LLC Converter Primary-Side Burst Mode Control
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Solution Overview
Problem
LLC power converters experience poor light load efficiency due to their voltage gain control characteristic, which prevents the use of conventional Burst mode control methods that rely on a direct link between the optical feedback signal and delivered power.
Innovation Solution
An inner feedback loop is introduced to adjust the LLC operating frequency, making the power drawn proportional to the optical feedback signal, allowing for Burst mode operation by setting thresholds on the feedback signal to initiate and terminate switching action, ensuring controlled power delivery and improved light load efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional Burst mode control is used in LLC converters, then light load efficiency is improved, but the voltage gain control characteristic prevents direct implementation of Burst mode
Solution Approach 1:
An inner feedback loop is introduced that senses the optical feedback signal and uses it to adjust the LLC operating frequency. This creates a direct link between the optical feedback signal level and the power delivered to the load, enabling conventional Burst mode control thresholds to be effectively applied. The feedback mechanism transforms the voltage gain control characteristic into a power-proportional control characteristic.
Solution Approach 2:
The operating frequency of the LLC converter is dynamically adjusted based on the optical feedback signal level. By changing the frequency parameter in response to feedback, the power delivery becomes proportional to the feedback signal, allowing Burst mode thresholds to control power delivery effectively. This parameter change enables the converter to achieve light load efficiency improvements through Burst operation.
2Reliability
If LLC operates in continuous mode, then stable power delivery is maintained, but switching power losses reduce overall efficiency at light loads
Solution Approach 1:
The converter operates in Burst mode, switching between active and standby states periodically. During active bursts, the converter delivers power at high frequency to charge the output capacitor. Between bursts, the converter remains off, allowing the output capacitor to supply load current. This periodic operation eliminates continuous switching losses while maintaining stable power delivery through capacitor discharge during off-periods.
Solution Approach 2:
Power delivery continuity is maintained through the output capacitor that charges during active bursts and discharges during idle periods. The capacitor acts as an energy buffer, ensuring uninterrupted power supply to the load even when the LLC converter is switched off between bursts, thus maintaining continuous useful action without continuous switching losses.
Data Source
AI summary
An LLC converter having a primary side burst control circuit, and a secondary side regulator circuit. The primary side burst control circuit has an inner feedback loop circuit for adjusting the LLC operating frequency to control the power drawn by the LLC converter proportional to an optical feedback signal.