MOSFET cannot switch on/off normally.

[Electronic Swear]

I have a PWM circuit to control the motor output.
The Ope-Amp 741 will send out a PWM signal and control
a MOSFET to chop the DC to control a universal motor.
Schematic is as following:

12V 120DC
| |
LM741 | (MOTOR)
Clock Pulse ----|\ |/ |
| >--| 2N2222 __|
Control signal --|/ |\> ||
|--------||<-| IRF740
| ||__|
(R=2.2K) |
| |
| |
GND AGND

I have noticed that the IRF740 cannot function correctly
to switch on/off to control the chopped DC. I have already
add a fast diode on motor terminal to avoid the back emf
feeding back to IRF740.

Is there any connection that I am not correct?
Thank you very much.

 

[Ban]

I have a PWM circuit to control the motor output.
The Ope-Amp 741 will send out a PWM signal and control
a MOSFET to chop the DC to control a universal motor.
Schematic is as following:

12V 120DC
| |
LM741 | (MOTOR)
Clock Pulse ----|\ |/ |
| >--| 2N2222 __|
Control signal --|/ |\> ||
|--------||<-| IRF740
| ||__|
(R=2.2K) |
| |
| |
GND AGND

I have noticed that the IRF740 cannot function correctly
to switch on/off to control the chopped DC. I have already
add a fast diode on motor terminal to avoid the back emf
feeding back to IRF740.

Is there any connection that I am not correct?

Almost all:
741: very bad choice if using as a comparator. Better check LM339, which is
a similar chip but optimized for comparator use.
Your transistor can switch the FET on very fast, but it takes long time for
removing the charge from the gate through that 2.2k. In fact it never gets
completely removed, so the fet stays mostly on.


12V o-----+-----------+---+
| | |100n
.-. | ---
| | | ---
| |1k | | |
'-' 2N3906 | === |
|\ | ___ |< GND |
-|+\ +--|___|--| |
| >--+ 1k |\ 22R +-----+
-|-/ | | ___ | |
|/ | +-|___|-+ ||-+ |
1/4 LM339 | ___ | ||<- -
| +-|___|-+---||-+ ^
| | 22R | |
| ___ |/ +-----+
+--|___|--| 2N3904 |
1k |> |
| |
| |
=== ===
GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
Take a fast diode across the fet S/D or use a power schottky.

Thank you very much.

You're welcome.

 

[terry]

Almost all:
741: very bad choice if using as a comparator. Better check LM339, which is
a similar chip but optimized for comparator use.
Your transistor can switch the FET on very fast, but it takes long time for
removing the charge from the gate through that 2.2k. In fact it never gets
completely removed, so the fet stays mostly on.

not only that, but when you switch your FET off, the drain voltage begins to
rise. This dV/dt causes current to flow through the so-called "miller"
capacitance (drain-gate capacitance) and into the gate circuitry (gate
capacitance in parallel with driver impedance). This current will basically
"fight" your gate driver, and attempt to turn the FET back on when you
switch it off. Likewise when you switch the FET on, miller capacitance tries
to turn the FET off again. This is what gives rise to the flat spot seen in
gate voltage curves, the width of which is directly related to the rise- or
fall-time of the drain voltage.

In your case driver impedance is (perhaps - a 741 is terrible!) fairly low
for turn on, but 2.2k for turn off. I would expect the FET to take a month
of sundays to turn off, if at all - if the PWM period is fast compared to
this, the FET will NEVER turn off.

12V o-----+-----------+---+
| | |100n
.-. | ---
| | | ---
| |1k | | |
'-' 2N3906 | === |
|\ | ___ |< GND |
-|+\ +--|___|--| |
| >--+ 1k |\ 22R +-----+
-|-/ | | ___ | |
|/ | +-|___|-+ ||-+ |
1/4 LM339 | ___ | ||<- -
| +-|___|-+---||-+ ^
| | 22R | |
| ___ |/ +-----+
+--|___|--| 2N3904 |
1k |> |
| |
| |
=== ===
GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
Take a fast diode across the fet S/D or use a power schottky.

