The Stability Criteria Applied to Switching Power Supplies

3 The Stability Criteria Applied to Switching Power Supplies 

Before starting the design for the compensation of the error amplifier, it is desirable to know what constitutes a stable closed-loop system. The rule is elementary  

The closed loop phase shall never exceed -330 degrees of lag whenever the gain of the closed-loop system is greater than i (or OdB). 

Actually, a total closed-loop phase lag limit of 3i5 degrees is commonly used by designers any closer to 360 degrees would constitute a metastable system. This could result in the power supply breaking out into periods of oscillation when large loads or line transients are experienced. 

Phase margin. This is the value of the phase of a closed-loop system at the gain cross-over frequency (G(s) = OdB). 

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