Switching Power Supply Soft-Start Circuit Using PWM-Synchronized Charging
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Solution Overview
Problem
Existing switching power supply soft-start techniques require a large circuit area and suffer from inrush current and overshoot issues due to sudden voltage increases, which can lead to reliability problems in circuit elements.
Innovation Solution
A control circuit for a switching power supply that includes a soft-start circuit with a capacitor charged intermittently by a current source synchronized with a pulse modulation signal, allowing for a low rate of change in the soft-start voltage and enabling the capacitor to be integrated within the control circuit, reducing circuit area and improving soft-start time precision.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a D/A converter is used to generate a soft-start voltage that gradually rises over time, then the soft-start function is achieved and output voltage increases gradually, but the circuit area is increased
Solution Approach 1:
The patent extracts the soft-start voltage generation function from the D/A converter and implements it using a simple capacitor charged by a constant current source. This removes the complex D/A converter while preserving the essential soft-start functionality, significantly reducing circuit area.
Solution Approach 2:
The patent uses a capacitor to copy the voltage ramping effect that would otherwise require a D/A converter. By charging the capacitor with a constant current, the voltage across the capacitor increases linearly over time, replicating the gradual voltage rise function of the D/A converter with much simpler circuitry.
2Duration of action of moving object
If the capacitance of the capacitor is increased to extend the soft-start time, then the soft-start time is increased, but the circuit area is increased when the capacitor is built-in
Solution Approach 1:
The patent changes the charging parameters of the capacitor by using a constant current source that can be precisely controlled. By adjusting the constant current value, the soft-start time can be extended without increasing capacitance, thereby avoiding the need for larger built-in capacitors and reducing circuit area.
3Duration of action of moving object
If the constant current is reduced to increase the soft-start time, then the soft-start time is increased, but the precision of the constant current deteriorates
Solution Approach 1:
The patent employs periodic switching of the constant current source synchronized with the PWM signal. This periodic operation allows the use of higher current values while still achieving extended soft-start time through accumulated charging cycles, thereby maintaining constant current precision while extending the soft-start duration.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution allows for a long soft-start time with a small capacitance, reducing the need for large capacitors and maintaining precision, while integrating the capacitor within the control circuit to minimize circuit area and costs.
Implementation Method 1
a capacitor arranged such that one terminal thereof is set to a fixed electric potential; and a current source configured to generate a charge current that flows intermittently in synchronization with the pulse modulation signal so as to charge the capacitor
Data Source
AI summary
A soft-start circuit generates a soft-start voltage Vss having a voltage level that rises over time. A pulse width modulator generates a PWM signal having a duty ratio adjusted such that a feedback voltage Vout′ that corresponds to the output voltage Vout of the switching power supply matches the soft-start voltage Vss. A driver circuit controls a switching element according to the PWM signal. A capacitor is arranged such that one terminal thereof is set to a fixed electric potential. A current source generates a charge current that flows intermittently in synchronization with the PWM signal so as to charge the capacitor. The soft-start circuit outputs, as the soft-start voltage Vss, a voltage that occurs at the capacitor.