Soft-off Control Circuit for Power Converter Over-voltage Prevention
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Solution Overview
Problem
Switching power converters face issues when shutting off, particularly in no-load conditions, where a large output capacitor can cause the input voltage to increase, potentially exceeding safe operating levels and damaging the converter due to negative low-side switching currents charging the input capacitor.
Innovation Solution
A soft-off control circuit that regulates the shutdown process by generating a variable reference signal based on the input voltage and an over-voltage threshold, controlling the charging and discharging of a reference signal generation capacitor to prevent over-voltage and ensure safe shutdown.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the power converter is switched off in no-load condition, then the shutdown process is initiated, but the input voltage increases due to negative low-side switching current charging the input capacitor, potentially exceeding safe operating voltage and damaging the converter
Solution Approach 1:
The patent implements a feedback mechanism through a comparator that continuously monitors the input voltage and compares it against a reference voltage. When the input voltage exceeds the safe threshold during shutdown, the comparator detects this condition and generates a control signal to adjust the switching duty cycle, thereby regulating the input voltage back to safe levels and preventing damage to the converter
Solution Approach 2:
The patent dynamically adjusts the switching duty cycle during the shutdown process based on real-time input voltage conditions. Rather than using a fixed shutdown sequence, the system modifies the duty cycle on-the-fly in response to detected over-voltage conditions, allowing the converter to adapt its shutdown behavior to prevent input voltage from exceeding safe operating limits
2Ease of operation
If the zero-crossing detection circuit is disabled during shutdown to permit negative low-side switching current, then the shutdown process can proceed, but the input capacitor charges cycle by cycle causing input voltage to increase until it exceeds allowable safe operating voltage
Solution Approach 1:
The comparator-based feedback system continuously monitors input voltage during shutdown and provides real-time feedback to the control circuit. When input voltage rises above the safe threshold due to capacitor charging, the feedback signal triggers duty cycle adjustment, creating a closed-loop control that prevents uncontrolled voltage increase while maintaining the shutdown sequence
Solution Approach 2:
The patent changes the switching duty cycle parameter dynamically during shutdown based on input voltage conditions. By adjusting this key operating parameter in response to over-voltage detection, the system prevents the input capacitor from charging to dangerous levels while still allowing the shutdown process to complete
Data Source
AI summary
A power converter having a soft-off control circuit and a variable reference signal generation module and associated method for controlling the power converter. The variable reference signal generation module is configured to provide a variable reference signal to the power converter. The soft-off control circuit is configured to determine whether an input voltage of the power converter exceeds an over-voltage threshold, and to control the variable reference signal to increase once the input voltage reaches the over-voltage threshold and to decrease when the input voltage is lower than the over-voltage threshold. The power converter can regulate an output voltage to increase with the increase of the variable reference signal and to decrease with the decrease of the variable reference signal so that during a soft-off procedure the input voltage may not exceed the over-voltage threshold, enabling the power converter to be safely shut off.