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The Q factor in Buck VM converter

Victoria Qin
Intellectual 1840 points

Hi Team,

In voltage mode Buck converter control to output transfer function , here is a Q factor. I 'm not able to find the related documents clarifying Q. What does Q mean physically in Buck converter and what influence it could make to system? 

Best regards,

Victoria

  • +1
    TI__Expert 7605 points

    Hi Victoria, 

    Q in a buck converter is related to the quality factor of the output LC filter.

    Strictly speaking, a higher Q would give you a faster transition in the slope of the phase of the power stage bode plot, that could give some trouble for the compensation.

    You can refer to this White Paper for further details (in particular pages 7 and 8): Switch-mode power converter compensatin made easy

    Thanks,

    Stefano

  • 0 in reply to Stefano Panaro
    TI__Intellectual 1840 points

    Hi Stefano,

    Many thanks for this detailed document. I know about higher Q would increase the phase slope which may cause low PM in cross frequency. I have a new question . As the document says,to decrease the bad influence of high Q,  we can put double zeros of err amplifier on the LC double poles which makes VM mode open loop gain -20dB/dec in cross frrequency(picture as below). So anyway we could eliminate the bad influence of high Q on phase, what's the point to talk about Q? Does Q have any influence on system which is cruel to system performance? 

    About 

    Stefano Panaro said:
    that could give some trouble for the compensation

    Could u please explain more?

    When we design a output LC filter, should we have any consideration about Q? 

    Thanks & BR

    Victoria

  • 0 in reply to Victoria Qin
    TI__Expert 7605 points

    Hi Victoria,

    The Q will be proportional to how fast it is the phase transition around the double poles (higher Q means higher transitions).

    However, as you noticed, this is usually not a significant factor in a real design. Usually, the power stage is designed to meet efficiency, power and other requirements.

    Moreover, most of our devices doesn't use VM anymore, solving this double complex conjugate poles problem.

    Thanks,

    Stefano