Resonant Converter Soft-Start Circuit Inrush Current Control
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Solution Overview
Problem
Conventional resonant DC-to-DC converters face issues during startup, including abrupt output voltage reaching maximum value, leading to large inrush currents and voltage overshoot, which can damage the converter and load, and require complex control schemes that conventional PFM controllers do not support.
Innovation Solution
A start-up circuit that provides a drive signal with a variable duty cycle and frequency to the switching transistor, initially set to maximum frequency and minimum duty cycle, which gradually decreases frequency and increases duty cycle, using a voltage-controlled oscillator and feedback signals to determine the drive signal parameters, thereby mitigating the issues of inrush currents and voltage overshoot.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the resonant converter operates in conventional PFM mode during startup, then the control scheme is simple, but the output voltage abruptly reaches maximum value causing large inrush current and voltage overshoot
Solution Approach 1:
The patent applies preliminary action by implementing a soft-start circuit that pre-charges the output capacitor through a controlled current path before the main power switch is activated. This preliminary charging action prevents abrupt voltage rise and large inrush current when the converter starts up, while maintaining simple PFM control logic.
Solution Approach 2:
The patent introduces an intermediary soft-start circuit consisting of a current source, capacitor, and control logic that mediates between the input voltage and the output capacitor. This intermediary circuit controls the charging current to prevent overshoot and inrush current, while the main PFM controller remains simple and unchanged.
2Loss of time
If the output voltage is controlled to rise quickly to reach maximum value, then the startup time is short, but the voltage overshoot and inrush current increase causing damage to components
Solution Approach 1:
The soft-start circuit performs preliminary charging of the output capacitor at a controlled rate before the main power stage is fully activated. This preliminary action reduces the subsequent inrush current when the main switch turns on, preventing component damage while achieving reasonable startup speed.
Solution Approach 2:
The patent implements beforehand cushioning by using a current source with limited current capability to charge the output capacitor during startup. This current limitation acts as a cushion that prevents excessive inrush current and voltage overshoot, protecting components from damage during the startup transient.
3Reliability
If a complex control scheme is used to prevent inrush current and voltage overshoot, then the component reliability is improved, but the device complexity increases
Solution Approach 1:
The patent uses an intermediary soft-start circuit that handles the complex protection functions separately from the main PFM controller. The intermediary circuit includes a current source, capacitor, and simple control logic that provides over-current protection and voltage overshoot prevention without requiring complex modifications to the main control scheme.
Solution Approach 2:
The patent segments the control function into two independent parts: a simple PFM controller for normal operation and a separate soft-start circuit for startup protection. This segmentation allows each part to remain simple while collectively providing both protection and efficient operation.
4Object-affected harmful factors
If the resonant converter operates at high switching frequency to reduce output ripple voltage at light load, then the output voltage ripple is reduced, but the switching losses increase and component stress increases
Solution Approach 1:
The patent implements dynamic switching frequency adjustment where the switching frequency varies with the load condition. At light load, the frequency increases to reduce output voltage ripple, while at full load, the frequency decreases to reduce switching losses. This dynamic adaptation optimizes the trade-off between ripple reduction and efficiency.
Solution Approach 2:
The patent changes the switching frequency parameter based on load conditions to optimize performance. The control circuit monitors the load and adjusts the switching frequency accordingly, allowing the converter to operate at higher frequencies when needed for ripple reduction and at lower frequencies when efficiency is the priority.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The soft-start control scheme allows for a monotonic increase in output voltage during startup, reducing the risk of damage and eliminating the need for complex control schemes, thereby improving the reliability and longevity of the resonant converter and its load.
Implementation Method 1
The frequency of the drive signal is preferably determined by a voltage-controlled oscillator that outputs a signal having a frequency dependent on an input voltage.
Implementation Method 2
The duty cycle of the drive signal is preferably determined by comparing a feedback signal from the resonant converter and a sawtooth signal.
Data Source
AI summary
A start-up circuit for a resonant converter is arranged such that, during start-up of the resonant converter, the start-up circuit provides a drive signal that is to be applied to a switching transistor of the resonant converter and that has a variable duty cycle and a variable frequency. A converter includes a voltage source, a capacitor connected to the voltage source, a first switching transistor connected to the voltage source, a transformer connected to the capacitor, and a start-up circuit arranged to drive the first switching transistor with the drive signal during start-up of the converter. A start-up method for a resonant converter including a first switching transistor includes driving the first switching transistor with a variable duty cycle and a variable frequency.