Totem Pole question

Discussion in 'The Projects Forum' started by MatheusLPS, Nov 14, 2012.

  1. MatheusLPS

    Thread Starter Member

    Jul 16, 2011
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    Good morning all.

    I am learning how to use a toten pole circuit to drive a MOSFET. At this time, I can make it work like the image below:

    http://img.photobucket.com/albums/v222/ahhh/toten_teste.jpg

    The input signal is a PWM signal. On the image above, i am not sure but I guess it has 10% in duty cicle.

    So, the problem is that the circuit is working in reverse.

    With 10% of the PWM I have 90% of the current flowing on the FET.

    I want to have 10% on the PWM and 10% of the current, 90% on PWM, 90% in current. That need to be direct proporcional.

    I tried to use a PNP transistor but did not worked.

    bye
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Remove R4 and Q4 and put your signal to the bases of Q2,Q3 via R2, this should invert your signal.
     
  3. MatheusLPS

    Thread Starter Member

    Jul 16, 2011
    34
    0
    I tried that but when i removed that resistor and transistor, my gate signal is 5V and not +12V.

    I want to keep the +12V to avoid the ohmic region.

    bye
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You need to invert your input signal.
    Where does your PWM signal originate?
    What is the frequency?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Just a nit, the circuit shown is a push-pull stage, not a totem-pole. A totem-pole uses two transistors of the same polarity. ;)
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Carl, you're an old guy like me. I had the same thought. A little Googling convinced me that a push-pull emitter follower has come to be known as a totem pole stage.
    International Rectifier even does it.
    I know, it ain't right, but the same thing has happened to a lot of other terms over the years.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Dang nab it. I just can't keep up with all this newfangled stuff. :rolleyes:
     
    Ron H and SgtWookie like this.
  8. vk6zgo

    Active Member

    Jul 21, 2012
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    It's because young people are dumb!:D
     
    Ron H likes this.
  9. williamj

    Active Member

    Sep 3, 2009
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    Oh to be young and dumb again.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    I was smart when I was young.
    Now things have changed and by brain farts sometimes.
     
  11. MatheusLPS

    Thread Starter Member

    Jul 16, 2011
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    0
    If i invert the signal, the output is inverted too.

    The PWM is from a microcontroler PIC @ ~ 25KHz

    Yeahhh. You are right. push-pull. I googled it...

    Sure I can make it work the way it is. I just need to remember that when I apply 10%, i will have 90% on the load. OK...

    But if I can make it right......
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    That's what you need. This is because Q4 in your circuit is an inverter.
    You should be able to output an inverted signal from your microcontroller.
     
  13. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Apparently IR is fast and loose with their definition. Wiki agrees with us old farts ...

     
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  14. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
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    I see your dilemma.. No matter what, the output current is inverted.
    Because, the Mosfet always inverts it.
    I think you need a P-channel Mosfet with the load 'below'. ??
     
  15. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Type too fast, think too slow. (Happened again Ron) :)

    What I was thinking of was a follower circuit. So, the N-channel would work with the load 'below'. But then I think you need an extra 4-5 volts positive to drive the Mosfet fully.

    I started in electronics with tubes (of the vacuum type).
    :D I still like them. :D
     
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