You're welcome.

Bans circuit solves this problem by providing a nice low impedance for
turn-on AND turn-off. Using an LM339 comparator provides nice sharp edges
(certainly compared to the terrible slew rate of a 741) to the
impedance lowering transistor circuit.

In this case, Bans circuit can be "improved" thusly:

12V o-----+-----------+---+
| | |100n
.-. | ---
| | | ---
| |1k | | |
'-' 2N3904 | === |
|\ | |/ GND |
-|+\ +---------| |
| >--+ |> +-----+
-|-/ | | | |
|/ | | ||-+ |
1/4 LM339 | | ___ ||<- -
| +-|___|-+---||-+ ^
| | 22R | |
| |< +-----+
+---------| 2N3906 |
|\ |
| |
| |
=== ===
GND GND


The "improved" is due to:
1) faster operation - the transistors now dont saturate, they are emitter
followers.

2) less bits

I have built hundreds of thousands of versions of this circuit, driving FETs
and IGBTs in power supplies ranging from 100mW to 1MW (thats a LOT of really
big IGBTs in parallel)

cheers
Terry

 

[Frithiof Andreas Jensen]

Is there any connection that I am not correct?
Thank you very much.

Your driver has to be able to Sink and Source current; the one you have
shown will swith off very slowly so if the PWM frequency is high enough, it
will not switch off at all! Some mosfet application notes will explain all!!

Why the 741? -

If you want a chip in there for some reason, there are dedicated driver
chips that can drive the mosfet properly. If you want to use transistors, it
is simple to build a three-transistor totem-pole driver that will work.
Chips tend to pull in requirements on power supplies and whatnot, discretes
increase component count in return for circuits that can be made to run on
*whatever* power is available. Here you have both!

 

[Frank Bemelman]

The speed is around 10000 rpm.

I have changed a little bit circuit and have a better result,
but I still cannot use the IRF740. I suppose to not change the
LM741 as I haven't any LM339 on hand.

Now, the circuit is like this:


12V 120DC
| |
LM741 | (MOTOR)
Clock Pulse ----|\ (R=10K) |
| >----+ __|
Control signal --|/ | DIODE ||
+---|>|--||<-| 2SK2508
| ||__|
(R=10K) |
| |
| |
GND AGND

Any comment, or what another solution can be provided?
Thank you very much.

A floating gate doesn't seem a good idea. Can't you move
that bottom 10K between gate and GND ? Or remove all
resistors and diode?

The speed of the motor is not of interest. It's the speed
of the clock signal that is. I hope it is slow. Is that
a sawtooth clock? Is the control signal a DC level signal to
control the width of the PCM?

 

[Fritz Schlunder]

In your case driver impedance is (perhaps - a 741 is terrible!) fairly low
for turn on, but 2.2k for turn off. I would expect the FET to take a month
of sundays to turn off, if at all - if the PWM period is fast compared to
this, the FET will NEVER turn off.



Bans circuit solves this problem by providing a nice low impedance for
turn-on AND turn-off. Using an LM339 comparator provides nice sharp edges
(certainly compared to the terrible slew rate of a 741) to the
impedance lowering transistor circuit.

In this case, Bans circuit can be "improved" thusly:

12V o-----+-----------+---+
| | |100n
.-. | ---
| | | ---
| |1k | | |
'-' 2N3904 | === |
|\ | |/ GND |
-|+\ +---------| |
| >--+ |> +-----+
-|-/ | | | |
|/ | | ||-+ |
1/4 LM339 | | ___ ||<- -
| +-|___|-+---||-+ ^
| | 22R | |
| |< +-----+
+---------| 2N3906 |
|\ |
| |
| |
=== ===
GND GND

Greetings.

Although this circuit is indeed superior to Ban's implementation (namely bad
things happen when the LM339 output is not actively trying to drive low),
this circuit too is not without its flaws.

