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Strange Mosfet Results

Discussion in 'Electronic Basics' started by Peter, Aug 2, 2004.

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  1. Peter

    Peter Guest

    I'm trying to fade some lights off slowly using a mosfet (see separate
    posting Fading Lights from 22 July) driven from a PIC and 74HC595
    shift registers. I've encountered some strange results.

    I actually have 7 identical mosfets with 7 strings of lights attached,
    each string of lights drawing a different amount of current. Each
    74HC595 output runs via a diode to one mosfet gate. The lights get
    turned on at different times then I want them all to fade off
    together.

    To create the fade I added a small cap to each mosfet gate (0.001uF).
    The first thing I noticed was that the higher the load current the
    faster the lights turned off. I therefore had to vary the size of the
    caps to get a uniform fade time across the 7 strings.

    The smallest string of lights however was turning off too slowly even
    using a 0.0001uF cap. I therefore removed the cap completely. To my
    surprise that INCREASED the turn off delay - so much so that the
    mosfet overheated badly. Why the increase in turn off time? Would it
    be due to the leakage through the external cap actually draining off
    the mosfet gate cap once the external cap discharges?

    Next problem. Increasing the size of the gate caps does not increase
    the period of the fade. It simply keeps the lights on at almost full
    brightness for longer, then they die very rapidly. The actual fade
    time remains constant at about 0.7 seconds no matter how big the caps.
    (I need about 1.5 seconds). I am therefore thinking of reverting to
    transistors instead to try to get a slower fade.

    (I am driving it from an 8-pin PIC (Picaxe) but I can't do PWM for
    various reasons.)
     
  2. John Jardine

    John Jardine Guest

    If I've read it right and you've a diode feeding each FET gate then I can't
    even see how they can turn off at all :). Would not something like a 100ohm
    resistor be better?.
    regards
    john
     
  3. John Jardine

    John Jardine Guest

    100"k"
     

  4. You are missing the resistor to ground, in parallel to the capacitor.
    The thing should work by keeping the mosfet completely on, but then
    limiting the current as Vgs drops. This won't really happen unless
    there is a path to ground, since the mosfet doesn't pass any current
    through the gate.

    The only thing draining the current circuit is reverse current leakage
    through the diode. Thats why removing the capacitor caused it to take
    longer; the diode was leaking away only the capacitance of the MOSFET,
    not the sum of this and the tiny capacitor you are using.

    Check the Vgs curve for your mosfet on the datasheet, and choose the
    capacitor/resistor so that the gate goes from Vcc to Vgs(th) in 2
    seconds. You can do that by using the following formula:

    Vgs(th)/Vcc = exp(-2/(R*C))

    so

    R*C = -2/ln(Vgs(th)/Vcc)

    For a Vgs(th) of 4V, and a Vcc of 4V, you have

    R*C = -2/ln(4/5) = 9

    Thus, you could pick C to be 10uF, and R to be 910k.

    Unfortunately, the Vgs(th) in the datasheet is not a very reliable
    parameter for most power MOSFETs. Thus, you may have to experiment and
    adjust the values.

    Regards,
    Bob Monsen
     
  5. Activ8

    Activ8 Guest

    ^^^^^^^^^^^^^^
    Yup. Use it. It'll work.
     
  6. Active8

    Active8 Guest

    BTW, John mentioned that the diodes blocked the discharge.

    |
    ||-+
    ___ ||<-
    -|___|------+-------||-+
    | |
    |
    ---
    ---
    |
    |
    ===
    GND


    That'll work, too, and the same equation applies. Just don't let
    Vcc/R exceed the maximum sink current of the SRs.
     
  7. colin

    colin Guest


    rather than conect the capacitor to ground u cld make use of the miller
    efect and conect it from gate to drain. this wld make any delay much more
    related to diming rather then delay before starting to turn off. basicaly
    the miler efect multiplies the efective capacitance by the gain of the
    circuit, wich is probably 30 or more at a very wild gues, its basicaly gm x
    r load. wich both vary with curent.


    Colin =^.^=
     
  8. Peter

    Peter Guest

    My first attempt was simply connecting a big resistor from gate to
    ground with no cap. But that gave almost instant turn-off, even when
    using a 10M resistor! That was why I removed it and used a cap
    instead. But I think I can now see that if I use a resistor and a
    bigger cap then it will ramp the voltage down more slowly.

    Colin can you please explain the Miller effect a bit more. And are you
    saying a cap from gate to drain (which is sitting at 40V) plus also
    the resistor to ground?
     
  9. Peter

    Peter Guest

    PS. See also my separate posting "Peak current to charge a capacitor".
    I was also concerned that adding too big a cap to the chip outputs
    might overload them when the cap is charging. How big a cap is too
    big?
     
  10. Active8

    Active8 Guest

    Those specs that define max load capacitance apply to a faster
    digital output where the cap is constantly being charged and
    discarged. Also, if you try to drive a big cap it takes longer to
    charge it up with a limited charging current.
     
  11. Active8

    Active8 Guest

    Sure. The gate cap is so small that you won't notice an increase in
    turn off time because it's down there in the uS range.
     
