Allegro A3941 noise & control issues

Discussion in 'General Electronics Chat' started by skyrat76, Apr 16, 2014.

  1. skyrat76

    Thread Starter New Member

    Apr 16, 2014
    3
    0
    I have designed a schematic and PCB which has 2 H-bridge drivers using the Allegro A3941 full bridge MOSFET driver.

    I am having problems when changing the PHASE of the H-bridge, because excessive noise appears on the control inputs (PWMH, PWML, PHASE) which causes the H-bridges to erratically turn on and off.

    I am only using the PWMH and PHASE control inputs. With reference to the attached schematic, the DIP switches are configured as follows:

    SW1&2a - PWMH and PWML short = OFF
    SW1&2b - PWMH 47k pull down resistor = ON
    SW1&2c - PWML 47k pull up resistor = ON
    SW1&2d - PHASE 47k pull down resistor = ON
    SW1&2e - SYNCHRONOUS RECTIFICATION 47k pull up resistor = ON

    Therefore the driver is configured for the "slow decay, SR active, high-side PWM" mode.

    The circuit behaves correctly when the PHASE input is LOW (0V) but behaves erratic when the PHASE input is HIGH (5V).

    The PCB is a 4 layer board and each layer is attached for reference, in addition to the Bill of Materials.

    Can anyone offer any suggestions what the issue could be ?

    Before using this PCB I designed, I was using the "Pololu High-Power Motor Driver 18v25 CS" which worked without any issues. The Pololu driver uses the same Allegro A3941 driver and has the same pull up and pull down resistors on the control inputs.
     
    Last edited: Apr 16, 2014
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I'm not sure what drives your inputs. I can see for example the pull down on phase, but is there a pull up or circuit driving it when it is not pulled down?
     
  3. Moon968

    New Member

    Apr 14, 2014
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    These are the most serious issues and you need to find a solution for them.
     
  4. skyrat76

    Thread Starter New Member

    Apr 16, 2014
    3
    0
    The inputs are driven by a national instruments digital output module for a compacRIO.

    I just don't know how to locate the source of the issue.

    One clue.... it works when driving resistive loads but not inductive.

    I also had to change the op-amps (U2&U3, LMH6611) for the current sensor to a low bandwidth op amp because these generated a huge amount of noise with the same consequences.
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Ahh, inductor. Add flyback diodes like attachment. They should be rated at at least half the motor current and higher voltage than the supply voltage.
     
  6. skyrat76

    Thread Starter New Member

    Apr 16, 2014
    3
    0
    I thought there were already fly back diodes intrinsic to the MOSFETS?
     
  7. kronerti

    New Member

    Jan 18, 2016
    1
    0
    Hello,
    You solved the problem .. What was the solution.

    THX.
     
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