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Overvoltage protection

Discussion in 'Electronic Design' started by Piotrne, Feb 11, 2013.

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  1. Piotrne

    Piotrne Guest

    Hi,

    I have a question about controlling a large current in an experimental
    welder. The welder should generate a short pulse of current, about
    50 ms long. Additionally, it has to be regulated with a simple PWM.

    Now, I have following circuit: ATMega32 microcontroller which generates
    the PWM signal (2 kHz square wave, turned on for 50 ms);
    then an optocoupler; then a MOSFET driver IRS2001 driving
    (through a 47 Ohm resistor 47 Ohm) the gate of IRFP4468PbF.
    The transistor acts as a key turning on an off the welding current
    (source to ground, drain to the burner, the burner to the positive pole
    of a rectifier connected to a large transformer: 300A, 50V).
    Additionally, there it is also a ionizer with a filter system (designed
    by someone else). The microcontroller is powered from its own power
    supply, the rest of the control system (driver) is galvanically
    separated and also supplied with its own power supply.


    The are (at least) two problems with this circuit:
    (1) the microcontroller is resetting from time to time,
    (2) the transistor is damaged after a single pulse...

    The problem (1) is probably caused by the ionizer. I suppose, it can be
    solved by closing the circuit in a metal housing.

    The problem (2) is more serious. When I turn on the transistor for
    the assumed period of 50 milliseconds, it does not turn off.
    I had to quickly disconnect the transformer (as the trinsistor does not
    stand 300A for a long time). However, the transistor has been damaged...

    I suspect that the reason was a high voltage induced at the time of
    disconnecting the high current (such moments occur 2000 times/second in PWM).
    The sudden power interruption can induce high voltage, which probably
    caused the destruction of the transistor.

    Does anyone have experience how to avoid such interference? Varistors,
    filters, etc.? Experiments are a bit expensive ... The high current part
    (transformer, rectifier, ionizer) is ready, I would "only" like
    to switch on/off a pulse of current, with adjustable parameters
    (hence the microcontroller instead of something simpler).

    Regards
    P.
     
  2. tm

    tm Guest

    You need to consider the "safe operating area" graph for that device. See
    the data sheet.

    You are outside of the limits.


    Regards
    tm
     
  3. Piotrne

    Piotrne Guest

    Of course that is the reason: I'm out of the limits.
    But how to prevent it? The transistor is for sure able to work
    for 50 ms at 300 A (being fully "on"). So, the problem may
    occur at the end of the pulse. I'm not able to measure it,
    because (1) it is too short, (2) it may damage my oscilloscope,
    (3) each try damages a transistor (10$). And I think, that designers
    of welders know how to solve the problem...

    Regards
    P.
     
  4. Robert Baer

    Robert Baer Guest

    Methinks the operation of the FET is beyond the I*I*T rating as well
    as the voltage rating at transformer turnoff.
     
  5. mike

    mike Guest

    Not clear what you're trying to accomplish. The devil will be in the
    details.

    I spent several years tinkering on and off with a welder for battery
    tabs. Didn't get very far.
    Then I acquired a digital storage scope and a current probe.
    Had it working a lot better in a day.
    There's no substitute for making measurements to see what is
    actually happening.

    Again, depends on what you're welding, but for me, constant current
    was not what I wanted. The resistance of the weldment and the contacts
    and...and...and make a significant difference is how much energy ends
    up where you want it.

    Switching to constant energy by discharging high voltage from a cap
    thru an SCR
    into a transformer dramatically increased the reliability of the welds.
    That's how the CD welders do it. Of course, the transformer
    is an art.
     
  6. tm

    tm Guest

    Save some money while you are learning about power electronics :)

    http://www.ebay.com/itm/IRFP4468-Or...865?pt=LH_DefaultDomain_0&hash=item5d39edf611

    Less than $1.50 each. Maybe they are real.

    I don't think you are turning the device fully on fast enough. It has an
    input C of about 0.02 uF. Can you reduce the 47 ohms?


    Good luck.
     
  7. Jamie

    Jamie Guest

    Not sure what you mean by the "Burner"? I can only assume you are
    referring to the electrodes?

    In any case, I am willing to bet that you are suffering from induction
    kick back.

    Basically you are using 50VDC, but the release of this current with
    your pulse generating many more times that on the drain of your MFET..

    Don't get fooled with that ZENER symbol for the foot print.. It does
    not do what you may think and the current in this return pulse of HV
    does not need to be high to damage the fet. Looking at the Cap rating
    on the gate, I would say it would be easy for it to short through the
    gate.

    You may want to report in which manner the fet shorted? Gate to drain?

    Source to Drain only ? etc.. This would better tell the story.

    Maybe you should be considering a snubber around the FET.

    P.S.

    THe pulse current ratings of over 1k amps are only good for less than
    400us.

    Jamie
     
  8. Piotrne

    Piotrne Guest

    Electrodes, of course (they are not called "burner" by welders?)
    I have simulated it in PSpice (SwitcherCAD) and it shows me pulses
    of about 45kV... Very bad. It also destroyed 4 drivers (IRS2001) in
    2 experiments - a bit strange, because not all drivers were used.
    Well, the transistor from the second experiment has shortened D to S only,
    the first (which I did not expect to destroy and turned off the circuit
    a few seconds later) is totally "thermally" destroyed.

    I have replaced the driver with an IR2151 (which required to modify
    the generated PWM), reduced the resistor to 10 Ohm and asked
    the owner of the current source to complete it with an overvoltage
    protection (varistor + capacitor). I'll report results...

