Low voltage H-bridge

Discussion in 'General Electronics Chat' started by MartinLarsson, Oct 24, 2010.

  1. MartinLarsson

    Thread Starter Member

    Feb 25, 2009
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    Hey!
    I'm motorizing a small truck (1:87) and I need to make a H-bridge of some sort that can be powered with just 2.4V (two NiMH cells). It will be controlled by an AVR micro controller, which runs on 5V from a boost converter that can't supply the motor. I haven't actually measured what current the motor takes but I guess it would be about 200mA running freely and perhaps 1A holding it still.

    Now, space is sort of an issue and since I want all sorts of other functions, I think I'll use SMT. I really just have about 1x2 inches or so, perhaps two layers with pin headers in between if the components aren't that high.

    I've looked at some MOSFETs and the only one I found that might work and was available was the IRF7105. I've looked at using "normal" transistors but found that the SOT-23 package would get to hot (generalization), since the Rthj-amb≈500°C/W. I could probably cram in some SOT-223 (eg BCP55) but I'd prefer something smaller. And then I would need some schottky diodes as well...

    So, I'm open to suggestions, MOSFET/BJT, SMT/through-hole, low-voltage/more NiMH cells, cooling?

    Thanks! :)
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Do you HAVE to use an H bridge? Perhaps this can be done with fewer components, say , just a single TO220 case(for wattage)switch on the motor. You could still get PWM control over the motor that way, and maybe even could use one of those SOT23 transistors to drive the switch. :)
     
  3. SgtWookie

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    Jul 17, 2007
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  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    If you use mosfets, you will need to get adequate gate voltage. "Standard" mosfets require about +/- 10V for gate relative to source (Vgs). Logic level versions require about 5 V and some newer ones are even lower. The STS5PF20V, for example, turns on almost completely at -2.5V and can be used at even little lower Vgs differences. Second, since your current needs are small, you might consider P-channel for the top and N-channel for the bottom to simplify the gate drive components.

    If you consider driving the mosfets directly from the MCU, be sure to account for the current rush as the gates are powered. Use adequate decoupling capacitors.

    John
     
  5. MartinLarsson

    Thread Starter Member

    Feb 25, 2009
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    Without the H-bridge there would be no problem at all, really. The problem is that I have such a low voltage that few power chips accepts. I've already looked into all this. Thanks, though.

    But can anyone tell me if the IRF7105 would work? Cause if it would I'd just need that right? Or do I need a MOSFET driver as well? The data sheet says the gate threshold voltage is 1.0 - 3.0V, so a voltage of 2.4V is kinda risky. And can I use 5V on the gate when there's 2.4V at the drain. Emm... no that's no good...

    Unless anyone has a super great idea I think I'll just settle with one direction for the motor.
     
    Last edited: Oct 24, 2010
  6. SgtWookie

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    Jul 17, 2007
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    No, the IRF7105 is a standard-level MOSFET. It would not work properly.
     
  7. Kermit2

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    Feb 5, 2010
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  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    The specified "gate threshold voltage" is the point at which the FET is just barely starting to open up at the very bottom of its linear region.
     
  9. sage.radachowsky

    Member

    May 11, 2010
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    I would say you could do this with four zxmn2b14 N-channel MOSFETs. You have 5V for the gate driver, so the top of the H-bridge which is at 2.4V will get 2.6V of gate charge.

    This one is specified for Ohmic resistance at 1.8V and 2.5V Vgs. The one you asked about it not spec'd below 4.5V Vgs, I think. Drain current rating is also adequate, from the graph on page 5.
     
  10. nickelflipper

    Active Member

    Jun 2, 2010
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    The Fairchild Portable Solutions (3MB pdf) Guide is an interesting compilation of low gate voltage spec'd mosfets.
     
  11. MartinLarsson

    Thread Starter Member

    Feb 25, 2009
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    Thanks! It was quite interesting and helpful. However, my supplier doesn't have any of those components. I only found one in their assortment that had a low enough gate-source threshold but I can't really use it anyway...

    The zxmn2b14 would have been perfect, really, but of cause - my supplier and others in this region doesn't have it.

    I think I'll just use some SOT-223 transistors and see if it fits or scrap the idea with an H-bridge. And perhaps I find some other supplier...

    Thanks for all, though!
     
  12. nickelflipper

    Active Member

    Jun 2, 2010
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    Up the voltage! Have you looked at using rechargeable 3.2V AA LifePo4 batteries. If there is a Walmart nearby, they may have the Westinghouse 14500, or 18500, LifePo4 batteries in the garden/solar light section.
     
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