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bypass for TC4427

Discussion in 'General Electronics Discussion' started by lotec, Sep 19, 2016.

  1. lotec

    lotec

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    Apr 15, 2013
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    Hi
    Im trying to build a variable frequency square pulse generator for driving and testing different power pulse transformers. I would like it to operate between 1-100 Khz.

    So far I have been using an IRF 540N driven straight from pin three of an NE555P, but the problem I have been having is that, at too higher freqency or too bigger load the fets are burning out.

    I have just got some TC4427 fet drivers and I am hoping to use them. I think that I understand most of the stuff that I need to know from the data sheet. But when it comes to the bypass capacitor I have some questions.

    From what I can gather it goes from the input pin to ground and is for bypassing too higher frequencies, but not sure. How critical is this or how can I calculate it so that things can run at the above frequency range.

    Thanks in advance.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,362
    1,901
    Nov 17, 2011
    From the datasheet I cannot see the requirement for any bypass capacitor on the input pin. What the datasheet shows on page 2 is the effective input capacitance that is inherent to the chip. No need to add another capacitor. On the contrary: adding another capacitor will distort the edges of the on/off signal thus leading to longer switching time hence higher power dissipation.

    What is "higher frequency" in this context? It seems that the FETs switching time is too long (possibly due to not enough gate drive current from the 555). During turn-on and turn-off the MOSFET dissipates quite a lot of power since Vds rises from ~0 V to Vmax while current is flowing.
    At "low frequencies" the ratio of switching time to (on-time+off-time) is probably small enough to allow cooling of the MOSFET during the on/off period of the switching cycle. At "higher frequencies" this ratio becomes higher, meaning less time is available to cool the transistor.
    Remedies are:
    • use a stronger gate drive (what you're looking for by using the MOSFET driver IC)
    • use a transistor with lower power dissipation (less rdson)
    • use a transistor with better thermal capabilities (to remove excess heat effectively)
     
  3. lotec

    lotec

    31
    0
    Apr 15, 2013
    Thanks for the reply.

    Your right I looked on the data sheet and could not find anything about a bypass capacitor. I made a mistake about where I saw that, it was something I read today in a Q and A about a someone having problems using that chip, while I was trying to google a tutorial. I guess I took that on board to seriously. If you say its not needed thats good enough for me. That solves that problem. I havent hooked it up just yet but that makes things easier. I dont have a scope at the moment and just wanted to eliminate any problems that might affect the efficiency of what I am doing.

    As for the exact frequency of the transformer that cooked the fet, I cant say, I was distracted by the smoke and it all happened so fast. At a guess maybe 50 – 60 K drawing 400 mA, Im pretty sure the NE 555 N used to be more forgiving but I think its obselete now so I thought Id try the driver chip, with the NE 555 P mainly for the reasons you stated above.

    Im working with 12 Volts at the moment and I am in the market for a Fet with low on resistance and as small gate capacitance as possible that can switch up to 10 amps comfortably. Perhaps someone can recommend a good one.

    Thanks again
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,362
    1,901
    Nov 17, 2011
    When you control a transformer by the FET, you have to provide a way to dissipate the back-emf that occurs at turn-off. This back-emf is generated by the energy stored within the coil whcih creates high reverse voltaeg spikes at turn-off. Typically a snubber network including a diode is used, but details vary and depend on your circuit design.
     
  5. lotec

    lotec

    31
    0
    Apr 15, 2013
    Thanks, Im still very much a learner, I was hoping the energy from the collapsing field would be inducted into the secondary and not cause too many problems, but that could be why Im having dramas as well. I hadnt given that much thought, Maybe a 15 volt zenner and high speed diode back to back from the drain to the positive on the battery or a neon across the drain and source? Now that you mention it Ill be looking into that aswell,
    [​IMG]
     
  6. lotec

    lotec

    31
    0
    Apr 15, 2013
    Thanks for the links that should keep me busy for a while thats been very helpful
     
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