Digitally controlled LED drivers offer better dimming in lamps

- Oct 09, 2018-

Most often the dimming performances of LED-based lamps are not up to the mark. Conventional drivers designed for use in LED replacement lamps face problems in delivering satisfactory dimming performance at an affordable price. Digital control in the driver electronics can solve this problem. It can remove the wasteful bleeder resistors, used for enhancing operation with triac dimmers, and allow flicker-free dimming down to low lighting levels. The use of dimmers in LED lights for domestic lighting can save energy and money and is comfortable and convenient also. This requires smooth, flicker-free dimming, covering a wide range of light levels.

How dimmers function

Standard triac-based, phase-cut dimmers are designed to drive the resistive load of an incandescent bulb. By triac dimmer, dimming is achieved by blocking the leading edge of the applied AC voltage signal using the triac. When the firing pulse is applied, the triac turns on, and remains so for the remainder of the cycle, if the current remains above the specified holding current for the device.
READ ALSO: LED drivers that ensure flicker-free dimming of tunable LED bulbs

The dimmer circuit has an electromagnetic interference (EMI) filter, which encloses inductor and capacitor components that can produce ringing noise to the current waveform. If the inductor is a low-quality component, the ringing can be sufficient to cause the current immediately after turn-on to spike below the triac holding current. This allows the device to turn off, causing visible flickering of the lamp. If a low-cost triac, with a high holding current is used, the dimmer will not be able to sustain dimming levels where the current is below the holding current of the triac.

Advantages of digital control

It allows complex dynamic control schemes that enable better use of the energy required to keep legacy dimmers operating correctly. Apart from eliminating flicker or dropout when used with a conventional triac dimmer, a suitable LED-driver circuit should also manage AC-cycle inrush current to avoid momentary overloads, minimize audible noise that results from interactions between the line cycle and internal magnetic components, and comply with regulatory standards for power factor and electrical noise (EMI). They should also be compatible with the largest possible range of dimmer types.

This technology provides a much better solution compared to conventional analog circuitry and resistive bleeders, which enables manufacturers to meet high expectations.

Dimmers with LED lights

The problems faced during the operation of dimmers at low load currents are worsened when the load is an LED replacement bulb. LED bulb is not totally resistive. Its impedance is reactive, which can?t allow the current from rising sufficiently above the triac holding current threshold to allow the device to remain on after the firing pulse is removed. Low levels of dimming is quite difficult to achieve, as the LED generally consumes less power in comparison to an incandescent bulb. Therefore, the current drawn through the triac is very low at the phase angles needed for maximum dimming. The current can drop below the minimum holding current of the triac, which causes the light to flicker or stop working suddenly when dimming is adjusted for very low light levels.

Dimmers need much greater levels of dimming than the casual observer can appreciate. Since our eyes compensates for low light levels by dilating the iris, the bonding between the electronic dimmer setting and the apparent dimming effect is not linear. If one adjusts the dimmer to reduce the light to 10% of its maximum measured level, one?s eyes will perceive a reduction only to around 30%. To get a perceived dimming level of 10%, the dimmer must reduce the measured light level to only 1%.

A bleeder circuit can be used to maintain proper current flow in the triac at lower dimming levels. This could be a passive circuit using a resistor or an active bleeder using power transistors to block current in the bleeder resistors when not needed. A passive bleeder has two disadvantages. As current continuously passes through the resistor, the efficiency advantage achieved through using LED technology is eroded. Thermal management is very important for LED lamps that must operate following the existing industry-standard form factors. The LED emitters are able to operate for life spans of 25,000 to 50,000 hours at high temperatures.