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parallel FETs

V

V8TR4

Jan 1, 1970
0
Hello,

I want to make a controller for a golf cart motor and am considering using
paralleled MOSfets or HEXfets. I know that there can be problems with the
individual xsistors not sharing the load equally. Would it help if I had a
seperate line driving each one rather than hooking up to some common rail to
activate them? Any info would be much appreciated.

I am looking to control a PWM with a current up to 300ampere surge and
80ampere running.

Thanks
 
K

Ken Smith

Jan 1, 1970
0
Hello,

I want to make a controller for a golf cart motor and am considering using
paralleled MOSfets or HEXfets. I know that there can be problems with the
individual xsistors not sharing the load equally. Would it help if I had a
seperate line driving each one rather than hooking up to some common rail to
activate them? Any info would be much appreciated.

I am looking to control a PWM with a current up to 300ampere surge and
80ampere running.

When used as switches MOSFETs tend to share rather than hog the current.
Imbalances are less likely to run away, as a result. You are doomed to
having some resistance in the wiring. If you are clever, you can make the
wiring resistance improve the sharing by having each MOSFET see part of
it.

During switching, the MOSFETs will not be timed perfectly. The faster the
gate drive the better. The differences in the threshold voltages is part
of this timing error. The gate drive wiring needs to be done carefully to
ensure that all the MOSFETs see the same drive. Don't run the drive to
one MOSFET and then trough a long squiggly trace to the next etc.
 
J

John Smith

Jan 1, 1970
0
V8TR4 said:
Hello,

I want to make a controller for a golf cart motor and am considering using
paralleled MOSfets or HEXfets. I know that there can be problems with the
individual xsistors not sharing the load equally. Would it help if I had a
seperate line driving each one rather than hooking up to some common rail
to
activate them? Any info would be much appreciated.

I am looking to control a PWM with a current up to 300ampere surge and
80ampere running.

Thanks


Here are some thoughts:

* Take a look at the Fairchild ISL9N302AP3. Rds less than .002 Ohms (25C,
10V). One of these can handle all your current in the silicon, but the
TO-220 package is only rated for 75 Amps. Use several in parallel or look
for this sort of device in a better package (or both).

* I would not add any resistance. Yes, I would use individual wires to the
battery (motor, whatever) for each device. You need to consider how you're
going to sense current so you can protect the devices. Something I've seen
in some PWM controllers is that they watch Vds. You could possibly do that,
maybe where the drain wires get connected together. There will be noise on
this point.

* Give some thought to how you're going to handle the ringing you're going
to have on the drains. Even a couple of feet of wire can cause a nasty spike
if you shut off the current fast enough. You may want to consider tailoring
the gate drive to help this problem.

* I would probably not depend on the integral body diodes in the FETs.

* Drive each gate through its own resistor.

* Layout and wire dressing will be important.

* Be sure you know what you mean by 300 Amps surge and pay attention to the
peak current vs duration curves for the device(s) you choose.

Good luck.

John
 
T

Terry Given

Jan 1, 1970
0
V8TR4 said:
Hello,

I want to make a controller for a golf cart motor and am considering using
paralleled MOSfets or HEXfets. I know that there can be problems with the
individual xsistors not sharing the load equally. Would it help if I had a
seperate line driving each one rather than hooking up to some common rail to
activate them? Any info would be much appreciated.

I am looking to control a PWM with a current up to 300ampere surge and
80ampere running.

Thanks

The key to successful paralleling of FETs is tight thermal coupling. The
+ve tempco of Rdson has a self-sharing kind of effect: the FET carrying
more current gets hotter, thereby increasing Rdson, which in turn
decreases the current thru that particular FET. If all the FETs share
the same thermal environment (eg closely spaced symmetric layout) then
direct paralleling can and will work. Normally one uses individual gate
resistors. The same approach works for IGBTs - I have direct-paralleled
6 x 600A IGBT.

Or you could buy a semikron SK300MB075 300A 75V dual FET in a SemiTop
package. Great thermal coupling, good thermal interface etc.

