Mosfet question -- substrate connected to source

Tom McAndrews · Jan 20, 2004

This question is geared toward the enhancement mode mosfet devices
that have the substrate connected to the source.

Since the device is not symmetrical, what is the downfall of driving
current from the source to the drain and effectively switching their
roles when the substrate is connected to the old source?

Thanks in advance.

Dave Platt · Jan 20, 2004

This question is geared toward the enhancement mode mosfet devices
that have the substrate connected to the source.

Since the device is not symmetrical, what is the downfall of driving
current from the source to the drain and effectively switching their
roles when the substrate is connected to the old source?

If you try to do this, the intrinsic substrate diode starts to
conduct. As I understand it, it's not a terribly great diode in terms
of either switching speed or forward voltage drop, but its presence
pretty much eliminates the ability to use the MOSFET in its usual way.
Remember, when you switch the connection roles around you do not
magically change the internals of the MOSFET - the substrate is still
connected to the "source" pin.

I have seen one interesting application for a MOSFET wired up in this
"backwards" fashion, in "Troubleshooting Analog Circuits" by Bob
Pease (page 164). It makes a nifty polarity-reversal protector for
power-supply-to-load hookups. For example, you install an NMOSFET on
the ground side of the load, with the drain grounded and the source
towards the load, and drive the gate via a resistor or voltage divider
from the high side of the power supply.

If the power supply is hooked up correctly, the positive voltage
biases on the MOSFET, _and_ forward-biases the substrate diode, so the
MOSFET switches on and allows power through to the load. If you use a
MOSFET with a low Rds[on], you end up with less voltage drop than
you'd have from a simple silicon-diode protector.

If the power is accidentally hooked up backwards, the substrate diode
is reverse biased (doesn't conduct) and the gate is pulled several
volts negative with respect to the source and drain and switches the
MOSFET off quite hard. The reverse-voltage standoff will be that of
the MOSFET - probably a lot more than you could get with a
low-forward-voltage-drop Schottky diode.

To quote Pease, "Yes, this circuit looks funny. Yes, it is correct,
and yes, it works well."

--
Dave Platt <dplatt@radagast.org> AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
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John Popelish · Jan 21, 2004

Tom McAndrews wrote:

This question is geared toward the enhancement mode mosfet devices
that have the substrate connected to the source.

Since the device is not symmetrical, what is the downfall of driving
current from the source to the drain and effectively switching their
roles when the substrate is connected to the old source?

Thanks in advance.

Since the drain to source path is paralleled with a diode, using the
fet as a conductive path only makes sense if it will always be turned
on when the drain and source are reversed or the applied voltage will
be less than about 300 millivolts. They are used this way as
synchronous rectifiers.
--
John Popelish

Ted Wilson · Jan 21, 2004

justlearning1@hotmail.com (Tom McAndrews) wrote in message news:<3064eebe.0401201300.6e3e5b4f@posting.google.com>...

This question is geared toward the enhancement mode mosfet devices
that have the substrate connected to the source.

Since the device is not symmetrical, what is the downfall of driving
current from the source to the drain and effectively switching their
roles when the substrate is connected to the old source?

Thanks in advance.

Another notable example of an instance where we make good use of the
integral diode is in totem-pole MOSFET drives for inductive loads.

In these applications, there has to be a delay between switching OFF
the conducting device and switching ON the device that was previously
OFF, in order to prevent shoot-through. What then happens is that the
inductor current commutates into the body diode of the device that was
previously OFF, thereby providing a continued path for the current,
and the diode is then shunted by the low Rd-s on of the MOSFET when it
is later turned ON. (Yes, it does conduct in both directions).

You kids have it easy these days though. Back in the eighties, you
had to be really careful using power MOSFETs in totem-pole
configuration. If one of the devices was OFF, but the current flowing
through its body diode, you couldn't simply turn ON the other MOSFET
to take over the current as this would 'pop' either the body diode of
the OFF MOSFET, or the d-s channel of the MOSFET that you had just
turned ON, due to the reverse recovery limitations of the in MOSFET's
body diode.

Regards

Ted Wilson

Veronique · Jan 21, 2004

Why is everybody talking about diodes? Power? Goodness.. there is only one
thing that this will do. If the voltage on the drain of the FET is more
negative then the source, then it's topologically the SAME as inverting the
device (exchanging the drain and source) and tying the substrate to the
drain. What that does is that it requires less voltage on the gate to turn
the device on. In your question, you forgot to mention something.. it this
pFET, or nFET?

Unless I'm wrong, the device is symmetrical (It's a FET afterall). The only
thing that changes is that you're effectively increasing the potential on
the substrate, thereby lowering the Vt of the device.. the rest depends on
whether you have a pFET or an nFET.

Veronique

"Tom McAndrews" <justlearning1@hotmail.com> wrote in message
news:3064eebe.0401201300.6e3e5b4f@posting.google.com...

This question is geared toward the enhancement mode mosfet devices
that have the substrate connected to the source.

Since the device is not symmetrical, what is the downfall of driving
current from the source to the drain and effectively switching their
roles when the substrate is connected to the old source?

Thanks in advance.

Jim Thompson · Jan 21, 2004

On Wed, 21 Jan 2004 16:27:47 GMT, "Veronique" <sunshyne@austin.rr.com>
wrote:

Why is everybody talking about diodes? Power? Goodness.. there is only one
thing that this will do. If the voltage on the drain of the FET is more
negative then the source, then it's topologically the SAME as inverting the
device (exchanging the drain and source) and tying the substrate to the
drain. What that does is that it requires less voltage on the gate to turn
the device on. In your question, you forgot to mention something.. it this
pFET, or nFET?

