BJT Totem-Pole Power MOSFET driver (Low side) – How and why?

Discussion in 'General Electronics Chat' started by Dyslexicbloke, Sep 13, 2010.

  1. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    Hi folks,
    I suspect this has been asked and answered many times but I couldn’t find anything definitive when I searched.

    I am trying to understand how this works, I am using an N-Channel FET …
    I sort of expected, when I started looking that the PNP to be at the top with its collector to the gate and the NPN below it in a similar fashion.

    How wrong can you be with just the one head?

    I found several examples, pulled a couple of GP transistors from my box (2N3906 / 2N3904)
    And shoved them onto a breadboard ….
    Well it works, but it occurred to me that it might be a really good idea to find out why and how. Particularly as I want to make sure it keeps working.

    I would appreciate a leg up and some recommended reading, BJT’s for dummies perhaps!!

    Thanks in advance … looking forward to learning.
    Al
     
  2. Ghar

    Active Member

    Mar 8, 2010
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    Post some schematics?

    I think I know what you're talking about but it'd be good to be sure...
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Are you talking about this?

    [​IMG]

    This?

    [​IMG]

    or this?

    [​IMG]
     
    Last edited: Sep 13, 2010
  4. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    This is the power board for an AVR I am trying to build, the origional is toast.

    The 317 puts out about 6V at startup and as soon as the fet begins to turn on that will quickly rise to 10.4v

    The FET hold its source at about 25v, zener - VGS(th)

    In theory at this point the PWM would have kicked in (b) is the output of an opamp with a max 50mA sink capability, no source to speak of.

    This board isnt finished, no fet driver yet, but I have an earlier itteration with the fet gate pulled up by resistors which is whats driving the genny now, badly I might add.

    Al
     
  5. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    Yes Bill thats about the size of it ....
    I am still struggliong with remembering which symbol is which, PNP NPN, never mind what an emitter follower is or if this has one.
    Sorry to be so dim .... just realy never learned the basics.
    I usuully use line drivers for current buffering .... someone else has already done the tricky bit for me that way.
    Al
     
  6. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    'O' the 1uF on the 317 is actually 0.1uF .... typo
     
  7. Wendy

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    Mar 24, 2008
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    MOSFETs make lousey followers, your first MOSFET won't work like you think. Try a BJT or Darlington Pair (better) instead. The configuration name is common collector.

    Chapter 4: BIPOLAR JUNCTION TRANSISTORS

    MOSFETs generally don't work well under 10V. They are generally not meant to be used in a linear fashion, BJTs do that much better.
     
  8. Ghar

    Active Member

    Mar 8, 2010
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  9. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    Bill since your edit my answer dosnt make much sense .... OK even less sense.
    It would be the top configuration that I found elsewhere .....
    Just off tom check BJT sysbols ... again.
    what are to other configurations .... or were you just asking what I had done.
    Al
     
  10. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    I just reread the wikipedia entry on BJT comon colector (emitter follower).

    I think I understand that the collector is 'common' to load and source.
    and it called an emitter follower because the load 'follows the emitter' voltage.

    If thats correct I cant get my head around why the totem pole would work at all.
    Isnt the bias current always Base Emitter ..how can that work with one only transistor on at once .. just not getting it.
    I must be making some fundamental mistake.
    Al
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    Doesn't invalidate anything I said, the design is flawed from the gitgo.

    I will repeat, MOSFETs are not linear devices. If you try to use them as such you will not have good results. I could be wrong, but I don't think I am.

    BJTs are linear, not all the way across, but for wide region of their curve. This is why they work well for emitter followers (the other name for common collectors).

    [​IMG]

    It is obvious you are trying to use it as a voltage follower feeding a LM317. Funny thing, a BJT would be a drop in replacement, a Darlington BJT even more so. Have you trying wiring this concept?

    The LM317T must have a 120Ω minimum resistance between the output and adjust, to satisfy the 10ma minimum current requirement.

    What is the target output of the LM317 regulator? 12VDC?

    And do you have the bridge circuit connected to the mains? This is not a good idea, but I can't interpret the schematic that far.
     
    Last edited: Sep 14, 2010
  12. Wendy

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    Mar 24, 2008
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    Quick side lesson.

    [​IMG]

    The first illustration shows a classic Common Collector configuration (AKA, voltage follower, emitter follower).

    A emitter follower will match the voltage of the base minus the BE drop. The input resistance at the base will be the gain of the transistor (beta, or β) times the emitter resistance. So if β = 150 (not an uncommon value) then the input resistance of this circuit is 150KΩ. This is a common voltage regulator for many designs.

    Darlington Pairs have the gain of both transistors multiplied. So if both transistors have a β of 150, the total gain is 22,500, and the input resistance of the circuit is 22.5MΩ. This kinda makes up for the extra BE drop.

    With BJTs the arrow points to the negative side, always. If the bias is reversed they are turned off, otherwise they look a lot like diodes with extra bonus points.
     
