parallel MOSFET driver, driver amp (parallel MOSFET bank driver)

Discussion in 'The Projects Forum' started by Tom_001, Mar 9, 2011.

  1. Tom_001

    Thread Starter New Member

    Mar 9, 2011
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    Hello, My name is Tom and I am looking to tap onto some knowledge of the members of this forum! I have some learning, but i'd like to expand on it.

    The final goal is to build a speed controller for a light ev motor bike, nothing to complex, no reverse, regen braking, motor braking or soft start. Just to control the speed of a series wound motor, much the same as the older electronic rc esc's.

    The planed motor will be in the values of 100v and 120A continuous so about 430A intermittent.

    The way I intended on doing this is with a "bank" of parallel 100v power mosfets.
    The problem is driving the "bank" I can not find any mosfet drivers that will handle that kind of load.

    The first idea was to use a simple small low voltage PMW motor speed controller to drive the "bank", I'v seen this done but I doubt it was to affective as the square wave patten will have already been through a set of mosfets of some kind and not be so sharpe because of the charge and discharge capacitance and that will mean the "bank" will spend a much longer time in the half on-off stage wasting power and making lots of heat, as far as I can tell.

    The next idea is to use a normal 20khz ish mosfet driver with potentiometer, to drive a simple audio style amp to then drive the "bank".

    The question is this, will the wave pattern from a PMW speed controller have the effect i think it will have if i then use it to drive more MOSFETS?

    Will an audio style amp keep the wave form from a MOSFET driver usable for driving the "bank" as I'm not familiar with different types of amp's and the full on full off speeds.

    One last thing, I have seen many 100v power mosfets with high current ratings say 50-70, witch is fine, but they mesure about 1cm high, and the legs are 1mm x 0.5mm, I can not see how the legs can handle any usable current at all, these are from rs, I'v studied the data sheets but perhaps I'm missing something.

    Any help or ideas are grately appreciated.

    Thanks Tom.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    For that current level you might want to look into IGBT instead of MOSFET.
     
  3. Tom_001

    Thread Starter New Member

    Mar 9, 2011
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    Hi, thing is MOSFET's seem to be used 600v and under and IGBT 600v and up, also the high current IGBT have low switching hz compaired to MOSFET's (well the ones i can buy and afford) so runing IGBT at near 20khz will be near there max and spend to much time in the half on-off state making heat?.
    IGBT are used in banks alot, do you know depending on type are the drivers cross usable?
    The IGBT drivers must some kind of transister amp to drive them and keep the signal clear?

    Thanks tom
     
  4. Tom_001

    Thread Starter New Member

    Mar 9, 2011
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    Last edited: Mar 10, 2011
  5. wayneh

    Expert

    Sep 9, 2010
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    You're beyond my expertise but here's how I understand it: You'll need to fully charge each gate - essentially a capacitor - on each cycle, to the required gate voltage. The datasheet should give the capacitance and voltage, and you choose the frequency. From these parameters you can calculate farads per second, or current in amps required to get the job done. Obviously you need to scale that to the number of MOSFETS you have in parallel.

    My point is, you don't need to guess. You can calculate an estimate and design for it with current limiting gate resistors and such.

    ps: Here's another MOSFET that might interest you. IRFS4010
     
    Last edited: Mar 11, 2011
  6. Tom_001

    Thread Starter New Member

    Mar 9, 2011
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    Yes that mosfet looks more the part, the one i looked at was the NTB6411AN and mentioned the legs, 3 normal legs (0.3mmx0.6mm) g d s and the back is connected to drain, so 100v 72a on 1 source leg... i'd love to know what current that leg can take at 100v.

    The IRFS4010 as you mentioned, 100v 130a has 5 source legs that are slightly bigger (0.1mm) and the back heat sink is drain (no paste then) so thats a mosfet only has 1.8x the current but over 5 timed the leg size. I think the NTB6411AN has a cheap inadequate package, I know they arent used to there max current ratings but still it must be worth the little extra money.

    The thing is I'm not convinced that that capacitance value is what current the mosfet gate needs, mosfets as I understand are voltage switched, but it need some current to charge the gate, a potential current? maby it needs to be there but is drawn on so quick it has the affect of not drawing it? I'm finding learning about this very fun. thanks for the input.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Not sure I follow you but yes, the MOSFET is switched by the charge (and resultant voltage) on the gate, which I think of as simply a capacitor that you have to charge and discharge to turn the FET on and off. At low frequency, there is essentially no current involved in charging the tiny cap/gate, except that the instantaneous peak must still be limited to avoid a damaging spike. At high frequency, charging and discharging a cap many times requires a high average current, and this too must remain within specifications. I guess at a high enough frequency, the gate current of a FET would approach the base current of a traditional BJT.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Better read through this: http://www.mcmanis.com/chuck/robotics/projects/esc2/FET-power.html
    It's a good tutorial on MOSFET power ratings.

    Then you'll might look at synchronous buck controllers; a standard PWM-type controller will dissipate far too much power in reverse-EMF diodes.

    A National Semiconductor LM25116 is one such type synch. buck controller device, but not really suited to your application.

    I'm afraid I don't have the time to help you with this; just trying to give a couple pointers.
     
  9. Tom_001

    Thread Starter New Member

    Mar 9, 2011
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    This is what i'm trying to get my head around:
    http://cgi.ebay.co.uk/12-40V-30A-36...Electrical_Test_Equipment&hash=item2561e6a56d

    I know its simple, but it turns out so am I! all I want to do is to run 3 of these IRFS4010-7PPBF (100v 190A) fets in this simple scooter like set up.

    Can I use this board to directly drive the 3 big fets, its only 30khz so the fets wont spend a high % of the time in transition?
    If not can I cannibalize it in some other way, say replace the fets for some sort of amp to drive the 3 big fets.

    Thanks for ant help, Tom.
     
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