Re: Driving Dual MOSFETs From A NPN Transistor


Michael wrote:
"PeteS" <peter.smith8380@xxxxxxxxxxxx> wrote in message
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Michael wrote:
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Michael wrote:
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Michael wrote:
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Michael wrote:
"jasen" <jasen@xxxxxxxxxxx> wrote in message
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On 2006-12-12, Michael
<mqiqcqhqaqeqlqhqiqmqsq@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Hi,
I'm working on a circuit that uses a NPN transistor to drive (i
think that's
the right word) two MOSFETs. Could someone please explain how to
work out
the theory behind it? For example what value resistors I need
between the
gates and emitter, etc. (i presume I need some). I'd like to be
able to work
it out myself as opposed to asking you guys for a 'solution' but
sadly I'm
struggling.....
what you describe suggests that you have the NPN transistor
manipulating the
voltage on the gates of the mosfets, that effects their
resistance.

not much more can be said without seeing the types of transistors
and how it's
all hooked up.

So far I've googled: driving mosfet, switching mosfet, mosfet,
mosfet
introduction, transistor introduction but to no avail....If it
really just a
case of wiring the MOSFETs' gates together and connecting them
directly to
the emitter? Any 'rules' to doing this sort of thing?
plenty of rules, the more info you give the better.



--

Bye.
Jasen
Hi,

Sorry for not getting back sooner,

I've drawn you a schematic and uploaded it to:
http://www.mhims.co.uk/MOSFET%20Schematic.png

I'm trying to work out a suitable transistor (Q3) to use and it's
base resistor (R1) value... From what I've read in the mosfets
datasheet (http://www.rapidonline.com/netalogue/specs/47-0530.pdf)
2-4V needs to be applied to the gates so that's where I get the 3V
value from and the 36V figure is from the onboard batteries (it's
for a robot).

Where should I go from here?

Thanks,

Michael
Looking at this, you could do the following:

1. Replace Q3 with a logic level MOSFET (the venerable VN2222LL
would be perfect), and change the sense completely. As shown, the
circuit really won't work that well.

Either use a PNP with emitter to positive rail or use a P-channel
MOSFET (Source to positive rail). If you need to drive with 3V or
3.3V logic, then there are solutions. The simplest fix here
(assuming 3.3V logic) and the fact you need 5V for gate drive (see
3) is to

a. Remove Q3 emitter from the gates and tie it to ground
b. Remove Q3 collector from supply, and pull it to the supply via a
10k resistor
c. Connect a 2N3096 with collector to gates, base to Q3 collector
via a 1k resistor, emitter to 5V

2. Add a pulldown from the MOSFET gates of 10k or so. There has to
be a DC return, and a resistor to ground is the simplest.

3. You need to provide a minimum of 4V to the gates of Q1 and Q2.
This is the only way you are guaranteed to exceed Vgs(th). Better
would be 5V, so change the power supplied to the collector [drain if
you take the advice in 1] to that level.
Note your TTL circuitry could be operating on 3V and the VN2222LL
would work fine as it would if it were a bipolar device.

4. I assume M1 and M2 are motors. As inductive loads, you need to
put diodes across them; anode to the MOSFET drain, cathode to
positive power for the motor. If you don't, the MOSFETS will operate
once, and at the first turn off, they will be toast (literally).



Cheers

PeteS
Ok thanks Pete....

Just to make sure I understand you correctly...are these right?:
http://www.mhims.co.uk/MOSFET%20Schematic2.png
OR
http://www.mhims.co.uk/MOSFET%20Schematic3.png

Michael



In http://www.mhims.co.uk/MOSFET%20Schematic2.png

1. Change Q4 to a 2N3904 or a VN2222LL

2. Remove R2 or R3 (only one needed)

3. D1 and D2 should be across the motors, not the FETs

Then we're starting to get there.

Cheers

PeteS
Thanks, so this ok then?:
http://www.mhims.co.uk/MOSFET%20Schematic4.png

Cheers,

Michael
Your 2N3904 shows a PNP - it should be an NPN :)

Apart from that, you'll need some decoupling - put a 0.1uF cap from the
5V rail. The amount necessary on the 36V rail depends on the motors.

The DC ratings:

Use 10V or better rated for the 5V rail
Use 63V electrolytics on the 36V rail.

As a first cut, I would use 47uF on the 36V rail.

Cheers

PeteS

PS : I hope you are learning something :)

lol, I appreciate the help :-)

This better?:
http://www.mhims.co.uk/MOSFET%20Schematic5.png

Where do the values of 10K and 1K for the resistors come from? I
presuming they aren't 'rigid' values.....

Cheers,

Michael
Looks good, but to know the proper values for the decoupling/mass storage
for the motors I would need more information about the motors.

You are correct, they are not rigid values. Let's take R4 / R5

When Q5 turns on, there will be a few 10s of milliamps of current to
charge the gates, and assuming a worst case hfe of 10, a milliamp or so of
base current into Q5 during this time. It will stabilise at about 500uA,
with a base current of 1/10 of that max.

R4 + R5 provides that current into the base. We could probably increase
the values significantly, but the trick of 'make it work' errs on the side
of plenty of current for bipolar devices.

Hope that helps too!

Cheers

PeteS

Ok thanks Pete, that makes sense...

The motors are pretty heavy duty and rated at 1.8Kw (I'm thinking the cap
value will need to be increased a fair amount...?)

Could you (or Chris) explain why your circuit has decoupling caps and Chris'
doesn't?

Cheers,

Michael

Hi, Michael. Wow. A 50 amp, 36V motor. Quite a load there.

First off, your motor current will exceed the MOSFET rating. You might
want to consider running each motor off two or three of your MOSFETs in
parallel. Since they're ohmic when fully on, they'll share current
quite nicely. The driver circuit I showed can run four or six MOSFETs
as easily as two. You see, you just can't run the motor off one MOSFET
in the real world. That's because it's rated to 50A at a junction temp
of 25C, but is derated to 35A at a junction temp of 100C. Even with
the magical 18 milliohms (it is to dream), you'll still have nearly 50
watts boiling off one MOSFET at 50A. It's gonna be very difficult to
even keep the die temp down to 100C, without a continuous spray of
freon. Use at least two MOSFETs -- I'd recommend three in parallel for
each motor.

Second, you're going to have to be very careful about layout. Try to
remember that any wire has some resistance as well as inductance. The
best thing to do might be to keep the line between the MOSFET source
pin and GND as short as possible, and have the GND for the logic power
and the motor power meeting at one and only one point.

Use a big heat sink. Make sure you include the big honker diodes
(rated for 50 amps repetitive surge). The avalanche diode internal to
the MOSFET can handle a meager 340 millijoules -- and they won't run in
parallel. Your motor can exceed 340 millijoules on turnoff with both
hands tied behind its back, hopping on one foot and humming "Dixie"
while typing the Gettysburg Address from memory on a Palm PDA with the
stylus gripped between its teeth. You turn it off and depend on the
internal diode, you let out the magic smoke. And if you run MOSFETs in
parallel, it will destroy them one at a time until, within a second,
they're all toast.

Borrow a storage scope if you can, to actually see what's happening on
turn-on. Aristotelian theorizing only takes you so far, and it's
depressing to pay a buck apiece times 2 or 3 just to have another WAG
at what's wrong.

Now, if you can do this, you will have learned something. You might
want to start out switching at a lower voltage. This will give you the
advantage of lower power and lower current. As you get a handle on the
switching, you can bump up the power supply until you're riding at 36V.

Best of luck, and Godspeed
Chris

.

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