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Small-Signal Analysis and Design of Constant Frequency V2 Peak Control (2)

Fig. 1. Constant Frequency V2 Peak Control with External Ramp.
To solve the instability issue which occurs either in the large duty cycle case or with the small capacitor time constant case, two types of compensations (either by adding an external ramp or a current ramp) are adopted. However, the effects of the external ramp and current ramp need to be further clarified for design purposes to obtain a well-damped dynamic performance. By examining the small-signal model with ramp compensations, design guidelines for the ramp compensations are proposed. It is found that the external ramp is good enough to get a well-damped performance when feedback strength is strong (for example, when employing OSCON caps). However, a current ramp is necessary to achieve a good dynamic performance when feedback strength is weak (for example, when employing Ceramic caps).

Fig. 2. Stability criterion with external ramp.
Fig. 3. Design Guideline for External Ramp with OSCON Cap.

Fig. 4. Hybrid Ramp Compensation for Ceramic Cap.
Fig. 5. Performance Comparison for Ceramic Cap with Hybrid Ramp and External Ramp.