Startup Circuit Soft Start Voltage Ramp Control
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Solution Overview
Problem
Conventional startup circuits for power supply systems experience abrupt voltage changes during the startup period, leading to unstable operation and potential overcurrent issues due to rapid voltage rise, which existing soft starting functions do not adequately address.
Innovation Solution
A startup circuit design that includes an error amplifier with an enable/disable function, a PWM comparator, a soft start circuit, and a detecting circuit to block PWM signals until a predetermined terminal voltage is reached, along with a level shift circuit to reduce the target voltage, preventing abrupt voltage changes and enabling gradual output voltage increase.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional soft start circuit charges a capacitor to generate a ramp voltage for gradual output voltage increase, then the output voltage rises slowly preventing inrush current, but the error amplifier continues to operate during startup causing abrupt voltage changes and unstable operation
Solution Approach 1:
The detecting circuit detects the terminal voltage of the soft start capacitor in advance and generates an enable signal before the error amplifier operates. This preliminary detection ensures that the error amplifier only becomes active after the terminal voltage reaches a predetermined threshold, preventing abrupt voltage changes during startup while maintaining the gradual voltage rise characteristic
Solution Approach 2:
The enable signal acts as an intermediary between the soft start circuit and the error amplifier. It mediates the interaction by controlling the activation timing of the error amplifier based on the terminal voltage level, ensuring smooth transition from soft start mode to normal operation mode without causing voltage fluctuations
2Power
If the error amplifier operates continuously during startup period, then it provides continuous voltage regulation, but it causes abrupt voltage changes and overcurrent states due to fast voltage variation
Solution Approach 1:
The detecting circuit performs preliminary detection of the terminal voltage and generates an enable signal that prevents the error amplifier from operating during the critical startup period. This preliminary anti-action blocks the harmful effect of abrupt voltage changes before they can occur, while still allowing the soft start circuit to perform its voltage ramping function
Solution Approach 2:
The system waits for the terminal voltage to reach a predetermined threshold through the soft start capacitor charging process before enabling the error amplifier. This preliminary waiting period ensures that the voltage has stabilized and the inrush current has subsided, preventing harmful voltage fluctuations when the error amplifier becomes active
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
The solution ensures a slow and stable rise in output voltage during the startup period, preventing abrupt voltage fluctuations and maintaining stable power supply operation by disabling error amplification until the terminal voltage exceeds a predetermined threshold, thus avoiding overvoltage and ensuring smooth power delivery.
Implementation Method 1
a capacitor CS for soft starting... the capacitor CS is charged with a constant current from the constant current circuit IS. The terminal voltage VCS of the capacitor CS is a ramp voltage increasing on a slope
Data Source
AI summary
In a startup circuit, an error amplifier receives a target voltage in a startup period that is a terminal voltage with a shape of a slope generated by charging a capacitor for soft starting with a current from a constant current circuit. A detecting circuit monitors variation of the terminal voltage and blocks pulse from a logic circuit until the terminal voltage, which has been zero volts at the moment of startup, reaches a predetermined threshold value. In this period, the detecting circuit disables the function of the error amplifier. It is not until the terminal voltage VCS reaches the predetermined threshold value and the PWM pulse begins to be delivered that the error amplifier is enabled. At this moment, startup control begins based on the difference between the terminal voltage and the feedback signal. Therefore, the output voltage never rises abruptly.