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High power PNP question

Discussion in 'Electronic Basics' started by kostas, Jan 5, 2005.

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  1. kostas

    kostas Guest

    Hello

    I want to design a PWM to control the duty cycle of o big fan (80watt).
    I have seen a nice circuit at

    http://www.cpemma.co.uk/pwm.html

    The nomad's with the kick start function. The maximum current i want to
    control is 6.6 Amber and in order for the kick start to work i should
    use a PNP transistor, correct? Can i use a N channel MOSFET like irf540?
    Do i have too use any special driver after 555 and before MOSFET or PNP?

    Thanks for your time
     
  2. If you are trying to build something like Nomad's 555 circuit, yes, it
    uses a PNP transistor, but the design is capable of nothing like a 6
    ampere output. If you use a good, high current PNP transistor, say a
    TIP36 or D45H. But this circuit will not provide enough base drive to
    switch one of these with a 6 ampere collector current. to get good
    saturation (less than .3 volts collector to emitter) one of these may
    need a base current between 1/20th and 1/10th of the collector current
    (.3 to .6 amperes). The design shown has a 470 ohm resistor limiting
    the base current. After you discount the 1 volt base voltage and the
    1.5 volt loss through the 555, this leaves 10 volts across that
    resistor for a base drive of 10/470=.021 amperes. This resistor would
    have to be lowered to about 20 ohms to get the needed base drive, and
    then the 555 would overheat.

    So the output capability of the 555 has to be increased. Adding a
    second PNP (say, a 3 amp rated TIP32) with its base connected directly
    to the 555, its collector connected directly to the negative rail, and
    a 15 ohm 5 watt resistor between its emitter and the big PNP's base,
    you get the needed base drive. Then you need to put a 10 ohm resistor
    between the base and emitter of the big PNP to suck the stored charge
    out of it when the 555 output goes high, to have a clean turn off.

    All these designs are also missing the diode that should be across the
    motor, cathode up. Without this, the high voltage pulse you get when
    you interrupt the motor current is likely to damage the switching
    transistor. For a 6 ampere motor, this diode should be fast (turns
    off fast when reverse bias is applied by the transistor) and rated for
    at least 3 amperes, average. Candidates would be the SR503 (30 volt 5
    ampere schottky) and 80SQ045 (8 ampere, 45 volt).

    If you want to simplify the circuit quite a bit, replace the entire
    PNP and resistor network with a big P-channel mosfet (gate directly
    connected to the 555). Something like IRF5305 (0.06 ohms on
    resistance, 55 volt).
     
  3. kostas_

    kostas_ Guest

    You have been very detailed, thank you. If i put a P-channel mosfet the
    kick start function won't work, right? I think without that i'm gone
    have problem starting the fan because it's very loud and i'm gonna work
    it with small duty cycle. If i put an N channel won't the kick start
    work (and i think i will not have to add a second transistor, i can
    driver it directly from 555, or i'm wrong?)
     
  4. I don't understand what you mean by 'kick start'. Either the PNP or
    P-channel switch will allow the motor to see zero to almost 12 volts,
    average. If the PWM frequency is high enough, the vibration should be
    acceptably low.
    I see no functional difference between N and P channel switches from
    the standpoint of motor operation. One switches the positive side of
    the motor and one switches the negative side. The motor current does
    not know the difference.
     
  5. The 'kickstart' thing is done by keeping reset low for a few seconds,
    which powers the fan during that time. Using a mosfet like JP suggested
    doesn't affect that at all


    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  6. kostas_

    kostas_ Guest


    With the kick start i mean the 555 is a in reset mode for about 2 second
    in order for the fan to take 12V directly for the first spin and then
    then 555 comes in the game. If the 555 is in reset condition the output
    will be 0 so i need a PNP in order to drive the fan for 2 second when
    the 555 is off. If i put an NPN then the kick start won't work. So i
    think i can put an irf540 n channel mosfet to do the job. I don't know
    exactly how mosfets work. Some explanation and a link for additional info?
     
  7. I see.
    An N-channel mosfet and an NPN transistor do approximately the same
    thing. Both turn on when the input pin (base for the NPN and gate for
    the mosfet) is more positive than the common pin (emitter for the NPN
    and source for the mosfet). The difference is that there is a diode
    junction in parallel with the NPN base-emitter, but the mosfet gate to
    source is open circuit (insulated). so to keep the NPN on, you need
    to supply that diode junction with a current, while no current is
    needed to keep the mosfet on. It does take a lot more voltage (most
    of the 12 volts) to turn the mosfet completely on. If you reverse
    voltages, the same applies to PNP and P-channel mosfets.

    Since the reset signal pulls the 555 output low, that would turn on
    either a PNP or P-channel output. If you wanted the same function
    with either an NPN or N-channel output, you would have to invert the
    555 output, using either a PNP small signal transistor or a small
    P-channel mosfet or a cmos inverter (CD4049 CD4069 or some inverting
    gate).
     
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