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cascode switching

Discussion in 'Electronic Basics' started by kell, Jan 25, 2006.

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  1. kell

    kell Guest

    I tried firing a car ignition coil by connecting
    a 400 volt transistor in cascode, like this:

    +12 v
    |
    |
    +--+-------+
    | |
    | |
    10R )
    | ignition)
    | coil )
    | )
    | _______)
    | c/
    +--| MJE13009
    e\
    |
    |
    points
    |
    |
    gnd

    but it blew out the transistor after a few cycles.
    I have fired ignition coils using igbt's and mosfets
    (although not in cascode)
    with similar voltage ratings to the MJE13009 and nothing
    ever went wrong.
    Recently I also tried the MJE5852,
    which is a 400 volt pnp transistor, in the following configuration,
    and didn't have any problem

    +12 v
    |
    |
    )
    ignition)
    coil )
    )
    )
    |
    |
    e/
    +----| MJE5852
    | c\
    | |
    points |
    | |
    | |
    +---+--+
    |
    |
    gnd

    Did I get a bad MJE13009 or is there something wrong with
    the arrangement in the top diagram?
     
  2. Guest

    Yes, the top diagram is completely wrong.
     
  3. You don't show any ignition condenser in your schematic. Did you
    remove the condenser from the points (you should)? I think the
    stresses on the transistor would be reduced quite a bit if you
    reconnected the condenser to the collector.
     
  4. Guest

    There are 2 problems with this circuit. The first is it doesnt make
    sense. The tr does nothing of any use. Lose the tr, connect coil
    direct, and it'll work fine.
    The 2nd is the b/e jn will see massive ac voltages as the coil LC
    rings, so no wonder it died.

    Thats hard to imagine, connecting c to b is generally regarded as a Bad
    Idea.

    No, yes, and in both I think.


    NT
     
  5. kell

    kell Guest

  6. kell

    kell Guest

    Let me show you and example of cascode switching. Look at figure two
    in the datasheet in this link.
    http://www.st.com/stonline/products/literature/ds/10854.pdf
    "high voltage bipolar low voltage mosfet" -- I'm using the same
    switching topology, with mechanical points where they have a mosfet in
    the pictured circuit.
     
  7. kell

    kell Guest

    Let me show you and example of cascode switching. Look at figure two
    in the datasheet in this link.
    http://www.st.com/stonline/products/literature/ds/10854.pdf
    "high voltage bipolar low voltage mosfet" -- I'm using the same
    switching topology, with mechanical points where they have a mosfet in
    the pictured circuit.
     
  8. The schematic is not all that bad. The transistor has a current gain
    less than 1, in that the points have to carry both the coil current
    and the base current, but when the points open, the points have to
    interrupt way less voltage than when they are connected directly to
    the coil. So the transistor provides voltage gain.

    At that moment the points open, all the coil current is detoured
    through the base as reverse current, turning the transistor off very
    quickly. This is great for fast switch off, but may be so fast that
    the voltage will peak extremely high, because not all the coil primary
    inductance is coupled to the secondary, so the collector voltage will
    be proportional to the secondary voltage, but will have an additional
    leakage inductance component. Slowing the rise of the coil voltage
    with some capacitance (possibly a little less than the original point
    condenser) will help the transistor get fully switched off before full
    voltage is reached (lowering its switching losses), and lowering the
    peak voltage the collector-base junction has to withstand.
     
  9. kell

    kell Guest

    I removed the condenser from the points of course. I didn't put a cap
    across the transistor because I never had to do that on any of the
    other ignition circuits I built. I wonder if there's something about
    cascode that escapes me. First time using it.
     
  10. Jim Thompson

    Jim Thompson Guest

    Put the cap across the COIL.

