Driving Dual MOSFETs From A NPN Transistor

[Michael]

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.....

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?

Thanks In Advance,

Michael

 

[Phil Allison]

"Michael"

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.....

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?

** You must supply more info.

Are the MOSFETS wired in parallel to increase current ?

Are the MOSFETS switching or linear types ?

Is you circuit switching or linear ?

Etc.


......... Phil

 

[jasen]

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.

 

[Michael]

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.


plenty of rules, the more info you give the better.

Hi,

Sorry for not getting back sooner,

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

 

[PeteS]

Hi,

Sorry for not getting back sooner,

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

 

[Michael]

Hi,

Sorry for not getting back sooner,

I've drawn you a schematic and uploaded it to:
http://www.mhims.co.uk/MOSFET Schematic.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 Schematic2.png
OR
http://www.mhims.co.uk/MOSFET Schematic3.png

Michael

 

[PeteS]

On 2006-12-12, Michael
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 Schematic.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 Schematic2.png
OR
http://www.mhims.co.uk/MOSFET Schematic3.png

Michael

In http://www.mhims.co.uk/MOSFET Schematic2.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

 

[Michael]

Michael wrote:
On 2006-12-12, Michael
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 Schematic.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 Schematic2.png
OR
http://www.mhims.co.uk/MOSFET Schematic3.png

Michael

In http://www.mhims.co.uk/MOSFET Schematic2.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 Schematic4.png

Cheers,

Michael

 

[PeteS]

Michael wrote:
On 2006-12-12, Michael
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 Schematic.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 Schematic2.png
OR
http://www.mhims.co.uk/MOSFET Schematic3.png

Michael

In http://www.mhims.co.uk/MOSFET Schematic2.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 Schematic4.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

 

[Chris]

Hi,

Sorry for not getting back sooner,

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

Hi, Michael. I'd guess you're overthinking this a bit.

First, according to the data sheet (ST page 4/18), the gate threshold
voltage of the MOSFET is between 2V and 4V. That means that, with a
gate voltage of less than 2V, 100% of test devices will be OFF. At a
gate voltage of 4V, 100% of test units will be ON. It gets a little
more complex than that, though, because it kind of depends on what you
mean by "ON". You'll have to look at the graphs for that info.

If you look at the graph titled, Figure 8. Static drain-source on
resistance (ST p. 7/18), you'll see that the juicy Rds(on) spec of 0.02
ohms is with a Vgs of 10V. Since you've got a 36V supply to play with,
that should be easy. You can do this with a couple of resistors like
this (view in fixed font or M$ Notepad):

36V
+ 36V 36V 36V 36V
| + + + +
.-. | | | |
10K| | 36V | | | |
| | + / \ - / \ -
'-' | ( M ) ^ ( M ) ^
| |< \_/ | \_/ |
o-| | | | |
| |\ | | | |
.-. | 2 X STB55NF06 o----' o----'
10K| | .-. | |
| | | |6.8K ||-+ ||-+
'-' | | ___ ||<- ||<-
TTL Signal | '-' .----|___|--||-+ .---||-+
___ |/ | | 100 | | |

-|___|-| o----o === | ===

10K |> | | GND | GND
| .-. | ___ |
| | | '----|___|----------'
=== | |2.7K 100
GND '-'
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

This is somewhat easier than your original circuit, because you don't
have to supply another regulated voltage. It would be even easier if
you could live with an active low TTL signal (TTL low = ON) instead of
the active high signal shown in your application, but there it is.

When the TTL signal is high (typically 3.5V unloaded), it will source
1/2mA or so into the base of the first transistor, turning it on. That
will turn on the second transistor, making about 10VDC available to
turn on the two MOSFETs. The 100 ohm resistors are typically used to
isolate the gates and help avoid oscillation. Be sure to use diodes
rated for the motor current, so they don't smoke to save the MOSFETs on
turn-off. Be sure to use transistors rated for the voltage (such as
the 2N4401 NPN / 2N4403 PNP).

Hope this has been of help. Not the most elegant thing, but it will
work very well.

Good luck
Chris

 

[Michael]

Michael wrote:
Michael wrote:
On 2006-12-12, Michael
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 Schematic.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 Schematic2.png
OR
http://www.mhims.co.uk/MOSFET Schematic3.png

Michael



In http://www.mhims.co.uk/MOSFET Schematic2.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 Schematic4.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 Schematic5.png

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

Cheers,

Michael

 

[PeteS]

Michael wrote:
Michael wrote:
On 2006-12-12, Michael
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 Schematic.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 Schematic2.png
OR
http://www.mhims.co.uk/MOSFET Schematic3.png

Michael



In http://www.mhims.co.uk/MOSFET Schematic2.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 Schematic4.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 Schematic5.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

 

[Michael]

