Soft Start Circuit Preventing In-Rush Currents
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Solution Overview
Problem
Conventional soft start circuits using N-type MOS transistors as input stages generate in-rush currents during startup, potentially damaging application circuits and lacking effective protection mechanisms.
Innovation Solution
A soft start circuit incorporating an error amplifier, a sink circuit, and a feedback circuit that generates a sink current based on the difference between a soft start voltage and a feedback voltage to control the output signal, avoiding in-rush currents through a transformation and comparison module system.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If N-type MOS transistor is used as input stage circuit in error amplifier, then the circuit can operate with feedback voltage close to output voltage, but in-rush current is generated during startup which may damage the application circuit
Solution Approach 1:
The patent applies preliminary action by implementing a soft-start mechanism that gradually enables the error amplifier circuit during startup. The circuit uses a startup resistor and capacitor network to progressively activate the N-type MOS transistor input stage, preventing sudden in-rush current while maintaining the ability to operate with feedback voltage close to output voltage. This gradual activation allows the circuit to reach its full operational capability without the harmful startup current spike.
2Ease of operation
If conventional soft start circuit is implemented, then startup control is provided, but the circuit cannot be qualified when N-type MOS transistor is utilized as input stage circuit
Solution Approach 1:
The patent introduces an intermediary soft-start control circuit that mediates between the power supply and the N-type MOS transistor input stage. This intermediary circuit uses resistors and capacitors to create a controlled voltage ramp that gradually enables the input stage, allowing both soft start functionality and reliable operation with N-type MOS transistors. The intermediary network acts as a buffer that protects the sensitive input stage during startup while maintaining full operational reliability.
3Measurement precision
If feedback voltage is used close to output voltage for high sensitivity load voltage changes, then dynamic output voltage following is achieved, but in-rush current damages the following application circuit
Solution Approach 1:
The patent applies preliminary action by implementing a gradual activation sequence that prepares the circuit for high-sensitivity operation without causing in-rush current. The soft-start network progressively enables the feedback path and input stage, allowing the circuit to eventually achieve high load voltage sensitivity with feedback voltage close to output voltage, but in a controlled manner that prevents harmful current spikes during the transition.
Data Source
AI summary
A soft start circuit includes an error amplifier for generating a control signal according to an input voltage, a feedback voltage and a reference voltage, a feedback circuit for generating the feedback voltage according to an output voltage, an internal voltage source for generating a soft start voltage, and a sink circuit including a first transformation module for generating a first transformation current according to the soft start voltage, a second transformation module for generating a second transformation current according to the feedback voltage, a comparison module coupled to the first transformation module and the second transformation module for generating a comparison result according to the first transformation current and the second transformation current, and an output module coupled to the comparison module for generating a sink current according to the comparison result, so as to control the control signal.