The real killer is switching - at which point stray inductance tends to
dominate. if dI = 300A and dt = 100ns say, then dI/dt = 3kA/us. 10nH
will drop 30V! Symmetry is a saviour here - if the layout is symmetric
then all the strays will be the same. If it is not, they wont.

Cheers
Terry
 
S

Spehro Pefhany

Jan 1, 1970
0
On Tue, 16 Nov 2004 22:31:28 +0000 (UTC), the renowned
When used as switches MOSFETs tend to share rather than hog the current.

Note that they share better if given individual heat sinks rather than
one big heatsink, so that the temperatures can vary a bit.


Best regards,
Spehro Pefhany
 
B

Boris Mohar

Jan 1, 1970
0
Hello,

I want to make a controller for a golf cart motor and am considering using
paralleled MOSfets or HEXfets. I know that there can be problems with the
individual xsistors not sharing the load equally. Would it help if I had a
seperate line driving each one rather than hooking up to some common rail to
activate them? Any info would be much appreciated.

I am looking to control a PWM with a current up to 300ampere surge and
80ampere running.

Thanks

IEEE Transactions on Electron Devices Vol. ED-31, No.7, July 1984

An Analysis and Experimental Verification of Parasitic Oscillations in
Paralleled Power MOSFETS.

Also appears in Siliconix MOSPOWER APPLICATIONS book
ISBN 0-930519-00-0

http://www.smpstech.com/books/booklist.htm
http://www.smpstech.com/severns/papers.htm


Regards,

Boris Mohar
Got Knock? - see:
Viatrack Printed Circuit Designs http://www3.sympatico.ca/borism/
 
T

Terry Given

Jan 1, 1970
0
Boris said:
IEEE Transactions on Electron Devices Vol. ED-31, No.7, July 1984

An Analysis and Experimental Verification of Parasitic Oscillations in
Paralleled Power MOSFETS.

Also appears in Siliconix MOSPOWER APPLICATIONS book
ISBN 0-930519-00-0

Bwahahahahahaaaa! I have a hard-cover of that :)

The first time I EVER saw a Routh-Hurwitz stability analysis performed
on a circuit. A lovely bit of maths.

Cheers
Terry
 
W

Winfield Hill

Jan 1, 1970
0
Boris Mohar wrote...
IEEE Transactions on Electron Devices Vol. ED-31, No.7, July 1984
An Analysis and Experimental Verification of Parasitic Oscillations
in Paralleled Power MOSFETS.
Also appears in Siliconix MOSPOWER APPLICATIONS book ISBN 0-930519-00-0
http://www.smpstech.com/books/booklist.htm
http://www.smpstech.com/severns/papers.htm

V8TR4 is on target. Most parallel switching FET oscillations are
easily solved using John Smith's advice, "Drive each gate through
its own resistor." And it would be wise to heed the rest of his
advice as well. The FETs in the article mentioned above (no author)
had their gates directly tied in parallel. That's a real no-no.
 
R

Rich Grise

Jan 1, 1970
0
The key to successful paralleling of FETs is tight thermal coupling. The
+ve tempco of Rdson has a self-sharing kind of effect: the FET carrying
more current gets hotter, thereby increasing Rdson, which in turn
decreases the current thru that particular FET. If all the FETs share
the same thermal environment (eg closely spaced symmetric layout) then
direct paralleling can and will work.

Oddly, Spehro Pefhany says exactly the opposite a couple of FUs back.
He says, put them on individual sinks, so each can find its own
thermal equilibrium, or something like that.

I'm not well-versed enough in this area to figure out which is
"right." I have encountered a system with banks of MOSFETs on
common heat sinks, with very carefully calculated individual
source resistors, so each would settle at a _different_ current,
the excuse being some mumbo jumbo about air flow over the heat
sink. And I know even _less_ about thermal transfer stuff. :)

Thanks,
Rich
 
A

Active8

Jan 1, 1970
0
Bwahahahahahaaaa! I have a hard-cover of that :)

The first time I EVER saw a Routh-Hurwitz stability analysis performed
on a circuit. A lovely bit of maths.

