Controlling LED lighting with triacs and quadrilaterals

Light-emitting diodes ( LEDs ) are quickly becoming the most popular lighting choice. The industry predicts that due to the huge future growth in the industrial and commercial lighting field, the market is expected to continue to expand at a rate of 20% per year from 2011 to 2016. Incandescent lamps are often replaced with LEDs in response to the US government's directive on energy conservation, as incandescent lamps are outdated and LEDs have a long service life (typically 25,000 hours) and are suitable for many different sockets and shapes.

　　However, LED lighting control presents some issues that incandescent lamps do not encounter. For example, since the current of LED load is much less, the normal type of triac switching element will face challenges in terms of latching current and holding current characteristics. The triac element is the heart of the AC dimming controller. The triac dimmer is usually specially developed according to the characteristics of incandescent lamp load. It has a higher rated current and surge current resistance under steady-state conditions and at startup, and a very high terminal surge current resistance when the filament breaks.

　　Because they are diodes, LEDs have much lower steady-state currents than incandescent bulbs, and their initial on-current is much higher than the AC line voltage per half cycle for a few microseconds. Therefore, a current spike can be seen at the beginning of each AC half cycle. Figure 1 shows a typical current spike. Typically, the AC bulb that needs to be replaced has a current spike of 6A to 8A, and the subsequent current after the spike steady state is less than 100mA.

　　LED lamps for home lighting fixtures can run up to 7.5W (450 lumens for an A19 bulb) or perhaps more, as a chandelier typically has four to ten spotlights. In contrast, a string of 50 Christmas lights draws only 4.8W of power. An LED diffuser designed for a recessed ceiling fixture can replace a typical filament fixture that produces 750 lumens at only 13W (BR30), while older filament fixtures typically consume 65W of power. When using the new Q6008LH1 LED or Q6012LH1LED series triacs, designing an AC circuit to control the LED light output is simple because only a few components are needed, all that is needed is an ignition/trigger capacitor, a potentiometer, and a voltage-on trigger device.

　　By using two reverse parallel sensitive gate silicon controlled rectifiers (SCR) S4X8ES1 as the voltage-on trigger device, the control circuit can produce a variety of different light level outputs.

　　Alternatively, low hysteresis control can be achieved by using two anti-parallel sensitive gate silicon controlled rectifiers (SCRs) as the trigger device, since the two SCRs form a complete anti-shoot-through trigger. Figure 2 shows a control circuit for the above situation, which may be well suited for LED diffuser lamps (such as BR30 LED lamps ), which can be dimmed to produce a low-level light output or turned to nearly 180° for full light output.

　　This circuit allows the lamp to turn on nearly 180° per AC half cycle, and in addition, the RC timed turn-on may be delayed to a small conduction angle per half cycle with very low light output. The Q60xxLH1LED triac series features low holding current and latching current characteristics to keep the triac switching element at very low current levels.

　　The gates of two anti-parallel photosensitive gate SCRs (S4X8ES1) are tied together to create a very low voltage trigger device with full blown breakdown voltage and produce very low hysteresis. This allows the potentiometer to be set to have a low conduction angle for instant turn-on when the line is switched (for more information and to understand hysteresis, see Littelfuse application note AN1003: Phase Control Using SCRs).

　　The circuit schematic of Figure 3 improves upon the old phase control/dimming circuit, which had only slight hysteresis. As shown, with the exception of the C1 ignition capacitor around the steering diode, the hysteresis can be completely eliminated.

　　If a wide control range (from full brightness to very dim) and low hysteresis are not critical to the application, a simple variable light control can be designed using the new Littelfuse Q6008LTH1LED or Q6012LTH1LED series of QUADRAC devices (QUADRAC devices are a special type of thyristor that combines a single bidirectional switching diode and a triac in one package). The circuit shown in Figure 4 can further reduce component count by combining the trigger and triac in a single TO-220 package. This control circuit can be used with a lower full on voltage because it has a higher VBO trigger device, providing a dimming function that can adjust from 175° to 90° per AC half cycle.

　　The potentiometer is 250kΩ with a built-in fixed-end 3kΩ minimum resistor. The model of the four-terminal bidirectional thyristor switch element (QUADRAC device) is QxxxxLTH1 LED , which has a more sensitive thyristor chip (low gate current and holding current characteristics). The minimum LED load of RL is 10W, and the minimum LED load of the circuit is also 10W, which is the trigger voltage of the built-in bidirectional switch diode module.

The LED lamp load 　　in an AC circuit may be very simple, as shown in Figure 5, or there may be additional components, such as filter capacitors. The added components will determine whether the LED lamp load is dimmable. The simpler the LED lamp load, the greater the possibility of dimmability.

　　Because the filter capacitor may enhance the minimum DC current, the thyristor latch can operate normally even if there is no change below the minimum thyristor holding current.