New Techniques for Measuring
Feedback Loop Transfer Functions
in Current Mode Converters
In power supplies with current mode control topology, the current feedback
forms an internal digital loop that cannot be directly measured. The phase
margin of this loop is a function of duty cycle, and this loop can become
unstable for duty cycles greater than fifty percent. Although techniques
such as slope compensation can be used to adjust the stability of this
internal feedback loop, measurement of this loop is essential to verify
the effectiveness of the adjustment. This paper describes techniques for
indirect measurement of the gain-phase characteristics (Bode plot) of this
feedback loop. If also describes how to use this information to optimize
the performance of the overall voltage feedback loop. Finally, the paper
describes how to measure the feedback loop of power supplies where no
single signal injection point is available.
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