Argh. That paper isn't in my old data book. How long is it? I've got
some papers on that analysis technique, but haven't seen it used
aside from that.
 
T

Tony Williams

Jan 1, 1970
0
Terry Given said:
The key to successful paralleling of FETs is tight thermal
coupling. The +ve tempco of Rdson has a self-sharing kind of
effect: the FET carrying more current gets hotter, thereby
increasing Rdson, which in turn decreases the current thru that
particular FET. If all the FETs share the same thermal
environment (eg closely spaced symmetric layout) then direct
paralleling can and will work.

I can't make sense of that Terry.

If all FETs are forced to run at the same temperature
then the current (or power) ratio will be constant.

Shouldn't there be some thermal elasticity between FETs,
to allow the device with the lowest RdsON to run slightly
warmer?
 
S

Steve

Jan 1, 1970
0
V8TR4 said:
Hello,

I want to make a controller for a golf cart motor and am considering using
paralleled MOSfets or HEXfets. I know that there can be problems with the
individual xsistors not sharing the load equally. Would it help if I had a
seperate line driving each one rather than hooking up to some common rail to
activate them? Any info would be much appreciated.

I am looking to control a PWM with a current up to 300ampere surge and
80ampere running.

Thanks

I see all the comments about paralleling *FETs. (All well and good, if you
insist on paralleling.) Am I missing something by suggesting that the given
peak and continuous ratings are well within the capabilities of a single
device?? Probably less cost, too.

Thanks, Steve
 
L

legg

Jan 1, 1970
0
IEEE Transactions on Electron Devices Vol. ED-31, No.7, July 1984

An Analysis and Experimental Verification of Parasitic Oscillations in
Paralleled Power MOSFETS.

Also appears in Siliconix MOSPOWER APPLICATIONS book
ISBN 0-930519-00-0

http://www.smpstech.com/books/booklist.htm
http://www.smpstech.com/severns/papers.htm

At the time the Siliconix article was published, you could still buy
multi-chip fet modules from IR that were miswired, preventing
predictable switching performance.

other refs

AN918 Motorola
http://www.datasheetarchive.com/download.php?pi=29568

AN7513, AB-9, Fairchild
http://www.fairchildsemi.com/an/AN/AN-7513.pdf#page=1
http://www.fairchildsemi.com/an/AB/AB-9.pdf#page=1

AN941 IR
http://www.irf.com/technical-info/appnotes/an-941.pdf

PESC'01 Huang
Characterization of Paralleled Super Junction MOSFET Devices under
Hard- and Soft-Switching Conditions link dead

EPE'99 Jeannin
http://manuales.elo.utfsm.cl/conferences/seminarios/EPFL/pc/papers/614.pdf

RL
 
J

John Smith

Jan 1, 1970
0
Steve said:
I see all the comments about paralleling *FETs. (All well and good, if you
insist on paralleling.) Am I missing something by suggesting that the
given
peak and continuous ratings are well within the capabilities of a single
device?? Probably less cost, too.

Thanks, Steve


Which one or ones do you favor, Steve? I don't have a project right now, but
I'd be really interested in archiving a copy of the specs for the ones you
suggest.

Thanks,
John
 
J

Joerg

Jan 1, 1970
0
Hi Winfield,
V8TR4 is on target. Most parallel switching FET oscillations are
easily solved using John Smith's advice, "Drive each gate through
its own resistor." And it would be wise to heed the rest of his
advice as well. The FETs in the article mentioned above (no author)
had their gates directly tied in parallel. That's a real no-no.

It depends on the size of it all. With very small FETs that are right
next to each other you can reach stability without. When the FETs are
large, on a heat sink and far apart that is, of course, a different
matter and resistors are a must. If have seen PWM converters where the
turn-on-off loss increases due to these series resistors were
responsible for more than a 1% penalty in overall efficiency.

