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MOSFET help?

Discussion in 'Electronic Design' started by Ed, Jun 27, 2004.

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  1. Ed

    Ed Guest

    Hi All possibly quite a simple question but I would like some advice.

    I have been looking at ways to use an IRFZ44N as a solid state 'switch'
    to switch on and of the positive supply of a 12 to 24 volt supply. As
    far as current requirements go the IRFZ44 is ideal, but I'm sturggling
    to find a good way to do this.

    To fill in more info its for a high current serial bus of 3 wires,
    12-30v Ground and Data.
    I need to beable to control the Data and Supply lines from a 5 volt
    logic level as simply and as ruggidly as possible, yet both must be
    capable of high currents, and minimal on resistance. I looked at using a
    P channel fet, such as the IRF5210 thinking this may be a solution, but
    I think I am going totally the wrong way.

    Any suggestions would be a MASSIVE help.


  2. I guess your problem is to supply the gate drive voltage to switch the
    MOSFET on. As it is an N-channel device this voltage must be above the
    source voltage. You're switching the "high side" of the supply, so the
    gate drive voltage must ride above the power supply. There are ways to
    do this, but it is not the simplest approach.
    Using a P-channel MOSFET does indeed simplify things considerably, as
    your gate drive voltage needs to be lower than the source voltage. It
    still can not be driven directly from a logic output as the logic levels
    are grond referenced whereas the MOSFET doesn't see any ground voltage.

    So my recommendation would be to use a P-channel MOSFET together with a
    level-shifter between the logic output and the MOSFET gate. This level
    shifter can be as simple as a transistor (NPN) and a couple of resistors.
    Here's a sketch of the idea (to be read using a monospace font):


    Power in ------o------+^+----- Switched power out
    | |||
    | ===
    | R1 |
    | ___ |
    ___ |/
    Control signal-----|___|---| NPN
    R2 |

    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    R1 ensures that the MOSFET turns off when the NPN turns off. Its value
    should be low enough to make the MOSFET switch off quickly enough and
    high enough not to exceed the NPN's ratings. R2 limits the base current
    of the NPN.
  3. Ed

    Ed Guest

    Thanks!!!! This is EXACTLY what I wanted to see! I'm new to using
    mosfets and their tricks, my head was spinning from looking through so
    much data the other day, it seemed such a daft thing to be stuck on.
    Thanks for this info, its greatly appreachiated - I'll order some
    devices now :) thanks.

    Ill let you know how I get on :)

  4. Stefan Heinzmann wrote...
    This configuration may be useful for switching fixed low-voltage
    power supplies, but if the supply voltage is above 15 to 20V, the
    FET's rated gate voltage will be exceeded. Furthermore, the gate
    voltage is not predictable. A good general purpose configuration
    is to develop fixed p-channel FET gate voltages by switching fixed
    currents into the gate resistor, like this:

    | p-type FET
    | Power in ------o------, ,----- Switched power out
    | 15 - 50V | S_|__|_
    | (65V max) | ,---
    | | |
    | '- R1 -+ for 10V gate drive, use
    | 1.0k | R1 = R2 (10/4.3) = 2.3 R2
    | |
    | |/
    | 5V logic-control -----| 60V NPN BJT
    | |\V 2n4401
    | |
    | R2 level-shift current-sink
    | 430 high-side FET switching
    | |
    | GND

    The fixed gate drive current of this configuration means that it's
    easy to add a current-limit feature, unlike the simple CE switch.

    | p-type FET
    | Power in ---o-- Rs --+----, ,----- Switched power out
    | 15 - 50V | | S_|__|_
    | (65V max) | 2n4403_|_ ,--- current limit
    | |______>/ \___| I_cl = 0.65 / Rs
    | | |
    | '---- R1 -----+
    | 1.0k |
    | |
    | |/
    | 5V logic-control -----| 60V NPN BJT
    | |\V 2n4401
    | |
    | R2
    | 430 ohms
    | |
    | GND

    Actually, it's a wise idea to use foldback current limiting to
    reduce the C.L. dissipation for the FET. Although this takes only
    two more parts (as we show in AoE), two parts are time costly in
    s.e.d. ASCII drawings, and one's intrinsic laziness prevails.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  5. Ban

    Ban Guest

  6. Robert Baer

    Robert Baer Guest

    As far as using an N-channel FET as a series power switch, i would use
    a "high-side" driver, like the IR2117 or IR2118 (one is inverting, the
    other is not).
    They will accept a logic level for input, and the isolated gate driver
    is ideal for this useage.

    For multiple channels in one IC, look at what Supertex has: HV20220 8
    channel, HV209 12 channel, or the HV20822 analog switches.
    They also have 16 and 32 channel switches, some with latches, etc.
    Something they mkae might be useable for your needs.
  7. Robert Baer wrote...
    One caution, those are for periodic (e.g. PWM) switching, and don't
    work for continuous switched FETs, unless the swinging gate-power
    diode & capacitor is replaced with a separate isolated power supply.
    Aren't those 200V signal switches, 22-ohm series resistance, etc.?

    - Win

    (email: use hill_at_rowland-dot-org for now)
  8. Ed

    Ed Guest

    Even more excellent information. So this looks like I can extend the
    supply range further IF needed which is great. I have found a high
    current P-Fet "SUP65P06-20". Which I have ordered a few samples.
    Hopefully armed with this info I shall now make some good progress.


  9. Ed wrote...
    If? You mentioned using a 24V supply. Since that's over 20V, which
    is the FET gate-voltage limit, you will need this configuration or
    an equivalent. If you're using a 12V battery, which is actually a
    12 to 14.5V source, with sag to 8V, Stefan's configuration would be
    more suitable. The circuit above provides 12-4.3 = 5.7V of FET gate
    drive for 12V input, and only 3.7V for 8V in.

