paul Mathews]

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

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

 

[Jim Thompson]

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

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

Posted originally in 2004....

http://analog-innovations.com/SED/PerfectDiodeForChargerIsolation.pdf

...Jim Thompson

 

[Peter S. May]

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

That doesn't look like an auto-select circuit to me.

I think I accidentally put a cross where there was supposed to be a
joint. The correction is where VREG crosses between the gate and the
33k resistor:

NDS352P 1N5817
[VBUS>---+--+--[S G D]---->|--+-[+5V>
| | | |
| | | |
[VREG>---|--|-----+------->|--+
\ | \ 1N5817
100k / =.1uF / 33k
\ | \
| | |
+--+-----+--| GND

The selector appears in context on page 54 of the board's datasheet:
http://ww1.microchip.com/downloads/en/devicedoc/51526a.pdf

Whoops
PSM

 

[Jan Panteltje]

I think I accidentally put a cross where there was supposed to be a
joint. The correction is where VREG crosses between the gate and the
33k resistor:

NDS352P 1N5817
[VBUS>---+--+--[S G D]---->|--+-[+5V>
| | | |
| | | |
[VREG>---|--|-----+------->|--+
\ | \ 1N5817
100k / =.1uF / 33k
\ | \
| | |
+--+-----+--| GND

The selector appears in context on page 54 of the board's datasheet:
http://ww1.microchip.com/downloads/en/devicedoc/51526a.pdf

Whoops
PSM

A safer way to draw is to avoid crossing wires altogether in case of a connection:
NDS352P 1N5817
[VBUS>---------[S G D]---->|----[+5V>
| | | |
| | |------->|--
[VREG>---|--|-----|
\ | \ 1N5817
100k / =.1uF / 33k
\ | \
| | |
------------| GND

 

[Allan Herriman]

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

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

Posted originally in 2004....

http://analog-innovations.com/SED/PerfectDiodeForChargerIsolation.pdf

Similar concept:
http://www.onsemi.com/PowerSolutions/product.do?id=NIS6111

It's a bit expensive though.

Allan

 

[Fred Bloggs]

I think I accidentally put a cross where there was supposed to be a
joint. The correction is where VREG crosses between the gate and the
33k resistor:

NDS352P 1N5817
[VBUS>---+--+--[S G D]---->|--+-[+5V>
| | | |
| | | |
[VREG>---|--|-----+------->|--+
\ | \ 1N5817
100k / =.1uF / 33k
\ | \
| | |
+--+-----+--| GND

That makes much more sense, the presence of Vreg turns off the Vbus
feed. You will find that the majority of enhancement mode MOSFET
applications are as "load switches" and, as the name implies, they are
used as a simple switch. The foremost parameters are the VGS,th,
gate-to-source threshold voltage, and RDS,on, the on state
drain-to-source channel resistance. Apply a gate-source bias well in
excess of VGS,th and the MOSFET state changes from a leaky drain-source
open circuit to a low resistance channel. N-channel devices require a
positive VGS, to attract negatively charged electrons into the N type
channel, and P-channel devices require a negative VGS, to attract
positively charged holes into the P type channel. A zero or reverse VGS
puts the MOSFET in a high impedance state.