Michael wrote:
Michael wrote:
Michael wrote:
On 2006-12-12, Michael
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 Schematic.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 Schematic2.png
OR
http://www.mhims.co.uk/MOSFET Schematic3.png

Michael



In http://www.mhims.co.uk/MOSFET Schematic2.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 Schematic4.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 Schematic5.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

 

[PeteS]

Michael wrote:
Michael wrote:
Michael wrote:
On 2006-12-12, Michael
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 Schematic.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 Schematic2.png
OR
http://www.mhims.co.uk/MOSFET Schematic3.png

Michael



In http://www.mhims.co.uk/MOSFET Schematic2.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 Schematic4.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 Schematic5.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

 

[Chris]

Michael wrote:
Michael wrote:
Michael wrote:
On 2006-12-12, Michael
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 Schematic.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 Schematic2.png
OR
http://www.mhims.co.uk/MOSFET Schematic3.png

Michael



In http://www.mhims.co.uk/MOSFET Schematic2.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 Schematic4.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 Schematic5.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

 

[Chris]

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

Michael, all things considered, it might be best just to use a couple
of contactors if you're not switching too fast or frequently (or one
contactor with two N.O. contacts).

If you place the honker diodes across the motor, you won't have to
worry about arcing welding the contacts.

Suitable special purpose contactors with DC coils are available from
many sources. You might just want to use a 24VDC coil (they're pretty
common), and put it in series with a power resistor that has half the
resistance of the coil. This will divide up the 36V pretty well. Be
sure to use a diode across the contactor coil to protect your power
transistor or MOSFET.

Actually, your MOSFET might be a good choice to drive the contactor.

Good luck
Chris

 

[Michael]

Michael, all things considered, it might be best just to use a couple
of contactors if you're not switching too fast or frequently (or one
contactor with two N.O. contacts).

If you place the honker diodes across the motor, you won't have to
worry about arcing welding the contacts.

Suitable special purpose contactors with DC coils are available from
many sources. You might just want to use a 24VDC coil (they're pretty
common), and put it in series with a power resistor that has half the
resistance of the coil. This will divide up the 36V pretty well. Be
sure to use a diode across the contactor coil to protect your power
transistor or MOSFET.

Actually, your MOSFET might be a good choice to drive the contactor.

Good luck
Chris

Thanks Guys,

Chris,

It's going to be used on two 'sets' of motors - one 12V 200W motor pulsed at
33% max and the 1.8kW one at the full 100%. But even so I'm happier with the
'mosfets only' way
Could you explain this please:

AND

Do you mean the internal avalanche diodes won't survive if I havn't got the
'big honker diodes' in place?

Cheers,

Michael

 

[Chris]

Thanks Guys,

Chris,

It's going to be used on two 'sets' of motors - one 12V 200W motor pulsed at
33% max and the 1.8kW one at the full 100%. But even so I'm happier with the
'mosfets only' way
Could you explain this please:

Do you mean the internal avalanche diodes won't survive if I havn't got the
'big honker diodes' in place?

Cheers,

Michael

Yes. The internal diodes of your MOSFETs are rated for 340 millijoules
non-repetitive. This is far less than the back EMF generated by the
1.8KW motor when it turns off -- they won't survive. And diodes in
parallel tend not to share current very well. The one with the lowest
forward voltage will hog the current.

No matter what you do to turn off the motor (contactor, solid state,
whatever), this back-EMF needs to be dissipated. Placing a big diode
across the motor will cause the energy to be dissipated through the
motor itself. Probably a good idea no matter what.

Good luck
Chris

 

[Michael]

Yes. The internal diodes of your MOSFETs are rated for 340 millijoules
non-repetitive. This is far less than the back EMF generated by the
1.8KW motor when it turns off -- they won't survive. And diodes in
parallel tend not to share current very well. The one with the lowest
forward voltage will hog the current.

No matter what you do to turn off the motor (contactor, solid state,
whatever), this back-EMF needs to be dissipated. Placing a big diode
across the motor will cause the energy to be dissipated through the
motor itself. Probably a good idea no matter what.

Good luck
Chris

Thanks Chris,

Actually I was playing with the circuit today and hit something that's
puzzling me (I wired it as a h-bridge)...

Could you explain why when one the inputs (the little squares at the base of
Q1 and Q17) is high, the motor turns as expected but only 5.51V reaches it
with the 30.49V being dropped accross the top mosfets?

I'm guessing it has something to do with the load connected to the source as
opposed to the drain.....

Schematic:
http://www.mhims.co.uk/MOSFET Schematic5.png

Cheers,

Michael

 

[Michael]

<snipped>

Oops, apologies:

http://www.mhims.co.uk/MOSFET Schematic6.png

(I realise the diode isn't there as well)

Cheers,

Michael