    The SUB65P06-20 is a large-die-area FET, rated at 250W mounted on a
    25C heat sink. Used as a switch it's capable of continuously passing
    up to 40A. For load currents above 5 to 10A, fault protection for
    the FET is certainly in order, such as a foldback current limit, and
    perhaps a thermal cutout mounted on the heat sink as well. Ed, tell
    us more about application, your load, and its current requirements.

    Some may wonder at the low-value of the resistors, creating a 10mA
    gate-switching current. But large FETs have high gate capacitance,
    e.g. Ciss = 4500pF for the SUP65, where even a low 1k gate resistor
    still yields a relatively slow 5us time constant.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  10. Joerg

    Joerg Guest

    Hi Winfield,
    If the power line out has any length this might need an additional cap
    from gate to source or some other means of suppressing oscillation.

    Regards, Joerg
  11. Ken Smith

    Ken Smith Guest

    Oscillation smoscillation!

    When the MOSFET is fully on, the VDS is near zero and hence so is the
    gain. When the MOSFET is fully off, the Id is near zero and hence so is
    the gain. While the MOSFET is turning off or on, the FCC isn't likely to
    be looking.

    It would be more important to limit the gate voltage swing.

    BTW: Has anyone suggested one of the photovoltaic isolators for this job?
    It would turn on an N channel FET.
  12. Joerg

    Joerg Guest

    Hi Ken,
    That's right, Ken. The FCC might not see it. But when the TV glitches
    out of sync everytime the FET switches that can cause some nasty
    outbursts. Been there when I was a kid, me happily switching that brand
    new hot rod FET and my father trying to watch a soccer game :) When
    this happened at a friends house his father actually kicked us out for
    that afternoon. Seriously.

    Regards, Joerg
  13. Ken Smith

    Ken Smith Guest

    There are two things your reply indicates:

    You aren't as old as me: FET <-> kid unless it was a glass FET, this
    wasn't too long ago.

    You were dumb enough to get caught.

    But that aside:

    The best way to prevent oscillation in MOSFETs is to use a lossy element
    like a resistor or RF bead. The source to gate capacitance idea can just
    serve to change the frequency unless it is large enough to needlessly slow
    the switching.
  14. Joerg

    Joerg Guest

    Hi Ken,
    I am 46. We didn't have really big FETs in those days, at least not ones
    we could afford. But we made big ones by scrapping BF245 and other stuff
    from reject boards and placing tons of those in parallel. That in itself
    was an invitation to become an RF generator unless in the on or off
    state. Tubes were much easier and had a lot more oomph per Dollar so
    that's really what I grew up with.
    Yeah, the TV caught us :-(
    Yes, beads are great. We used to scrap those out of TV sets when I was a
    kid but of course back then I didn't have a clue that you could slide
    them over transistor pins to quiet things down.

    Regards, Joerg
  15. Ken Smith

    Ken Smith Guest

    Yes for about $6, or a dumpster dive after an old radio, you could get a
    6V6 or 6L6. They could make about 14W at 1MHz which isn't too bad when
    you think about it.

    I know some reader just said "so that was you!". Honest it wsn't, honest.

    Tubes had the advantage that failure was either thermal or flashover. I
    learned quickly to cleans the outsides of the tubes. An RF stage has a
    peak voltage about twice the supply so the tube may have 2000V on it.
  16. Ed

    Ed Guest

    Yes the fact that a battery is as much as 14.5 when charged is why I
    wanted to manage voltages as much as 30 since thats pretty close to what
    two series fully charged would be. The components I ordered are now
    here, and I shall put something together after I am back from lunch.
    The application is simply this. I'm building a microcontroller based
    pyrotechnic firing system. The fet is needed to switch on and off the
    supply to isolate the modules from the pyro. Since typically these
    systems will be running from battery, and it involves reasonably long
    wire lengths I want to introduce as little voltage drop as possible.

    I would imagine I could create a zener gate voltage limiter with a
    suitable resistor? This should solve the problem I would imagine, and
    surely allow me to operate the fet at a suitable gate voltage level
    across a wide range of supply rails?

  17. Ed

    Ed Guest

    How amusing :) I was going to add this anyway. I have a 250meg spectrum
    analyser which I shall check things on if I suspect anything strange is
    happening :) If I'm still not sure I can get hold of a 8gig Advantest
    one, but I'm sure I'll have no problems :)

  18. Joerg

    Joerg Guest

    Hi Ed,
    If there are any oscillations it'll be very short bursts while switching
    so a spectrum analyzer might not show much. But a fast digital (high
    single-shot sample rate) scope would.

    Regards, Joerg
  19. Joerg wrote...
    I routinely examine the FET switching waveforms in my designs
    with a 500MHz 2GS/s scope when debugging them, and have not
    seen this problem. Perhaps that's because I use short wiring
    leads along with low-inductance ceramic bypass caps. I *have*
    seen severe RF oscillation in paralleled linear power MOSFETs.
    This is a major issue, especially with high-voltage FETs.

    Note, the circuit I suggested had rather low resistor values,
    creating a 1 to 2us switching time. This is fairly fast and
    provides a rapid slew rate through any vulnerable regions.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  20. Ed

    Ed Guest

    Hmm I have access to a 100mhz digital scope. Ive no idea if this would
    be suitable. I shall just do my best with stopping such things happening
    in the first place. Thanks for the advice though, I totally forgot that
    the time scale of these oscillations if they happen would be very short

    (Boy this newsgroup makes me feel dumb!)


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