LLC Resonant Converter Secondary-Side Reverse Current Prevention
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Solution Overview
Problem
Conventional switching power supply devices with synchronous-rectification LLC resonant converters face issues such as increased complexity and cost due to the need for current transformers and high-accuracy comparators, and can generate reverse current when the switching frequency is lower than the resonant frequency, leading to inefficiencies and potential reverse current flow from the secondary side to the primary side.
Innovation Solution
A switching power supply device with a converter transformer and series resonant circuit, featuring complementary on/off control of switching elements and a controller that performs PFM control based on output voltage, limits the turned-on time of secondary-side switching elements to prevent negative current flow and reverse current generation, using a configuration that includes a primary winding, secondary windings, and switching elements connected in a half or full bridge type configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a current transformer and high-accuracy comparator are provided to control secondary-side switching elements, then the synchronous rectification can be achieved, but the device complexity and cost increase
Solution Approach 1:
The patent extracts and eliminates the current transformer and high-accuracy comparator from the circuit by utilizing the inherent current detection capability of the resonant inductor and excitation inductor. The control circuit directly monitors the current through these inductors to determine when to switch the secondary-side elements, achieving synchronous rectification without the additional components.
Solution Approach 2:
The resonant inductor and excitation inductor serve dual functions: they perform their primary role in the resonant circuit operation while simultaneously acting as current sensors for controlling the secondary-side switching elements. This multi-functionality eliminates the need for dedicated current detection components.
2Use of energy by moving object
If the switching frequency is lower than the resonant frequency, then the power transfer efficiency may improve, but reverse current flows from secondary side to primary side
Solution Approach 1:
The control circuit proactively monitors the current through the resonant inductor and excitation inductor before reverse current can occur. When the current drops below a threshold indicating approaching reverse flow, the control circuit preemptively turns off the secondary-side switching elements, preventing reverse current from establishing.
Solution Approach 2:
The control circuit continuously monitors the current status through the resonant inductor and excitation inductor, using this feedback to dynamically control the secondary-side switching elements. This real-time feedback mechanism ensures that switching actions are synchronized with the actual current flow direction, preventing reverse current even at low switching frequencies.
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 reverse current flow from the secondary side to the primary side, reduces heat generation in the transformer, and allows for a more efficient and cost-effective implementation of synchronous-rectification LLC resonant converters by properly controlling secondary-side switching elements, even when the switching frequency is lower than the resonant frequency.
Implementation Method 1
series resonant circuit including a resonant inductor Lr and a resonant capacitor Cr that are connected in series with the primary winding L1
Implementation Method 2
converter transformer T1 having a primary winding L1, a first secondary winding L21, and a second secondary winding L22
Data Source
AI summary
A switching power supply includes a series resonant circuit that includes a resonant inductor and a resonant capacitor connected in series with a primary winding of a converter transformer. By controlling turning on and off of first and second switching elements in a complementary manner, current is supplied to the series resonant circuit. A third switching element connected on the secondary side of the converter transformer is synchronized with the first switching element, and a fourth switching element is synchronized with the second switching element. If a switching frequency is less than a resonant frequency, turning on of the third and fourth switching elements is synchronized with turning on of the first and second switching elements, and turning off of the third and fourth switching elements is controlled, without being synchronized with turning off of the first and second switching elements, after half a resonant period has elapsed.