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What Driver Voltage to Drive a Mosfet? (SMPS app)

Discussion in 'Electronic Design' started by D from BC, Jul 30, 2007.

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  1. D from BC

    D from BC Guest

    My power mosfet has an absolute gate spec of 30VDC.
    Most of the Mosfet datasheet gate behavior graphs only go to 10V.

    Is there a drive voltage that'll provide low switching loss?
    Looks like there's a 6V to 30V range to choose from.

    Mosfet Specs
    Gate charge 40nC
    Input capacitance 1500pF
    Absolute max gate voltage 30V
    Id=10amps @Vgs=7V

    Circuit Specs
    f=600khz
    Id=2amps
    Mosfet driver can be powered up to 36V.
    D from BC
     
  2. Andy F Z

    Andy F Z Guest

    Do you mean by loss the energy dissipation on the Drain-Source side of
    the MOSFET, during the switching? It will largely depend on how fast the
    MOSFET goes from the "open" to "closed" state. I would start by choosing
    first the Vgs_1 high enough to get the Vds under load as low as you need
    it for your "charge" part of the cycle, and then would go on optimizing
    the circuit on the Mosfet drive side to get Vgs from 0 to Vgs_1 and back
    as fast as possible. Then simulate and validate the energy dissipation.

    -- Andy
     
  3. default

    default Guest


    International Rectifier has a good application note on driving
    mosfets.

    Application Note AN-937
    Gate Drive Characteristics and Requirements
    for HEXFET Power MOSFETs

    I don't have the link but a search ought to do it.

    When they give you the characteristic curves, that tells you what it
    takes to use it as a saturated switch - what you are doing . . .

    The ten volts is probably turned on fully so there's no point in
    carrying the graph out further.

    The only things to watch for is to get enough current to the gate fast
    enough to charge and discharge the 1500 pf of gate capacitance - or
    you'll be in the linear region and creating heat. They may have a
    very high gate impedance - but that pesky gate capacitance requires
    current to keep it from spending time in the linear region during the
    transitions - so the driver impedance has to be low

    Keep transients on the gate (and drain) subdued or they might easily
    exceed the 30 volt maximum and cause a failure. That usually means
    short wire runs from driver to gate and/or gate resistor or snubbing
    network. If you rely on diodes for protection - they have to be fast
    switching diodes.
     
  4. D from BC

    D from BC Guest

    Yup..it's the gate peak voltage for low drain source switching loss.
    I'm letting the mosfet do whatever it can do in the way of rapid
    current rise and fall times for reducing Pds switching loss.

    Are you saying I should use the lowest gate voltage possible for
    sufficient drain current?
    That way there is less gate charging and discharging?
    A gate charged to say 6V can be discharged faster than say a gate
    charged to 20V.

    D from BC
     
  5. D from BC

    D from BC Guest

    I'm using a IXDD414 mosfet driver with a impressive speedy 14A peak
    current rating.
    Probably overkill but just using overrated parts for assurance.
    I'll check out the app note and take care with the PCB layout for the
    gate trace.
    Thanks.
    D from BC
     
  6. Andy F Z

    Andy F Z Guest

    The lowest gate voltage (under the range of the working conditions and
    the part-to-part variations) to get Vds at the given Id (or Rds) as low
    as you need.
    Yes. The spec that you cite "Gate charge 40nC" is valid for specific
    starting and ending Vgs only.
    -- Andy
     
  7. D from BC

    D from BC Guest

    Ooops..I just realized I forgot a word in my last post * *
    Rewrite:
    "I'm letting the mosfet *driver* do whatever it can do in the way of
    rapid
    current rise and fall times for reducing Pds switching loss."
    That probably caused some confusion...

    Thanks for the info.
    Gonna juggle some parameters now. :)
    D from BC
     
  8. Tim Williams

    Tim Williams Guest

    Why such a high F? Trying to push gate driver dissipation in exchange for
    better...something?

    I don't see any advantage over 100 or 200kHz, but that's me.

    Tim
     
  9. D from BC

    D from BC Guest

    I'm still learning smps design..
    I'm trying out a smps design of mine at 600khz (150Watts) to reduce
    the size of the magnetics.

