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TIDA-00365: Brushed DC motors: above 90% PWM noisy output at the bridge.

Jaffer Mohammed Ismail
Prodigy 20 points
Part Number: TIDA-00365
Other Parts Discussed in Thread: SM72295,

Hi, 

I have used the reference design TIDA-00365 to design motor controller for brushed DC motors, I ran the motors with 16KHz PWM signal via SM72295 and through the H-Bridge at different duty cycles.

My observations are that upto 90% duty cycle the output is a nice square wave but above 90% duty cycle the output of the H-Bridge starts to become noisy, I am not able to understand why this is occurring, is this some kind of limitation with this design?

here the reference schematic, Q6 highlighted in RED.

CH-1 is Gate of n-CH Mosfet Q6

CH-2 is source of n-CH Mosfet Q6

Math: CH2 - CH1

here is the output at 90% duty cycle nice square wave in RED

Here is the output at 93% duty cycle, poor quality and noisy square wave in RED

  • 0
    TI__Expert 3580 points

    Hello,

    thank you for using TIDA-00365 and reaching out to us.

    The TIDA-00365 use a bootstrap supply for the high-side gate drivers. The bootstrap capacitor is charged, when the low-side FET of the corresponding half-bridge is on, and discharged (driving the high-side FET gate to source), when the high-side FET of the corresponding half-bridge is turned on. If the duty cycle is close to 100%, the time to charge the capacitor (C6 or C7) is too small and the supply voltage across C7 and C6 drops. In general bootstrap supplies won't support 100% duty cycle. How close we can get to 100% depends on how fast the bootstrap cap can be recharged during the low-side FET on-time.

    Your issue might be related to that. To find out, could you test, please?  

    - Could you measure the supply voltage across C6 and C7 to validate if it drops from 12V? You typically need a scope which can handle high common mode transients. 
    - You may try with a larger cap, e.g. change C6 and C7 from 100nF to 470nF.  This may help if the time you run >90% duty cycle is small.

    Could you let me know if this addresses the problem you see? 

    Thanks. 

    Regards, 
    Martin Staebler

  • 0 in reply to Martin Staebler
    TI__Expert 3580 points

    Hello,

    I assume our above explanation resolved your problem. If not please get back to us.

    Thank you. 

    Regards,

    Martin Staebler