Resonant Converter Bridge Midpoint Voltage Control
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Solution Overview
Problem
Conventional LLC resonant converters experience excessive resonance energy and voltage overshoot in the resonant capacitor when the lower transistor is switched on, leading to inefficient operation.
Innovation Solution
A control circuit that controls the bridge midpoint voltage before the lower transistor is switched on, using a bridge midpoint voltage control circuit, driving voltage generation circuit, and clamp circuit to prevent excessive voltage on the resonant capacitor by managing the on-off state of the second power switch transistor based on voltage thresholds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the lower transistor Q2 is switched on when the bridge midpoint voltage is high, then the resonant converter can operate, but the voltage of the resonant capacitor CR resonates to an excessively high voltage causing overshoot
Solution Approach 1:
The control circuit performs preliminary action by detecting the bridge midpoint voltage before switching on the lower transistor Q2. When the bridge midpoint voltage exceeds a preset threshold, the control circuit prevents Q2 from being switched on, thereby avoiding the harmful resonance voltage overshoot in the resonant capacitor CR before it can occur.
Solution Approach 2:
The control circuit implements feedback by continuously monitoring the bridge midpoint voltage and using this information to control the switching state of the lower transistor Q2. When the bridge midpoint voltage is high, the feedback mechanism keeps Q2 off; when the voltage drops below the threshold, Q2 is allowed to switch on, thus maintaining reliable operation while preventing voltage overshoot.
2Productivity
If the lower transistor Q2 is constantly on to maintain operation, then the resonant converter can function, but larger resonance energy occurs in the resonant network formed by inductor LR and capacitor CR
Solution Approach 1:
The control circuit applies dynamics by making the switching state of the lower transistor Q2 variable rather than fixed. Q2 is dynamically controlled based on the real-time bridge midpoint voltage: off when voltage is high to reduce resonance energy loss, and on when voltage is low to maintain operation continuity, thus balancing productivity and energy efficiency.
3Device complexity
If conventional control is used without bridge midpoint voltage management, then the circuit is simple, but voltage overshoot of the resonant capacitor occurs during the time period when the lower transistor is on
Solution Approach 1:
The control circuit introduces an intermediary mechanism that mediates between the simple conventional control and the need for voltage stability. By adding voltage detection and conditional switching control, the circuit maintains operational simplicity while ensuring the bridge midpoint voltage remains within safe ranges, preventing resonant capacitor voltage overshoot.
Data Source
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AI summary
A control circuit, a control method and a resonant converter are provided. The resonant converter includes a first power switch transistor and a second power switch transistor. The second power switch transistor and the first power switch transistor are coupled in series sequentially between a ground potential terminal and a high potential terminal at an input side of the resonant converter. The control circuit includes a bridge midpoint voltage control circuit. The bridge midpoint voltage control circuit is configured to control a bridge midpoint voltage in a process of starting the resonant converter, to prevent a voltage of a resonant capacitor from being excessively high in a time period during which the second power switch transistor is on. The bridge midpoint voltage represents a voltage at a common terminal of the first power switch transistor and the second power switch transistor.