    At that frequency, it probably becomes important to carefully select
    the mosfet driver peak gate voltage (or driver supply rail).

    My actual built and tested design works great at 100khz...I just want
    to find out if I can push the design further. :)
    If it goes well at 600Khz...I hope to take the design into the Mhz
    next.

    So..yah..the mosfet D-S switching loss is something I'm concerned
    with.
    Are you saying that power conversion above 200khz gets ugly?
    D from BC
     
  10. Eeyore

    Eeyore Guest

    As others have said, the trick is to turn it on *FAST*. Age proportion of the
    losses are during turn-on and turn-ff as opposed to conduction.

    So... what you *should* be asking (given a sensible choice of voltage drive) is
    gate CURRENT ! The more amps the better.

    Graham
     
  11. MooseFET

    MooseFET Guest

    Include this in the current spec of the driver. This much charge must
    be moved quickly to switch the part.
    Include this in the current spec of the driver. This gives a bunch of
    extra charge you have to move. It isn't all right at the instance of
    switching though.

    You sould also look for the capacitance from the drain to the gate.
    This makes a bunch more charge for you to move with the driver.
    Stay way below this number.
    Swing up to about 10V to stay well above this number.
     
  12. D from BC

    D from BC Guest

    I'm picturing driving a mosfet like this..

    +----Rsource---+
    | |
    | |
    Vdriver Cgate
    | |
    | |
    +--------------+

    V is from the mosfet driver rail voltage.
    Rsource is mostly internal mosfet driver resistance.

    Here's my confusing thing...

    Given that the mosfet driver can change state fast with currents up to
    14A peak and the objective to fast charge/discharge the gate for low
    Pds switching loss ...It kinda looks like one should make V (the
    mosfet driver rail) as high as possible for the fastest charging.


    D from BC
     
  13. default

    default Guest

    That is an impressive driver IC

    I found this http://www.classeradio.com/driver.htm with layout
    suggestions for driving mosfets up to 7 MHZ
     
  14. Wimpie

    Wimpie Guest

    Hi "D from BC"

    Basically driving the MOSFET with high positive and negative voltage
    will result in faster switching times

    Driving your MOSFET with high voltage (for example 20 V) will turn-on
    the mosfet faster, but increases the drive losses significantly (so
    your driver will consume more power).

    The gate of the MOSFET has internal resistance (the gate of standard
    SMPS MOSFETs is made of polysilicon instead of metal). Even when you
    would use a driver with infinite current capability, the drive current
    is limited by the MOSFET's internal gate resistance (and dI/dt is
    limited by inductances).

    You can get some idea of the internal Rgate by looking into the
    simulation model of the device you are going to use.

    If you want to speed up turn-off, you may use a negative gate drive.
    In that case you can pull more current out of the gate (but with
    higher gate drive loss).

    Before designing your driver circuitry I would recommend you to first
    evaluate the losses for you SMPS topology. Maybe you are spending
    money (and power) in an almost perfect drive circuitry, while the
    reduction in switching loss is insignificant with respect to other
    losses.

    Unnecessary fast switching will generate more HF noise, so you have to
    spend more components to meet EMC requirements.

    You mentioned 600 kHz (and higher). Are your using a (semi) resonant
    or a zero-voltage-switching topology?

    The driver IC has been (probably) designed for the IXYS MOSFETS. They
    have several types that have very low internal gate resistance and can
    be used into the HF/VHF frequency range.

    Best regards,

    Wim
    PA3DJS
    www.tetech.nl
     
  15. D from BC

    D from BC Guest

    Hey...that's different.. It's been suggested in this thread to only
    charge up the gate to a voltage for needed drain current.
    That way there's less to discharge.
    Little charges and little discharges can be done fast.

    But... Every Rseries and Lseries (driver internal,trace and internal
    mosfet R&L) limits the current therefore limits the charge rate which
    in turn sets the mosfet switching speed.
    The slower the switching speed...the more Pds switching loss..

    Someday I'll learn how to balance the mosfet driver heat, the mosfet
    heat and the EMI.
    I guess when I burn my finger on the IXYS (IXDD414) mosfet driver,
    it's max'd out. :)
    (Assuming it's worthy to get the mosfet driver that hot.)

