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Paralleling hexfets

Discussion in 'Electronic Design' started by Bart, Jun 20, 2004.

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  1. Bart

    Bart Guest

    Dear,

    I have a question about paralleling hexfets.
    I already searched a lot, but I did not find yet the rigth answer to
    my question.

    Let me explain what I want to do:
    I created a circuit to drive four fans. (I want to regulate the fans
    RPM) Explained in short it is as follows:
    The gate of my FET is driven by a fan driver (MAX 6651), the source is
    connected to ground and the drain is connected to the first wire of
    the fan, the other wire of the fan is connected to 24V.
    So by regulating the voltage at the gate, the 24V power voltage is
    divided in 2 parts, one part is over the fet (Vds) the other part is
    over the fan.

    If I connect four fans to 1 fet (which is theoretical right) there is
    a lot of heat dissipated in the hexfet. I measured temperatures of
    130 °C. So I thougt to place four fets in parallel. Then the current
    would divide into 4 equal parts and the dissipated heat in the fets
    will decrease.

    By measuring temperatures after placing them in parallel, the result
    is as follows: One fet is getting very hot (120°C), 2 other fets
    about 70°C and the fourth has 40°C. How is this possible?

    I tried already several things. I tried with 2 fets. Then the first
    ons has 100°C and the second 75°C. On the internet I found that you
    have to place serial gate resistors (typical value = 100 ohm) when you
    place fets in parallel. Also when I try this, the temperature does
    not decrease.

    Question: is there anyone who can tell me what I am doing wrong. I
    would like to have a circuit by which the maximum temp is about 40 or
    50 °C (instead of 120 °C). The number of fets doesn't matter.

    Is it useful to place a serial source resistor?
    If so, what is the typical value about that?

    Please can anyone help me?
     
  2. Tim Wescott

    Tim Wescott Guest

    If you parallel hexfets of the same type and drive them so they are full
    on then they should share current fairly well. It sounds like you are
    using them as linear devices (i.e. you aren't turning them full on),
    which will make them much more sensitive to variations in their
    threshold voltages.

    Putting individual source resistances on will help. Decide how much
    voltage drop you can afford when your fans are running flat out, and
    figure the allowable source resistance from that and the fan current.

    Buy why not just use a single honking big FET instead?
     
  3. Tim Wescott

    Tim Wescott Guest

    If the fan controller is pulsing, which it doesn't sound like from his
    description.

    If it is, then ignore my response...
     
  4. So they don't all have exactly the same resistance.
    Since the gates do not pass DC the resistor does not have any voltage
    drop and does not change the voltage on the gates.
    Use each fet to run one fan, with all their sources and gates tied in
    parallel, but each drain going to only one fan. That way, each fet
    sees the current from one fan.
    This will help. A resistor with a value that is about the same as the
    typical drain to source resistance will cut the variations at least in
    half.
     
  5. Ken Smith

    Ken Smith Guest

    [...]

    If you are running the HEXFETs under its rated current and the linear
    mode, the drain current has a positive temperature coef. Which ever fet
    has the lower threshold voltage will try to take almost all of the current
    and get hot. You need a source resistor or to use the fets as source
    followers to drive the fans.
     
  6. artie

    artie Guest

    Suggestion--

    Back off to a simpler design, running one fan with one FET, and get
    that working. Then add complexity.

    From the 6650/1 spec sheet (page 6), the controller is running the FET
    as a linear device -- that means you need some kind of heat sink for
    the FET. Figure out worst-case power dissipation, and size both the
    FET and its heat sink to handle that.

    The spec sheet (page 14) shows controlling 3 fans using one FET. Four
    fans is going to require fooling the controller, such as using the
    expansion circuitry shown in Fig. 8 (page 15), or by not connecting one
    fan's tach output to anything, and assuming its speed will follow the
    other fans.

    You also mention using +24 volts to power the fans. The spec sheet
    shows an absolute max voltage on the feedback and tach pins of +13.2V.
    Page 13 of the spec sheet shows how to calculate the value of
    protection resistors for these pins.
     
  7. mook johnson

    mook johnson Guest

    Go PWM and solve all of these problems. one FET switching on and off with
    variable duty cycle will dissipate far less power than linear mode.
     
  8. Ken Smith

    Ken Smith Guest

    Or, since there are several fans, only turn on as many fans as needed.
     
  9. legg

    legg Guest

    Mosfets will only share linearly if Vgs variations are overcome. The
    source resistor on each device is one way of doing this.

    The fan loads are also not characterized to run in parallel at
    anywhere near the same rpm, particularly at their starting or stalling
    speeds. At that time, individual fans will expect large pulsed
    currents to be available to start, and the actual voltage to rpm
    relationship of each device may exhibit considerable hysterisis about
    the start/stop point.

    RL
     
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