COT Power Converter Soft-Start With Adjustable On-Time Control
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Solution Overview
Problem
Conventional Constant On-Time (COT) converters experience current transients during the soft-start phase, leading to potential damage and instability due to electromagnetic interference, thermal stress, and undesired voltage fluctuations, especially when the output voltage is high and the converter has a low inductance value or operates with a high duty cycle.
Innovation Solution
A method and system for COT converters that progressively increase the pulse width and limit the switching frequency during the soft-start phase, using a feedback processing circuit and timers to control the power switch, thereby reducing current spikes and ensuring a stable output voltage trajectory.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the converter operates with a fixed on-time period during soft-start, then the structure remains simple and fast transient response is maintained, but current transients occur causing electromagnetic interference, thermal stress, and voltage fluctuations
Solution Approach 1:
The patent applies dynamics by transitioning from a fixed on-time period to a variable on-time period during soft-start. The on-time is dynamically adjusted based on the output voltage level, initially using a longer on-time to limit current transients and then switching to the standard fixed on-time once the output voltage reaches the target level. This resolves the contradiction by making the converter structure adaptive rather than static.
Solution Approach 2:
The patent changes the on-time parameter from a constant value to a time-varying value during soft-start. Specifically, the on-time is extended beyond the normal fixed duration when the output voltage is below the target level, and then reverted to the standard fixed duration when the target is reached. This parameter change eliminates current transients during startup while maintaining the simplicity of the fixed on-time structure during normal operation.
2Productivity
If the converter uses a standard fixed on-time period, then regulation is simple and fast, but voltage overshoots and excessive settling times occur during soft-start
Solution Approach 1:
The patent applies preliminary action by extending the on-time period before the output voltage reaches the target level during soft-start. This extended on-time allows the output voltage to approach the target more gradually, preventing overshoot. Once the target voltage is reached, the converter switches to the standard fixed on-time, maintaining fast regulation. This preliminary adjustment of the on-time parameter ensures both stability during startup and speed during normal operation.
3Speed
If the converter operates with high duty cycle during soft-start, then the output voltage can reach target quickly, but current transients become more severe causing overcurrent conditions
Solution Approach 1:
The patent applies dynamics by making the on-time period variable during soft-start rather than fixed. When the output voltage is below the target level, the on-time is extended, which effectively limits the duty cycle and prevents severe current transients. Once the target voltage is reached, the converter transitions to the standard fixed on-time, allowing normal high-speed operation. This dynamic adjustment ensures both safe startup and reliable operation.
Data Source
AI summary
In accordance with an embodiment, a method for controlling a constant on-time switched mode power converter includes: generating a pulse signal having an on-time; during a soft-start phase of the power converter, successively increasing the on-time from an initial duration to a final duration, wherein the final duration is larger than the initial duration; sensing an output voltage of the power converter and generating a feedback signal proportional to the output voltage; adjusting a frequency of the pulse signal based on the feedback signal and a reference signal; and driving a power switch with the pulse signal to control the power switch and regulate the output voltage.