In offline LED dimming applications, phase cut dimmers are one of the most popular dimmers in the market. Two types of phase-cut dimmers exist: "leading edge" or "trailing edge" dimmers. All TRIAC (Triode for Alternating Current) based dimmers are leading edge dimmers. A TRIAC dimmer operates by delaying the outset of each half-cycle of alternating current (AC) power, which is often called "phase control." By delaying the outset of each half-cycle, the amount of power delivered to the load (e.g., the lamp) is dropped, causing a dimming effect in the light output by the lamp. Triac dimming circuits perform extremely well when employed to dim incandescent bulbs since these lamps draw more than the minimum holding current from the AC power source to allow reliable and consistent operation of a TRIAC.
However, when operated with a TRIAC dimmer an LED luminaire does not appear to dim smoothly and the variation in perceived light intensity doesn't have intuitive relation to the dimmer's knob position. Flickering in LED lamps may occur due to the fact these devices are generally driven by LED drivers utilizing regulated power supplies that supply regulated current and voltage to the LED lamps from AC utility lines. TRIAC dimmers necessitate a certain minimum holding currents to operate. However, the comparably low currents drawn by LEDs from efficient power supplies will most likely not match the minimum load necessary to keep the TRIAC switches conducting for reliable operation. Often there are capacitors in the LED load that have to be charged which may trigger transient but large spikes in current generally known as "repetitive peak current." As a result, the TRIAC may trigger inconsistently. Eventually, the effect of such a current on the circuit relies not just on the magnitude of the peak inrush current but in addition on the duration. These repetitive peak currents may destroy circuit components in the dimmer or, in some circumstances cause the LEDs to flicker or flash. They are furthermore a significant cause of audible buzzing of lamps and dimmers. Increasing the power consumption of LED lights is one strategy which has been utilized to adapt LED lights to a range of LED dimmers. Nevertheless, increasing the power consumption can result in thermal issues in the LED lamp and the associated circuitry and offsets many of the benefits that drive the switch to energy efficient LED lamps.
Unless the regulated power supplies that drive the LED lights are made to recognize and react to the voltage signals from TRIAC dimmer in a suitable way, the TRIAC dimming circuits are prone to produce undesirable results, including limited dimming range, flickering, blinking, and/or color shifting in the LED lights. For an LED luminaire to respond accurately to a phase-control dimmer, it is necessary to incorporate several functional blocks into the driver electronics. For instance, a sensor is usually included for monitoring the AC input waveform before the power-factor-correction (PFC) stage and outputting an output signal proportional to the amount of phase cut. But this may increase the product cost and come with design tradeoffs for space or complexity contributing to different performance from different lamps.
