pwn signal getting spikes when connected to gate driver

Discussion in 'General Electronics Chat' started by ak52, Jul 13, 2016.

  1. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    Hello Everyone,

    My pwm signal gets very spiky when i connect it my my gate driver,i'm not able to deduce why.
    I have tried reducing the period of the pwm signal ,but no effect.
    The gate driver circuit is as followes:
    gatedriver.JPG

    At the moment Jumpers J8,J9 and J11 are connected.

    When i disconnected J9,the pwm signal from the controller is like this:
    pwm_without_j9.jpg

    When i re-connected J9,the pwm signal from the controller is like this:
    pwm_with_j9.jpg

    I do not know what is causing these sudden spikes ,it is the gate driver issue?
    The same spikes are also carried to the output of the gate driver.

    Thanks,
    Arun
     
    Last edited: Jul 13, 2016
  2. Papabravo

    Expert

    Feb 24, 2006
    10,145
    1,791
    There are a number of possibilities, but without seeing the layout it may be difficult to speculate. It looks like you are driving a digital input with an opamp. This may or may not be a good idea depending on the source and termination impedance of the connection. Are you sure the part is suitable for single supply operation, and will it swing rail-to-rail? The first thing to do is check your probe calibration/compensation. The next thing to do is check for unexpected inductance in your signal and ground paths. If the spikes are narrow enough it is possible they are artifacts of the measurement process.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    Where exactly are you measuring this signal on that schematic?
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,145
    1,791
    Should be the Gate Driver Input, according to the scope trace settings. Looks like one opamp is connected and the other is not.
     
  5. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    I am measuring the pulse on the second pin of J9.
    Also i am not using the opamp at the moment,Jumpers J7 and J10 are always open.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,800
    1,104
    Why a pull-down resistor on pin 12 but not 10?
    :confused: In post #1 you said J10 was connected.
     
  7. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    Apologies i meant J11(edited the post).
    I am designing a non-synchronous buck ,so i don't need the bottom pwm signal.,but in the future when i use the bottom FET for a synchronous buck then the bottom pwm signal will be used.(and R22 will be removed)
    As of now i am pulling down the low side pwm signal so that it remains grounded permanently.
     
  8. Papabravo

    Expert

    Feb 24, 2006
    10,145
    1,791
    So the opamp input impedance is in parallel with the driver impedance? That doesn't seem like a good thing to do. How long is the signal trace driving the driver chip?
     
  9. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    Trace length is approximately 4 inches.
    Should i de-solder the opamp and try.
     
  10. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    never mind guys ,it was the probe after proper contact the spikes vanished. :)

    Thanks,
    Arun
     
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