Hello!

I would need an PWM controller for a 24V 50W brushed DC motor. I made one
like on this link, but it's working very bad, it work good when is alive
but
MOSFET's gone dead quick.

http://www.ozitronics.com/docs/k166.pdf


Does maybee someone have somethink that could drive mine motor in both
direction, so like this: When potmeter is in middle motors stops, and then
going speed up when potmeter go to one side motor goes in one direction
and
opposite?

Thanks for help,

Marin

The most obvious problem is that this circuit does not have protection,
wheeling or ring-off diodes to keep the inductive kick from the motor from
damaging the FETs when switched off. Any inductor, including a motor, when
switched off will try to maintain the flow of current into the off
circuit.
This creates a high voltage spike that is the probable reason for FET
failure.

The FETs have an internal body diode that is totally unsuited for this
function. They are too slow and retain too much charge.

Place four fast diodes, reversed biased, from M1 and M2 to the plus
voltage
rail and to ground. This should protect the FETs from inductive spikes.

Secondly, insure that at no time are the FETs of either side of the bridge
EVER on together. This situation creates a dead short from the power rail
to
ground and will blow FETs.

Driving the FETs are LM324 op-amps used as comparators. These are lousy
amps
and a worse comparators. They may not turn off fast enough to insure that
no
on time overlap occurs. It's hard to find this kind of stuff without good
test equipment like a fast storage scope.

Try the diode fix first, then dig deeper if necessary.

Hello!

I would need an PWM controller for a 24V 50W brushed DC motor. I made one
like on this link, but it's working very bad, it work good when is alive but
MOSFET's gone dead quick.

http://www.ozitronics.com/docs/k166.pdf

I allready did place shotky on every mosfet, still burning, also I did try
with some other gate resistors and so....problem is in dead time then....but
this design end then.

Hello!

I would need an PWM controller for a 24V 50W brushed DC motor. I made one
like on this link, but it's working very bad, it work good when is alive
but MOSFET's gone dead quick.

http://www.ozitronics.com/docs/k166.pdf


Does maybee someone have somethink that could drive mine motor in both
direction, so like this: When potmeter is in middle motors stops, and then
going speed up when potmeter go to one side motor goes in one direction
and opposite?

Thanks for help,

Marin

On Sat, 10 May 2008 18:15:24 +0200, "mtech"
marin.zorica(nospam) at (no spam) zd.t-com.hr> wrote:

I allready did place shotky on every mosfet, still burning, also I did try
with some other gate resistors and so....problem is in dead time then....but
this design end then.

Um, this crcuit PWMs at 270Hz.

10K gate drive resistors.10mA gate drive currents. Give me a break.

As long as there's no actual shoot-through or gate overstress failure,
and as long as the 324 can actually shut the Pchannel parts off,
theres no reason why the circuit shouldn't work below 20V. It
obviously does, to a certain extent.

Some kind of ripple current, though.

If all you're doing is introducing better drivers, you could probably
retain the electronics to generate the control signals, providing that
they're troubleshot before applying juice.

I was just mulling over the possibility of substituting a better quad
op-amp when
I noticed that Q3 and Q5 will never be properly fully turned on since
there's no
high side drive. Heck, Vcc for the LM324 ins't even as high as V+ due to
D3 and
R15.

It's a truly very poor design.

Graham

It sure could use some ceramic low esr caps across the mosfet bridge. The
mosfets have some antiparellel diodes in them already albeit mediocre
ones. when the freewheeling currents attempt to go back into the bus
you're getting a spike on the bus. A couple of caps with low ESR and ESL
(think x7r ceramic) with a couple hundred uF electrlytic in parallel as a
bulk capacitor and you might be in business. These go diretly across the
mosfets (drain of the high side to the source of the low side.

That said hte circuit design is poor at best. The 10K gate resistors will
be SLOW turnon and turn off causing heat and shootthrouh.

I'd scrap that and find a Brushed motor driver PWN IC (TI, onsemi,
fairchild) that had real mosfet drivers in it. you'll be much better off.

I was just mulling over the possibility of substituting a better quad
op-amp when
I noticed that Q3 and Q5 will never be properly fully turned on since
there's no
high side drive.

It's a truly very poor design.

On Sat, 10 May 2008 16:58:14 +0200, "mtech"
marin.zorica(nospam) at (no spam) zd.t-com.hr> wrote:


Hello!

I would need an PWM controller for a 24V 50W brushed DC motor. I made one
like on this link, but it's working very bad, it work good when is alive but
MOSFET's gone dead quick.

http://www.ozitronics.com/docs/k166.pdf



That's a stunningly bad circuit, but then most circuits are stunningly
bad.

These work nicely:

http://www.national.com/mpf/LM/LMD18400.html

but I'd add some clamp schottkies for luck.


John