High current switching MOSFETs
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Posts: 46,084
I am very interested in this topic, so maybe some of the experts can
enlighten us.
I can see that using a simple darlington or some 2222's could use the
12 V to drive the MOSFETS on the referenced schematic, but in my
case, I am looking for ultra fast switching, but have not found
MOSFETS in the uS ranges.
In my case, around 80 volts at around 20 amps. or possilby around 5
volts at 100 amps, but that would use multiple MOSFETS to get the
amperage.
I did figure to use a totem pole to allow the cleanest on/off
switching.
if you can elaborate on making this a better circuit, I think
comments would be welcomed.
Dave
enlighten us.
I can see that using a simple darlington or some 2222's could use the
12 V to drive the MOSFETS on the referenced schematic, but in my
case, I am looking for ultra fast switching, but have not found
MOSFETS in the uS ranges.
In my case, around 80 volts at around 20 amps. or possilby around 5
volts at 100 amps, but that would use multiple MOSFETS to get the
amperage.
I did figure to use a totem pole to allow the cleanest on/off
switching.
if you can elaborate on making this a better circuit, I think
comments would be welcomed.
Dave
Comments
than microsecond pulse widths. Not an expert, just done it a bunch<G>.
The main problem in the speed is the miller capacitance. This is the
capacitance between the gate and drain. It is relatively large, and at
high Voltages, there's a bunch of charge that has to be removed or
added. You are lucky in that you are using small Voltages.
However, the Voltage drop across your fets at that current will be a
problem.
There are fet driver circuits out there that you can buy cheap. But if
you want to roll your own, use a standard transistor push pull setup to
drive it. Also, put bypass capacitors right at the transistor
connections. 1000 pf, 0.1 uf, 10 uf, all of these. The small
capacitors will help take care of the fast switching times, and the
larger ones will help take care of the slower switching times.
Also, if you want fast switching times, drive the gate with much more
than is needed to turn it on and off. For example, you need only 2-4
Volts to turn the fet on and off. But if you drive it with +/- 15
Volts, then you have a higher Voltage source, and the limiting
resistance of the driving circuit (which you can't do much about once
you have built it) will allow more current into the gate.
In addition, you can put fets in parallel. However, you have to choose
them for the circuit. For a variable DC 0 to 200 Amp 12 Volt load that
we designed and constructed, we bought IXFN200N07 from Digikey. These
are rated at 200 Amps, 0.006 Ohm!!!, and 70 Volts. However, each is 520
Watts. We used 5 in parallel for our circuit.
The first 12 or so were close in on resistance. The next several
batches were seriously different, so that when we turned on our 5 in
parallel, one hogged all the current and died. Then the others decided
they were working too hard as a result, and started smoking to show
their protest<G>.
To test them, I pulsed them with gate Voltages and noted the current
through them from a very large capacitor (so that I didn't have to worry
about a large power supply) and found ones that were closely matched. I
marked each with a "serial number" and kept records so that I could
match more up later without rechecking.
In paralleling high power like this, either you will have to check for
closely matched fets, or design a circuit so that they all draw the same
amount of current when in the linear region. In the linear region when
they are near full on is when the most problems occur. Still, when they
are full on, you can have non-sharing of current. In many circuits they
design in resistors in series with each fet to equalize the current
sharing. However, if you want ON ON dogone it ON, adding resistors
isn't productive<G>.
Also, it's valuable to put a 10 Ohm resistor in series with each gate,
close to the gate, and a ferrite bead on the resistor leg closest to the
gate. If you get oscillations, sometimes you can hear them in the
wiring<G>.
Make sense?
Original Message
From: Dave Mucha [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=nFgiHaoxSzu94o5GLdsBwgpazh76aKMfWp63s7KaoVv9Db8_PDVzSOsATxJGpSF9Vjbo7h927JU]davemucha@j...[/url
Sent: Monday, September 08, 2003 7:06 PM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] Re: High current switching MOSFETs
I am very interested in this topic, so maybe some of the experts can
enlighten us.
I can see that using a simple darlington or some 2222's could use the
12 V to drive the MOSFETS on the referenced schematic, but in my
case, I am looking for ultra fast switching, but have not found
MOSFETS in the uS ranges.
In my case, around 80 volts at around 20 amps. or possilby around 5
volts at 100 amps, but that would use multiple MOSFETS to get the
amperage.
I did figure to use a totem pole to allow the cleanest on/off
switching.
if you can elaborate on making this a better circuit, I think
comments would be welcomed.
Dave
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