A 1kW H-bridge is not a good choice for a high-power-novice
copy-the-schematic project (making no assumptions about you).
I'd begin with my favorite HIP4081A H-bridge driver IC, add
some large MOSFETs, evaluate snubbers and filters, and move
on to considering protection and controller schemes. Hmm,
perhaps that's backward, and one should start by evaluating
recent motor-controller ICs first, and move on FETs, etc.
Heh. Stephen King just wrapped up a seven-book series with a polite
request to readers to refrain from looking up his address and driving
over for a visit to debate the merits of the ending.
For your own peace of mind, this habit you have of citing "bad" circuits
without revealing their flaws may require a similar notice in the
preface to AoE 3rd ed.
Just sayin', 's all.
Note: My E-mail address has been altered to avoid spam
Nah, folks are more than welcome to come visit me anytime at my lab
and debate circuits, lift a brew, or whatever! But as for Stephen
King, after stringing along as a paying customer on his online novel
experiment, one delayed chapter at a time, and getting jilted unable
to read the ending, I realize he has a very serious crybaby problem.
On Wednesday 29 September 2004 03:13 pm, Winfield Hill did deign to grace us
with the following:
Was Stephen King the one who did "Pet Sematery?" Or was that Spielberg?
I saw "It," and was really disappointed with that sewer clown/spider thing.
Other than that, I think the crowning moment of King's glory was the line:
"I'll be dipped in shit if that ain't a meteor!"
If thats an open invitation i might be interested in taking you up on exept
for the fact that ive not been wel enough to go out for the last few years,
however i find im only realy interested in circuits that are on the edge of
posibility, wich means i dont often produce anything that usefull, just as
well its only a hobby now i gues, unless of course i managed to actualy do
something that realy is (was) imposible.
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
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.
OK, I'll bite! I can't see the fatal flaw.
Why is this circuit a poor performer?
A pal of mine up in Newcastle, is gathering
parts to build this particular motor controller,
so if there is anything flawed, we would like
to know about it.
Is it the choice of parts that you don't like?
Remember, here in Australia and NZ the
parts available to choose from are limited,
compared to the vast array of components
available at low prices in the USA.
Getting back to that particular circuit, I thought
the designer in New Zealand hand done a
good job. The Fet driver circuit looks good,
there is a safety interlock and over current feature.
What don't you like about this controller?
I'm not the original poster that said to be a flaw and never came back
with the flaw, but took some minutes to take a look at the circuit and
saw 2 possible points of failure: the low speed diodes in the bridge and
the high side driver. Both could burn the FET's. I really think it would
be better to use some bootstrap from IR or some other (IR2104 comes to
mind, but it may be too weak for driving mosfets with 40A). Most of the
times the MOSFET internal diode is enough. IGBT can be purchased with
internal diode as well. If the internal diodes aren´t enough, be sure to
use high speed ones and to give dead time to the switches longer than
the reverse recovery of the diode and the turnoff+fall time of the switch.
The fonts used in that schematic made my eyes go funny, and double and
triple lines here and there made me feel a bit drunk, also for some reason
adobe seemed to take forever just to scroll acros it .. so i havnt managed
to look for any flaws.
Further looking at it revealed C5 wrong way round,lower gate drive circuits
could be a lot simpler with just 2 transistors each, top ones cld be done
with 3 or posibly 2, overcurent should have some sort of hystereseis/time
latch on it, otherwise the transistors could be switched on and off very
rapidly under overcurent conditions for wich the slow antiparalel diodes
used would cuase serious problems. im not sure about the wiseness of
paraleling such a lot of the circuitry for more power.
As a note, reverse and forward seem to be negative logic inputs, and it
might be preferable to force an all off/fualt condition if both are active.
Someone else mentioned the slow diodes in the bridge and the high side
driver, but tbh i assume this isnt going to be switched at a very high
rate, - or sudenly switch from forward to reverse instantly, its just so
obviously not designed for it, but maybe it could use some dead time
control, just in case, a few tens of microseconds at least, then the diodes
would have plenty of time to recover before they were reverse biased, is
obviously not designed to handle the kenetic energy of the motor being
dumped back into it, so the bridge diodes just have to handle inductive turn
off transient, in fact i asume the mosfet internal diodes will be able to
handle that anyway, although thinking about it again the emf from the free
wheeling motor might reverse bias them imediatly after the curent has droped
to zero, im not sure the 0.1 uf capacitors would help much with the curents
involved, but its not like its 100v or more.
The supply to the high side driver could do with being modified slightly,
high transient curents flow through c8/c11 also d1/d4 wich is probably not
Apart from the choice of power transistors im not sure what else there is
left to criticise perhaps it could do with an on/off switch maybe. or maybe
im missing something bigger? the 100k resistors in series with the scmit
trigers seem dubious, and perhaps some supply decoupling.. there doesnt seem
to be any at all for the main supply.
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.
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