Resonant LLC Converter Control for Soft Switching Stability
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Solution Overview
Problem
Resonant converters face challenges in maintaining stability and preventing hard switching when operating near the capacitive mode, leading to increased switching losses and instability, especially during transients in supply voltage or load changes.
Innovation Solution
Implementing a control algorithm that monitors state variables such as primary current and capacitor voltage to ensure inductive mode operation by turning off switches before current reversal in the resonant tank, allowing for soft switch-on and cycle-by-cycle near capacitive mode protection, eliminating the need for frequency regulation loops and preventing hard switching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the switching frequency is decreased to operate near capacitive mode, then the converter can achieve higher efficiency at light loads, but the system becomes unstable and hard switching occurs
Solution Approach 1:
The patent implements a control algorithm that continuously monitors state parameters (current and voltage) of the resonant converter and provides feedback to adjust switching timing. This feedback mechanism detects when the converter approaches capacitive mode operation and adjusts the switching instant to prevent instability and hard switching, while still allowing operation near the capacitive mode border for improved efficiency.
Solution Approach 2:
The patent employs dynamic adjustment of switching parameters based on real-time operating conditions. The control algorithm dynamically modifies switching timing and frequency according to the instantaneous state of the resonant tank, allowing the converter to adaptively operate near capacitive mode when beneficial while automatically retreating to stable inductive mode when necessary, thus resolving the stability-efficiency tradeoff.
2Loss of energy
If the non-overlap time is increased to prevent hard switching, then switching losses are reduced, but the converter may enter capacitive mode causing instability
Solution Approach 1:
The control algorithm performs preliminary detection of state parameters before hard switching conditions occur. By monitoring current and voltage in advance, the system predicts when increasing non-overlap time might cause entry into capacitive mode, and takes preliminary corrective action by adjusting switching timing to maintain inductive mode operation while still achieving soft switching and reduced losses.
3Reliability
If frequency regulation loops are used to maintain inductive mode, then stability is improved, but the control complexity and response time increase
Solution Approach 1:
The patent extracts the essential stability function from complex frequency regulation loops and implements it through a simplified control algorithm that directly monitors state parameters and adjusts switching timing. This extraction eliminates the need for elaborate frequency regulation machinery while maintaining stability, reducing control complexity and improving response time.
Solution Approach 2:
The patent replaces traditional mechanical/analog frequency regulation mechanisms with a digital control algorithm that processes state parameter data and generates switching control signals. This substitution simplifies the control system architecture, reduces component count, and enables faster computational response while maintaining the stability function.
Data Source
AI summary
The invention deals with the control of a resonant LLC converter by setting up criteria for state parameters of the resonant converter, so that the converter may be operated in a near capacitive mode. The current flowing in the resonant tank and optionally the voltage at the a predetermined point in the resonant tank are monitored, and wherein a switch (a high side switch or a low side switch) is turned off when a first criterion is fulfilled together with a second criterion or optionally a third criterion, the first criterion ensuring a minimum time has lapsed after the switch is turned on, the second criterion being that the absolute value of the current is reaching a predetermined current level, the third criterion being that the voltage at the predetermined point reaches a predetermined voltage level.