Understanding Op Amp Dynamic Response In A Type-2 Compensator (Part 1): The Open-Loop Gain

Focus:

In most studies, a power supply compensator is an active circuit built around an ideal op amp. If this approach suffices in low-bandwidth systems, nowadays power converters cross over at or beyond 100 kHz to ensure a transient response that is fast enough to limit the output voltage drop in spite of a small output capacitive bank. In these applications, calculations considering a perfect op amp induce severe gain and phase distortions. But by accounting for the effects of the finite open-loop gain and the two low- and high-frequency poles of the op amp on the compensator’s response, you can select the right op amp without altering the gain and phase you need at crossover. This part 1 of two article analyzes the impact of the finite open-loop gain on the compensator’s response, purposely ignoring the low- and high-frequency poles. This part begins with a review of the three types of compensation circuits used in power supplies followed by an explanation of a circuit analysis method known as Fast Analytical Circuits Techniques (FACTs), which the author uses to determine the response of compensators.

What you’ll learn:

• How to understand the impact of an op amp’s finite open loop gain on the response of power supply compensation circuits
• How to derive a transfer function for a type 2 compensator that accounts for the open-loop gain of the op amp
• How to design a type 2 compensator for high bandwidth applications

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Author & Publication:

Christophe Basso, ON Semiconductor, Toulouse, France, How2Power Today, Jan 15 2017

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