Lighting designs with flicker-free LED dimming
LED lighting is becoming increasingly prominent in both residential and commercial lighting markets. To enhance the success and adoption of solid-state lighting in retrofit lamps, it is essential for LED lamps to support dimming with existing controllers and wiring. However, efforts to dim LED lamps have faced several challenges, leading to issues such as flickering and other unwanted behaviors. This article discusses the typical TRIAC dimmer, the challenges associated with its use in LED lighting, and two innovative power management solutions that address these problems while adhering to new energy, power factor, THD, and EMI specifications. Incandescent-bulb-replacement LEDs consist of an LED array arranged in a series string to ensure an even distribution of light. The LED string must be powered by a constant current supply that is precisely regulated to maintain the desired luminous flux. The leading-edge TRIAC dimmer is a commonly used lighting dimmer today. For an LED lamp to be dimmable, its power supply must interpret the variable phase angle output from the TRIAC controller to adjust the constant current drive smoothly. Achieving this while maintaining proper dimmer functionality can lead to flickering, audible noise, and blinking during light level adjustments. These issues typically arise from false triggering or premature shutdown of the TRIAC, as well as inadequate control of the LED current. For reliable dimming at low levels, the TRIAC must remain conductive until the AC voltage nearly reaches zero; the holding current required is generally between 8 to 40 mA. Since LED lamps consume less than 10% of the power of equivalent incandescent lamps, the current can easily drop below the TRIAC's holding current, resulting in inconsistent performance. Additionally, Energy Star specifications mandate a minimum power factor of 0.7 for residential applications and 0.9 for commercial or industrial applications. Strict requirements for efficiency, output current tolerance, and EMI must also be fulfilled, and the power supply must safely respond to short or open circuits in the LED load. Recent advancements by semiconductor companies have demonstrated solutions to these challenges. For instance, a 14-W LED driver utilizing the LNK406EG IC is presented. The LinkSwitch-PH and LinkSwitch-PL IC families from Power Integrations integrate a 725-V power MOSFET and a continuous conduction mode primary-side PWM controller in a single package. This controller offers both power factor correction and a constant current output of up to 50 W. The primary-side control technique of the IC ensures highly accurate constant current control, eliminating the need for an optocoupler and the associated circuitry typically required in isolated flyback power supplies. Furthermore, the control technique provides power factor correction while removing the necessity for a high-voltage aluminum electrolytic bulk capacitor. LinkSwitch-PH ICs are applicable in both isolated and non-isolated supplies, as well as in designs for either non-dimming or dimming (TRIAC or PWM). In TRIAC dimming applications, these devices enable flicker-free operation over a broad dimming range. The constant current control allows for a ±25% voltage swing, negating the need to bin LEDs based on forward voltage drop, while a current accuracy of ±2.5% guarantees consistent luminous flux. The 14-W LED design achieves compatibility with standard leading-edge TRIAC AC dimmers, featuring a dimming range of 1,000:1 (500 mA to 0.5 mA), high efficiency (greater than 85%), and high power factor (greater than 0.9). For low-power non-isolated designs that require power factor correction, the LinkSwitch-PL family can deliver up to 16 W in both dimming and non-dimming configurations. The minimal component count of this single-stage solution makes it suitable for space-constrained enclosures. A schematic for a 5-W dimmable power-factor-corrected LED driver using the LNK457DG is also provided. In this design, no bias winding is necessary on the transformer, and the constant current mode setpoint is determined by the voltage drop across R18, which is connected to the feedback pin of U1. A dimming range of 100:1 is achieved without flickering or blinking with all types of dimmers, including leading-edge TRIAC dimmers.LED lighting, and the potential that it brings, is moving to the forefront of residential and commercial lighting markets. However, to maximize the success and adoption of solid-state lighting for retrofit lamps, the LED lamps should be capable of dimming when used with existing controllers and wiring.
Attempts to dim LED lamps have encountered a number of problems resulting in flickering and other undesirable behavior. This article explores the typical TRIAC dimmer, some of the challenges of using it with LED lighting, and two interesting new power management solutions that solve these issues while meeting new and emerging energy, power factor, THD, and EMI specifications. Incandescent-bulb-replacement LEDs contain an LED array connected in a series string and arranged to provide an even spread of light.
The LED string must be driven by a constant current supply, which must be tightly controlled to ensure the level of luminous flux. A common lighting dimmer available today is the leading-edge TRIAC dimmer. For an LED lamp to be dimmable, the lamp`s power supply must interpret the variable phase angle output from the TRIAC controller to monotonically adjust the constant current drive.
The difficulty of achieving this while keeping the dimmer working correctly may result in flickering, audible noise, and blinking as the light level is adjusted. These are generally caused by a combination of false triggering or premature shutdown of the TRIAC and inadequate control of the LED current.
For reliable dimming down to low levels, the TRIAC must remain conducting almost to the point where the AC voltage falls to zero; the required holding current is typically in the range of 8 to 40 mA. With LED lamps consuming less than 10% of the power of an equivalent incandescent lamp, the current can easily fall below the TRIAC`s holding current with resulting inconsistent behavior.
There are other issues too. Energy Star specifications require a minimum power factor of 0. 7 for residential and 0. 9 for commercial/industrial applications. Tight requirements for efficiency, output current tolerance and EMI must also be met, and the power supply must respond safely in the event of a short or open circuit of the LED load. Recent developments by semiconductor firms have shown how the challenges of driving LEDs can be addressed.
