Soft-Start Control Circuit for Switching Power Supply Overshoot
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Solution Overview
Problem
Current soft-start control methods for switching power supplies are not applicable to fast-response types, as the feedback voltage immediately rises during startup, making it unusable for controlling the soft-start circuit and resulting in voltage overshoot.
Innovation Solution
A control apparatus for a current resonance type switching power supply that includes a soft-start control circuit which increments the on-width of the drive signal by a prescribed step per prescribed switching number to reduce output voltage overshoot during startup, independent of the response speed of the power supply.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If feedback voltage control is used for soft-start, then output voltage overshoot is reduced, but the method becomes inapplicable to fast-response switching power supplies because feedback voltage immediately rises during startup
Solution Approach 1:
The patent introduces an intermediary signal (counter output signal) between the switching frequency control and the duty cycle control. The counter counts switching cycles and generates an on-width signal that indirectly controls the duty cycle based on switching frequency rather than directly using feedback voltage, enabling soft-start functionality in fast-response power supplies where feedback voltage rises too quickly
2Reliability
If switching frequency is controlled based on output voltage, then output voltage overshoot is reduced, but the switching frequency needed in advance is uncertain at startup when output voltage rises from almost 0 volt
Solution Approach 1:
The patent performs preliminary action by setting a predetermined counting value in the counter before startup occurs. This predetermined value determines the initial on-width of the drive pulse signal, allowing the switching frequency to be controlled in advance without needing to know the uncertain output voltage conditions at startup
3Stability of the object's composition
If duty cycle is gradually increased based on feedback voltage, then output voltage rises smoothly, but the feedback voltage is not usable as control signal in fast-response power supplies
Solution Approach 1:
The patent implements dynamic control by making the on-width (duty cycle) change according to the switching frequency through the counter mechanism. Instead of static feedback voltage control, the system dynamically adjusts the duty cycle based on the number of switching cycles, adapting to fast-response power supply characteristics while maintaining smooth voltage rising waveforms
Data Source
AI summary
A control apparatus of a current resonance type switching power supply that has a switching element and generates an output voltage. The control apparatus includes a drive circuit configured to generate a drive signal to drive the switching element of the switching power supply, and a soft-start control circuit connected to the drive circuit, the soft-start control circuit being configured to send a signal to the drive circuit to cause the drive circuit to increment an on-width of the drive signal by a prescribed step per prescribed switching number, to thereby reduce an overshoot of the output voltage when the switching power supply is started.