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High side FET drive question

Discussion in 'Electronic Design' started by Chris Carlen, Sep 16, 2003.

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  1. Chris Carlen

    Chris Carlen Guest

    Greetings:

    I am considering to build an H-bridge around a solenoid coil with the
    capability of delivering 150V at 30A for up to 500us pulses, followed by
    about 40V at 20A (PWMed) for up to about 4.5 ms, with up to about 5%
    duty cycle of solenoid pulses (58.5W average power, 4500W peak power).
    The H-bridge would be part of a PWM constant current drive, controlling
    solenoid current to be proportional to some command voltage. This is a
    deisel fuel injector driver. I want to achieve about a 500us current
    rise/fall time from 0-30A into a 2ohm 2mH load.

    I am looking at a employing four of IRFP260N 200V 50A 0.04ohm 300W
    devices. They seem beefy enough to not need to parallel devices.

    The question is how to accomplish the high side drive with a 150V B+
    rail? I am considering using a DC-DC converter to provide isolated 12V,
    and powering a FET driver such as TC4426 with that. The input of the
    FET driver would be driven by the output of a fast optocoupler powered
    by the DC-DC converter as well. The input to the optocoupler is then
    accessible at the ground level, and can turn my FET on and off. The
    problem with this is that a bunch of circuitry (the DC-DC output
    circuitry, and everything to the right of the optocoupler LED) must
    swing with the output voltage. I guess there is nothing inherently
    wrong with this, but it must radiate a lot of EMI as well as presenting
    capacitance to the output.

    Is this the right approach, or is there a better way?

    I think it is impossible to use a pulse transformer in this situation,
    because there are times when the solenoid must be switched on for a
    solid 500us. But I haven't evaluated this option in depth yet, as I
    have only recently settled upon the H-bridge approach after realizing
    the factors that make a 1/2 bridge impractical, as well as a zener catch
    diode approach.

    But my intuition as well as a few cursory calculations suggest that any
    pulse transformer capable of holding the FET on for 500us would have too
    high of an inductance, and thus too high of a leakage inductance, to be
    able to turn the FET on and off quickly enough.


    Comments appreciated.


    Good day!




    --
    _______________________________________________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
    -- NOTE: Remove "BOGUS" from email address to reply.
     
  2. Chris Carlen

    Chris Carlen Guest

    Whoops, I think that since the coil current is always flowing in the
    same direction, I really need only two NMOS devices, in the upper left
    and lower right corners of the H-bridge, with a freewheeling diode in
    each of the lower left and upper right corners.

    Hmm, that makes things a little easier! But I still need one high side
    driver circuit.


    Good day!



    --
    _______________________________________________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
    -- NOTE: Remove "BOGUS" from email address to reply.
     
  3. Chris Carlen wrote...
    Listen up, grasshopper, and learn. Your goal is to turn on the FET,
    which requires speed and power and then leave it on, followed later
    by turning it off, which also requires power. Here's one easy way
    to achieve these tasks without requiring a low-frequency transformer.
    In fact, the technique requires a high-frequency pulse transformer.

    .. D1
    .. ,---+--|>|---+-- gate
    .. | | |
    .. |( Rx Q1 |/ source
    .. |( | ,--| |
    .. | | | |\V |
    .. | | | | |
    .. '-------+----------'
    .. |________|

    Positive transitions turn on the FET, afterwhich it remains on due to
    its gate capacitance. The negative transition at the end of the 500us
    pulse turns off the FET by means of Rx and the NPN. Actually, I use
    a 4-layer construction for Q1 to insure rapid and complete turnoff.

    Thanks,
    - Win
     
  4. Jim Thompson

    Jim Thompson Guest

    Very similar to what I used for a satellite spinner/launcher about 15
    years ago, except I used a PNP in place of D1 and I had a zener in
    there to set VGmax.

    IIRC I also put it in the Hubble Telescope controls.

    ...Jim Thompson
     
  5. Jim Thompson wrote...
    How long did you rely on the FET's gate voltage remaining charged
    for your space app?

    Thanks,
    - Win
     
  6. James Meyer

    James Meyer Guest

    I'm probably missing something vital, but if the current never reverses,
    why do you need two switch elements?

    Jim
     
  7. Chris Carlen wrote...
    Yes. I've used your method (required for really-long ON time)
    but it's not only more complicated, it's much much slower.

    Thanks,
    - Win
     
  8. Chris Carlen

    Chris Carlen Guest


    In order to get the current fall time to be as fast as the rise time, it
    is necessary to be able to reverse the polarity of voltage applied to
    the coil. I had considered a single switch with zener, but that
    requires a physically unrealizable zener which if it has a voltage of
    150V and the current is 30A, must dissipate 4500W peak power:

    http://home.earthlink.net/~crobc/misc/images/sol-zen.png

    Then I considered a 1/2 bridge, but that leads to problems with the DC
    nature of the coil current. The midpoint of the caps and resistor
    voltage divider can't get back to 1/2 the supply volts until an
    inordinate amount of time, unless I am willing to use heating elements
    for the resistors. Bad idea:

    http://home.earthlink.net/~crobc/misc/images/sol-halfb.png

    A full bridge gets me the desired results:

    http://home.earthlink.net/~crobc/misc/images/sol-fullb.png


    Now if *I'm* missing something vital, please let me know.

    Thanks for the response.


    Good day!
     


