Re: Ganging H-Bridges

Terry Given wrote:
bill.sloman@xxxxxxxx wrote:

On 8 apr, 21:59, "Jon Slaughter" <Jon_Slaugh...@xxxxxxxxxxx> wrote:

<bill.slo...@xxxxxxxx> wrote in message

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On 8 apr, 15:26, "Jon Slaughter" <Jon_Slaugh...@xxxxxxxxxxx> wrote:

"Terry Given" <my_n...@xxxxxxxx> wrote in message


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bill.slo...@xxxxxxxx wrote:

On 8 apr, 00:25, "Jon Slaughter" <Jon_Slaugh...@xxxxxxxxxxx> wrote:


"Jon Slaughter" <Jon_Slaugh...@xxxxxxxxxxx> wrote in message


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Is it common to parallel h-bridges?


It isn't usually a good idea - the tolerance on the gate threshold
voltages usually means that one side of the pair carries the bulk of
the current, and as that device gets hotter its gate threshold voltage
will drop, leading it to carry even more.


BTW, I can't seem to find any in-expensive H-Bridges for 180W@12V so I
was
thinking of using two of these


http://www.fairchildsemi.com/ds/FD%2FFDD8424H.pdf


At currents above 30A the postive temperature coefficient of the
channel resistance of these parts beats out the negative temperature
coefficient of the gate source voltage, so if you are looking to
switch more than 60A you might get away with it.


Otherwise you'd need to add a small resistance in series with each
source to force current sharing.


--
Bill Sloman, Nijmegen


sounds like a recipe for disaster at currents below 30A. But its also
why
IGBTs can be direct paralleled (nice +ve tempco). Extremely tight
thermal
coupling can get around a lot of the problems though.


the easiest way to parallel H-bridges is with interphase reactors to
soak
up all the little variations. Depending on the load, split the first
inductor into N inductors for N bridges, each N times more henries and
1/Nth the current so each one is N*(1/N)^2 = N times smaller. join the
ends of the inductors together, then continue with the rest of your
circuit.


I don't see any difference between parallel H-bridges and discretizing
the
H-bridge and paralleling the individual mosfets... which is no problem.


If you don't think it is a problem, you haven't been doing it for long
enough or on a large enough scale.


If you want to parallel MOSFETs or discrete transistors you almost
always have to add components to make sure that each active device
carries more or less the same current. Production tolerance is not
your friend.


But this contradicts AOE and many other sources I have read that say
paralleling them is no problem. MOSFETS have negative temperature
coefficients rather than positive like BJT's. (hence as one gets hotter it
gets more resistive and less current will flow through it and through the
other.. they should ultimately balance out, in proportion, if it is not too
bad)



Go back to my original response (the third one in the list) and read
it to the end. Then take a careful look at the data*** that you
posted. MOSFETs only had a positive temperature coefficient for high
drain currents - higher than you are likely to be using. Check out the
drain current versus gate-voltage curves in the data *** you posted,
rather relying on Win Hill's thirty year-old observation about a much
smaller MOSFET than you will be using - the 2N4351 data in his figure
3.13 switches to a positive temperature coefficient at 2mA, which the
Fairchild part you are contemplating has a negative temperature
coefficient up to 30A.


dead right if you are using these as linear amps.

now stick 15V up the gate, and drive a SMPS-style load (Id set by load rather than Rdson). how much use is that figure now?

And MOSFETs have fairly large gate threshold voltage tolerances, so
you are quite likely to start off with all the current going through
one of your parallelled MOSFETs, which isn't a good start.


Choose Vg >> Vt


I assume then you mean that one mosfet might take a little more current than
another because they are not exactly the same. Ok, that might be true but
then you just add one more mosfet to the mix and it should compensate enough
(assuming they are not that much different, which I imagine they aren't).



Using your imagination is a poor substitute for reading the data ***
carefully


The only issue it says is that the more you parallelize the more gate cap
you have hence its harder to drive(and eventually becomes impossible).



It never becomes impossible - the switching times just grow in direct
poroportion to the number of MOSFET's.


hit them harder. For really big devices I use a current-limited bipolar driver (emitter resistor and feedback transistor) along with a stupidly low Rg. A set of large paralleled gate caps can charge up fairly quickly with, say, 30A.

Especially when you consider that all that is really important is the region around Vt, the transition thru which one wants to proceed AFAP.


Of course that stuff is for discrete mosfets and I'm not sure about
h-bridges(specially since they probably have more circuitry in it for other
things, in general).



Dream on. If they do incorporate current limiting or thermal
protection, the data *** will tell you about it, and you won't want
either to come into action in normal operation.


hell yeah! I can see those "features" leading to days, weeks, months of despair :)

--
Bill Sloman, Nijmegen


Cheers
Terry

and you could always read "paralleling of power mosfets for higher power output" J. B Forsythe (nice to see you, to see you...nice), IR

Cheers
Terry
.

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