24v upto 40Amp H bridge DC motor driver schematics

[classd101]

Harris and IR probably both do drivers and app notes.

Graham

I'm able to spot a few things I don't like in this one as well, at a
glance.


First I think the rectifier diodes across the mosfets are supposed to
be schottkys, of a few amps but that can block full rail to rail
voltage.

Second is the method I see used to control dead time, slowing the turn
on rate of one side over the other by using different sized gate
resistors. I think you'd want to have one side to switch just as fast
as the other, only delayed in time. What we have here is switching
like: | on one side and / on the other, so one side will be alot less
efficient as well.

Perhaps this was done in a misunderstood attempt at speeding up turn
off?

Those same resistors should be removed from the circuit during turn
off to allow fast discharge of the gate capacitances. Easiest done
with a back diode in this case, possibly better done by only using one
resistor per driver, placed at the emitter of the NPN turn on
transistor. That will also help with gate step induced spurrious turn
on by presenting a lower impedance to the reverse miller charge, it'll
have a straight path to ground through the PNP's emitter. Said PNP
could also be beefed up in size over the turn on NPN transistors. The
more current it can sink the better.

Last, for everyone who said something about "less transistors on the
bottom" etc, I would think you'd want matched drivers for all FETS in
order to preserve any delays induced?

I don't think there's much reason to double up the output mosfets,
just select the right ones for the job, and it will switch that much
faster for the given driver.

What else, would have been nice to look over the rest of it more but
that text made my eyes bleed.

Cheers
Chris

 

[John Crighton]

colin wrote...

There's more, much more.


--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)

Colin and Ricardo have pointed out a few flaws,
reversed electrolytic capacitor which is a typo,
slow and puny protection diodes. Right Oh.
No large electrolytic capacitor on the controller
board. Right Oh.
Someone criticised parallel power fets.
Is that so bad?

The over current protection is not latched off
permanently until overload is removed.
OK not so good.

I still don't see the fatal flaw to cause it to go bang.

My friend is intending to control an auto pilot motor
in a small fishing boat. The motor will run for a few
seconds in one direction, stop for a second or so
and run for a second or more in the other direction.

No speed control but later on my friend will add a
soft start circuit to produce narrow pulses around
300/400 Hz, gradually increasing in width to full on,
over a period of a second or so to prevent the
steering gear being knocked around too much
by the motor. That is just an added refinement
that would be nice but not essential at this
stage because the main concern now is

The fatal flaw that we can't see!

I give up. Do tell.

Regards,
John Crighton
Sydney

 

[Rich Grise]

Colin and Ricardo have pointed out a few flaws,
reversed electrolytic capacitor which is a typo,
slow and puny protection diodes. Right Oh.
No large electrolytic capacitor on the controller
board. Right Oh.
Someone criticised parallel power fets.
Is that so bad?

The over current protection is not latched off
permanently until overload is removed.
OK not so good.

I still don't see the fatal flaw to cause it to go bang.

Have you tried it with the listed fixes fixed?

My friend is intending to control an auto pilot motor
in a small fishing boat. The motor will run for a few
seconds in one direction, stop for a second or so
and run for a second or more in the other direction.

No speed control but later on my friend will add a
soft start circuit to produce narrow pulses around
300/400 Hz, gradually increasing in width to full on,
over a period of a second or so to prevent the
steering gear being knocked around too much
by the motor. That is just an added refinement
that would be nice but not essential at this
stage because the main concern now is

The fatal flaw that we can't see!

The fatal flaw is your bang-bang on/reverse cycle.

Since it's only software, do the softstart and deadtime
now, so you quit breaking stuff, and implement the other
suggestions before you fire it up some more.

Good Luck!
Rich

 

[Daniel Rudy]

At about the time of 9/30/2004 6:24 AM, colin stated the following:

I think the bad circuits idea in AoE are a good idea and not having an
explanation gets ones mind working till the flaw is found, and also maybe
gives you an idea of how good you are if you can find each ones flaw without
any help.

Colin =^.^=

Looks to me like the two H-Bridges are not independantly controlled.
Two motors at the same speed is a straight line. I could think of a
couple of applications for this, but nothing really practical.

 

[colin]

"Daniel Rudy"

At about the time of 9/30/2004 6:24 AM, colin stated the following:


Looks to me like the two H-Bridges are not independantly controlled.
Two motors at the same speed is a straight line. I could think of a
couple of applications for this, but nothing really practical.

As i understood it, the output of the two bridges are suposed to be
paraleled together to deliver more curent to a single motor,
Although i hadnt thought about two motors driven at the same time.

Colin =^.^=