The 1k pull up resistance is much to low and results in a significant
probability of MOSFET destruction. The LM339 datasheet is here:

http://cache.national.com/ds/LM/LM139.pdf

Unfortunately, the LM339 blows big chunks. Admittedly the device is cheap
and is still useful, but it has a rather pitiful output driver. It claims
to be capable only of sinking around a minimum of 6mA with a one volt
overdrive on the inputs and with the output saturation voltage being less
than or equal to 1.5V. Obviously this is not good enough for operation at
12V since the pull up resistor will provide 12mA should the LM339 be able to
pull the output voltage to ground. As a consequence the MOSFET gate will
likely be driven somewhere in the linear region and unfortunately get
toasted.

This is of course the most severe problem, but other caveats should be
mentioned when considering using the LM339 in this way.

The LM339 sucks hard in its instability during output edge transitions. It
tends to oscillate on output transitions. The application hints of the
datasheet addresses this issue to a limited extent. Indeed it has been my
personal experience that the LM339 is quite prone to edge transition
oscillations even when some hysteresis is used and otherwise seemingly
reasonable layout techniques are used. These oscillations will naturally
lead to significantly increased MOSFET and motor antiparallel diode
dissipation, as well as increased EMI (which is likely of genuine concern
with a 120V motor).


For a more robust and trouble free solution it is probably easier to use
some dedicated PWM controller such as the venerable TL494 (
http://focus.ti.com/lit/ds/symlink/tl494.pdf ) or a microcontroller with
built in PWM functionality driving a dedicated MOSFET gate driver IC such as
the TC4427 ( http://ww1.microchip.com/downloads/en/DeviceDoc/21422b.pdf ).
Of course these types of solutions also have their own problems (such as
microcontroller susceptibility to upset by EMI) which may or may not need to
be addressed.

 

[colin]

I have a PWM circuit to control the motor output.
The Ope-Amp 741 will send out a PWM signal and control
a MOSFET to chop the DC to control a universal motor.
Schematic is as following:

12V 120DC
| |
LM741 | (MOTOR)
Clock Pulse ----|\ |/ |
| >--| 2N2222 __|
Control signal --|/ |\> ||
|--------||<-| IRF740
| ||__|
(R=2.2K) |
| |
| |
GND AGND

I have noticed that the IRF740 cannot function correctly
to switch on/off to control the chopped DC. I have already
add a fast diode on motor terminal to avoid the back emf
feeding back to IRF740.

Is there any connection that I am not correct?
Thank you very much.

dont forget about the ne555, it has high drive op and wil directly drive a
mosfet quite quickly. it is very versatile and can be used a s a simple
comparator with built in hysterisis or of course as a pulse width modulator
/ timer etc ...

Colin =^.^=

 

[HARRY DELLAMANO]

You fellers are NOT driving your MOSFETS fast and clean until you replace
your 2N390x's with a Zetex complementary transistor pack found here:
http://www.zetex.com/3.0/b1-21.asp
Check out the ZXTDB2M832, drives 10A in 30nS with <5nS delay. Small enough
to place at the Gate-Source connection, with a 0.47uF cap. No gate current
loops. Controller IC stays on a nice clean ground. Much cleaner than the
TC442x series over temperature.
Now that's smoking!
Cheers,
Harry

 

[Fred Bloggs]

I have a PWM circuit to control the motor output.
The Ope-Amp 741 will send out a PWM signal and control
a MOSFET to chop the DC to control a universal motor.
Schematic is as following:

12V 120DC
| |
LM741 | (MOTOR)
Clock Pulse ----|\ |/ |
| >--| 2N2222 __|
Control signal --|/ |\> ||
|--------||<-| IRF740
| ||__|
(R=2.2K) |
| |
| |
GND AGND

I have noticed that the IRF740 cannot function correctly
to switch on/off to control the chopped DC. I have already
add a fast diode on motor terminal to avoid the back emf
feeding back to IRF740.

