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Logic level HV fet...

Discussion in 'Electronic Design' started by Mike Deblis, Jan 27, 2005.

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  1. Mike Deblis

    Mike Deblis Guest


    I'm looking for a 200 - 250V FET with low Qg, low RDSon (only needs maybe 3A
    peak Ids, but low RDSon for efficiency) and LOGIC LEVEL GATE for use in an
    SMPS that only has a 9V supply - the SMPS is in boost configuration to give
    180V @ 20mA out, probably going to use the LTC1872 if I can.

    Any ideas would be appreciated - I've not had a lot of luck so far - I know
    its a tall order. An SMD footprint would also be good.


  2. PeteS

    PeteS Guest

    At voltages such as this, Qg (and Ciss, Coss) are more important than
    Rds(on) although obviously it has to be reasonably low. The reason is
    the large voltage swing of the source when the Switch turns on. It will
    go from Vout to Vin, and capacitively couple that delta to other parts
    of the SMPS with possibly harrowing results. At the load level you
    suggest, a device in the hundreds of milliohm range would probably be
    suitable (but check the SOA curve).

    Most FETs at this breakdown are normally specified at 10V Vgs. If 9V is
    not sufficient, you can always make a simple capacitive charge pump to
    run the boost driver (Linear tech has excellent app notes on this sort
    of thing).

    For a typical device list, see this page
    which lists the excellent range of IR HexFets(tm). Similar devices and
    capabilities are also available from Vishay-Siliconix and Fairchild
    (although the selection from fairchild is more limited).

    As for footprints, many high voltage devices come in D-PAK or D2-PAK
    surface mount footprints, to name the more common ones.


  3. Genome

    Genome Guest

    Use a voltage boosted boost converter. You tap the inductor so that the FET
    sees a fraction of the output voltage.


  4. Hi there. Unfortunately the pickings for 200V or more logic level MOSFETs
    is exceedingly slim. For the most part your only potentially feasible
    choices I'm aware of are the IRL620, IRL630, IRL640 (TO-220 devices) and the
    D2Pak versions of those same parts: IRL620S, IRL630S, IRL640S. Here is the
    datasheet for the IRL630S which is a 200V 0.4 ohm Rds(on) D2Pak part.

    Digikey sells all of these device types. They aren't really very good,
    noticeably inferior to their standard gate drive newer cousins.

    The LTC1872 is a nice part, but the switching frequency is really
    unreasonable for this application. Efficiency would be hideous at 550kHz
    operation, no matter your MOSFET selection. Without doing any major
    calculations before spouting off conclusions, I would bet efficiency would
    be so low as to preclude the use of an SMD MOSFET for thermal reasons alone,
    despite your low 3.6W output power. In addition to abysmal efficiency, you
    could also expect tremendous EMI production, likely way beyond FCC
    limitations. If you listen to the radio at all, you quite possibly might
    find that all but the most powerful AM and FM band stations will have become
    too static filled to listen to.

    The problem is high dV/dt radiates. A little 3.3V to 5V step up converter
    can operate at high switching frequency with very fast transitions without
    producing very much EMI. The drain of the MOSFET would only swing from
    0-5V. With 180V output the swing suddenly becomes 0-180V. Unless the
    switching speed is reduced drastically, the dV/dt will be greatly higher and
    hence much stronger EMI radiated. At first it may seem strange that a
    switcher operating at 550kHz could interfere with the FM band which operates
    around 100MHz. It is important to remember that a 550kHz squarewave with
    fast edges contains large amounts of higher frequency content.

    I would strongly recommend using something much closer to 20kHz as your
    switching frequency. Make sure to use discontinuous conduction mode since
    the control loop gain gets really large (IE: impossible to make stable) for
    large step up ratios (which 9V to 180V certainly qualifies) in continuous
    conduction mode. With a 20kHz switching frequency and 9V input an inductor
    value of vaguely around 150uH would seem to be reasonable.
  5. Mike Deblis

    Mike Deblis Guest

    Hi Fritz,

    Thanks for the reply. Normally, I use a MAX1771 which runs at 50kHz and
    gives me about 87% efficiency in this configuration - really a very nice

    However, getting the MAX chips in Europe and elsewhere is often a pain, and
    I was looking at emulating its performance using LTspice when an LT FAE
    mentioned their devices, which are emulated for you in LTspice, and he
    suggested I look at those as well, hence this investigation.

