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IGBT in a MOSFET application ?

Discussion in 'Electronic Design' started by Mike Diack, Sep 19, 2004.

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  1. Mike Diack

    Mike Diack Guest

    I need a 1Kv 200ma regulated variable supply, and thought that the
    reference design in Win Hill's "A of E" was a good place to start. Trouble
    is that MOSFETs hit the wall at 1Kv, wheras IGBTs are readily and cheaply
    available up to 1200v (more than enough, the raw supply will be around
    1160v). I cannot see any reason why the IGBT should not make a suitable
    series pass element in a linear regulator like this - unless I missed
    something.
    Any Gottchas ?
    cheers
    M
     
  2. James Meyer

    James Meyer Guest

    I think there's one big "gottcha" up front. Using a 1200 Volt rated
    device with "around" 1160 Volts applied is asking for trouble. You're only 40
    Volts away from disaster.

    Jim
     
  3. John Larkin

    John Larkin Guest


    If they didn't think it was reliable at 1200 volts, they wouldn't rate
    it such. Discretes usually have lots of margin.

    John
     
  4. Mike Diack wrote...
    I have a nice stock of 1200V MOSFETs, so wouldn't consider IGBTs for
    the task. The problem I've faced in making fast amplifiers has been
    getting small enough transistors. In the old days 1n120 parts (small
    1.2kV fets) were available, but these were supplanted by 1n100 (1kV)
    parts, and by 3n120 parts (1.2kV, but larger die size). The problem
    with big FETs is their higher capacitance, which forces one to run
    at higher operating currents. Now when adding IGBTs into the mix, we
    observe that for a given current rating they have smaller dies, and
    hence lower capacitance than similar MOSFETs. But sadly, the IGBTs
    generally aren't offered in truly low-current versions, so the "small"
    IGBTs may end up less appealing the larger MOSFETs they might replace.
    Another factor, important for higher-power linear amplifiers, is that
    good heat removal capability => good power dissipation capability, is
    in direct proportion to the die size => higher capacitance. Another
    serious aspect is which parts can you get from a distributor somehow?
    They may be "active" parts, but non-stocked. In addition, there are
    the 1700 volt IGBTs to consider. Finally, tests on individual FETs
    will reveal that many entire runs work well to several hundred volts
    above their rated values. And that some in fact fail to reach their
    rated values. (Motorola discontinued all FETs above 900V for a time
    until they got their fab process back on track, and meantime shipped
    non-compliant parts. Later they discontinued 1.2kV parts, but their
    1kV parts in fact were the same and as good as the older 1.2kV parts.
    Finally ON Semi discontinued the entire high-voltage line.)

    Alternately one can make cascode high-voltage MOSFET amplifiers.

    Yep, Mike, it's all a giant trade-off game.

    So good luck, have lots of fun, and let us know how it works out.
     
  5. John Larkin wrote...
    That's right, but as I mentioned in my other post, they can be all over
    the place, and it's worthwhile when making high-voltage linear mosfet
    amplifiers to check all the FETs. E.g., with a curve tracer. It's a
    useful fact that the "failure" mode is a innocuous avalanche leakage.
     
  6. Mike Diack

    Mike Diack Guest

    Ah ha - the curve tracer would indeed be a nice thing - such a nice
    thing that it is what, in fact, I'm making :) Wasn't it Confucius that
    said "To make a pair of tongs, you need a pair of tongs"
    cheers
    M
     
  7. John Larkin

    John Larkin Guest

    Aren't there some 1400-volt fets somewhere? I know there are 1400 volt
    bipolars, horizontal output thingies.

    John
     
  8. Pooh Bear

    Pooh Bear Guest

    1N parts are diodes.

    1N100 & 120 are germanium to boot !


    Graham
     
  9. Pooh Bear

    Pooh Bear Guest

    Simple answer.

    I don't think you missed anything. They should be fine.

    Graham
     
  10. Some MOSFETs follow a numbering scheme that consists of some
    characters followed by a number indicating the Id current rating in
    amperes, followed by an 'N' and then a number indicating the voltage
    rating.