    ...Jim Thompson
     
  11. kell

    kell Guest

    Thanks for reminding me I could do that.
    If anybody's interested, I went with this circuit because it has the
    lowest voltage drop of several circuits I tried. Coil driver IGBT's
    are robust but the ones I've experimented with seem to have a much
    higher voltage drop than bipolar transistors. The difference as much
    as a whole volt. And I didn't want to drop any voltage because this is
    going on a six volt vehicle. A volt is a terrible thing to waste!
     
  12. kell

    kell Guest

    Thanks for reminding me I could do that.
    If anybody's interested, I went with this circuit because it has the
    lowest voltage drop of several circuits I tried. Coil driver IGBT's
    are robust but the ones I've experimented with seem to have a much
    higher voltage drop than bipolar transistors. The difference as much
    as a whole volt. And I didn't want to drop any voltage because this is
    going on a six volt vehicle. A volt is a terrible thing to waste!
     
  13. Bobscar

    Bobscar Guest

    I remember Maplin Electronics publishing a book for about ten pounds.
    The circuit consisted of a BU205 tv line output transistor driven by
    BC461. The BC461 is in turn driven from the breaker points/ trigger
    head via a 555 timer wired as a scmitt trigger. Unfortunately this book
    is no longer in print so try see if local library has it if in UK. The
    circuit also contains RC/ varistor filtering to protect the driving
    components, notably the 555 (max +Vcc = 18v).
     
  14. kell

    kell Guest

    They're a bunch of nervous nellies if they think they have to use a
    Scmitt trigger for something like this.
     
  15. Jasen Betts

    Jasen Betts Guest

    debounce might be handy though.
     
  16. Pooh Bear

    Pooh Bear Guest

    No you're not ! Look at what the base is connected to.

    Graham
     
  17. Carrying your logic one step further, do we not come across a second
    mode that might have destroyed the transistor:

    Once the voltage produced by the inductive current collapse peaks, it
    starts back down through zero on the negative swing of the ringing
    wave form. At the point where the voltage at the collector drops 0.7
    volts negative (referenced to its other end at B+), the collector-base
    junction of the transistor becomes forward biased through the 10 Ohm
    resistor. The collector-base junction becomes a 0.7 volt voltage clamp
    for all the stored energy with only a 10 Ohm resistor for a load.
    It would explain why this particular topology failed where the others
    didn't.

    I'd be interested to hear your response.

    Rick
     
  18. I understand the situation you describe, so let's think about the
    magnitude of the current pushed through the base by this mechanism.
    The maximum forward biased emitter to base junction current was (6
    volts - ~0.7 volts)/10 ohms = 530 mA. But the collector current peak
    is (6 volts - Vcesat)/ primary resistance, which is almost certainly
    quite a bit higher than 530 mA. The current peak on the first
    rebound will be lowered from the original peak, by the energy that
    went into the spark, so it is a bit of a guess for me to know how much
    current we are talking about.

    Hooking a scope probe to the base lead would be instructive.

    I still think the worst thing the circuit does to the transistor is to
    try to force it off while the primary is applying maximum voltage to
    the collector. The load line shaping provided by a bit of parallel
    capacitance really unloads this power dump into the collector each
    pulse by getting the transistor almost completely turned off while
    there is just a modest voltage on the collector, and then limiting the
    peak voltage as the energy in the leakage inductance charges the cap,
    instead of being consumed as heat by breaking down the transistor with
    over voltage.
     
  19. 2 things:

    That the points are likely to bounce and wiring i.e. inductance.

    A cascode is FAST, when the emitter opens the collector current is directed
    through the collector-base junction until the collector-base storage has
    been swept out. This takes a couple of usec. After that, the transistor will
    switch of in a few nsec causing the collector voltage to shoot up.

    Antisaturation diodes helps with the stored charge and a decoupling
    capacitor on the base-ground will let the charge be dumped cleanly without
    "exciting" long wires with usec current pulses of many A and fast risetimes.

    If he switch bounces, the transistor will switch a high voltage few dosen
    times every cycle - it might not like that.

    An oscilloscope with decent probes would be useful.
     
  20. Guest

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