Flicker Free Dimming LED Driver IC Scheme

Today, LED lighting has become a mainstream technology. LED flashlights, traffic lights and lamps are everywhere. Countries are promoting the replacement of incandescent and fluorescent lamps in residential, commercial and industrial applications powered by main power supply with LED lamps. After switching to energy-efficient LED lighting, the energy savings will be amazing. In China alone, according to government estimates, if a third of the lighting market turns to LED products, they will save 100 million kwh of power consumption and 29 million tons of carbon dioxide emissions each year. However, there is still one obstacle to overcome, that is dimming.

Incandescent lamps can easily realize dimming by using simple and low-cost frontier thyristor dimmer. Therefore, this dimmer can be seen everywhere. If solid-state lighting replacement lamp is to be truly successful, it must be able to realize dimming using existing controllers and circuits.

Incandescent bulbs are ideal for dimming. Ironically, it is their low efficiency and the resulting high input current that are the main factors for the dimmer to work well. The thermal inertia of the filament in an incandescent bulb also helps to mask any instability or oscillation caused by the dimmer. In the process of trying to dimming LED lamps, a lot of problems are encountered, which often lead to flicker and other unexpected situations. To find out the reason, it is necessary to understand the working principle of thyristor dimmer, LED lamp technology and the relationship between them.

Figure 1 shows a typical leading-edge thyristor dimmer and its voltage and current waveforms.

Figure 1. Front thyristor dimmer

Potentiometer R2 adjusts the phase angle of triac. When vc2 exceeds the breakdown voltage of diac, the triac will turn on at the leading edge of each AC voltage. When the thyristor current drops below its holding current (IH), the thyristor turns off and cannot be turned on again until C2 is recharged in the next half cycle. The voltage and current in the bulb filament are closely related to the phase angle of the dimming signal, which varies from 0 degrees (close to 0 degrees) to 180 degrees.

LED lamps used to replace standard incandescent lamps usually contain an LED array to ensure uniform illumination. These LEDs are connected together in series. The brightness of each LED is determined by its current. The forward voltage drop of the LED is about 3.4 V, usually between 2.8 V and 4.2 v. The LED lamp string shall be driven by constant current power supply, and the current must be strictly controlled to ensure high matching between adjacent LED lamps.

In order to realize dimmable LED lamp, its power supply must be able to analyze the variable phase angle output of thyristor controller, so as to adjust the constant current flowing to LED unidirectionally. It is very difficult to do this while maintaining the normal operation of the dimmer, which often leads to poor performance. The problem can be slow start-up, flickering, uneven illumination, or flickering when adjusting the brightness. In addition, there are some problems, such as inconsistency between components and unnecessary audio noise from LED lamps. These negative conditions are usually caused by factors such as false triggering or premature shutdown of thyristor and improper LED current control. The root cause of false triggering is the current oscillation when the thyristor is turned on. Figure 2 graphically illustrates this impact.

Figure 2. Thyristor current and voltage oscillation in LED lamp power input stage

When the thyristor is turned on, the AC mains voltage is applied to the LC input filter of the LED lamp power supply almost at the same time. The voltage step applied to the inductor will cause oscillation. If the dimmer current is lower than the thyristor current during oscillation, the thyristor will stop conducting. The thyristor trigger circuit charges and then turns on the dimmer again. This irregular multiple thyristor restart can make the LED lamp produce unnecessary audio noise and flicker. Designing simpler EMI filters helps to reduce such unnecessary oscillations. To achieve successful dimming, the inductance and capacitance of the input EMI filter must also be as small as possible.

The worst condition for oscillation is a 90 degree phase angle (at this time, the input voltage reaches the sine wave peak and is suddenly applied to the input of the LED lamp) and a high input voltage (at this time, the forward current of the dimmer reaches the lowest level). Premature shutdown occurs when deep dimming is required (e.g. the phase angle is close to 180 degrees) and the input voltage is low. To reliably lower the luminosity, the thyristor must be monotonically turned on and stay at the point where the AC voltage is almost reduced to zero volts. For thyristors, the holding current required to maintain conduction is usually between 8 Ma and 40 ma. Incandescent lamps are relatively easy to maintain this current, but for LED lamps whose power consumption is only 10% of that of equivalent incandescent lamps, the current can be reduced below the thyristor maintenance current, resulting in premature shutdown of thyristors. This will cause flickering and / or limit the dimmable range.