Users quickly notice shimmer and flicker in LED lighting. The strobing effect that is generated by misfiring phase-cut dimmers is unacceptable to end-users and dimmable bulbs that don’t work with a majority of local dimmers will not be successful.
Dimming range – the minimum (and maximum) output light that can be delivered by the lamp when connected to a dimmer – is also an important characteristic for LED light bulbs. At the bottom end of the dimming range an LED bulb will have either turned off, or reached minimum light output. Dimming range is usually expressed as a ratio of maximum to minimum light output. Driver manufacturers tend to look at the output current ratio as an approximation of dimming range.
The typical dimming range requirement for leading-edge TRIAC dimmers is a 10:1 current step; for trailing edge dimmers the figure is above 5:1. To understand what controls dimming range and why the accepted performance for leading-edge and trailing-edge dimmers is different, it is necessary to understand how minimum output current is achieved in a phase-cut dimmer.
Figure 1. The effect of TRIAC dimming angle on LED driver output current
Trailing-edge dimmers use internal logic circuitry to control the dimming angle which requires power. This power is delivered to the driver when the TRIAC is turned-off (not delivering power to the lamp). To ensure this happens, trailing-edge dimmers tend to have higher dimming angle than equivalent leading-edge types.
Typically this was not a problem with incandescent bulbs because their light output changes exponentially with power at low brightness levels (the bulb gives very little light output at low conduction angles) which means low brightness occurs significantly above the limit the power storage requirement imposes.
Figure 2. Maximum and minimum dimming angles for highline dimmers (Source: Power Integrations) – The range of maximum and minimum dimming angles makes wide dimming range challenging
Controlling factors for dimming range
The red trace in figure one shows an LED driver output current that is directly proportional to the conduction angle. The LED load takes a high load current even at relatively low conduction angles, but does not reduce current sufficiently by the time the minimum conduction range for TRIAC dimmer is reached. The high load current delivered using this approach means that TRIACs will see high holding currents, reducing the likelihood of shimmer or flicker.
The designer can elect to increase the dimming-slope of the LED driver to arrive at a lower output current at a higher dimming angle (blue trace in figure 1). This allows the bulb to dim to a lower brightness but risks causing shimmering and/or flickering as the load current drops. In a practical design this means that a significant amount of extra power must be drawn by a bleed circuit to keep the TRIAC from misfiring (this region is shown in red area in figure 1). This reduces driver efficiency making the need for heat sinking or potting materials more likely in the final design.
An alternative approach is to employ an adaptive bleeder circuit such as shown in the driver in figure 3. The bleeder circuit draws more current in deep dimming to compensate for the reduced output current but preventing excess heating at full brightness
Figure 3. Dimmable bulb driver with smart (lossless) bleeder function. The driver also shuts off at very low output current (deep dimming) to reduce the risk of TRIAC dimmer misfiring and causing shimmer (http://led-driver.power.com/sites/default/files/PDFFiles/der409.pdf
Dimming range, like so many of the design considerations for a TRIAC dimmable LED driver, cannot be considered in isolation. A design requirement that pushes for excellent dimmer compatibility and extended dimming range will (with presently available driver circuitry) lead to increased solution cost.