Regards, Joerg
 
T

Tom Seim

Jan 1, 1970
0
Tony Williams said:
I can't make sense of that Terry.

If all FETs are forced to run at the same temperature
then the current (or power) ratio will be constant.

Shouldn't there be some thermal elasticity between FETs,
to allow the device with the lowest RdsON to run slightly
warmer?

Even if they are mounted side-by-side on the same heatsink their can,
and will be, temperature differences between them. Heatsinks, too,
have thermal resistance.
 
T

Tom Seim

Jan 1, 1970
0
V8TR4 said:
Hello,

I want to make a controller for a golf cart motor and am considering using
paralleled MOSfets or HEXfets. I know that there can be problems with the
individual xsistors not sharing the load equally. Would it help if I had a
seperate line driving each one rather than hooking up to some common rail to
activate them? Any info would be much appreciated.

I am looking to control a PWM with a current up to 300ampere surge and
80ampere running.

Thanks

This may be of interest:

SCHÖNKNECHT AND DE DONCKER: PARALLEL CONNECTION OF SOFT-SWITCHING
HIGH-POWER HIGH-FREQUENCY INVERTERS
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 39, NO. 2,
MARCH/APRIL 2003

Abstract—Inductive heating applications like pipe welding or
steel strip annealing require electrical power ratings of several
megawatts at frequencies up to 100 kHz and higher. The large
power-frequency product represents a significant challenge for
today's semiconductor technology. As the absolute maximum
rating of a single-stage inverter is often far below rated power,
several inverters or several devices have to be connected in parallel.
This paper presents a novel topology, consisting of parallelconnected
soft-switching high-frequency inverters. Distinctive
features include flexible configurations, negligible shunt currents
between inverters, and equally shared power among inverters.
Furthermore, compared to a single-inverter system, no additional
reactive components are necessary for connecting inverters in
parallel and there is no need of a high-frequency transformer to
adapt the impedance of the load to the inverters.

A novel topology for connecting soft-switching inverters in
parallel is proposed in this paper. The following demands, which
often cause difficulties, have to be taken into consideration:
1) no or negligible circulating currents, even if inverters
switch nonsynchronous;
2) equal power sharing among parallel-connected inverters;
3) flexible variation of the number of parallel inverters, enabling
a wide power range and, thus, the possibility to
minimize the number of inverters for the given power;
4) no additional devices for connecting inverters in parallel;
5) minimization of losses in semiconductors by ZVS softswitching
operation at near zero-current.
 
S

Spehro Pefhany

Jan 1, 1970
0
Even if they are mounted side-by-side on the same heatsink their can,
and will be, temperature differences between them. Heatsinks, too,
have thermal resistance.

And the lower that thermal resistance between devices is, the poorer
the current sharing will be. All other things being equal.


Best regards,
Spehro Pefhany
 
K

Ken Smith

Jan 1, 1970
0
When used as switches MOSFETs tend to share rather than hog the current.

Note that they share better if given individual heat sinks rather than
one big heatsink, so that the temperatures can vary a bit.[/QUOTE]

Yes the currents are more equal but, unless I am missing something it
really doesn't matter that much whether you use one big or individual
sinks since the die temperatures end up the same in the two cases and it
is the die temperature that really matters to the survival of the devices.

BTW: at low currents in linear operation MOSFETs do hog current. As
temperature goes up Vth and gm decrease and Rds(on) increase.
 
W

Winfield Hill

Jan 1, 1970
0
Ken Smith wrote...
Spehro Pefhany wrote:
[...]
Note that they share better if given individual heat sinks rather than
one big heatsink, so that the temperatures can vary a bit.

Yes the currents are more equal but, unless I am missing something it
really doesn't matter that much whether you use one big or individual
sinks since the die temperatures end up the same in the two cases and it
is the die temperature that really matters to the survival of the devices.

BTW: at low currents in linear operation MOSFETs do hog current.
As temperature goes up Vth and gm decrease and Rds(on) increase.

Unless one is fortunate enough to be using the elegant lateral power
MOSFETs from Hitachi, etc.
 
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