LLC Converter Adaptive On-Time Control
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Solution Overview
Problem
Current methods for controlling diode conduction time in LLC converters are costly, challenging to implement, and sensitive to parasitic losses, leading to inefficiencies and current inversion issues, especially in low output voltage applications.
Innovation Solution
An adaptive algorithm implemented by a secondary side controller that monitors voltage and adjusts the on-time of transistors to optimize diode conduction time, increasing or decreasing the on-time based on comparisons with a target time to minimize losses and prevent current inversion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If current sensing approaches are used to regulate transistor on-time, then diode conduction losses can be reduced, but parasitic losses increase and measurement precision decreases
Solution Approach 1:
The patent introduces a current mirror circuit as an intermediary mechanism that copies the primary side current to the secondary side without direct sensing. This current mirror (comprising transistors Q1-Q4 and resistors R1-R2) acts as a mediator that transfers current information across the transformer isolation barrier, enabling precise on-time control without the parasitic losses associated with direct current sensing approaches.
Solution Approach 2:
The patent replaces direct electrical current sensing with a magnetic field-based current mirror mechanism. By utilizing the transformer's magnetic coupling to replicate current characteristics on the secondary side, the system eliminates the need for physical current sensors that introduce parasitic losses, substituting a field-based approach for direct electrical measurement.
2Loss of energy
If transistor on-time is extended to reduce diode conduction losses, then efficiency improves, but current inversion events increase
Solution Approach 1:
The patent implements a feedback mechanism where the secondary side controller continuously monitors the current mirror output and adjusts the transistor on-time accordingly. When the current mirror indicates that the primary current has fallen below a threshold, the controller terminates the on-time, preventing current inversion. This closed-loop feedback ensures optimal efficiency while maintaining reliability.
Solution Approach 2:
The patent employs dynamic adjustment of transistor on-time based on real-time current conditions. Rather than using fixed timing, the system dynamically extends or reduces on-time according to the actual load conditions and current characteristics, allowing the converter to adapt to varying operating conditions and prevent current inversion events.
3Loss of energy
If rectifier diodes are replaced with transistors to reduce conduction losses, then efficiency improves, but device complexity increases
Solution Approach 1:
The patent makes the secondary side transistors serve multiple functions: they act as both the rectifying switches and the current sensing elements through the current mirror mechanism. This multi-functionality reduces the need for separate sensing components and simplifies the overall circuit architecture while maintaining the low conduction losses of transistor-based rectification.
Solution Approach 2:
The patent merges the current sensing function with the rectification function by using the same transistor components for both purposes. The current mirror circuit combines the secondary side transistors with the transformer winding to create an integrated sensing mechanism, eliminating the need for separate sensing circuitry and reducing overall device complexity.
Data Source
AI summary
A circuit for use in an LLC converter to control diode conduction time includes a secondary side controller, the secondary side controller configured to monitor voltage, measure a diode conduction time for the LLC converter, in response to determining that the diode conduction time is greater that a target time, increase the on-time for the first switch, and in response to determining that the diode conduction time is less than a target time, decrease the on-time for the first switch.