Snubbers....?

Discussion in 'General Electronics Chat' started by mwalden824, Feb 17, 2014.

  1. mwalden824

    Thread Starter Member

    Mar 6, 2011
    51
    2
    I have a quick question for you guys and gals.

    I have built a circuit that can control a Brushless DC motor called an Electronic Speed controller. I was told that I should put snubbers on each phase. I will attach the circuit below. The problem is, I don't know how to calculate the values for these RC snubbers that were recommended.

    When I run the motor in open loop (I don't want to add my code for closed loop commutation yet until I fix this problem) and I attach the oscilloscope to one of the phase leads of the motor with respect to the ground of the circuit, I see high spikes in the PWM waveform. I am driving the highside N-channel MOSFETS with PWM while leaving the lowside MOSFETS completely on or off so that I can control the current, torque, and speed of the motor. Every time the voltage goes high in the PWM signal, there is a significant spike which I am assuming is from the inductive kick of the electromagnetic stators of the motor. I was told that snubbers would solve this.

    I kept damaging motors over time and other components. So my question is, how do I calculate the values of the snubber components...?
    ...or do I just play around with the values and/or RC time constant and view the waveform on the oscilloscope until I reduce the spiking completely or to a bearable minimum?

    I only showed the section of the circuit I thought was relevant but if you need me to upload more let me know. I have RC snubbers (R8, C15; R9, C16; R10, C17) on each of the 3 phases but I don't know what values to use or even know how to come up with these values. So if anyone could guide me in the right direction I would be thankful.

    Thanks for any help,
    Michael Walden
     
    Last edited: Feb 17, 2014
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    Couple of links.
    Max.
     
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  3. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Google is your friend!

    Simply enter "calculate values for snubbers" in the search window and read your heart out.
     
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  4. eeabe

    Member

    Nov 30, 2013
    59
    9
    In my experience, snubbers are used to reduce oscillatory ringing. If you are just seeing spikes on one or both edges of the pulsed power to a phase it may be some other issue such as back emf feeding the power supply rail without adequate capacitance or some phase sequencing or timing issue or the need for some flyback diodes that are better than the mosfet body diodes. It would be very helpful if you could post some oscilloscope images along with your associated low and high phase signals.
     
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  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Do you happen to have the inductance and resistance of the motor?
     
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  6. mwalden824

    Thread Starter Member

    Mar 6, 2011
    51
    2
    Not off hand, but I have the tools to measure it. Couldn't I just use a DMM and LCR meter for this? Then model the MOSFETs as simple switches then draw the circuit for all the possible combinations of open and closed switches for one phase, then add the RC snubber circuit and solve for R and C and the RC time constant? I was thinking this, but wasn't sure if it would work correctly because even though the MOSFETs act like switches they have their own resistance and capacitance which I guess I could look up in the datasheet..??

    By the way, I will post a picture of the oscilloscope screenshot and show you what I am talking about real soon. My friend borrowed my oscilloscope. I'll get in back within the next few days. I don't know why I didn't take the screenshot already.

    Thanks,
    Michael Walden
     
    Last edited: Feb 17, 2014
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I think if we had those 2 values we could simulate it.
     
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  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Another critical function performed by snubbers is they reduce switching losses in the FET. Essentially, when the FET snaps off and the current through the inductance (transformer or motor winding) does not want to stop quickly, the R-C snubber provides a path for winding current so it doesn't stop as fast. It does also reduce ringing (and resulting EMI) if the snubber is designed properly.
     
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  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    It won't model accurately since the snubber's working against the parasitics like the leakage inductance of the transformer winding (or motor winding) and inter winding capacitance. I doubt that there is an accurate model for it.
     
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  10. ronv

    AAC Fanatic!

    Nov 12, 2008
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    There are all kinds of second order effects, but I've never seen any equations or calculators that take them into account either.
     
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  11. mwalden824

    Thread Starter Member

    Mar 6, 2011
    51
    2
    I think I am just going to look at the frequency content with the FFT function on my scope and design the snubber as an RC filter to filter out the high frequency spiking..?

    This would work??

    I might try modeling it also like ronv is suggesting in a simulator. I may not be able to capture all of the effects, but just the main ones which would give me a general idea of what the values should be. Then I could just play with different values to try and make it better....?

    Thanks,
    Michael Walden
     
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    When all else fails...... dump the simulator and do the thinking yourself. You might even learn the theory behind what they do and how they work.

    http://www.cde.com/tech/design.pdf
     
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  13. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Your problem may not be snubbers at all. Check the voltage on the 11.1 volt rail at your board. I suspect you need a big good quality electrolytic from 11.1 to ground close to the FETs. Say 1000 Ufd. low esr.
     
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