Ganging H-Bridges

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

But I'd like to find a full bridge with some protection in it(current limiting, temperature, etc...).

Any ideas?

Thanks, Jon

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.

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.

Cheers Terry

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

tight thermal & electrical coupling and it can work well. H-bridges tend to be larger and thus further apart than the discrete case (pun intentional) so might make that a lot harder - especially if they have crappy Rtheta-jc

Cheers Terry

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.

-- Bill Sloman, Nijmegen

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)

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).

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). 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).

essentially you are both right. The trick is to keep them in very close proximity, with extremely tight thermal coupling. And dont forget to use one Rg per FET. Symmetry is your ally here; visual disharmony is the enemy.

Cheers Terry

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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 datasheet 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 sheet 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.

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.

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

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

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

-- Bill Sloman, Nijmegen

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Diplomatic, but wrong.

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Good advice, but it doesn't help current sharing, for which you need a source resistor per part if you aren't operating at high enough currents for the positve temperature coefficient of the channel resistance to swamp the negative temperature coefficient of the gate threshold voltage.

-- Bill Sloman, Nijmegen

LOL

Hi Bill,

do an electrothermal model, then think again. tight thermal coupling ensures they stay at the same temperature regardless of Rdson. yes one might hog current and heat up, but that heats up the other die(s) so you end up with them sharing anyway (as well as can be expected given Rdson variations between devices, which will inevitably lead to some derating)

loosen up the thermal coupling and your argument will indeed hold, and they wont share very well (if at all).

linear people might toss watts away into source resistors to force sharing, but SMPS people *never* do. And I have paralleled a lot of FETs and IGBTs. A 2500W 16:400V converter I worked on had a block of 12 TO-263 FETs for the H-bridge, which took up 2x as much space as the transformer itself.

I challenge you to find a single commercial SMPS that has paralleled FETs with source resistors. you might find some hobbyist designs that do that.....

I also used to work with a techgineer (he was bloody good but lacked the eng. degree) who built stupidly large (1-6kW) audio amps. Coming from a motor drive background, he optimised the thermal coupling and found that he could use stupidly low source resistors.

and of course when he cranked the sound up (we flatted together) he could make the letterbox flap rattle (and that was a good 5m from the house). Toccata & Fugue in D minor was pretty good for that!

Cheers Terry

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?

Choose Vg >> Vt

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.

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

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

I suspect Bill is thinking about linear applications.

Cheers Terry

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 datasheet 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 sheet 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.

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.

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

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

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

--------

I suggest you read

because you seem to think MOSFETS = BJT's.

for example,

"Differential RDS (on) will cause current unbalance and extra conduction losses as expected, but these are limited due to the

positive temperature coefficient for MOSFET resistance. The thermal 'runaway' characteristic of other semiconductor technologies

does not apply to MOSFETs."

"Gain factor differentials (DGF) result in limited current unbalance. In the extreme, which is difficult to realize in practice,

the current unbalance is limited to the gain ratio. Since turn-on differentials are very easy to control, the predominate loss

differential occurs during turn-off."

And the pdf just about contradicts everything you have said so far. (except maybe in the rare case where there is a complete parameter mismatch). Of course its not only the pdf but other sources too.

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Current differentials during turn-on and turn-off do depend on the differences between the device gate-threshold voltages. For the Fairchild parts you nominated, the worst case limits are 1V and 3V. A

1V part would be carrying about 50A before a 3V part started to turn on. Paralleling two transistors with that level of mismatch would put almost all the switching disipation in the lower threshold part.

Forsythe weasels around this point - his job is selling MOSFETs - but it's in his paper, if you read it carefully.

-- Bill Sloman, Nijmegen

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I suspect that neither Terry nor Jon has thought about the increased dissipation in MOSFET switches during switching. My 1996 Peltier junction thermostat paper talked about setting the switching frequency for the PWM output stage at around 200kHz to get roughly equal static and dynamic power dissipiations in our switching MOSFETs - more recent circuits switch appreciably faster and this dissipation would presumably be dominant in these applications.

-- Bill Sloman, Nijmegen

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Don't guess. Dig out a calcular or dig into LTSpice and work out what would actually happen.

People are good at ignoring issues they don't want to think about, or aren't important in the area of their immediate interest. For the Fairchild FDD8424H that you were thinking about using, the 1V to 3V range tolerance on the gate threshold voltage - if manifested between two parallelled switching transistors - would have the lower threshold transistor carrying some 50A more of the switching current than its high threshold partner.

How much extra switching dissipation this would generate in the lower threshold part depends on the complex impedance of the load being switched, but it could well be that the lower threshold part would be doing all the heavy switching.

-- Bill Sloman, Nimegen