  12. Peter

    Peter Guest

    Well that had a less than interesting effect!
    First I tried the idea of just connecting a capacitor and resistor in
    parallel between the gate and ground. Tried a few combinations but the
    result was exactly the same as I was experiencing with just a cap on
    the gate, namely that increasing the RC did not increase the "fade"
    period. It just stretched the fully-on time then it faded off rapidly.
    That seems to be indicating that the mosfet is very efficient. ie. it
    is fully on over a fairly wide range then rapidly goes to fully off.

    I therefore tried the idea of using the so-called Miller effect. ie. I
    connected a cap from gate to drain plus the resistor from gate to
    ground. Very interesting. The mosfet promptly died (internal short so
    always hard on). It had gotten fairly hot so I then installed a new
    mosfet with heat sink attached. Same result. Died immediately. It did
    not even wait for a signal to the gate and did not bother getting hot.
    Just died the moment power was applied to the drain (indicated by
    coming hard on even though no gate signal). The cap I used was a 0.1uF
    (104) with a 10k resistor from gate to ground.

    As I said before the source is at 40v (actually rectified 38vAC). The
    max rating specs for the mosfet (2SK2175 - equivalent to STP16NE06)are
    as follows:
    "Vdgr Drain-Gate voltage (Rgs=20k) 60V
    Vds Drain-Source voltage (Vgs=0) 60V
    Vgs Gate-Source voltage +/- 15V."
     
  13. Peter

    Peter Guest

    This is getting even stranger! I remembered reading about gate ringing
    in mosfets and found a ST datasheet that recommended inserting a 4k7
    resistor on the gate to stop it. I therefore got another new mosfet
    and hooked it up that way. At least then it did not die, but it acted
    even stranger. As soon as any of the OTHER mosfets came on, the one I
    was testing turned on spontaneously without any input from the chip. I
    put a multimeter between the diode (coming from the 595 output) and
    the 4.7k resistor and it read 2v. The 595 output was still at 0v.

    Oops, I spoke too soon. I was using a 0.27 400V cap a minute ago as a
    trial. I just changed it back to a little 0.01 "metalised ceramic
    disc" and the mosfet just died as soon as I switched on. Could it be
    that the 40v is too much for those little caps?

    Anyway this whole idea of a cap between gate and source doesn't seem
    to be helping.
     
  14. Peter

    Peter Guest

    Anyway this whole idea of a cap between gate and source doesn't seem
    ....I mean between gate and drain.
     
  15. Active8

    Active8 Guest

    IS your light string attached to the source, or the drin?
     
  16. Peter

    Peter Guest

    35VAC > Bridge rectifier > Lights > Mosfet > Gnd.
     
  17. Active8

    Active8 Guest

    Damn sam. No simple error.

    I was also thinking that the Vgs(th) was a loose spec and (I'm
    talkin parallel RC on the gate to ground/source here - forget that
    drain - gate cap) that even though the gate voltage is dropping
    slowly, the mosfet is being a sensitive prick. They're nice that way
    and it's a useful feature. They are used as on/off switches more
    often than not.

    So your thought to try a bipolar tranny might just be the ticket but
    then you'd have to deal with the RC net discharging though the base.
    Bummer, eh? A Darlington pair migh help a bit. I'd also use a
    resistor in series with the gate so that the cap discharges into the
    595 and skip the diode.

    The prob I see with either case is the exponential charge/discharge
    nature of the RC net. If RC is the time constsnt, that's the time it
    takes for a cap to charge to 63%, IIRC of the charging voltage or
    discharge to 37% of it's initial charge. Then for the next RC
    interval, it only changes that same percentage of what's remaining.
    It's not a linear thing so the control voltage of your switching
    active is probably going to rapidly shoot right past that point
    where the active is in its linear region.

    I'd probably forget this RC approach. It's subject to mfg process
    variations and everything else.
     
  18. Active8

    Active8 Guest

    Best idea I can come up with without using separate *real* dimmer
    circuits for aech string is that which you don't seem to want to
    hear. I don't know what picaxes can do, but if you can get a single
    pwm output, you can and that with the on signal. 7 outputs... like
    so:

    |
    __ ||-+
    +----| | ||<-
    | |& |---------||-+
    D0 -------------|__| |
    |
    | __
    +----| | |
    | |& |- |
    D1 -------------|__|
    | |
    | |
    | | |
    | |
    | |
    | |
    | |
    | | |
    | __ |
    +----| |
    | |& |-
    D7 -------------|__|
    |
    |
    |

    pwm
    created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de

    Then you just dim the lights and turn off the PIC outputs with a

    and 0x00, port

    and reset the pwm register or control voltage. and you can get your
    pwm from a 555 timer if you don't want to do it in the pic, but
    yuo'll need a control voltage.
     
  19. How are you arranging the 5V logic supply? Battery?

    Regards,
    Bob Monsen
     
  20. Active8

    Active8 Guest

    Good question. He mentioned that turning on one string causes them
    all to turn on under whatever incarnation of the circuit he was
    talking about. IIRC it was the one with the gd cap.
    ^^^^^^^^^^^^^^^^ add a hyphen or two and readers will clip your
    title block for replies.
     
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