Triac Dimmable LED Power Supply with Damping and Bleeder Circuits
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional power-factor-corrected (PFC) flyback converters experience issues when used with triac dimmers, including current ringing, inconsistent timing, slow startup, and flickering when driving LEDs, due to high input impedance and minimum holding current requirements.
Innovation Solution
Incorporating a damping circuit to suppress ringing, a bleeder circuit to stabilize triac dimmer operation, a fast startup circuit to ensure quick voltage stabilization, a dimming slope control circuit to emulate incandescent lamp dimming, and a dummy load circuit to maintain triac holding current, all of which are integrated into the PFC power supply to address these issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional PFC flyback converter is used with a triac dimmer, then the circuit structure remains simple, but current ringing occurs causing the triac to turn off and leading to LED flicker
Solution Approach 1:
A damping circuit is introduced as an intermediary component between the triac dimmer and the PFC converter. This damping circuit absorbs the harmful current ringing and prevents it from causing triac turn-off, thereby stabilizing triac operation without significantly increasing overall circuit complexity
Solution Approach 2:
A feedback control mechanism is implemented where the controller monitors the input voltage and dimmer state, then adjusts the MOSFET switching timing accordingly. This feedback loop compensates for the phase shift caused by the damping circuit and maintains stable operation across different dimming levels
2Power
If the input capacitance is kept small to maintain input voltage close to rectified sinusoidal wave, then the power factor is improved, but the startup time becomes slow due to insufficient voltage buildup
Solution Approach 1:
A bleeder circuit is activated during the startup phase to pre-charge the input capacitance and build up voltage before the main power conversion begins. This preliminary action ensures that sufficient voltage is available for immediate operation while maintaining the small capacitance value needed for good power factor
Solution Approach 2:
The bleeder circuit operates periodically during startup, charging the input capacitance in controlled intervals until the threshold voltage is reached. This periodic charging approach balances the need for fast startup with the requirement to maintain low input capacitance for high power factor
3Ease of operation
If a triac dimmer chops the load voltage to reduce power, then the dimming function is achieved, but the sharply increasing input voltage causes current ringing that reverses several times during the half cycle
Solution Approach 1:
The damping circuit is designed to convert the harmful current ringing into a controlled energy dissipation mechanism. By placing the damping resistor in parallel with the triac, the harmful ringing currents are redirected through the resistor where they are safely dissipated as heat, preventing triac turn-off and LED flicker
Solution Approach 2:
The damping circuit acts as an intermediary that absorbs the voltage spikes and current ringing generated by the triac chopping action. This intermediary circuit protects the PFC converter from the harmful effects of voltage chopping while allowing the dimming function to operate effectively
4Stability of the object's composition
If the PFC controller regulates output current to maintain constant average current, then the LED current stability is improved, but the narrow bandwidth of the feedback loop makes it insensitive to double-utility-frequency ripple at the LEDs
Solution Approach 1:
The feedback control is segmented into two independent loops: a wide-bandwidth current loop that handles high-frequency ripple and a narrow-bandwidth voltage loop that maintains average power factor correction. This segmentation allows each loop to optimize its bandwidth for its specific function, with the current loop responding quickly to ripple while the voltage loop maintains overall stability
Data Source
AI summary
A power-factor-corrected power supply adapted to supply power to one or more light emitting diodes (LEDs), comprises: a triac dimmer electrically connected between an alternating current source and a bridge rectifier; a damping circuit electrically connected between the alternating current source and the bridge rectifier; a bleeder circuit configured to conduct current between a supply terminal of the bridge rectifier and ground only when a triac in the triac dimmer is not conducting current; a fast startup circuit configured to conduct current between the supply terminal of the bridge rectifier and a voltage supply terminal of a power-factor-corrected controller when the triac dimmer is initially turned on until a supply voltage capacitor coupled to the voltage supply terminal of the power-factor-corrected controller has charged; a dimming slope control circuit configured to reduce a first voltage sensed at a current sensing terminal of the power-factor-corrected controller, such that a reduced amount of current is supplied to the one or more LEDs at a given firing angle, wherein the first voltage is representative of a current flowing through a primary winding of a flyback transformer; and a dummy load circuit provided in parallel with the one or more LEDs to draw a holding current for the triac only after the triac is turned on.