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Topic: Help with a High voltage, High current H-Bridge (Read 11610 times) previous topic - next topic

tuxgeek22

I need a H-bridge capable of driving up to 125A at 36v, and I have an idea of what I might need, but I am not sure. I am looking at IXTP200N055T2 mosfets (N-CH 55V 200A TO-220) and driving them with a HIP4081A h-bridge driver. I already know I will need a regulated 12v feed to run it, but will it work for my application? If so, what size bootstrap capacitors do I need?
Thanks!

RuggedCircuits

125A is pretty scary stuff. At 36V, you're looking at a load with a resistance of 36/125 = 0.288 ohms. Is you wiring going to be up to the size necessary to be negligible relative to this this? What are you going to mount the MOSFET on? It ain't going to be a breadboard or a regular PCB.

Your MOSFET has an on-resistance of 4.2 milliohms, quite respectable, but that means it will dissipate 125^2 * 0.0042 = 65W of power at 125A. Nuh-uh. It'll put on a brief light and sound show once, unless (maybe) it's well bolted onto a massive heat sink or air/liquid cooled.

When switching such large currents, too, transients can easily (well...conceivably) couple into the drivers so you have to be really careful using IC's like the HIP4081A and keep it protected.

If you're not perfectly sure of what you're doing here, I'd stay away from anything that requires 125A at 36V: that's a 4500W power driver you're trying to build. Only experts do this. Beginners end up building sources of fire.

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tuxgeek22

The motors are going to be driven at 24v, but I would like the ability to overvolt in the future. The constant current of the motor is 40A, but it peaks at around 100A. I have heatsinks for the mosfets for this, and I can dissipate up to 200W of power from the heatsinks, so I think I will be good there. I never plan on pushing the motors hard enough to even draw 40A. I will probably run them more in the 5-25A range. Will this driver run the mosfets that I am looking at?

RuggedCircuits

Yes, the HIP4081A can source/sink 2.5A peak currents so should be able to handle 7nF gates like your MOSFET, though you don't want the switching frequency to get higher than necessary. I'd maybe start in the audio range (1 kHz?) and monitor how hot the driver is getting, and turn up the frequency beyond 20 kHz if it will handle it.

At those currents, though, I'd still recommend some driver protection, like Schottky diodes across the driver outputs to prevent negative transients.

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tuxgeek22

Ok, that makes sense. I was wondering about protection of the driver. Now, what bout bootstrap caps? I have never played with anything running bootstrap capacitors before, and was wondering what size I should use. Thanks

RuggedCircuits

Those aren't critical, 1uF ceramic should do fine.

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tuxgeek22

Ok. Is that mosfet good for the application, or is there a better one that would work? To me, they looked good for the task... \ Also, would 90v 3A schottky diodes be ok? I have enough sitting here for this.

RuggedCircuits

Depends on your budget :)   If you've got money to burn you could go for this bad boy:

http://www.irf.com/product-info/datasheets/data/irf7749l2pbf.pdf

90V 3A Schottky diodes for protecting the driver are overkill. They're only ever going to see the MOSFET gate voltage (12V) so 20V-30V ratings are fine, and current will be negligible except during transients.

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tuxgeek22

If I had money and was going to build a board, HELL YEAH! But, I am using all hole mount stuff for a reason. Any REASONABLE recommendations? I would like to be able to reuse this motor controller for other high current projects (Hence the high specs) so the 200A is kinda nice. The heatsinks I have are nice aftermarket jobs build for the C2Q processors, so i can deal with some heat

RuggedCircuits

I doubt you can do better than 4.2 milliohms in through hole -- heck the part leads are going to have more resistance than that. Oops...here's a 2.2 milliohm one in TO-220:

http://www.nxp.com/documents/data_sheet/PSMN2R0-60PS.pdf

If your heatsink is for a C2Q processor then I wouldn't trust its thermal resistance rating for a TO-220 MOSFET. That rating was specified for an attachment to its full surface area. You're going to be mating to a much smaller area thus will not be able to remove heat nearly as fast.

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tuxgeek22

#10
Feb 27, 2012, 06:24 am Last Edit: Feb 27, 2012, 06:28 am by tuxgeek22 Reason: 1
If I am only running on average say 25A through them and I peak sometimes to 100A, will it be too much of an issue :~? I can watercool them if I need. I have a self contained water-cooling rig in my basement I can try it with.  :smiley-roll:
Also, now I will have to find out how to include a person in my source list for my research paper. List information gathered from you as directions on how NOT to blow myself up  XD

RuggedCircuits

"Peak sometimes" needs to be quantified in time duration, then you have to look at thermal capacitance properties....this is not going to be easy to predict, and even a prediction will have to be verified in practice. Whatever you do, turn it on under a minimum of loading (1A!) and slowly start turning up the current and monitor what's going on with all the components temperature-wise, rather than just turn it on full blast and hope for the best.

Better than watercooling is paralleling MOSFET's. Put two MOSFET's in parallel and they'll share their current load quite nicely, but of course your driver now needs to supply twice the current, or your MOSFET's will turn on/off half as fast (which will increase their switching power dissipation and they will get hotter).

I remember Infineon has some really nice application notes that show how to estimate MOSFET power dissipation beyond just computing I^2 * Rds(on), to include switching losses (which can be significant for large gate capacitances). For example here's their app note on MOSFET paralleling:

http://www.infineon.com/dgdl/Parallel_Operation_of_Power_MOSFET_.pdf?folderId=db3a304412b407950112b426db703ad9&fileId=db3a30431ed1d7b2011eee77009e547a

Here's another one on MOSFET selecton -- Chapter 11 is particularly relevant as it deals with switching power losses.

http://www.infineon.com/dgdl/highswitchingfrequency_C3.pdf?folderId=db3a304412b407950112b40acf480692&fileId=db3a304412b407950112b40acf580693

I wouldn't cite anything you found on an Internet forum in a research paper :)  Better to cite Infineon's app notes.

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tuxgeek22

Place it on your website, and i will cite it :D. I hope I dont have to parallel mosfets. It makes it more complicated and expensive and more junk to go wrong  :smiley-eek:...

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