Selecting some parts for an H-bridge.

Discussion in 'The Projects Forum' started by Zycron, Aug 13, 2008.

  1. Zycron

    Thread Starter New Member

    Aug 12, 2008
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    Hi everyone,

    I'm trying to build an h-bridge for an inverted pendulum type project (a segway, basically). I've been researching on how to build them (online and bought a book), and I think I'm kind of close to getting it, but I really don't know for sure. Any help would be greatly appreciated!

    The project will use two identical 250W 24v / 13.7amp (Unsure of stall current, it's not documented as far as I can tell.) dc motors; Model: MY1016. Total load will be in the vicinity of 200lbs, shared amongst the two motors. The battery is a NiMH 24v / 40amps continuous output at a rating of 4200mAh, so I assume I am within an acceptable range to have both motors run off it exclusively.

    The parts I have chosen are listed here: (I believe they all have a much higher rating than needed but I could be wrong, I'm a real newbie here.)

    High side (p-channel) MOSFETs: http://www.st.com/stonline/products/literature/ds/8177/stp80pf55.pdf
    Low side (n-channel) MOSFETS: http://www.vishay.com/docs/91328/91328irl.pdf
    Half-bridge driver ICs (synchronous, with dead-time built in): http://www.irf.com/product-info/datasheets/data/ir2104.pdf
    Diodes (really unsure if these will work correctly): http://www.diodes.com/datasheets/ds23001.pdf

    I didn't label the capacitors in my schematic because I have no idea what ratings to use. I have read about the back-emf and electrical noise. I've seen a few h-bridges use 1uF capacitors for C1 through C4 (referring to my attached schematic) to eliminate most of the noise, but they were for smaller h-bridges so I don't know whether or not I should use them. C5 is supposed to absorb voltage spikes when the motor switches directions (the magnetic field collapses, sending power back into the circuit, right? or is this the job of the diodes, or both :confused:), and give the circuit lots of power when it starts up again (also unsure on a good rating to use here, I just know it needs to be big), from what I understand.

    The circuit design program I used (Pulsonix) left out some pins on the driver ICs, but I don't think they are really needed to address this problem (they are in the datasheet if needed though). I'd also like some feedback on if those are good drivers to be using, if possible.

    If you need more information I'd be more than happy to get it for you. I tried not to leave anything out.

    Again, any help is much appreciated, the tutorials I have read can't seem to answer this for me.

    Thanks,
    -Keith
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Is that really 4200mAh? Or did you mean 4200Ah (which would also be incredible, but on the other end of the incredulity scale.) A 4200mAh battery would give you a run-time of perhaps a minute, if you were very easy on the "throttle".

    As far as the MOSFETs, you'd be better off to use N-ch power MOSFETS all the way around, and a high-side driver to take care of the high-side drive requirement.

    Back to the battery thing, dry cell/rechargeables will have a comparatively high internal resistance over lead-acid cells. This will cause a great deal of wasted power when the motor is in a stall condition. Unfortunately, lead-acid and SLA (sealed lead-acid) batteries are rather heavy and bulky. But without a decent-sized battery, you will have an extremely limited run time.

    If you're going with SLA's, plan on needing about 3x the capacity as the batteries are rated for. Discharging lead-acid or SLA batteries too deeply is hard on them; and as they age, their ability to accept and release a charge will gradually decline - until they get to the "open cell" stage.
     
  3. nanovate

    Distinguished Member

    May 7, 2007
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    The high-side mosfets are reversed...
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, the high-side and low-side MOSFETs are swapped - and the symbol for the P-ch MOSFETs wasn't made correctly as it's indicating an N-ch.

    If an existing library N-ch model was renamed to a P-ch part number, it sure won't work very well in simulation.

    C5 will save a lot of power, but not much useful work will get accomplished.

    I don't understand why you have the low-side drive lines swapped across the H. That would defeat any automatic "dead time" the driver IC might add - although I haven't looked at the driver's datasheet yet.

