Switching Power-Supply Circuit Soft-Start Discharge Control
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Solution Overview
Problem
Existing power-supply circuits face challenges in achieving reliable soft-start and independent turn-off operations without increasing the number of terminals and circuit complexity, and struggle to completely turn off output voltages due to parasitic resistance and floating soft-start voltages.
Innovation Solution
A switching power-supply circuit that includes a first driver for the first switching transistor, a second driver for the second switching transistor, a soft-start voltage generator, an error amplifier with an offset applying circuit, and a discharge transistor, which controls the drivers based on an amplified error voltage to ensure reliable turn-off even when the soft-start voltage does not drop to ground.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a switching element is connected across the capacitor for soft start to discharge it, then the circuit complexity is reduced, but the soft-start voltage cannot be reliably decreased to 0 V due to parasitic resistance
Solution Approach 1:
The patent introduces an intermediary discharge transistor connected between the soft-start voltage node and ground, which acts as a mediator to completely discharge the capacitor. This discharge transistor is controlled by a discharge control signal and provides a low-impedance discharge path that overcomes the parasitic resistance issue, enabling the soft-start voltage to be reliably decreased to 0 V without requiring additional external circuitry.
2Ease of operation
If switches and control terminals are added to achieve independent turn-off, then the turn-off function is achieved, but the number of terminals and circuit scale increase
Solution Approach 1:
The patent makes the existing switching element serve multiple functions: it acts as both the main power switching element and the discharge switch for the soft-start capacitor. By controlling the timing and state of this single switching element, the circuit achieves both power regulation and capacitor discharge functions, eliminating the need for separate discharge switches and control terminals.
Solution Approach 2:
The circuit uses its own internal switching element and control logic to discharge the soft-start capacitor, rather than requiring external discharge circuitry. The switching controller monitors the soft-start voltage and automatically activates the discharge function when appropriate, making the system self-sufficient and avoiding additional external components.
3Productivity
If the error amplifier outputs amplified error voltage during soft-start, then the output voltage control is achieved, but the output voltage cannot be effectively turned off when soft-start voltage floats above ground
Solution Approach 1:
The patent implements preliminary discharge action by actively discharging the soft-start capacitor before or during the turn-off sequence. The discharge transistor is activated to remove the floating voltage from the capacitor, ensuring that when the error amplifier stops outputting the amplified error voltage, the soft-start voltage is already at 0 V, enabling complete turn-off of the output voltage.
Data Source
AI summary
A switching power-supply circuit generates a predetermined output voltage by controlling a first switching transistor connected to a power supply and a second switching transistor connected between the first switching transistor and the ground. The switching power supply circuit includes a first driver, a second driver, a soft-start voltage generator and an error amplifier. The first driver drives the first switching transistor. The second driver drives the second switching transistor. The soft-start voltage generator generates a soft-start voltage that increases gradually after the time of start. The error amplifier amplifies an error voltage between a feedback voltage based on the predetermined output voltage and the soft-start voltage, and controls the first and second drivers in accordance with an amplified error voltage.