    I agree with everything you say about the LT chip - it is a very nice device
    in a very small package, but the maximum efficiency I've been able to get is
    in the order of 65%.... Also, unlike the MAX1771, the LT device can only
    handle up to 9.8V in, which really means logic level FETs - if I was going
    at add a charge pump or whatever for the gate drive, I might as well use the
    MAX device which is more forgiving anyway...

    Ho, hum... just a thought...


  6. Fred Bloggs

    Fred Bloggs Guest

    Although diode sees a multiple of reverse output voltage, the duty D can
    be significantly reduced from the 95% required of single inductor 20:1
    multiplication of 9V->180V. Good idea.
  7. Genome

    Genome Guest

    Yes, that's the caveat.


    the duty D can
  8. Hi Fritz,
    This is a great application for running a simple boost in Zero Voltage
    Switching mode. Add more capacity in parallel with the FET to control
    (resonate) rise and fall times to about 200nS. Operate the PWM in constant
    "off time" and current controlled "on time". Most control IC can be
    configured for this. I use this mode of operation so I can lisiten to the
    radio as I check it out. Piece of cake.
  9. John Larkin

    John Larkin Guest

    9 volts is plenty of gate drive for most regular mosfets. Drain
    current will be well below 1 amp, no big deal.

  10. john jardine

    john jardine Guest

    The 2SK2350 is 200V, 0.26ohms, with 4V gate drive.
  11. legg

    legg Guest

    Siliconix Si4490DY Nchannel 200V 0R09 in SO8, is characterized with

  12. mike

    mike Guest

    Once you've decided to tap the inductor, is there any practical
    difference between that and a transformer. Probably still want
    different size wire in the two sections anyway.

    Return address is VALID.
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
    Bunch of stuff For Sale and Wanted at the link below.
  13. I agree. Once you have decided to spring for a custom magnetic part,
    you have all sorts of choices that trade cost and complexity in one
    place for cost, efficiency and simplicity in others. I would consider
    a 3 winding transformer, with the primary rectified and both
    secondaries separately rectified, and all 3 stacked in series, so that
    the transistor would see only about 60 volts and schottky rectifiers
    could be used all 3 places. 100 volt fets are more common than 200
    volt units in SMT low gate voltage versions.

  14. Hey now that is an interesting part. I wonder how much it costs...
  15. I agree, ZVS would be very nice for this application. This is a good idea.
  16. legg

    legg Guest

    The last time I was fooling with 150V parts, top of the line SO8
    packages were about US$ 1.70, when they hit their intended volume.
    Small quantities and early parts could be three times that.

    Never inquired about Si4490. The 150V parts were 0R05 or less.
  17. Genome

    Genome Guest

    A) With one bit of wire. Start pin A wind 10, tap pin B wind 90 finish pin

    One wire, three terminations. One setup.


    B) With one bit of wire. Start pin A wind 10, terminate pin B.
    With next bit of wire. Start pin C wind 90 finish pin D.

    Two wires, four terminations. Two setups.

    If you want to be a purist..... but..... does the cost function work in your
    favour? Materials/Labour/Error.

    Is it your Mother or some Chinese bit of shit doing the winding for you?

    I know we've just celebrated fucking off 6 million and a bit Jews, and the
    idiots still let Woody Allens parents get away!!!!!, but your Mum versus an
    odd Billion Chinkies could be a powerful persuader.

  18. mike

    mike Guest

    Genome wrote:
    snipped all the bigoted stuff

    Why'd you have to go ruin a perfectly good suggestion with all that

    Of course, you have to do the math for all the options. But you can at
    least be objective about it.


    Return address is VALID.
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
    Bunch of stuff For Sale and Wanted at the link below.
  19. legg wrote...
    It's a boring part. Yawn.
  20. john jardine wrote...
    It appears the 0.26ohm rating is with 10V drive.
    That's a boring part. Nothing special. Yawn.
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