    Still, you're practically guaranteed to wind up with one side of the H-bridge turned completely off, and the other side turned on, resulting in "shoot-through" or a direct short across your supply. Batteries have a nasty habit of exploding when you do that to them. :eek:
     
  5. Zycron

    Thread Starter New Member

    Aug 12, 2008
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    I did rename the parts, since mine were not in the parts list--I guess that's misleading and screwing up the schematic, sorry.. first one ever. I swapped the line to the other side because the driver chip said they were synchronous... connected?, so I thought having a connection to high on one side and a low on the other would give me the diagonal effect to turn the h-bridge on, and prevent shoot through. For the battery, isn't 4200mAh = 4.2Ah, so it could supply 40 amps continuously for 4.2 hours? That's how I looked at it. It was also labeled for use in scooters/robots. Sorry for the mistakes.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    That's OK - no harm no foul. ;) However, it is a good idea to at least select a component that is the same basic type; in this case you would need to rename an enhanced P-ch power MOSFET of whatever type you may have in your library. Unfortunately, you chose an N-ch type, which lead to some momentary confusion.

    I confess that I haven't looked at the datasheet yet. However, all prior gate driver IC's I've looked at previously require the high and low side driver outputs to be connected to the same side of the H-bridge.

    Nope, sorry - that's 4.2 Amps for 1 hour. In reality, it would be more like 420mA for 10 hours to get maximum efficiency from the battery. But since you will be placing quite a heavy load on it (about 27A), you would be very lucky if the battery didn't immediately explode, as the motors would appear as less than a 1 Ohm load (nearly a direct short) across the battery. This would cause the battery to dissipate most of it's energy across it's internal resistance, which would cause very rapid heating; the hotter it got, the more the internal resistance, until the contents boiled, causing rapid expansion and forceful rupturing of the battery enclosure.
    Don't be sorry for the mistakes. Be glad that they were caught before anything bad or expensive happened :) It's a learning experience. The main thing is to learn what is correct and what is not, and how to figure out what the best way to do something might be.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    OK, the IR2104 includes a high-side driver. This means that you can use N-ch power MOSFETs on both the high and low side of the bridge. This is advantageous, as N-ch MOSFETs typically have a much lower Rds(on) than their P-ch counterparts. It also simplifies your bill of materials list, and your spare parts bin.

    The IRFZ44 is a pretty handy power MOSFET. However, it's really designed for logic-level inputs, and the IR2104 can output 15v. Frankly, I'm not sure what might happen to the IRFZ44 with such a high Vgs, but I suspect it won't be good.
    Instead, consider something like an IRF1404, or IRF3205. ST Microelectronics has some interesting N-ch items as well; have a look at their site. Your target is the lowest Rds(on) with a Vdss of at least 50.

    In order to control the direction of motor rotation, the IN of one IR2104 will need to be held high, and the IN of the other IR2104 held low.

    To control the speed of the motor, the SD\ of one of the IR2104's needs to be held high, and the other needs to have a PWM signal that alternates rapidly between a logic high and a logic low.
     
  8. Zycron

    Thread Starter New Member

    Aug 12, 2008
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    Digi-key didn't have IRF1404 or IRF3205 in stock, but http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=497-2774-5-ND from ST micro had the lowest Rds(on) for being Vdss being >= 50 volts.

    Thanks for the battery clarification, I was way off in left field there--makes a LOT more sense now--I think for now I'll use some large lead acids to test it because the smaller packs with as much Ah cost a fortune :O

    The pointers for doing the PWM also helped a lot, I'll be sure to refer back to this.

    Thanks for everything, I really appreciate all of your help! :D
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    You're welcome.

    The STP80NF55-06 looks like a decent choice for your application. The lower the Rds(on), the less that heat will plague you.

    Still, make room for heat sinks. When making things at home, one can hardly go wrong when planning for overkill - within reason, of course. ;) Keeping your MOSFETs nice and cool will keep their resistance low, helping to maximize their lifetime, and the efficiency of the device.

    Low resistance is not necessarily a friend of batteries. However, that is an entirely different subject.
     
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