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TMS320F28035: Symmetric vs Asymmetric PWM

rjb
Intellectual 470 points
Part Number: TMS320F28035

I am a working on Power Converter control using piccolo, I was reading TI's piccolo DSP ePWM guide and came across symmetric and asymmetric ePWMS, I have a good basic understanding about converters, DSP and all.

Long story short, My question : According to ti's note www.ti.com/.../spra278.pdf [page 10] symmetric PWM generates less harmonics, Can anyone please explain How? Seen from the power converter hardware's perspective isn't it the same thing?

Thanks

Rahul

  • 0
    TI__Guru 57255 points

    Hi Rahul,

    There are ways to work through this mathematically, but I'll help give you the conceptual reason.  I feel certain some internet searching would also help you find more papers on this.

    With duty cycle static, you are correct - asymmetric PWM and symmetric PWM could look the same.  However, for your question, it's helpful to think through what happens when you edit the duty cycle between periods, for each instance. 

    With symmetric, the ZRO and PRD point are fixed as the center points of the the respective pulses.  CMPA (and/or CMPB) values can then be configured to edit the PWM waveform on CMPAU & CMPAD events.  Because both edges are being moved in the case of an update, symmetry is maintained period-to-period, which helps with ensure that the majority of energy is kept with the switching frequency & ultimately the 1st harmonic of your system. 

    With asymmetric, the ZRO point is fixed - and this is often where the PWM waveform will be brought high. Period-to-period changes to compare events will then only affect the falling edge.  This would result in extra harmonics in the output.

    For the reasons mentioned, Symmetric is often best for inverter-type applications.  Asymmetric mode is often used in things like DC-DC converters - using this you are able to get an extra bit of resolution in your ability to control the duty cycle (vs Symmetric). 


    Thank you,
    Brett

  • 0 in reply to Brett Larimore
    Intellectual 470 points

    Thanks Brett, It clearly makes sense. Easy to visualize, especially the part "Because both edges are being moved in the case of an update, symmetry is maintained period-to-period, which helps with ensure that the majority of energy is kept with the 1st harmonic."

    So, If I understand it correctly what matters is the center of the pulse which defines the fundamental essentially. And Also, harmonics is low only when the duty cycle is being changed keeping the period the same.