Primary Feedback Control Circuit for LLC Resonant Converters
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Solution Overview
Problem
Existing series resonant converters, particularly those used in LED lighting, face challenges in accurately controlling power switches due to the complexity of LLC resonant circuits, where both excitation and resonant currents coexist on the primary side of the transformer, leading to inaccuracies in output current calculation and limited applicability, especially when the excitation current proportion is significant.
Innovation Solution
A primary feedback control circuit is introduced, which includes an excitation current simulation circuit to generate a voltage representing the excitation current and a feedback control circuit to control power switches based on this voltage and the resonant current voltage, ensuring they are equal when the secondary current is zero, thereby accurately simulating the excitation current across all frequency ranges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a primary feedback control approach is used in LLC resonant converters, then the control circuit structure is simplified and costs are reduced, but the precision of output current calculation deteriorates when the excitation current proportion is significant
Solution Approach 1:
An excitation current simulation circuit is introduced as an intermediary component that generates a simulated excitation current signal based on the primary current and transformer parameters. This simulated signal serves as a mediator to compensate for the excitation current's effect on output current calculation, allowing the control circuit to maintain simplicity while achieving accurate output current measurement across all operating conditions.
2Ease of operation
If the excitation current is ignored in output current calculation, then the control circuit becomes simpler, but the accuracy of control deteriorates especially at frequencies where excitation current proportion is significant
Solution Approach 1:
The patent implements a feedback mechanism where the simulated excitation current signal is fed back to the control circuit and subtracted from the primary current signal. This feedback approach allows the control circuit to automatically compensate for excitation current effects without complex manual calculations, maintaining ease of operation while significantly improving control accuracy across the entire frequency range.
3Measurement precision
If secondary feedback control is used to achieve constant current control, then output current control precision is improved, but the device complexity increases due to optocouplers and complex control signals
Solution Approach 1:
Instead of using complex secondary feedback control with optocouplers, the patent creates a copied or simulated version of the excitation current signal on the primary side. This simulated signal replicates the excitation current's characteristics without requiring physical coupling to the secondary side, thereby achieving accurate output current control while maintaining circuit simplicity and avoiding the complexity of secondary feedback implementation.
Data Source
AI summary
A primary feedback control circuit of a series resonant converter having a transformer, can include: an excitation current simulation circuit configured to sample an excitation voltage of the transformer, and to generate a first voltage representing an excitation current of the transformer; and a feedback control circuit configured to control on and off states of power switches of the series resonant converter in accordance with the first voltage and a second voltage representing a resonant current of the series resonant converter, where the first voltage is controlled to be equal to the second voltage when a secondary current of the transformer is zero.