Switch-mode Power Supply Dimming Control Circuit
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Solution Overview
Problem
Switch-mode power supplies struggle to achieve accurate current regulation and high power factor for LED lighting applications, especially during dimming, due to limitations in minimum-off time control and pulse skipping modes which result in flicker and increased harmonic distortion.
Innovation Solution
A control circuit for switch-mode power supplies that adjusts the minimum-off time period based on a compensation signal, using a combination of error amplifiers, multipliers, flip-flops, and minimum-off time control circuits to provide a control signal for the switch, allowing for inverse adjustment of the minimum-off time period with dimming signal levels, thereby maintaining high power factor and accurate current regulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If pulse skipping mode is used for dimming, then power factor is improved, but current regulation accuracy deteriorates and flicker increases
Solution Approach 1:
The patent implements dynamic adjustment of minimum-off time based on operating conditions. The minimum-off time is extended when the switch is about to enter pulse skipping mode, and shortened when operating in continuous conduction mode. This dynamic parameter adjustment maintains current regulation accuracy across different dimming levels while preserving the power factor benefits of pulse skipping mode through the compensation signal mechanism.
2Device complexity
If minimum-off time is kept constant, then circuit simplicity is maintained, but current regulation accuracy deteriorates at low dimming levels
Solution Approach 1:
The patent employs preliminary action by predicting when the minimum-off time needs adjustment before the actual dimming issue occurs. The compensation signal is generated in advance based on the dimming signal and feedback signal, and the minimum-off time is proactively extended or shortened based on this compensation signal. This prevents current regulation degradation before it happens, maintaining accuracy at low dimming levels without requiring complex real-time intervention circuits.
3Manufacturing precision
If minimum-off time is extended to improve current regulation, then current regulation accuracy is improved, but switching losses increase
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the minimum-off time parameter based on operating conditions rather than keeping it fixed. The minimum-off time is extended only when necessary (when compensation signal indicates approaching pulse skipping mode) and shortened when operating in continuous conduction mode. This selective parameter adjustment maintains current regulation accuracy while minimizing switching losses by avoiding unnecessary extensions of minimum-off time during normal operation.
4Use of energy by stationary object
If pulse skipping mode is used at low dimming levels, then power factor is maintained, but harmonic distortion increases
Solution Approach 1:
The patent implements feedback control by continuously monitoring the feedback signal and comparing it with the dimming signal to generate a compensation signal. This compensation signal is fed back to adjust the minimum-off time control, preventing the circuit from entering pulse skipping mode that would cause harmonic distortion. The feedback mechanism maintains power factor by keeping the circuit in continuous conduction mode through proactive minimum-off time adjustment, thereby eliminating the harmful harmonic distortion associated with pulse skipping operation.
Data Source
AI summary
A switch-mode power supply and associated control circuit for a light-emitting device are disclosed. The switch-mode power supply having a first switch which is turned ON or OFF by a control signal. The control circuit having an error amplifier, a minimum-off time control circuit and a logic circuit, wherein the error amplifier is configured to provide a compensation signal based on a dimming signal and a feedback signal. The minimum-off time control circuit is configured to provide a minimum-off time control signal to adjust a minimum-off time period of the first switch based on the compensation signal. The logic circuit is configured to provide the control signal based on the minimum-off time control signal.