LLC Resonant Converter Power Limit Protection
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Solution Overview
Problem
Existing DC-to-DC LLC resonant converter circuits inaccurately detect output power limits due to reliance on integrated primary-side current measurements, which do not account for changes in switching frequency, leading to inappropriate power protection activation.
Innovation Solution
A control circuit that estimates output power by comparing an integrated primary current to a constant slope threshold or dividing it by a switching period, and using polynomial approximations to accurately determine if the power limit is exceeded, thereby accounting for frequency changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If power limit detection is based on integrated primary-side current measurement, then the detection method is simple, but the detection accuracy deteriorates when switching frequency changes
Solution Approach 1:
The patent introduces an intermediary variable (switching frequency) that mediates between the primary-side current measurement and the actual output power. By incorporating frequency information into the power estimation calculation, the system accurately reflects the relationship P ∝ I_primary²/f, resolving the inaccuracy caused by frequency variations while maintaining measurement simplicity.
Solution Approach 2:
The patent changes the detection parameter from simple integrated current to a composite parameter that includes both current and frequency information. This parameter transformation (P ∝ I_primary²/f) allows accurate power limit detection across varying frequency conditions without significantly increasing system complexity.
2Speed
If power protection is activated based on integrated primary-side current value, then the protection response is fast, but false activation occurs when power limit is not exceeded
Solution Approach 1:
The patent implements a feedback mechanism that continuously monitors both primary-side current and switching frequency, then feeds this information back to the power limit detection logic. This closed-loop approach ensures that protection activation decisions are based on accurate real-time power estimation, preventing false activation while maintaining rapid response when actual power limits are exceeded.
Solution Approach 2:
The patent makes the power limit detection threshold dynamic by adjusting it based on switching frequency variations. Instead of using a fixed current threshold, the system dynamically adapts the detection criterion to account for frequency changes, ensuring reliable discrimination between normal operation and actual over-power conditions.
Data Source
AI summary
A circuit comprises an Inductor-Inductor-Capacitor (LLC) tank circuit and an energizing circuit. The LLC tank circuit includes first and second inductors, a capacitor, and a primary coil of a transformer. The first inductor is coupled in series with the second inductor, the second inductor is coupled in series with the capacitor, and the primary coil is coupled in parallel with the second inductor. The energizing circuit supplies power to the LLC tank circuit according to a switching period, and detects a power limit condition according to a value of an integrated current sense signal and a duration of the switching period. The integrated current sense signal corresponds to an integration over time of a current supplied to the LLC tank circuit. The circuit may be incorporated into a power converter to provide power limit detection according to an accurate real-time estimation of the power converter's output power.