Technology sharing: How to design dimmable LED lamps

　　Over the years, manufacturers have continued to bring LED lamps to the market with the ultimate goal of replacing incandescent and compact fluorescent lamps (CFLs). These bulb designs have evolved from very simple non-dimmable solutions, to advanced but expensive dimmable solutions, to more cost-effective dimmable solutions.

　　Many LED lights are claimed to be dimmable, but in reality, the performance of many LED lights is not very ideal and varies depending on the dimmer used and the circuit load. Sometimes, when LED lights are installed in a room with a dimmer, the LED lights will flicker and the brightness cannot be adjusted evenly.

　　These drawbacks are due to the fact that most dimmers in use in the United States today are based on two-wire leading-edge phase-cut circuits of bidirectional thyristors (TRIACs), which were developed in the 1960s for use with resistive incandescent lamps. A TRIAC is a bidirectional semiconductor power switch that is triggered by pulses generated by a variable timing circuit and remains on as long as the conducting current is above the holding current. There are many different types of dimmer circuits, using devices with different characteristics as well as different control circuits and filtering components.

　　The LED lamp driver circuit converts the AC input power into a low voltage DC power supply and maintains a stable current to drive the high brightness LED load to obtain a constant light output. To adjust the basic LED driver circuit through a bidirectional thyristor-based dimmer, additional components must be added to achieve stable dimmer operation and adjust the output current according to the dimmer phase angle.

　　Since dimmers vary widely, the performance of the connected LED dimming circuits will also vary. This issue is further complicated by the fact that there are no clear standards for classifying the performance of LED bulbs with dimmers. At most, some bulb manufacturers provide a list of dimmers that they believe are compatible with their products.

　　With the support of the U.S. Department of Energy (DoE), the National Electrical Manufacturers Association (NEMA) is working to develop dimming standards for LED lamps driven by phase-cut dimmers, including test procedures and indicators to determine whether acceptable performance is achieved. It is hoped that this standard will eventually help clear the market of products that claim to be dimmable but whose performance is far from the soft, stable incandescent dimming that end users expect.

　　Most LED bulbs use driver circuits that include buck, buck-boost, or flyback converters. In each case, the basic circuit can be modified to achieve acceptable dimming performance without increasing the cost and complexity of the device. This can improve the performance of dimmable drivers and meet the cost savings requirements of the consumer lighting market.

　　The compatibility issue lies in how the TRIAC dimmer circuit interacts with the LED driver input circuit.

　　The single-stage LED driver example circuit (Figure 2) replaces the resistive load that represented the incandescent lamp in Figure 1. Although this circuit simulates a resistive load due to its high power factor during stable operation, it also includes the capacitors necessary for EMI filtering on the front end. In addition, the LED bulb consumes less than 25% of the power of an equivalent incandescent lamp. As a result, the dimmer is primarily subject to a capacitive load during the AC line half-cycle before the TRIAC is triggered.

　　Figure 1: Typical dimmer schematic

　　Figure 2: Basic LED driver circuit block diagram

　　The bidirectional trigger circuit shown in Figure 1 also requires a resistive path to the neutral point if it is to operate as designed. If the load is capacitive, the circuit will not operate properly and will cause unstable triggering during the cycle transition, which is manifested as flickering light at the output. EMI filters in dimmers and LED drivers can also cause ringing due to the high dv/dt when the TRIAC starts.

　　When the oscillation reaches a certain amplitude, the current drops below the "holding current", turning the TRIAC off and unable to keep the TRIAC on until the next line zero crossing. This condition usually causes the trigger circuit to re-trigger the TRIAC, causing it to turn on and off multiple times in a single line half-cycle. In addition to stressing the components and possibly destroying the dimmer or LED driver, this will cause severe flicker and unpleasant noise.

　　Assuming that replacing the dimmer with a dimmer suitable for LED lamps is not an ideal solution, the above problem can be solved by modifying the LED driver so that it can be used with the standard dimmer.

　　Figure 3: Schematic diagram of dimmable LED driver

　　The example circuit (Figure 3) is a single-stage LED flyback converter, but the same techniques can be used for a buck-boost or adaptive buck converter. First, the input capacitor must be kept to a minimum when designing the input filter, which also helps achieve the best power factor. The next step is to introduce the active attenuator and passive bleeder circuit. The attenuator circuit limits the inrush current when the TRIAC is triggered, thereby greatly suppressing the ringing so that the TRIAC remains in the on state. After a short delay, the attenuator resistor is bypassed by a small MOSFET to prevent power loss during the remaining on period. To minimize the cost of low-power drivers, the bypass MOSFET and its associated drive circuit can be omitted, but this will result in heat dissipation in the resistor and the associated efficiency loss.

　　A passive bleeder circuit can be used in place of the active bleeder used in some dimming solutions. The series RC network conducts current from the trigger point long enough for the switching converter to start drawing current, which helps ensure that the current does not drop below the holding current during this period. A flyback or buck-boost converter operating with a constant on-time acts as the main resistive load to the DC bus to keep the dimmer TRIAC on until the next line zero crossing. The converter needs to draw enough current to stay above the TRIAC holding current. A single-stage PFC flyback or buck-boost converter can usually achieve this.

　　The circuit described here uses the IRS2983 controller IC, which operates in voltage mode. The DC voltage level on the COMP input determines the on-time of the switching cycle. Because the controller IC is often used with primary-side regulation to maintain constant output power, a Zener diode must be added to this input to clamp the COMP voltage. This sets a limit on the maximum on-time, so that when the DC bus voltage drops during dimming, the on-time cannot increase to compensate.

　　The result is that as the dimmer setting is lowered and the DC bus voltage drops, the output current also decreases. This allows the light to be dimmed to less than 20% by adjusting the dimmer control without the need for more complex circuitry to detect the dimmer phase angle or adjust the output. At the same time, the controller VCC supply must be released during the dimming off period to ensure that the IC operates only during the required period. A high-voltage diode is used to connect VCC to the DC bus for this purpose.

　　Active attenuators and passive bleeder circuits can also be used with a buck converter, but the results depend on the LED voltage. Since the converter cannot draw current when the line voltage is below the output voltage, the phase dimming operating range will be limited. For this reason, the LED voltage is best kept low, but not too low, otherwise the circuit will become ineffective and an oversized inductor will be required. For a 120VAC system to maintain a reasonable regulation range, the LED voltage is best between 20V and 40V. CCM buck LED controller ICs (such as the IRS2980) can maintain average current regulation of the LED lamp while the unsmoothed bus voltage is always higher than the total LED output voltage.

　　These simple tips, when used with the LED converter described, enable smooth, flicker-free dimming from most TRIAC-based dimmers.