  9. If you haven't already read Texas Instruments "Design and Application Guide
    for High Speed MOSFET Gate Drive Circuits" by Laszlo Balogh I would highly
    recommend it:

    http://focus.ti.com/lit/misc/slup169/slup169.pdf

    It isn't quite comprehensive but it makes an extremely good effort to be
    nevertheless. It is certainly the best MOSFET gate drive resource I've ever
    seen compiled into a single document.
     
  10. Chris Carlen wrote...
    Indeed you are.

    The inductance flyback nicely reverses the voltage as far
    as you want, there's no need to apply a reversed voltage
    source. And small zener diodes can handle surprisingly-
    large peak currents, or for truly impressive peak power
    capability use the silicon Transient Voltage Suppressor
    (TVS) type of zener diodes. These have massive blocks of
    copper on both sides of the silicon die to absorb large
    amounts of energy.

    Calculate the energy in the coil's inductance E = 1/2 LI^2
    and compare it to the capability of parts like the 5kp100A
    (5kp110C), which can repeatedly handle 5kW peak exponential
    pulses lasting 1ms (about 2.5J of energy). These can also
    handle 10kW lasting 250us, etc. The breakdown voltage at
    30A will be under 162V. Should you have a need to dissipate
    more energy, use multiple low-voltage TVS parts in series.

    Remember the inductor's discharge will be very fast if you
    let the flyback go to 150V. What's the inductance value?

    Thanks,
    - Win
     
  11. Chris Carlen

    Chris Carlen Guest



    Thanks for the link, Fritz. I'll definitely check that out and keep it
    in my collection.


    Good day!


    --
    _______________________________________________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
    -- NOTE: Remove "BOGUS" from email address to reply.
     
  12. Chris Carlen

    Chris Carlen Guest

    It's not to test injectors, but to drive them. What is under test is
    engine combustion. We have specially designed and modified production
    deisel engines with optical access to the combustion chambers. We
    perform laser induced incandescence and laser induced fluorescence
    imaging spectroscopy to study the processes of pollutant formation, as
    well as some investigations into alternative fuels and the effects of
    oxygenates on soot formation.

    We want to be able to perform up to 4 or 5 injections per engine cycle.
    Our existing "ECM" production engine Electronic Control Modules
    aren't able to issue more than 1-2 inject pulses per cycle.

    Perhaps I can use one FET with an amplified zener damper diode for the
    current ramp down as another respondent indicated.

    Yes, but it requires two power supplies, no? One sources while the
    other sinks. I tried it in simulation, like this:

    http://home.earthlink.net/~crobc/misc/images/sol-halfb.png

    but realized there are serious problems with keeping the voltage divider
    midpoint at V/2 without using very stiff resistors and wasting a lot of
    power.

    That's OK for a single injection. That's what the ECM boxes do.

    Yes. I will work on this approach.


    Thanks for the input!


    Good day.


    --
    _______________________________________________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
    -- NOTE: Remove "BOGUS" from email address to reply.
     
  13. Chris Carlen

    Chris Carlen Guest

    In the H-bridge, energy is removed from the capacitor when the current
    ramps up, and much of that energy is returned to the capacitor when the
    current ramps down. The loss of energy would be the integral of
    I^2*Rcoil, and the average power over time would be the energy loss per
    pulse divided by the repetition period. The power supply needn't sink
    any current. The duty cycle is low at about 5-10% so the average power
    is low, at about 90W max for the heaviest operating conditions.
    Yes, I will look into this. Maybe my first circuit was right after all,
    just missing some assistance to the zener.


    Good day!



    --
    _______________________________________________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
    -- NOTE: Remove "BOGUS" from email address to reply.
     
  14. Chris Carlen

    Chris Carlen Guest


    I expect about 2mH plus 2 ohms of coil R, maybe a little less
    inductance, so about 0.9J of energy at max current. The problem is that
    I have to sustain 20A of coil current for up to 5-10ms perhaps. It's
    not just a matter of dumping the coil energy once, then waiting a while
    to cool down. Thus, I will be PWMing the supply voltage, with a control
    loop implementing a constant current source. But the TVS would then
    have to disspate about 1100W for several ms during the 20A level.

    What do you think of a transistor assisted zener?

    A TVS (if the average power can be worked out) or transistor assisted
    zener isn't such a bad idea for a simpler design for one injector.

    But I may have to drive 4 injectors, so power supply consumption becomes
    an issue.

    The H-bridge results in less total power consumption from the power
    supply, since energy is returned to the supply's output cap during
    flyback, instead of being dissipated as with a zener.

    For instance, with a 1200RPM engine condition, with 5ms injects per
    cycle consisting of 1ms of 30A peak and 4ms of 20A hold current
    (assuming square current waveforms here) we have to supply only the 2
    ohm coil resistance power dissipation with a full bridge, or 100W
    average power per injector. (Actually 99W from the simulator.)

    But with a zener flyback damper, the power dissipation from the
    simulator is about 209W.


    Maybe I will be building that H-bridge after all.

    Thanks for the input.


    Good day!




    --
    _______________________________________________________________________
    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
    -- NOTE: Remove "BOGUS" from email address to reply.
     
  15. John Larkin

    John Larkin Guest

    Sure: transformer coupling followed by DC restoration is a nice drive
    trick, if the time constants don't get'cha.

    John
     
  16. R Adsett

    R Adsett Guest

    Why not use a high side gate drive like the IR 2110? If the high side on
    time is too long to sustain the drive you can always add a charge pump
    circuit. IR has an app note on doing just that. Two 2110's would give
    you all the gate drives needed for a full H.

    Robert
     
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