Voltimum - Dimming LEDs – the issues and the technologies

For example, an LED driver directly connected to a line-voltage incandescent dimmer may not receive sufficient power to operate at lower dimming levels. In addition, it could be damaged by current spikes.

Voltimum managing editor James Hunt takes a look at LED dimming issues and technologies:

Domestic incandescent lamp dimmers regulate the amount of power to the lamp filament using phase control techniques that incandescent lamps could handle reliably, while users could not see the inherent flicker because it was too fast. However, these electrical characteristics matter greatly when used with electronic devices such as compact fluorescent lamps (CFLs) and LEDs.

Certain LED lighting products can be used with such line-voltage dimmers, but both dimmer and LED driver electronics have to be carefully matched. This brings another difficulty in that because already installed dimmers often vary widely, it cannot be guaranteed that an LED fixture will automatically work with all dimmers.

More modern LED dimmers incorporate low-voltage (LV) controls connected separately to the electronic driver to which full AC power is supplied. This arrangement allows the electronic controls to operate at all times, in turn enabling LEDs to be uniformly dimmed to around 5% or lower. One drawback is that that this arrangement may need extra LV wiring for retrofit applications, which can increase the installation cost.

LED dimmer issues:

Issues that have to be addressed with LED dimming include colour temperature and efficiency changes, plus flicker.

• Colour temperature - When dimming incandescent lamps, the filament temperature is reduced, causing a colour shift from white through yellow to orange/red. Dimming a CFL has a generally similar effect, but much less pronounced. Where white LEDs are concerned, the light does not shift to red when dimmed - indeed, some can even appear more blue. However, depending on how the LEDs concerned have been designed, as white light can be produced by mixing red, green, and blue LEDs, colour temperature compensation is possible.
• Efficiency - A measure of lighting efficiency is luminous efficacy, which is significantly reduced for dimmed incandescent lamps. CFLs are much less affected by dimming, but LED luminaire efficacy can decrease with dimming because of reduced driver efficiency at low dimming levels. Looking specifically at modern LEDs however, unlike conventional light source technologies, LED performance improves when dimmed in terms of efficacy, lumen maintenance and longevity. For example, a 525mA LED dimmed to 75mA provides a third of the light output but uses only about a tenth of the energy.
• Flicker - Modern LED drivers typically use pulse width modulation (PWM) techniques to regulate power to the LEDs, which is a form of fast switching to achieve perceived dimming. To avoid perceptible flicker, the driver output frequency should be 120Hz minimum. One problem has been the finite resolution of the digital electronics sometimes causing apparent LED flicker at the lowest settings when the dimmer control is moved, but modern 12-bit or greater resolution electronic drivers get round this problem.

LED dimming techniques:

Lighting dimming is essentially the reduction of the voltage into, and luminance out of, a light source.

Commercial LED drivers commonly use one of two methods to dim mixed-colour RGB (red, green and blue) and phosphor-converted high-power white LEDs. These are continuous current reduction (CCR) - also called 'analogue', which decreases the forward current supplied to the LED, which in turn proportionally lowers the light output level, and pulse-width modulation (PWM) - as mentioned above - which changes the duty cycle very rapidly at a frequency high enough to eliminate any perception of flickering. Both techniques control the time-averaged LED drive current through the LED or LED cluster/string, which is proportional to the light output, but the two have their own advantages and disadvantages.

Note that LEDs can also be dimmed using 1-10V, DMX and DALI.

PWM dimming advantages:

• The important advantages of using PWM dimming is that because a PWM driver will only run the LEDs at the rated current level or zero, preventing colour and efficiency characteristics from changing as the load is dimmed, the same colour temperature is maintained throughout the dimming range.
• PWM provides a wide dimming range.
• PWM-based drivers can provide a very precise output, since the LEDs are always on at the same current level. Changing the operating current in a linear fashion should not result in a light output linear change. Therefore, the relationship between light output and duty cycle is linear.

PWM dimming disadvantages:

• To avoid any risk of perceivable flicker, the PWM operating frequency must be high; however, higher-frequency power supplies are typically more complex and expensive, especially when low light levels are desired.
• PWM operation can easily produce electromagnetic interference (EMI), which may not be suitable for a number of applications.
• Another problem is that PWM drivers can suffer performance loss if mounted remotely from the light source, because the capacitance and inductance of lengthy cable runs interfere with the fast rise and fall times needed for precision LED lighting control.
• Therefore, PWM dimming is typically suitable for LED lighting that must be dimmed below 40% while still maintaining a consistent colour temperature. It is also suitable for colour mixing applications.

CCR dimming advantages:

• CCR dimming is simple, requires the least control overhead, and generally is more efficient than PWM dimming because of the lower forward voltage of the LEDs at lower drive currents.
• A Class 2 power supply using the CCR technique provides a greater voltage than one using the PWM method.
• CCR dimming does not require complex and expensive higher-frequency power supplies, as PWM does.
• CCR dimming does not produce potentially damaging EMI.
• CCR dimmers can be mounted remotely from the LED light source without significant performance loss.
• With one analogue LED dimming method, the current is controlled via amplifier gain, and feedback allows current and thermal foldback for LED protection.

CCR dimming disadvantages:

• CCR dimming needs the analogue voltage to be generated using a separate voltage reference, typically using a digital-to-analogue converter or the output of an RC filter on a square wave input signal. Both can raise the price.
• The colour temperature may vary with the LED current, which for some applications - such as in retail outlets - cannot be allowed.

Therefore, CCR dimming is suited to applications in which there are lengthy cable runs between driver and the light engine. It is also suitable for applications where high performance dimming is essential, and where there are EMI restrictions. CCR is also suitable for lighting systems needing a higher rated output voltage than the PWM voltage level.

For the future:

As techniques become more fully developed and widely used, leading to lower first costs, it is likely that fully integrated LED dimming controls will become more widely used in new build homes. However, developments are also aimed at developing LED lighting products that can be used with existing dimmers to allow for more retrofitting to homes.