    Thank you
    P.
     
  9. tm

    tm Guest

    Is this for a TIG machine? If so, is it using HF?

    Can you put up a schematic of your design somewhere?

    tm
     
  10. Guest

    How is the transformer configured? You mention just one diode, so is it just halfwave rectified making for only DC loading of the secondary? Once youget past the transient overvoltage issue, you may be confronted with a saturated core short circuit issue.
     
  11. Piotrne

    Piotrne Guest

    It is a 3-phase transformer with a fullwave rectifier consisting of 90 diodes
    mounted on 6 aluminum plates (radiators). It provides a nice, slighlty
    wavy voltage which does not go to 0.

    The whole device should be welding small (thin) elements by melting only
    their surface. Therefore, such short pulses of current should be used.

    Today I have been trying to get rid of the interference caused
    by the ionizer. It simply resets the ATMega32 each time when
    the ionizer is turned on. It even influences separately the LCD
    display (2*20 characters) and the microcontroller: sometimes
    only the LCD shows strange thing, while the microcontroller
    keeps working and sometimes the microcontroller resets.
    Originally, the ionizer was turned on by the microcontroller
    (through a small, optically isolated circuit) and I suspected,
    that the distance between wires going to the ionizer and
    the microcontroller are too close (of course, the ionizer
    is turned on and off at the primary side, 230V).
    But even after completely disconnecting these two devices
    (microcontroller and ionizer) and keeping only common
    power supply, the problem persisted. Finally, a filter
    inserted between the mains and the ionizer removed the problem.
    Unfortunately, coming closer with other wires (the high current
    circuit) brought the problem back, even without turning on
    the current. So, it has to be solved too...

    I'll report the progress :)


    Regards
    P.
     
  12. Jamie

    Jamie Guest

    is this a spot or arc welder?

    Jamie
     
  13. Guest

    There are industrial line transient suppression devices based on the ckt shown below. The advantage is it's a non-magnetics solution, simply parallel your rectifier with another that charges a large filter cap to peak line, then that becomes your high voltage spike clamp level:

    Please view in a fixed-width font such as Courier.

    ..
    ..
    ..
    ..
    ..
    .. ---------
    .. | |
    .. >----------------+--|PH1 +|------------>
    .. | | | TO HIGH
    .. 3PHAC >-------------+--|--|PH2 | CURRENT
    .. | | | | LOAD
    .. >----------+--|--|--|PH3 |
    .. | | | | -|------+----->
    .. | | | | | |
    .. | | | --------- ---
    .. | | | ///
    .. | | |
    .. | | |
    .. | | | ---------
    .. | | | | |
    .. | | '--|PH1 +|-------.
    .. | | | | + |
    .. | '-----|PH2 | ---
    .. | | | ---
    .. '--------|PH3 | |
    .. | -|-------+
    .. | | |
    .. --------- ---
    .. ///
    ..
     
  14. Guest

    ....may be patent protected, but that's your headache...
     
  15. Fred Abse

    Fred Abse Guest

    How do you weld anything by melting only the surface?

    Sounds like the welding equivalent of a dry soldered joint,

    Welding needs penetration.
     
  16. Piotrne

    Piotrne Guest

    It is an experimental automatic welder, following the idea
    of TIG (with shielding gas - argon etc.). I don't know
    the details. My task was to design a controlling circuit
    which lets the user to set the duration of pulses,
    PWM parameters and some delays (the process should be
    repeated). Now, the controller generates all signals
    (this was checked separately - without the welding part,
    only turning on/off a small load). After connecting
    the ionizer, the microcontroller goes crazy after
    about 5 seconds of electric sparks.
    Here is a simplified schematic (hand drawn):
    http://piotrne.republika.pl/welcontroller1.jpg

    The "ionizer" is a factory made device to ignite electric
    arcs in welders. It should be connected through a transformer
    (e.g. 24:3 windings). The secondary winding (connected
    in series) adds HV pulses to the circuit. The circuit
    for these pulses should be closed by R and C (just above
    the ionizer on the schematic). Precise parameters are not
    known. But it generates "thick" sparks about 5 mm long, at the
    frequency of the mains (50 Hz). It may be about 7 kV.

    And, these HV pulses are getting somehow trough the
    "high current" wires to the IRFP transistor AND, through
    the optocoupler (!), to the microcontroller. And resets it.
    After disconnecting two wires (G and D of the transistor)
    the microcontroller does not reset.

    Regards
    P.
     
  17. tm

    tm Guest

    I'll add more to this later as real work invades play.

    Why don't you consider using three or six triacs in the AC input to the FW
    bridge? Or even as the bridge? Then you can stay away from the very high DC
    currents. Also, it would use pulse transformers to drive the gates, giving
    additional isolation from the HF junk.

    Or even better, use an inverter tig welding power source? Most of them have
    a low voltage control input that allows doing exactly what you are after.
    Not only that, the pulser device is readily available OTS. Some welding
    sources include the pulser built in.


    Regards,
    tm
     
  18. josephkk

    josephkk Guest

    Check into battery tab welders.

    ?-)
     
  19. josephkk

    josephkk Guest

    From a simplistic point of view, is there some way to move the ioniser to
    the other electrode? Placed on the FET switch side it will leak HV pulses
    into the FET no matter what you do.

    ?-)
     
  20. Fred Abse

    Fred Abse Guest

    Still needs full penetration through the tab, partial penetration into
    the terminal.

    Not exactly easy to control.
     
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