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Automotive Protection Circuit

Discussion in 'Electronic Design' started by eaglecbr, Dec 18, 2008.

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  1. eaglecbr

    eaglecbr Guest

    I'm designing a Automotive Regulator Circuit for a system that runs at
    about 100mA at 7V with a max current draw of around 600mA for 10secs.
    The problem i'm having is with the protection circuit. I'm using the
    RBO40-40GT(http://www.stmicroelectronics.com/stonline/products/
    literature/ds/5484.pdf). This is the best chip I have found that can
    easily suppress voltages up to 120V(highest that I've tested it
    against). The problem is that this chip only clamps the voltage to a
    max of 40V. I'd like to use a linear regulator if possible, due to
    various load changes. I've been using an LT1129 from Linear Tech, but
    the problem is that it has a max input of 30V. Most linear regulators
    that can handle this current usually have a max input of 30V. I'd
    also like to keep the circuit small and surface mount if possible.

    Any idea what I can do to clamp the voltage to 30V, while still using
    the RBO40-40GT? If I add another Zener diode between the regulator
    and the RBO, clamping the voltage to 30V - will this work? Any ideas
    would be great. Thanks
     
  2. gearhead

    gearhead Guest

    How about using a circuit that goes open circuit instead of clamping.
    You might have to do it with discretes.
    Actually Jim Thompson posted a link here to just such a circuit on his
    website a few months ago.
    Goes something like this:
    A p-channel mosfet in the top rail with source connected to the
    positive power supply and base tied to ground provides reverse
    protection.
    To get overvoltage protection you use a TL431, a transistor and
    another mosfet in the rail. With a voltage divider to the reference
    terminal of the TL431, an overvoltage condition will cause the TL431
    "cathode" to sink current, turning on a pnp transistor that connects
    the source and gate of the mosfet to turn it off.
    Plus the various resistors you would need in a circuit such as this.
    Remember the TL431 may sink some bit of current even when the
    reference pin sees low voltage, so you should put a resistor from the
    cathode to the top rail.
     
  3. For the benefit of the OP.

    This circuit is not suitable for your application as drawn. The FETs
    don't have the drain to source voltage rating and if you put in
    suitable FETs M1will go phut when you exceed its gate source voltage
    rating.
     
  4. IanM

    IanM Guest

    So replace M1 with a diode as its only there for the reverse protection
    and the OP has plenty of headroom with a 7V regulated output even if one
    needs to run right down to 10V in.
    I'm more concerned about M2. Given a suitable FET, I suppose it
    *should* survive provided the trigger point for the TL431 is less than
    its Vgs rating and the TL431 protect itself as it triggers but if you
    aren't muntzing it, a Zener connected gate-source would prevent
    transient stress on the gate oxide. If you *are* muntzing it, make M2
    the pass transistor in the regulator as well.
     
  5. nospam

    nospam Guest

    The first question is is it acceptable to shut down during a load dump. If
    it is you can buy LDO regulators with reverse battery and 60v load dump
    protection built in.

    I presume 60v is enough for 12v automotive systems else they wouldn't
    bother trying to sell them?
    --
     
  6. neon

    neon

    1,325
    0
    Oct 21, 2006
    I don't see how an automotive that uses a battery can experience spikes of any kind. a regulator of the kind of LM 317 HAS A 90DB REJECTION FROM THE LINE USE THAT.
     
  7. MIL-STD-1275 for 24V vehicles specifies a single fault spike of 250V
    for 70us with 100V surges of 50ms. So 251V is allowed to fry the
    electronics.
     

  8. I was thinking of making a load dump simulator/generator "one day".

    There is a circuit in the datasheet for the "LDP24M" (p4).

    <http://www.digchip.com/datasheets/download_datasheet.php?id=502684&part-number=LDP24>

    The energy and currents involved are huge!

    I have seen voltages specified up to ~150V.

    47,000uF charged to 150V dumped through 2 ohms is a pretty serious
    pulse. I've actually gone as far as snapping up a cheap box of 200V,
    1500uF electrolytics to build the capacitor bank required.
     
  9. nospam

    nospam Guest

    The 100v load dump requirement is intended to cover cases where the vehicle
    is operated with no battery connected.

    Higher voltages are low energy spikes which can be absorbed, the load dump
    tests can source about 200J. Your typical 1500W transient suppressor can
    absorb about 1.5J.

    We were talking about cars on 12v not military vehicles on 24v.

    If 100v is good enough for 24v military systems with no battery connected a
    60v requirement on 12v systems seems quite adequate.

    --
     
  10. I guess I am a bit too electronics orientated - because I have no idea
    what those words mean (when put in that order)!
     
  11. Oh I see! Maybe... I was wondering if it was possible to store the
    energy magnetically somehow but it seemed like there would not be
    enough in anything reasonable. It's 500-1000J as far as I can make
    out.

    Is the energy just in the inductance, or is it mechanical energy in
    the movement of the clutch (or compressor rotation)?
     
  12. Jasen Betts

    Jasen Betts Guest

    also you need to increase R3 to about 3K to save the TL431 from having
    to conduct 1A during faults.

    to compensate you may need to add some voltage drop (three 1m914's in series)
    in series with R4 (which should also be increased to about 3K)
     
  13. JosephKK

    JosephKK Guest

    Both as i understand it. Kind of like large solenoids.
     
  14. neon

    neon

    1,325
    0
    Oct 21, 2006
    spikes are not across the batterybut other places withing the system. You got 40kv switching for spark plugs and that you may see some but never across a battery. the MIL SPEC is precicely the design to eliminate those spikes in add on system like an APU.
     
  15. Cool. The "official" simulator circuit I referenced in the datasheet
    above contains a 12mH inductor. A simulation indicates it can see
    currents of >50A.

    12mH, 50A inductors are a bit thin on the ground... I found a 5mH one,
    it's a 10 inch diameter toroid :)
     
  16. eaglecbr

    eaglecbr Guest

    Thanks for the help everyone. Sorry I didn't respond earlier. It is
    not acceptable to power down the system during a large load dump. I
    need an always on system. I might end up putting like a battery
    backup system that will switch to backup battery during long load dump
    pulses. I would prefer a system that I described earlier, but don't
    know if its possible to clamp the voltage to 30V.
     
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