Unless I'm wrong, the device is symmetrical (It's a FET afterall). The only
thing that changes is that you're effectively increasing the potential on
the substrate, thereby lowering the Vt of the device.. the rest depends on
whether you have a pFET or an nFET.

Veronique

[snip]

480)460-2350 | |
| E-mail Address at Website Fax

Tim Wescott · Jan 22, 2004

I don't think that a MOSFET _has_ to be symmetric (you could build a
symmetric bipolar transistor, after all, and even "regular" ones have some
action when run in reverse). I just think that MOSFET designs tend to come
out more symmetric than bipolars do.

If you dig up a drawing of a MOSFET in a textbook you'll see that the
interfaces between the substrate and the source and drain are PN junctions
(a depletion-mode MOSFET will have one region that makes up the source,
drain and channel). This junction will have some bias on it, and will
conduct if it's a forward bias. So for a three-pin device you connect the
substrate to the source, and all of a sudden your device has a built-in
diode.

When they first started using power MOSFETs in switching supplies this was a
pain, because the diode is right where you put your snubber diode, only it
has first crack at turning on. Since the device is optimized for
MOSFETness, the intrinsic diode is usually a pretty bad snubber diode and
can have an extremely long reverse-recovery time. Everyone has learned how
to deal with this now, but it's still an issue to be ignored at risk.

"Veronique" <sunshyne@austin.rr.com> wrote in message
news:7MxPb.5900$6o4.876@fe2.texas.rr.com...

Why is everybody talking about diodes? Power? Goodness.. there is only one
thing that this will do. If the voltage on the drain of the FET is more
negative then the source, then it's topologically the SAME as inverting
the
device (exchanging the drain and source) and tying the substrate to the
drain. What that does is that it requires less voltage on the gate to turn
the device on. In your question, you forgot to mention something.. it this
pFET, or nFET?

Unless I'm wrong, the device is symmetrical (It's a FET afterall). The
only
thing that changes is that you're effectively increasing the potential on
the substrate, thereby lowering the Vt of the device.. the rest depends on
whether you have a pFET or an nFET.

Veronique

Walter Harley · Jan 22, 2004

"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:101040tbbilph33@corp.supernews.com...

When they first started using power MOSFETs in switching supplies this was
a
pain, because the diode is right where you put your snubber diode, only it
has first crack at turning on. Since the device is optimized for
MOSFETness, the intrinsic diode is usually a pretty bad snubber diode and
can have an extremely long reverse-recovery time. Everyone has learned
how
to deal with this now, but it's still an issue to be ignored at risk.

Can you talk some more about that? I noticed recently (while looking for
replacements for a blown MOSFET) that some of them claim "ultrafast
intrinsic diode" or the equivalent, so I assume that's one solution to the
problem. You make it sound like there are other solutions - what are they?

Thanks!

Roy McCammon · Jan 22, 2004

Tom McAndrews wrote:

This question is geared toward the enhancement mode mosfet devices
that have the substrate connected to the source.

Since the device is not symmetrical, what is the downfall of driving
current from the source to the drain and effectively switching their
roles when the substrate is connected to the old source?

Thanks in advance.

No problem when the device is used as a switch and is ON.
But when it is off, you have to contend with the the diode.

It is common to see two devices in series with their sources
tied together. That way, when they are off, only one
of them will have a forward biased substrate diode.

--
local optimization seldom leads to global optimization

my e-mail address is: <my first name> <my last name> AT mmm DOT com

John Woodgate · Jan 22, 2004

I read in sci.electronics.design that Tim Wescott
<tim@wescottnospamdesign.com> wrote (in <101040tbbilph33@corp.supernews.
com&gt

about 'Mosfet question -- substrate connected to source', on Thu,
22 Jan 2004:

you could build a
symmetric bipolar transistor,

Philips did; the OC140 was a silicon alloyed device. Used in video
processors, and designers were very annoyed when Philips suddenly
discontinued it.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Tim Wescott · Jan 23, 2004

The only other way that can remember is to use a Shottkey in parallel with
the MOSFET, pointing the same way as the intrinsic diode. Very fast
transients will want to go through the MOSFET anyway, so you have to be
careful that the diode sees a nice low-inductance path.

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:10105u51kbt7t27@corp.supernews.com...

"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:101040tbbilph33@corp.supernews.com...
When they first started using power MOSFETs in switching supplies this
was
a
pain, because the diode is right where you put your snubber diode, only
it
has first crack at turning on. Since the device is optimized for
MOSFETness, the intrinsic diode is usually a pretty bad snubber diode
and
can have an extremely long reverse-recovery time. Everyone has learned
how
to deal with this now, but it's still an issue to be ignored at risk.

Can you talk some more about that? I noticed recently (while looking for
replacements for a blown MOSFET) that some of them claim "ultrafast
intrinsic diode" or the equivalent, so I assume that's one solution to the
problem. You make it sound like there are other solutions - what are
they?

Thanks!

Ian Buckner · Jan 23, 2004

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:jBIN2tDleDEAFwu4@jmwa.demon.co.uk...

I read in sci.electronics.design that Tim Wescott
tim@wescottnospamdesign.com> wrote (in
101040tbbilph33@corp.supernews.
com&gt about 'Mosfet question -- substrate connected to source', on
Thu,
22 Jan 2004:

you could build a
symmetric bipolar transistor,

Philips did; the OC140 was a silicon alloyed device. Used in video
processors, and designers were very annoyed when Philips suddenly
discontinued it.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I recall there were some others, done like that for use as analogue
switches.

Regards
Ian

Mosfet question -- substrate connected to source

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