LCL-T Resonant Converter Soft Start for Inrush and Saturation Control
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Solution Overview
Problem
Conventional LLC resonant converters face challenges in controlling start-up mode, particularly due to high switching frequency variations, leading to potential damage from inrush currents and transformer saturation, which existing control methods fail to adequately address.
Innovation Solution
A soft-start strategy is implemented for LCL-T resonant converters, involving a controller that performs a soft start control to pre-charge bulk output DC link capacitors and balance capacitor voltages, using phase shifts to manage inrush currents and ensure zero voltage switching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If conventional LLC resonant converter control is used during start-up mode, then high switching frequency variations can control voltage gain, but inrush currents and transformer saturation occur causing potential device damage
Solution Approach 1:
The patent applies preliminary action by implementing a soft-start control strategy that pre-charges the DC link capacitor before full power operation. The controller gradually increases the duty cycle from zero during start-up, preventing inrush currents and transformer saturation before they can occur, thus resolving the contradiction between voltage gain control and harmful current effects
Solution Approach 2:
The patent applies dynamics by making the switching frequency and duty cycle variable during start-up mode. The controller dynamically adjusts the switching frequency to track the resonant frequency and varies the duty cycle to control power transfer, enabling safe start-up operation without fixed frequency limitations, thus resolving the contradiction between operational flexibility and device protection
2Adaptability or versatility
If stacked half bridge inverter is used to meet wide input voltage requirement, then each device blocks half of input voltage, but additional blocking capacitors are needed and DC link capacitor balancing becomes challenging
Solution Approach 1:
The patent applies self-service by implementing a capacitor balancing control strategy where the controller automatically detects and corrects DC link capacitor voltage imbalances during normal operation. The system uses feedback from capacitor voltage sensors to adjust switching patterns, enabling the converter to self-balance without external intervention, thus resolving the contradiction between wide voltage adaptability and system complexity
Solution Approach 2:
The patent applies feedback by implementing closed-loop control that continuously monitors DC link capacitor voltages and adjusts switching duty cycles to maintain balance. The controller uses voltage feedback signals to modulate the inverter switching, automatically compensating for voltage drift and ensuring equal capacitor voltages, thus resolving the contradiction between operational versatility and control complexity
3Productivity
If period doubling modulation is used for steady state operation, then natural DC link capacitor balancing occurs, but additional control effort is needed during transient modes and start-up
Solution Approach 1:
The patent applies continuity of useful action by extending the capacitor balancing control from steady state to continuous operation including transients and start-up. The controller maintains active balancing throughout all operating conditions, ensuring uninterrupted power transfer and consistent capacitor voltage balance, thus resolving the contradiction between steady-state efficiency and transient control requirements
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 solution effectively prevents inrush currents and transformer saturation during power up, enabling safe and efficient operation of the converter across a wide voltage range with zero voltage switching.
Implementation Method 1
a âTâ-type resonant network formed by inductors L1, L2, and capacitor Cr... there is only one resonant frequency, and the switching frequency of the converter is set to be equal to the resonant frequency
Implementation Method 2
there are additional blocking capacitors in the circuit diagram marked as CB, which provide DC blocking functionality
Implementation Method 3
The inverter includes a plurality of inverter switches... a stacked half-bridge inverter meets the wide input voltage requirement with each device blocking half of the input voltage VIN
Implementation Method 4
The rectifier is shown in a standard full bridge configuration where each device needs to block full output voltage VOUT
Data Source
AI summary
The present disclosure provides a control method of a converter system. The control method includes steps of: (a) performing a soft start control by the controller until a voltage of the blocking capacitor reaches half of a voltage of the DC voltage source; (b) performing an output current control loop and a voltage balancing loop for obtaining a first phase shift and a second phase shift by the controller; and (c) utilizing the first phase shift and the second phase shift for controlling the plurality of inverter switches and the plurality of rectifier switches.