    Picking a mosfet driver supply voltage has become a 3 bears story for
    me..
    Too big...too small...just right..

    About resonance...
    I'm not using a resonant design.
    The power inductors I'm using do ring a bit and I do have to try to
    keep inductor parallel capacitance to a min with proper windings.

    I could convert to a resonant design and switch at "dead" times but
    I'd like to experiment with non-resonant first.

    Ahh....you suspect IXYS drivers best with IXYS mosfets...I'll take a
    peak at the IXYS mosfets sometime.


    D from BC
     
  16. Wimpie

    Wimpie Guest

    Hi D from BC,
    Driving many MOSFET above 8 to 10V, doesn't reduce the Rds_on. So at
    that point I agree. Also most non-logic drive MOSFET switching
    behavior is specified with a 0 to 10 V gate drive (with certain series
    resistance).

    Regarding switching. One has to see the delay time apart from the rise/
    fall time. When you want to turn of a MOSFET that has its gate on 15V,
    this will result in a longer delay time than when the gate was at
    10V. But I believe, the fall time is not affected by the higher
    initial gate voltage.
    Right, when for a certain MOSFET the internal gate resistance is 2 Ohm
    and you are driving from a 0 to 10 V driver, is has no use to select a
    driver that can sink and source 10A.
    Not always

    The time between, for example 10% and 90% (the rise or fall time) of
    drain current is important. Besides this, you have to be careful when
    you turn on a MOSFET, while a diode is conducting (as in a buck
    converter), the MOSFET has to remove the reverse recovery charge of
    the diode. When you turn on the MOSFET very fast, with a high gate
    voltage, the peak drain current can be very high. Because of the diode
    reverse recovery charge, very fast switching does not always lead to
    lower switching loss. The diode recovery time was the reason for
    asking what type of converter you are designing.

    One of my clients blew-up some LM5010 buck converter ICs, just because
    of diode recovery, changing to a Schottky rectifier did solve the
    problem.
    These capacitances also increase the switching loss, faster switching,
    does not reduce the losses caused by winding capacitance. So look the
    complete design, not just switching loss.

    Depending on the topology, you may expect difficulties in getting a
    low leakage inductance, with low capacitance, and safety barrier.

    The energy stored in the leakage inductance, you have to dissipate
    also (for example in TVS diodes as snubbers). Resonant topologies are
    more forgiving at some points.
    I can imagine your statement. The control range of a single resonant
    converter is limited (with respect to, for example, a fly-back or non-
    resonant forward converter).
    They make MOSFETs in a very special case that you normally see in RF
    power devices (the terminations are strips with very low inductance).
    They claim switching speeds in the low nano-s range. link:
    http://www.ixysrf.com/app/switch_mode.html
    Best regards,

    Wim
    PA3DJS
    www.tetech.nl
     
  17. D from BC

    D from BC Guest

    You asked about the topology..
    I'm playing around with a Cuk based design (just for something
    different).
    I'm using Infinieon SiC power diodes or hyper fast power diodes such
    as the IR 15ETH06.

    Thanks for the pointers.
    Gotta go do some thinking now..
    D from BC
     
  18. Wimpie

    Wimpie Guest

    Hi D from BC,

    I assume that you are going to build an AC mains powered switcher with
    input output isolation (2 (loosely coupled) coils and one
    transformer)?

    Thanks for the info,

    Wim
    PA3DJS
    www.tetech.nl
     
  19. D from BC

    D from BC Guest

    Just 2 inductors not coupled.
    No transformer therefore no output isolation.

    It's a pain to test and the entire cct. is a shock hazard.

    Just plain ol textbook Cuk.

    +---L-----+----C----+----L---+
    | | | |
    | D A Rload
    VDC Mosfet Diode |
    | S K |
    | | | |
    G G G G

    I think I read somewhere that the inductors can be coupled but I'll
    look into that at a later time.
    D from BC
     
  20. neon

    neon

    1,325
    0
    Oct 21, 2006
    the 1500 pfd doesn't change with gate voltge someone sugested a low voltage the charge will be faster and i don't see how. ?????
     
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