As an example, let`s look at Fig. 1, which shows a 14-W LED driver built around the LNK406EG IC. The LinkSwitch-PH and LinkSwitch-PL families of ICs from Power Integrations incorporate a 725-V power MOSFET and a continuous-conduction-mode primary-side PWM controller in a single device. The controller provides both power factor correction and a constant current output up to 50 W. The IC`s primary-side-control technique provides highly accurate constant-current control, eliminating the need for an optocoupler and the supporting circuitry commonly used in isolated flyback power supplies, and the IC`s control technique provides the power factor correction while eliminating the need for a high-voltage aluminum electrolytic bulk capacitor.
LinkSwitch-PH ICs can be used in both isolated and nonisolated supplies, and for either nondimming or dimming (TRIAC or PWM) designs. For TRIAC dimming designs, the devices provide flicker-free operation over an exceptionally wide dimming range.
Constant-current control allows for ±25% voltage swing, eliminating the need to bin LEDs for forward voltage drop, and a current accuracy of ±2. 5% ensures consistent luminous flux. The 14-W LED design achieves compatibility with standard leading-edge TRIAC ac dimmers, a dimming range of 1, 000:1 (500 mA:0.
5 mA), high efficiency (>85%), and high power factor (>0. 9). For low-power nonisolated designs that require power factor correction, the LinkSwitch-PL family can provide up to 16 W in nondimming or dimming designs. The very low parts count of this single-stage solution allows its use in space-constrained enclosures.
Figure 2 is the schematic for a 5-W dimmable power-factor-corrected LED driver using a LNK457DG. In this design, no bias winding is required on the transformer and the constant-current-mode setpoint is determined by the voltage drop that appears across R18, which is fed to the feedback pin of U1. A dimming range of 100:1 is achieved without flickering or blinking with all dimmer types, including leading-edge TRIAC dimmers.
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Attempts to dim LED lamps have encountered a number of problems resulting in flickering and other undesirable behavior. This article explores the typical TRIAC dimmer, some of the challenges of using it with LED lighting, and two interesting new power management solutions that solve these issues while meeting new and emerging energy, power factor, THD, and EMI specifications. Incandescent-bulb-replacement LEDs contain an LED array connected in a series string and arranged to provide an even spread of light.
The LED string must be driven by a constant current supply, which must be tightly controlled to ensure the level of luminous flux. A common lighting dimmer available today is the leading-edge TRIAC dimmer. For an LED lamp to be dimmable, the lamp`s power supply must interpret the variable phase angle output from the TRIAC controller to monotonically adjust the constant current drive.
The difficulty of achieving this while keeping the dimmer working correctly may result in flickering, audible noise, and blinking as the light level is adjusted. These are generally caused by a combination of false triggering or premature shutdown of the TRIAC and inadequate control of the LED current.
For reliable dimming down to low levels, the TRIAC must remain conducting almost to the point where the AC voltage falls to zero; the required holding current is typically in the range of 8 to 40 mA. With LED lamps consuming less than 10% of the power of an equivalent incandescent lamp, the current can easily fall below the TRIAC`s holding current with resulting inconsistent behavior.
There are other issues too. Energy Star specifications require a minimum power factor of 0. 7 for residential and 0. 9 for commercial/industrial applications. Tight requirements for efficiency, output current tolerance and EMI must also be met, and the power supply must respond safely in the event of a short or open circuit of the LED load. Recent developments by semiconductor firms have shown how the challenges of driving LEDs can be addressed.
As an example, let`s look at Fig. 1, which shows a 14-W LED driver built around the LNK406EG IC. The LinkSwitch-PH and LinkSwitch-PL families of ICs from Power Integrations incorporate a 725-V power MOSFET and a continuous-conduction-mode primary-side PWM controller in a single device. The controller provides both power factor correction and a constant current output up to 50 W. The IC`s primary-side-control technique provides highly accurate constant-current control, eliminating the need for an optocoupler and the supporting circuitry commonly used in isolated flyback power supplies, and the IC`s control technique provides the power factor correction while eliminating the need for a high-voltage aluminum electrolytic bulk capacitor.
LinkSwitch-PH ICs can be used in both isolated and nonisolated supplies, and for either nondimming or dimming (TRIAC or PWM) designs. For TRIAC dimming designs, the devices provide flicker-free operation over an exceptionally wide dimming range.
Constant-current control allows for ±25% voltage swing, eliminating the need to bin LEDs for forward voltage drop, and a current accuracy of ±2. 5% ensures consistent luminous flux. The 14-W LED design achieves compatibility with standard leading-edge TRIAC ac dimmers, a dimming range of 1, 000:1 (500 mA:0.
5 mA), high efficiency (>85%), and high power factor (>0. 9). For low-power nonisolated designs that require power factor correction, the LinkSwitch-PL family can provide up to 16 W in nondimming or dimming designs. The very low parts count of this single-stage solution allows its use in space-constrained enclosures.
Figure 2 is the schematic for a 5-W dimmable power-factor-corrected LED driver using a LNK457DG. In this design, no bias winding is required on the transformer and the constant-current-mode setpoint is determined by the voltage drop that appears across R18, which is fed to the feedback pin of U1. A dimming range of 100:1 is achieved without flickering or blinking with all dimmer types, including leading-edge TRIAC dimmers.
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