Constant Voltage Circuit Startup Control
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Solution Overview
Problem
Constant voltage circuits face issues with high inrush currents and overshoot of output voltage during start-up, which can lead to performance degradation and transistor failure, especially when connected to low load resistance or high-capacity capacitors.
Innovation Solution
A constant voltage circuit design incorporating an output transistor, an error amplifier circuit, a ramp voltage generating circuit, and an amplifier circuit that controls the output transistor by amplifying the voltage difference between a ramp voltage and a proportional output voltage, allowing for controlled rise time of the output voltage and reducing inrush current and overshoot.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If the output capacitor has high capacity to ensure stable output voltage, then the output voltage stability is improved, but the inrush current at start-up increases excessively
Solution Approach 1:
The patent applies preliminary action by using a startup resistor connected in series with the output capacitor. This resistor is present during startup to limit inrush current, and then bypassed during normal operation. The bypass mechanism activates after startup, allowing the capacitor to be fully charged without excessive current while maintaining voltage stability during normal operation.
2Object-generated harmful factors
If the output transistor limits current to a low value to reduce inrush current, then the inrush current is reduced, but the output voltage rises slowly and may not reach the target level
Solution Approach 1:
The patent applies dynamics by making the output transistor's current limiting capability variable rather than fixed. The transistor operates in different modes: during startup, it limits current to a low value to prevent inrush; during normal operation, it can provide higher current to maintain proper output voltage levels. This dynamic adjustment resolves the contradiction between current limitation and voltage rise speed.
3Reliability
If a bypass circuit is added to handle inrush current to improve startup performance, then the inrush current problem is solved, but the circuit complexity increases
Solution Approach 1:
The patent applies merging by combining the bypass function with the existing output transistor. Instead of adding a separate bypass transistor and control circuitry, the patent utilizes the output transistor's inherent current limiting capability and controls it through the existing error amplifier circuit. This integration achieves inrush current protection without significantly increasing circuit complexity.
4Measurement precision
If the error amplifier controls the output transistor to maintain precise output voltage, then the output voltage accuracy is improved, but the inrush current increases due to rapid voltage rise
Solution Approach 1:
The patent applies feedback by using the error amplifier to monitor the output voltage and adjust the output transistor's gate voltage accordingly. During startup, the feedback mechanism naturally limits the rate of voltage rise by comparing the actual output voltage with the reference voltage and adjusting the transistor control. This maintains voltage accuracy while preventing excessive inrush current through controlled voltage rise.
Data Source
AI summary
A disclosed constant voltage circuit is configured to become active or inactive and convert an input voltage applied to an input terminal into a predetermined constant voltage for output from an output terminal. The circuit includes an output transistor for supplying, from the input terminal to the output terminal, an output current, an error amplifier circuit unit for controlling operations of the output transistor to make a first proportional voltage, which is proportional to the output voltage from the output terminal, equal to a predetermined reference voltage, a ramp voltage generating circuit unit for generating and outputting a ramp voltage whose voltage level increases at a predetermined speed from start-up, and an amplifier circuit unit for amplifying the voltage difference between the ramp voltage and a second proportional voltage, which is proportional to the output voltage, and outputting the amplified voltage difference to a control electrode of the output transistor.