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H Bridge

Discussion in 'Electronic Basics' started by Jonathan Leppert, Jan 22, 2004.

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  1. I'm designing a high current H-bridge to provide power to a thermoelectric
    cooler/heater. I have selected the International Rectifier IR3221 high side
    switch along with the suggested MOSFETs to form a low footprint H-bridge.
    However, it has some type of provision to start DC motors (its intended
    application) in a kind of soft-start mode, variable by some type of PWM. I
    really don't need this feature and in fact I'm confused by the explanations
    for it in the datasheet. Is there any way to bypass this -- in the example
    application circuit and demoboard they have all kinds of extra components
    for this feature. If anyone could give me an idea on what exactly this is,
    and what I can do with that pin (ss) as far as what it needs for the device
    to work properly. All I need is to be able to switch a current source on and
    off and to change the polarity with this H-bridge, what would be the
    simplest circuit using this device to do that? Is this even the best way to
    go about all this?

    The data sheet for the IR3221 can be viewed below:


  2. Tim Dicus

    Tim Dicus Guest

    Hi Jonathan,

    This refers to the schematic of the IRL3221 20A H-Bridge Demoboard on page
    10 of the datasheet:

    You can delete CP1 (2.2uf cap). It controls the soft start feature. It
    really does not need to be removed to disable the soft start, just leave the
    "Duty Cycle" line disconnected at the connector.

    Hope that helps,

  3. The soft start function is programmed by the RC network applied to the
    ss pin. You can make it as fast or as slow as you want, including
    eliminating that function. If all you need is full heat, full cool
    and off, this chip should do it. If you want to PWM regulate the
    current, you will need some additional components.
  4. Tim Dicus

    Tim Dicus Guest

    Hi Jonathan,

    My first post was a little lacking in information about the soft start
    feature. I was called away by business, so I sent what I had already
    written. So to continue...

    The block diagram on page 1 shows a resistor (R) and a capacitor (C). These
    components determine the time constant for the RC circuit and control the
    voltage rise time at the SS pin.

    The resistor from +Bat to R, and the diode (looks like it should be a 5v
    zener in your case), must remain. These limit the SS pin to 5 volts (5.5v
    max here!).

    The other capacitor(s) from +Bat to 0v is(are) for surge/sag/spike
    protection. The diagram on page 10 shows a couple of 2000uf caps and a 100nf
    cap in parallel. Inductive devices like motors are really noisy

    The "Duty Cycle" input on the connector (page 10 again) is is designed to be
    controlled by an open-collector device, like a 2N3904 or 2N2222. Otherwise
    the soft start won't work. It should pull the line low momentarily only,
    then release. The rise time on the RC circuit will control the soft start
    sequence from there.

    Hope that is more complete.

  5. The resistor from +Bat to R, and the diode (looks like it should be a 5v
    So to completely bypass the soft start function I just leave these
    components connected? No need for the caps/RC network since there will be no
    input on the pin to begin with right? Physical space is at a premium so I'm
    trying to eliminate as many components and complexity as possible, not to
    mention I have to control four separate TE devices.

    Sorry, bear with me, software person (trying) to do some hardware, your help
    is greatly appreciated.

  6. Tim Dicus

    Tim Dicus Guest

    Hi Jon,

    Leave the resistor from +Bat and the 5.1v zener diode connected as it is.
    That will produce 5.1 volts at the junction of the two. Connect SS pin to
    the junction of the two. In other words, replace R with a wire or trace and
    remove capacitor C.

    The two large caps may not be needed if your load is resistive rather than
    inductive. Leave the 100nf cap and maybe a single 1000uf surge/sag cap.

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