AC/DC Converter PFC Stage Control via Switching Frequency
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Solution Overview
Problem
Switched mode AC/DC converters introduce harmonic distortion into the mains supply due to non-unity power factor, leading to regulation challenges, and existing controllers' power control pins are prone to noise, resulting in inappropriate enabling or disabling of the Power Factor Correction (PFC) stage.
Innovation Solution
A method and controller that enable and disable the PFC stage based on the average switching frequency, using a timer and integrator to determine when the power exceeds predetermined levels, and incorporating a Schmitt trigger for rapid response to step changes, thereby bypassing the slow response to load changes and minimizing noise-induced errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the PFC stage is enabled based on the power control pin voltage, then the power factor is improved, but the noisy and unstable voltage causes inappropriate enabling or disabling of the PFC stage
Solution Approach 1:
The patent introduces an intermediary signal processing chain between the power control pin and the PFC enable/disable control. This includes a Schmitt trigger to clean the signal, a timer to detect frequency, and an integrator to average the signal over multiple cycles. This intermediary processing eliminates the direct coupling between the noisy power control pin voltage and the PFC stage control, resolving the contradiction by filtering out noise while preserving the intended control function.
2Loss of energy
If the PFC stage is disabled for low load operation, then the efficiency is improved, but the noisy signal may cause inappropriate disabling at non-ideal load thresholds
Solution Approach 1:
The patent employs periodic action by integrating the power control signal over multiple switching cycles (e.g., 5 cycles) before making the PFC enable/disable decision. This periodic averaging smooths out transient noise and provides a more stable threshold detection, ensuring that the PFC stage is only disabled when the load is genuinely low rather than during transient noise conditions. This resolves the contradiction by maintaining measurement precision while still enabling efficiency improvements at low loads.
3Reliability
If switching thresholds are set to accommodate noisy voltage, then the PFC control stability is improved, but the thresholds become non-ideal values
Solution Approach 1:
The patent replaces the direct voltage-threshold comparison mechanism with a frequency-based control mechanism. Instead of comparing the noisy power control pin voltage directly to fixed thresholds, the system uses a timer to detect the switching frequency and an integrator to average it. The PFC control is then based on comparing this averaged frequency signal to thresholds, which are set based on the known relationship between frequency and power. This substitution eliminates the need to accommodate noise in the threshold settings, allowing ideal thresholds to be used while maintaining stability.
Data Source
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AI summary
A method of controlling an AC/DC to converter is disclosed, the converter having a power factor correction stage and a signal (205) indicative of a required power and operating with a switching cycle having a switching frequency being the inverse of a switching period. The method comprises switching on the PFC stage, in response to a signal (215) indicative of an average switching frequency rising above a first threshold. The method further comprises switching off the PFC stage, in response to the signal indicative of an average switching frequency failing below a second threshold. The method may further comprise switching on the PFC stage, in response to a positive step change in the signal indicative of a required power, and switching off the PFC stage, in response to indicative step change in the signal indicative of a required power. A controller and an AC/DC converter operable according to such a method are also disclosed as is an LED lighting system comprising such a controller.