ELectrolytic Capacitor for Reducing Wiring Effective Inductance

Discussion in 'General Electronics Chat' started by Bronchos, Oct 2, 2013.

  1. Bronchos

    Thread Starter New Member

    Sep 8, 2013
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    Hi there,
    I cited it from Fuji IGBT Application Manual. It's about lump snubber circuit for IGBT protection.

    Overvoltage protection method:
    a. .....
    b. .....
    c. Place the electrolytic capacitor as close as possible ti the IGBT in order to reduce the effective inductance of the wiring. Use a low impedance capacitor.
    d. .....

    Can somebody explain me about point c?
    How electrolytic capacitor can reduce wiring inductance?

    Thanks alot guys.....!
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It doesn't reduce the inductance it reduces its effect on the circuit. The trace on the board has inductance so the further away from the IBGT you put the capacitor the more the voltage can rise before the capacitor can filter it out.
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Here is a picture of what often happens.

    Lets say you have a pwm driving an inductive load like a motor.

    You have a fly back diode to protect your FET.

    But, there is some wire from the controller back to the battery. This would allow the clamp voltage to rise to the point where your FET would blow so you add a big cap in the controller.
    The first circuit has the cap close to the FET while the second has it further away (more inductance). So you can see the voltage may still go to high for your FET.
    I guess I'm not very good at explaining this. :p
     
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    The green trace with the spikes to 30V and 55V would damage the FET?
     
  5. #12

    Expert

    Nov 30, 2010
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    Which FET are you using, shteii01, and what is its voltage rating?
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Ah, sorry, I was looking at the pic ronv uploaded with his post. I was just double checking that I was following his explanation correctly.
     
  7. ronv

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    It was just an example to point out why you put the cap close to the transistor. Not aimed at a specific circuit.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    In high current circuits the sudden stopping of that current, such as by an IGBT switch, can generate significant voltage due to the small stray inductance of the wiring. This is absorbed by a capacitor placed next to the transistor collector to prevent transistor over-voltage.
     
  9. WBahn

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    Mar 31, 2012
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    As others have at least hinted at, you are reading the statement incorrectly.

    They are not saying that the electrolytic capacitor reduces the effect of the inductance of the wiring. They are saying that placing it as close to the IGBT as possible reduces the effect of the inductance associated with the wiring between the IGBT and the capacitor.
     
  10. Bronchos

    Thread Starter New Member

    Sep 8, 2013
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    Thanks for all your response. I think I misunderstand the statement at point c about reducing inductance.

    I have another question. I attached the guide that I follow

    View attachment fuji_igbt_application_manual(REH984).pdf.pdf

    My conclusion from Section 2) Overvoltage Suppression Method is:
    I have to place 2 capacitor accros the IGBT (fill and capacitor).
    Am I right???
    So, if the film capacitor function is to bypass the surge current, what is the function of the electrolytic capacitor?

    Hoho, thanks guys.
     
  11. WBahn

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    Mar 31, 2012
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    Real-world capacitors are not ideal capacitors. They have parasitic resistances and inductances. In general, this results in a frequency response that falls in proportion to the size of the capacitor. Hence large capacitors can store a lot of charge, but can't make it available very quickly because of parasitic inductances. Conversely, small capacitors have smaller parasitic conductances and can access their charge storage abilities much more quickly, but can't store as much charge. In fact, all real capacitors have a "self-resonant" frequency above which they actually look like an inductor and no longer look like a capacitor. Hence, you often put several capacitors in parallel, particularly if you are dealing with noise-sensitive electronics, in order to create a combined capacitor that looks like a capacitor across a broader range of frequencies. This is often referred to as "decade-tiered" capacitances because, in the extreme case, you put capacitors in parallel that are spaced by factors of ten in size. In the surge-suppression case, you would likely only use a total of two or maybe three. An electrolytic to handle the large, slow surges and a smaller film or other capacitor to handle the small, fast surges.
     
    Bronchos likes this.
  12. crutschow

    Expert

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    Without a schematic its difficult to know whether the two capacitors are in parallel or connected at different points in the circuit.
     
  13. Bronchos

    Thread Starter New Member

    Sep 8, 2013
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    Thanks WBahn for the explanation


    Yeah, you are right. I'm still try to figure out how many caps needed and how they must be placed accros the IGBT switch, according to the guide that a attached.
     
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