Re: Driving Dual MOSFETs From A NPN Transistor

Michael wrote:
"PeteS" <peter.smith8380@xxxxxxxxxxxx> wrote in message news:yYFgh.6822$Dr3.3269@xxxxxxxxxxxxxxxxxxxxxxx
Michael wrote:
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Michael wrote:
"PeteS" <peter.smith8380@xxxxxxxxxxxx> wrote in message news:zlFgh.2206$v4.1702@xxxxxxxxxxxxxxxxxxxxxxx
Michael wrote:
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Michael wrote:
"jasen" <jasen@xxxxxxxxxxx> wrote in message news:elpis9$k2$2@xxxxxxxxxxxxxxxxxxxxxx
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?

Perhaps because we were trying to look at the basics first, and the practicalities afterwards :)


Cheers

PeteS





Cheers,

Michael


.

Relevant Pages

  • Re: Driving Dual MOSFETs From A NPN Transistor
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  • Re: Driving Dual MOSFETs From A NPN Transistor
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  • Re: Driving Dual MOSFETs From A NPN Transistor
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  • Re: Driving Dual MOSFETs From A NPN Transistor
    ... I'm working on a circuit that uses a NPN transistor to drive (i think that's ... voltage on the gates of the mosfets, ... Either use a PNP with emitter to positive rail or use a P-channel MOSFET. ...
    (sci.electronics.basics)
  • Re: Driving Dual MOSFETs From A NPN Transistor
    ... I'm working on a circuit that uses a NPN transistor to drive (i ... the right word) two MOSFETs. ... gates and emitter, etc.. ... Either use a PNP with emitter to positive rail or use a P-channel ...
    (sci.electronics.basics)