Dimming techniques for LED drivers

Resources International Rectifier, www.irf.com

The quest for energy efficiency has led manufacturers to investigate ways of dimming all kinds of lighting technologies, including those that usually can’t be dimmed. Consider, for example, fluorescent lamps. With relatively expensive dimmable electronic ballasts, fluorescent lamps can be dimmed to below 5% of maximum light output. But even with electronic ballasts, HID (high-intensity discharge) lamps cannot be dimmed to much more than half of their maximum light output. Dropping past that point risks experiencing noticeable color change and instability in their plasma arc.

To further complicate matters, most dimmable fluorescent and all HID systems are incompatible with standard triac-based phase dimmers. Instead, they use specialized dimming controllers, often demanding additional analog or digital dimming control cables.

Fluorescent and HID lamps are both types of arc discharge lamp. One reason they are so hard to dim is that the impedance of plasma arcs is non linear and varies considerably depending on current and temperature. In addition, there are operating points at which lamp impedance changes rapidly in response to small changes in arc current. This forces dimming circuitry to include a closed-loop current regulating system able to respond rapidly to such changes.

In contrast, it is much more straightforward to dim LEDs because of their makeup. LEDs consist of a solid state p-n junction with a fairly constant forward voltage drop. This constitutes a stable load that can be driven by a constant dc current source.

Off-line LED drivers are comprised of constant-current regulating switching power supplies generally equipped with dc outputs. LEDs, unlike discharge lamps, need no high-voltage ignition. So LED dimming can use pulse width modulation (PWM), in which the output current is switched on and off at a constant frequency with a variable duty cycle. This action adjusts the average current, which is proportional to the light output.

The PWM dimming frequency must be above 120 Hz to comply with Energy Star requirements by avoiding visible flicker. Alternatively, LEDs can be dimmed by reducing the dc current. However, this technique causes some white LEDs to change color and is more difficult to control at low dimming levels.

It is worth noting that the operating life of LED light sources depends on the operating temperature and current that the individual LED die see. Dimming reduces both of these parameters and thus potentially extends LED operating life.

Lumen maintenance for LEDs is specified by the L70 parameter, which indicates the average hours of operation until the light output deteriorates to 70% of its initial quantity. Either dimming technique described above will extend the L70 parameter by operating the LED at reduced output. One reason dimming capability is important for LED drivers is that the US Dept. of Energy has mandated such capability for any lamp hoping to gain an Energy Star rating.

Some early LED bulb replacement products are not dimmable. But legislation eliminating incandescent lights make it inevitable that dimmable LED products will eventually dominate the market.

There are several alternative approaches to dimming LEDs that apply in different segments of the market. LED-based replacements for incandescent or CFL lamps should be dimmable by standard wall dimmers. These are widely used and constitute the vast majority of all household dimmers. Wall dimmers use an extremely basic and cheap triac-based circuit originally designed to handle purely resistive incandescent light bulbs. (CFLs are capacitive, not resistive. Because they draw relatively little current from the ac line, they are fundamentally incompatible with triac-based dimmers.)

The triac is the switching element in the accompanying dimmer circuit. It fires at a certain point in the ac line cycle which can be adjusted by a potentiometer, allowing current to flow until the end of the cycle. The red waveform shows the ac line voltage at the dimmer input. The blue shows the phase cut voltage waveform coming out of the dimmer to the lamp.

The firing point of the triac determines the period of the ac line cycle over which the lamp gets current. In an incandescent lamp this would directly control the light level. But LEDs are driven by an ac-to-dc switching power supply, so dimming does not work in the same way. It is important to understand that the triac is turned on by a pulse and will continue to conduct until the current drops to a low level called the holding current, at which point it will switch off until fired again.

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