Techniques for enabling dimmable LED bulbs
Many LED lamps are advertised as dimmable, however in reality the performance of many of them is unimpressive with varying results depending on the dimmer used and loading of the circuit. In some cases the LED lamp has been taken out of its box and installed in a room with a dimmer switch only to discover flickering and uneven adjustment of the light level.
These deficiencies result from the fact that the vast majority of dimmers installed in the US are based on the triac-based two-wire leading edge phase cut circuit developed in the 1960s intended for use with resistive incandescent lamps. The triac, a bi-directional semiconductor power switch, is triggered by a pulse generated by a variable timing circuit and maintains conduction while its conducted current remains above a certain level termed as the holding current. There are many varieties of the dimmer circuit using devices with different characteristics combined with variations in the control circuitry and filtering components.
Figure 1: A typical dimmer schematic.
The LED lamps driver electronics converts AC input power to low voltage DC power and maintains a regulated current to drive the high-brightness LED load for constant light output. A basic LED driver circuit cannot be dimmed from a triac-based dimmer without some additional components being added to enable stable dimmer operation and then to adjust the output current based on the dimmer phase angle.
Since dimmers vary considerably it is inevitable that the performance of connected LED dimming circuits also varies. The issue becomes more confusing as there are currently no clearly defined standards for classifying performance of LED bulbs with dimmers. At most some bulb manufacturers provide lists of dimmers that they claim to be compatible with their product.
The National Electrical Manufacturers Association (NEMA) supported by the US Department of Energy (DoE) is now developing dimming standards for LED lights driven from phase cut dimmers, which will cover test procedures and metrics for determining whether acceptable performance has been met. Hopefully this much needed initiative will eventually remove products from the shelves that claim to be dimmable when performance is far below expectations of end users accustomed to smooth and stable incandescent dimming.
Driver electronics within most LED bulbs incorporate Buck, Buck-Boost or Flyback converters. In each case the basic circuit can be modified to make it dimmable with acceptable performance met without adding greatly to the component cost and complexity. The goal here is to provide dimmable drivers with good performance able to meet the aggressive cost constraints imposed by the consumer lighting market.
The problem of compatibility lies in how the triac dimmer circuit interacts with the input circuitry of the LED driver.
Figure 2: Basic LED driver circuit blocks.
The single stage LED driver example circuit above replaces the resistive load representing an incandescent lamp shown in figure 1. Although this circuit mimics a resistive load due to its high power factor during steady state operation, the front end includes capacitors necessary for EMI filtering. The LED bulb also consumes less than 25% of the power of an equivalent incandescent. The result is that the dimmer sees a mainly capacitive load during the portion of the AC line half cycle before the triac fires.
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