In 1986, Dr. Ridley tackled the challenging topic of current-mode control. His landmark dissertation compared all of the different modeling approaches of the time, and developed a new model that accurately predicted the behavior of current-mode power supplies. The current loop instability was properly predicted for the first time, using average models for the applicable part of the circuit, and sampled-data for the current loop phenomena. A simple approximation revealed a new, straightforward model.
The new model offered several insights to properly understand the system:
- The best representation of a second order converter is given by a third order transfer function. A dominant pole represents the current-source effect, and a double pole at half the switching frequency shows the subharmonic oscillation.
- Current-mode control can become unstable even at duty cycles below 50%, and a compensating ramp must be added for some converters that are limited to 50% duty cycle.
- The current feedback loop has two right-half-plane zeros in the transfer function that lead to instability.
- The PWM switch model, developed by Dr. Vorpérian, works perfectly well in the current-mode model. This allows a single model for current-mode, voltage-mode, and the important case where significant compensating ramp is used and the resulting system is somewhere between the two.
Since this work was done, there has been nothing published to improve the model, and the results are used widely in the industry and in research. This book is essential reading for all power supply designers.
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