Two-Stage Soft-Start Method for Switching Regulators
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Solution Overview
Problem
Conventional switching power regulators face challenges in limiting inrush current during startup, as operating in PFM mode prolongs output voltage rise time, while operating in PWM mode makes it difficult to maintain inrush current within tolerance ranges.
Innovation Solution
A two-stage soft-start method for switching regulators, where a control circuit provides a first gate drive signal with pulse skip modulation (PSM) during the initial stage to limit inrush current, and switches to pulse width modulation (PWM) when a mode switching condition is met, allowing the output voltage to reach a predetermined value within a shorter rise time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the switching power regulator operates under PFM mode during startup to limit inrush current, then the inrush current is limited and the load device is protected, but the output voltage rise time becomes longer
Solution Approach 1:
The startup process is divided into two distinct stages: a first startup stage using PFM mode to limit inrush current and protect the load device, and a second startup stage using PWM mode to rapidly increase output voltage. This segmentation allows each stage to optimize for its specific function, resolving the contradiction between protection and speed.
Solution Approach 2:
The switching power regulator dynamically switches between PFM and PWM modes based on the startup progression. The control unit transitions from PFM mode during the first startup stage to PWM mode during the second startup stage when the output voltage reaches a predetermined level, enabling adaptive optimization of both current limiting and voltage rise speed.
2Loss of time
If the switching power regulator operates under PWM mode during startup to reduce rise time, then the output voltage reaches the predetermined voltage faster, but the inrush current exceeds the tolerance range and damages the load device
Solution Approach 1:
The startup process is segmented into two stages with distinct control modes: PFM mode in the first stage for current limiting, and PWM mode in the second stage for rapid voltage rise. This segmentation ensures that PWM's speed advantage is utilized only after the load device is protected by PFM's current limiting.
Solution Approach 2:
The PFM mode operates as a preliminary action during the first startup stage to limit inrush current and protect the load device before the PWM mode is activated. This preliminary protection enables the subsequent rapid voltage rise without compromising load safety.
3Reliability
If the switching power regulator operates only under PFM mode during startup, then the inrush current is limited within tolerance range, but a longer period of rise time is required for the output voltage to reach the predetermined voltage
Solution Approach 1:
The startup process is divided into two stages: PFM mode for the first stage to ensure inrush current remains within tolerance range, and PWM mode for the second stage to rapidly complete the voltage rise. This segmentation combines the current control reliability of PFM with the speed advantage of PWM.
Solution Approach 2:
The control unit dynamically transitions from PFM mode to PWM mode based on output voltage reaching a predetermined level. This dynamic switching enables the system to maintain reliable current control initially, then maximize startup speed once the load is protected.
4Productivity
If the switching power regulator operates only under PWM mode during startup, then the required rise time is reduced, but it becomes difficult to limit the inrush current within the tolerance range
Solution Approach 1:
The startup process is segmented such that PFM mode handles the first stage with strict inrush current limiting, while PWM mode handles the second stage for rapid voltage rise. This segmentation allows PWM's high productivity to be achieved without compromising current control reliability.
Solution Approach 2:
PFM mode performs the preliminary action of limiting inrush current during the first startup stage, creating safe operating conditions that enable PWM mode to subsequently operate at full speed without risking current control violations.
Data Source
AI summary
A soft-start method for a switching regulator is provided. In the soft-start method, a first gate drive signal for limiting an inrush current is provided, by a control circuit, to a switching circuit in a first soft-start stage. After the control circuit determines that a mode switching condition is satisfied according to an output voltage and an output current, a second drive signal for increasing a rise rate of an output voltage is provided to the switching circuit so that the output voltage increases with a faster rise rate and reaches a predetermined voltage at a second soft-start stage.