    So, for example, a IXTP3N120 ('3N120') is a 3A 1200V MOSFET made by
    IXYS. A '1N120' is a short form way to refer to a 1A rated 1200V
    MOSFET- it's not the entire part number.

    Best regards,
    Spehro Pefhany
     
  11. Pooh Bear

    Pooh Bear Guest

    I've never seen that usage. Sounds kinda sloppy to me. You could drop the MTP
    off the front of On Semi parts too if you wanted.

    Part numbers beginning 1N, 2N, 3N etc... are JEDEC authorised. Any manufacturer
    can use their own letter prefixed part designation ( excepting Pro-electron part
    number designations of course ).


    Graham
     
  12. Depends on who "they" are - the brochurepeople; i.e. Sales clearly, would.
    As confirmed by the availablity of 1200V devices in TO220 packages!!

    However, There is nothing wrong with sticking two 800V devices in series -
    and supply the gate of the upper device with a resistive voltage divider
    backed up with capacitors to supply the gate current. The drive is via the
    lower device. A'la Cascode.

    IGBT's have a "buglet" in that it is possible to make them latch-up on a
    rapid overcurrent, which can happen easily in an HV supply, making is safer
    to series-connect MOSFET's.
     
  13. James Meyer

    James Meyer Guest

    But.. but... but.... Aren't curve tracers easier to make if you don't
    have to have regulated and/or programmable power supplies? Wouldn't the old
    standard half-wave rectified transformer output along with voltage and current
    measurements and a base or gate step generator be better?

    Jim
     
  14. Pooh Bear wrote...
    If there was any sloppiness, it was leaving off a leading mark, would
    '1n100 have satisfied you? As Spehro says, there're many suppliers of
    nearly identical high-voltage FETs and IGBTs, with different prefixes.
    A quick search in my computer shows these: ixtp, ixta, ixfh, fqp, fqd,
    fqa, mtp, mtb, fqpf, ssw, sth, ixcp, ixrh, stp, mty, mtw, mty, ixgp,
    irfp, irfb, spn, spw, skw, spd, spp, mgp, mgw, and mgy. There's more
    in there, all followed by xNxx specific identifiers, you get the idea.
    So when talking about power fets, it's clearly convenient to ignore the
    prefix! And few folks will think we're talking about germanium diodes!
     
  15. Sometimes I really wish there was a way to search with regular
    expressions on Google..

    Best regards,
    Spehro Pefhany
     
  16. Ken Smith

    Ken Smith Guest

    Some IGBTs are not rated for linear service. Check the data sheet of the
    one you use.

    Also IGBTs are slow to turn off. If the load is suddenly removed the
    voltage will spring up above the set point.
     
  17. Spehro Pefhany wrote...
    Hear! Hear!
     
  18. Terry Given

    Terry Given Guest

    ROTFLMAO!

    lies, damn lies, and things salesmen put in brochures. cf rating Rdson
    at 25V Tj.....or TO220 packages with 180W power ratings (and 1C/W Rthetajc)

    It is rare for ANY manufacturer to tell the whole awful truth. Normally
    they display only favourable characteristics, in whatever light makes
    them look best, regardless of how the device will actually be used.
    why not use a 4kV IGBT? (actually, because the "little" ones tend to be
    about 1,000 amps :)

    1700V IGBTs are as common as all hell.

    a 1200V IGBT is good in a switching app up to about 900Vdc, depending on
    stray inductance and dI/dt. Many IGBT-based products self-destruct when
    turning off under fault conditions, as dI/dt is often about 10x larger
    than normal - although a gatedrive that uses a much larger Roff under
    fault (ie desat) conditions helps a LOT (Semikron do this in their gate
    driver modules). In a linear app, dI/dt should be a hell of a lot lower,
    so you ought to be able to push it well over 1kV, but dont forget fault
    scenarios - shorts followed by opens etc.

    My tech once built me a 0-400V linear supply that ran from 900Vdc using
    a 1200V 600A IGBT - for doing splat tests on very large chokes (it
    allowed us to suck almost all the energy out of the HVDC bus caps whilst
    supplying constant voltage across the DUT). It worked quite well, BUT
    igbts are designed for switching rapidly, so can be prone to oscillation
    when used in a linear mode.


    cheers
    Terry
     
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