Help with a H-bridge FET driver circuit

Discussion in 'General Electronics Chat' started by oswaldonfire, Jul 15, 2010.

  1. oswaldonfire

    Thread Starter New Member

    Jun 21, 2010
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    Hi everyone,

    I need to build an H-bridge FET driver circuit. I've been looking at this circuit:

    http://www.discovercircuits.com/PDF-FILES/HBRIDGE2.pdf

    This fits my current requirements perfectly.

    Any ideas on how to build this circuit? I don't have the experience or resources to build a custom board to put this circuit on myself, and I think that this is a little too much to build "dead-bug style", without a board.

    Also, does anyone know how to find the frequency that this will run at?

    Any help is appreciated.

    Chris
     
  2. hspalm

    Active Member

    Feb 17, 2010
    201
    8
    The maximum switching speed of the transistors you will find in it's datasheets. If you use it as a simple full-speed on/off controller, you do not need to worry about its bandwidth, which you probably never need to consider in a simple application as this anyway.

    I suggest you just buy a ready made integrated circuit h-bridge driver on a single chip if you worry about building this circuit :)

    edit: If you give us the specifications of the application and motor you will be driving we are able to help you much better.
     
  3. sgardner025

    Active Member

    Nov 5, 2009
    79
    4
    You may want to look at a H-bridge driver IC. I just built a circuit using the one in the attached data sheet with pretty good results. Are you using this for one speed control or variable?
     
  4. oswaldonfire

    Thread Starter New Member

    Jun 21, 2010
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    0
    Thanks for the replies guys.

    Sorry, I should have included what I will be using it for. I will be using this to switch a coil at high speeds. The coil is wound like the primary of a transformer. I am interested in transferring the power in the coil to my work via induction, so I need to drive the coil with a high frequency, pulsed DC current. I want to use another circuit to provide the high-frequency pulsing, and then interface that to the H-bridge somehow to drive the coil. I'm talking about a pretty decent amount of power, too. Probably around 12v @ up to 10A.

    sgardner025, that looks good, except that it cannot handle the current that I am after. it is only good up to around 2.5A.

    Thanks

    Chris
     
  5. sgardner025

    Active Member

    Nov 5, 2009
    79
    4
    The IC won't be carrying the load current, you will have to use external N-channel mosfets. Are you sure you need to use an H-bridge for your application?
     
  6. oswaldonfire

    Thread Starter New Member

    Jun 21, 2010
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    Well, if you can think of anything else that will be able to switch my inductive load at high speeds and high currents, that would be helpful. An H-bridge was suggested to me as a way to start, but it could be any circuit as long as it gets the job done. I was originally thinking of a simple circuit that used a crystal to set a signal at a high frequency, which then drove a MOSFET or something to drive my coil. But I couldn't find any sort of circuits online that accomplished this. So then I was thinking of using a 555 to do the same thing as the crystal, driving the MOSFET at a high frequency, in the high kHz range. However, I don't have much experience in this subject area so thank you for sharing your knowledge.

    Chris
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    You don't say at what kind of speeds, nor what voltage and current you want to switch. You need to be more specific. "High kHz" isn't very specific.

    Dave Johnson's H-bridge would be quite slow, as the IRF4905 has a large total gate charge requirement (Qg=180nC) and that has to be discharged via a 2.2k resistor. There is no dead-time protection against the dreaded shoot-through condition (shoot-through is when both the high and low side MOSFETs are on at the same time, shorting out the supply, which usually results in lots of smoke). The 1N540x series diodes are very slow 3A diodes, and will not perform well at much more than a few hundred Hz.

    [eta]
    Trying to operate an inductor at high speed really is not a trivial task. For example, if you tried to drive the inductor at maximum current in both directions, you would very likely experience a phenomenon sometimes referred to as "flux walking". This occurs when the ratio of ON times for both directions is not perfectly matched, or not controlled via current sensing mechanisms. Flux walking leads very rapidly to inductor saturation, high voltages and high currents through the MOSFETs, and smoke.
     
    Last edited: Jul 16, 2010
  8. oswaldonfire

    Thread Starter New Member

    Jun 21, 2010
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    I intend on switching 12 volts @ 5 amps @ about 120 kHz. I could always modify this slightly later, if I allow a little leeway, right?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    At those kinds of speeds, if you don't make it properly the first time, you won't have to worry about modifying the existing circuit - as if it's not correct to start with, it'll be a puddle of burned/melted plastic. :eek:

    If you want this thing to have a reasonable chance to survive, you'll need current sensing, high-side/low-side drivers, and a pretty speedy microcontroller to time things precisely.

    Just to get an idea of a better H-bridge, take a look at Microchip's AN907: http://www.cs.uiowa.edu/~jones/step/an907a.pdf
    It's a stepper motor driver dual H-bridge, but you'd only need a single bridge.
     
  10. oswaldonfire

    Thread Starter New Member

    Jun 21, 2010
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    Is there any reason why I couldn't just use a microcontroller to create the timing signal, that then drives a large MOSFET? Maybe a xenon lamp or something in there to protect the MOSFET from the back emf?
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    You would need a snubber circuit, and the MOSFET on-time would have to be less than 50%.
     
  12. oswaldonfire

    Thread Starter New Member

    Jun 21, 2010
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    You mean that the duty cycle would have to be less than 50%? I plan on making it 50%. And by a snubber circuit, do you mean a simple doide in parallel to to load? Or a more complex circuit?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, the ON-time will have to be less than 50%.

    No, a diode in parallel to an inductor is typically used for reverse-EMF as a "flywheel diode" on a motor or the like. You need to stop the current flow without allowing the voltage to exceed the rating of the MOSFET. One way to do this is by using a Zener in series with a fast-recovery diode. However, in your application, that would require a Zener of about 15v, and about 20-25W power rating.

    There are components called TVS diodes that can be pressed into service.
    http://en.wikipedia.org/wiki/Transient_voltage_suppression_diode
     
  14. Erac

    New Member

    Aug 7, 2010
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