Typical LED dimmable circuit module design

　　Many LED lamps are advertised as dimmable, but in reality, the performance of many LED lamps is not very ideal and varies depending on the dimmer used and the circuit load. Sometimes, when LED lamps are installed in a room with a dimmer, the LED lamps will flicker and the brightness cannot be adjusted evenly. The driver circuit of the LED lamp converts the AC input power to a low-voltage DC power supply and maintains a stable current to drive the high-brightness LED load for constant light output. In order to adjust the basic LED driver circuit with a bidirectional thyristor-based dimmer, some additional components must be added to achieve stable dimmer operation and adjust the output current according to the dimmer phase angle.

　　The single-stage LED driver example circuit (Figure 2) replaces the resistive load that represented the incandescent lamp in Figure 1. Although this circuit simulates a resistive load due to its high power factor during stable operation, it also includes the capacitors necessary for EMI filtering on the front end. In addition, the LED bulb consumes less than 25% of the power of an equivalent incandescent lamp. As a result, the dimmer is primarily subject to a capacitive load during the AC line half-cycle before the TRIAC is triggered.

　　The bidirectional trigger circuit shown in Figure 1 also requires a resistive path to neutral to operate as designed. If the load is capacitive, the circuit will not operate properly and will result in unstable triggering during the cycle transitions, which is manifested as flickering of the light output. EMI filters in dimmers and LED drivers can also cause ringing oscillations due to the high dv/dt at the start-up of the TRIAC. When the oscillation reaches a certain level, the current drops below the "holding current", causing the TRIAC to turn off and not be able to keep the TRIAC on until the next line zero crossing. This condition usually causes the trigger circuit to re-trigger the TRIAC, causing it to turn on and off multiple times in a single line half-cycle. In addition to stressing the components and possibly destroying the dimmer or LED driver, this will cause severe flickering and unpleasant noise.