Is there any connection that I am not correct?
Thank you very much.

In most cases, this will be sufficient:

View in a fixed-width font such as Courier.

12V
|
LM741 | 120DC
Clock Pulse ----|\ |/ |
| >--| 2N2222 1N4148 (MOTOR)
Control signal --|/ |\> +---|>|----+ |
| | | __|
| | 1.0K | ||
+------+---/\/\---+--------||<-| IRF740
| | | ||__|
| | | |
| | e |
(R=1.0K) | |/ |
| +--------| 2N2907A AGND
| |\
| c
| |
+-----------------+
|
|
GND

 

[terry]

You fellers are NOT driving your MOSFETS fast and clean until you replace
your 2N390x's with a Zetex complementary transistor pack found here:
http://www.zetex.com/3.0/b1-21.asp
Check out the ZXTDB2M832, drives 10A in 30nS with <5nS delay. Small enough
to place at the Gate-Source connection, with a 0.47uF cap. No gate current
loops. Controller IC stays on a nice clean ground. Much cleaner than the
TC442x series over temperature.
Now that's smoking!
Cheers,
Harry

yep. I wuz gonna do a post with decent transistors (aint Zetex great), but
then I read the latest installment from the OP, who clearly doesnt
understand whats been said, so didnt bother.

The last time I did a big inverter I had (IIRC) FZT851 & FZT951 sitting on
the IGBT itself, with local decoupling, and a single stage of FMMT491/591
driving the source-terminated cable feeding my buffer amp. The buffer also
had desat detection with increased Rgoff (the damn IGBTs current limited at
6,000A so L*dI/dt became a bit of an issue unless Rgoff_fault = 10xRgoff (or
so). I also had a suitably monstrous transorb network arranged to blow the
gate bond wires in the event of a collector-gate short. This ensured that
only the buffer amp pcb (which screwed directly onto the IGBT) died in the
event of a failure - dynamic thermal stresses mean IGBT failure is
inevitable, and careful planning can greatly reduce MTTR (and cost). This
allowed me to integrate pretty much all my electronics onto a single PCB...

we did extensive short-circuit testing (directly across each half-bridge
with very low L) and it was solid as a rock. We routinely destroyed (oh
yeah, smoke, flames, explosions - the whole sheBANG) the prototype
machine(s) we were driving, but never broke an inverter. We had small output
chokes (1-2%), and the inverters happily controlled current into a dead
short on the output terminals, indefinitely.

Cheers
Terry

oh yeah, its worth mentioning that if you dont have to drive the mosfet
fast, then dont. lower EMI etc. But dont just increase Rg, shape the
waveform applied to the emitter follower, and drive the gate with a nice low
Rg. and/or use analog's dV/dt limiter.

 

[Tam/WB2TT]

The speed is around 10000 rpm.

I have changed a little bit circuit and have a better result,
but I still cannot use the IRF740. I suppose to not change the
LM741 as I haven't any LM339 on hand.

Now, the circuit is like this:


12V 120DC
| |
LM741 | (MOTOR)
Clock Pulse ----|\ (R=10K) |
| >----+ __|
Control signal --|/ | DIODE ||
+---|>|--||<-| 2SK2508
| ||__|
(R=10K) |
| |
| |
GND AGND

Any comment, or what another solution can be provided?
Thank you very much.

Outlook Express won't let me see your schematic like I would want to, but I
see only one driver transistor. You will want to use both NPN and NPN
drivers. Bases get tied together and connect to the 741. Emitters get tied
together and connect to the power FET. Collector of PNP goes to ground and
collector of NPN goes to VCC. You may also want to square up the signal
coming off the 741 to speed up the rise and fall times. Some even number of
CD4XXX inverters in series should help.If you use 1 inverter in series with
5 others in parallel, you might not need the transistors.

CHECK THIS: What is the low state output voltage of tghe 741? If you are
using a single supply, it might not be low enough - you want < +1V. The
4XXX will help that also.

Tam