Soft Start Circuit Reduces Battery Charging Inrush Current
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Solution Overview
Problem
Battery charging circuits experience a large inrush current during startup, which can damage both the battery and the charging circuit, as the feedback loop quickly tries to bring the current from zero to the target current, resulting in a current overshoot that is substantially larger than the target current.
Innovation Solution
A soft start circuit is integrated into the feedback loop of the battery charging circuit, which controls the battery charging current by selecting the lowest voltage between a soft start voltage and a controller output voltage, thereby reducing the inrush current by generating a soft start control voltage based on an external power supply voltage and an error voltage indicative of the difference between the measured and target charging currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the charging circuit quickly brings the current from zero to the target current during startup, then the charging speed is improved, but the inrush current becomes substantially larger than the target current, causing damage risk
Solution Approach 1:
The soft start circuit performs preliminary action by pre-charging the capacitor with a limited current before the main charging switch is fully activated. This preliminary charging phase gradually builds up the voltage, preventing the sudden inrush current that would occur if the full charging voltage were applied immediately. The circuit transitions from a soft start mode to a full charging mode after the capacitor is sufficiently charged.
Solution Approach 2:
The soft start circuit acts as an intermediary between the power supply and the main charging circuit. It includes a soft start switch and capacitor that mediate the power transfer, gradually connecting the main charging circuit to the power supply. This intermediary structure controls the rate of power transfer, preventing direct inrush current while still enabling fast charging once the soft start phase is complete.
2Object-affected harmful factors
If a soft start circuit is added to control the charging current, then the inrush current is reduced, but the device complexity increases
Solution Approach 1:
The soft start circuit is merged with the existing charging circuit components. The soft start switch is integrated with the main charging switch, and the soft start capacitor is combined with the battery capacitor. This merging approach allows the soft start function to be implemented using existing circuit elements, minimizing additional complexity while still providing effective inrush current protection.
Solution Approach 2:
The soft start circuit components serve multiple functions. The soft start capacitor not only limits inrush current but also acts as the battery charging capacitor. The soft start switch integrates the soft start control function with the main charging switch control. This multi-functionality reduces the need for separate dedicated soft start components, thereby limiting the increase in device complexity.
Data Source
AI summary
A soft start amplifier provides a soft-start function that controls a battery charging current in a feedback loop charging circuit by selecting the lowest voltage between a soft start voltage and an error derived control voltage. The lowest voltage is used as a control signal for controlling the battery charging current in the feedback loop charging circuit. The error voltage is a difference between a voltage proportional to the charging current and a voltage proportional to the target charging current while the soft start voltage is a voltage configured to ramp up with time. Using the lower voltage of the error voltage and the soft start voltage reduces the inrush current that may occur when the error derived control voltage spikes to the supply voltage in an attempt to correct the initial difference between the target charging current and a measured charging current.