  13. Ghar

    Active Member

    Mar 8, 2010
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    I was merely providing you with some background on his discussion to save you some time.
     
  14. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    You'r right the MOSFET is a lousy follower, I even think I understand why now thanks to all the input here. That said I have a FET, whereas I dont have any high voltage high power BJT's. Definatly a requirement of the next itteration I think.

    Its offset starts at about 2V and increases with input voltage to about 5V. The circuit is built, all but the totem-pole ... The lm317 has a 150/1100Ω divider giving about 10.5V once its input is at a couple of volts above that and the FET is limiting that input voltage to about 25V.
    Both caps are 0.1F and I have added an LED limited to 5mA at 10V across the output.

    This application has a power supply of 10 to 12VAC when it first powers up and as much as 130VAC once it running. Thats a real world problem that I cant avoid, if it doesn't work at 10V then it cant ever reach 120V and my generator will not work.

    Al
     
  15. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    OK Folks ..... Lets put the supply part of this to bead for the time being, I know it needs changing but frankly thats not as important, right now, as getting the PWM gate drive to work on the second FET.

    Am I correct with these asumptions:-

    1. Both transistors are working in comon collector mode.
    2. They can only be ON when the gate is charging or discharging because Ibe is what turns them on.
    3. Ib will be inherrently limmited because once the turn on threshold is reached any aditional 'potential' current will be Iec.
    4. The final phase of gate charge / discharge will always rely on Ibe because the transistor will be off.

    I am desperatly in need of education here ... please help if you can.

    Al
     
  16. Wendy

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    Mar 24, 2008
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    I notice you keep mixing transistor terms. They are pretty different, and there is no relation between them. Are you using your original schematic, or is there one not shown?
     
  17. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    I'm mixing them because I dont understand, sorry, I wish I did.

    I am certainly not trying to be difficalt or obtuce, I apriciate the help however silly I look, don't go pulling any punches to spare my feelings, that won't get me anywhere.

    The schematic has altered slightly as I said .. The new one is attached, again, sorry if I am confusing things ferther.

    Al
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    I'll say it again, no one is born knowing this stuff. I would suggest some side experiments before getting into high power circuits. The latter tend bite harder, and the sparks are bigger.

    BJT stands for Bipolar Junction Transistor (hence the name BJT transistor is an oxymoron). It has two junctions, like a sandwich. You can put the N material on the inside, or the P material on the inside, so there are two kinds, NPN and PNP. They act just alike except they are opposite in polarity (like antimatter to matter). They have three leads, base, emitter, and collector.

    Our text book is very good on the subject. It is worth reading and rereading. Any time you see Ibe, or Ice, or Vbe drop it is a good bet we are talking a BJT.

    MOSFETs are much newer devices, and spun off of conventional FETs. They have excellent turn on resistances (very low values), and can usually handle more power and current than the leads. Given how nonlinear they are they tend to be used quite effectively for digital designs, stuff that is either one or off but not between. They have a gate, a source (which is usually the currents exit, go figure), and a drain (again, opposite the name). They also have polarities, a N channel and a P channel, but for technical reasons the N channel tends to work better.

    BJT transistors handle analog quite well, which includes audio and analog power supplies (while MOSFETs handle SMPS (switching mode power supplies, which is digital)) quite well.

    I've messed with BJTs both professionally and for pleasure for well over 30+ years. I've messed with MOSFETs for around 20, and for pleasure for about 5 or so years.
     
  19. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    I wish I had a choice ...
    MY generator AVR is dead .... Parts arn't available.
    No water, because I cant pump any and power only when the sun shines.... Not good all in all.

    I am just about to update the thread on AC Sensing and generator control with my current circuit and build.

    I do understand the very basic principles and therfore differences between BJT's and FET's I even looked up JFET's which appear to work in depletion mode, wondering if they may be a better choice for the totem-pole MOSFET driver.

    My problem is real world implimentation of my extreemly sketchy knolidge.
    Are my asumptions about how the BJT totem-pole correct particulaly in respect of controlling the base currents?
    I expect it will soon die if I am wrong.

    The FET I am mis-using in a linier fashon is getting very hot even with a substantial heatsink. Is this likly to be a product of its misuse or is it simply disipating too much power?

    Al
     
  20. Wendy

    Moderator

    Mar 24, 2008
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    Do you have it mounted on a heat sink? It is absolutely required.

    Here is what I am talking about...

    [​IMG]

    I would like to find a higher voltage transistor, but TIP102 is all I can come up with for the moment. I'll ask SgtWookie to check in and suggest better.

    There is an assumption on my part this will work. Truth is, I don't have a clue. I don't have the total picture, and if I did I'm not sure I'd know how to fix it.

    The 50Ω resistor (47Ω actually, since that is the nearest value) is needed, and should be soldered as close to the gate as possible.

    .
     
    Last edited: Sep 15, 2010
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