Secondary-Side Soft-Start Circuit for Isolated Switching Regulators
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing isolated switching regulators face challenges in tightly controlling output voltage during startup due to the use of open-loop soft-start schemes, which can result in overshoot and are ineffective for lower output voltage values, especially when the secondary-side controller requires a minimum voltage of 1.24V for operation.
Innovation Solution
A closed-loop soft-start circuit is implemented, utilizing an error amplifier, pull-down transistor, and low-voltage bandgap circuit to regulate the output voltage from as low as 1.8V, with a secondary side soft-start ramp and minimum detection circuit, allowing for precise control and integration of the secondary side controller within a power management IC.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If open-loop soft-start schemes are used to control startup voltage or current, then the circuit can be simplified and implemented with discrete components, but the output voltage cannot be tightly controlled during startup resulting in overshoot
Solution Approach 1:
The patent implements a closed-loop soft-start circuit that uses feedback control to regulate output voltage during startup. The error amplifier continuously compares the output voltage with a reference voltage and adjusts the control signal accordingly, enabling precise voltage control while eliminating the overshoot problems associated with open-loop schemes.
Solution Approach 2:
The soft-start circuit is activated before normal operation begins, gradually ramping up the output voltage from zero to the target value. This preliminary controlled action prevents sudden voltage jumps and allows the system to reach steady state smoothly before full power operation commences.
2Reliability
If shunt regulator (TL431) is used as secondary-side controller, then the circuit can provide voltage regulation, but it requires minimum 1.24V operation voltage which causes problems for lower output voltage applications
Solution Approach 1:
The patent employs a low-voltage bandgap reference circuit that can operate at voltages below 1.24V, fundamentally changing the minimum operating voltage parameter of the secondary-side controller. This enables the system to regulate output volt as low as 1.8V while maintaining stable reference voltage for the error amplifier.
Solution Approach 2:
Instead of using a traditional shunt regulator that requires high minimum voltage and then trying to adapt it for lower voltages, the patent inverts the approach by designing a low-voltage bandgap reference from the ground up, specifically engineered to operate in the sub-1.24V regime, thereby naturally achieving compatibility with low voltage outputs.
3Ease of manufacture
If discrete components are used for soft-start circuit, then the circuit can be implemented without integration, but closed-loop control during startup cannot be achieved
Solution Approach 1:
The patent merges the soft-start functionality with the secondary-side controller by integrating the error amplifier, reference voltage generator, and control logic into a single integrated circuit. This combination enables closed-loop control during startup while maintaining manufacturing efficiency through monolithic integration, eliminating the need for separate discrete soft-start components.
Data Source
AI summary
An isolated switching regulator has a closed-loop soft-start feature that allows tighter regulation of the output voltage and eliminates or reduces overshoot. It also has an optional reset feature which will resoft-start the regulator during recovery from a fault on the output voltage.