DCDC: Feed forward capacitor Cff Feed forward capacitor enhances loop stability
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This post was last edited by qwqwqw2088 on 2024-1-3 21:59
参考文章：Optimizing Transient Response of Internally Compensated dc-dc Converters With Feedforward Capacitor
1. Basic principles
Adding a feedforward capacitor to the FB network of the internally compensated converter can adjust the control loop crossover frequency and phase margin, thereby improving control performance.
Zero frequency:
Pole frequency:
Bode plot before compensation:
Bode plot after compensation:
The purpose of Cff is to increase the gain and phase of the feedback loop while maintaining an acceptable phase margin.
2. How to calculate the feedforward capacitor
The first is the selection of compensation frequency. Here, an approximate method is used to set the frequency after compensation equal to the geometric mean of fz and fp, that is, the average frequency of fz and fp in the logarithmic domain, refer to the following formulas 3 and 4. At the geometric mean frequency, the phase improvement brought by the Cff capacitor is the largest.
The geometric mean frequency is selected to be the crossover frequency when there is no Cff capacitor. However, since the Cff capacitor will increase both the phase and gain, the new crossover frequency will be greater than the geometric mean frequency. The optimal feedforward capacitor can be calculated according to the above formula (6). Optimization means that the best compromise is achieved between the crossover frequency and the phase margin.
3. Going further, how to select the feedforward capacitor according to the needs
If the feedforward capacitor is larger than the calculated capacitor, the crossover frequency will be higher (faster response speed), but the phase margin will be reduced. If a smaller value is selected, the crossover frequency will be lower, and the phase margin will be larger. It needs to be selected according to the actual situation and verified by testing.
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