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DIMMING LEDs, WITHOUT THE FLICKER

2010-10-11  POWER INTEGRATIONS

LED lighting is a mainstream technology now. LED flashlights, traffic signals, and vehicle lights are commonplace, and the push is on to replace incandescent and fluorescent lamps in mains-powered residential, commercial, and industrial applications. The energy savings to be earned by moving to energy-efficient LED lighting is simply staggering. In China alone, authorities estimate that by moving one-third of their lighting market to LEDs, they will save 100 million kilowatt hours of electricity and reduce carbon dioxide emissions by 29 million tons each year. However, there is a snag – dimming.

Incandescent lights are easy to dim with a simple, low-cost, leading-edge TRIAC-based dimmer. As a result, these dimmers are everywhere. For solid-state lighting retrofit lamps to be truly successful, they must be capable of dimming when used with existing controllers and wiring.

Incandescent bulbs lend themselves very well to being dimmed. Ironically it is their low efficacy, and consequent high current draw, that allow the dimmer to work well. The thermal inertia of the filament in an incandescent bulb also helps mask any instability or oscillation created by the dimmer. Attempts to dim LED lamps have encountered a number of problems, often resulting in flickering and other undesirable behavior. To understand why, it is necessary to explain how TRIAC controllers work, the technology of LED lamps and how they interact with each other. Figure 1 illustrates a typical leading-edge TRIAC dimmer and the voltage and current waveforms it generates.

The potentiometer R2 adjusts the phase angle of the TRIAC, which fires on each leading AC voltage edge when VC2 exceeds the breakover voltage of the DIAC. When the TRIAC current falls below its holding current (IH), the TRIAC turns off and must wait until C2 is recharged in the next half cycle to turn on again. The voltage applied across and the current through the lamp filament is a function of the phase angle of the dimming signal which can vary from almost zero to 180 degrees.

An LED lamp intended to replace a standard incandescent bulb typically contains an LED array arranged to provide an even spread of light. The LEDs are connected in a series string. The brightness of each LED is a function of the current through it, and the LEDs have a forward voltage drop of approximately 3.4 V, which can vary from 2.8 V to 4.2 V. The LED string should be driven by a constant current supply, which must be tightly controlled to ensure matching between adjacent lamps.

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 to the LEDs. The difficulty of achieving this while keeping the dimmer working correctly can result in poor performance. Problems can show up as slow startup, flickering, uneven illumination, or blinking as the light level is adjusted. There are also issues with unit-to-unit inconsistency and unwanted audible noise emanating from the lamp. These effects are generally caused by a combination of false triggering or premature shutdown of the TRIAC and inadequate control of the LED current. The root cause of false triggering is current ringing when the TRIAC fires. Figure 2 illustrates this effect.

When the TRIAC fires, the AC mains voltage at that moment is applied almost instantaneously to the LC input filter of the LED lamp power supply. The voltage step applied to the inductance results in ringing. If during this ringing the current through the dimmer falls below the TRIAC trigger current, the TRIAC stops conducting. The TRIAC trigger circuit recharges and then re-fires the dimmer. Such errant multiple TRIAC restarts can cause undesirable audible noise and flickering in the LED lamp. Less complex input EMI filters help minimize this undesirable ringing. For successful dimming, it is critical also that the input EMI filter inductors and capacitors be as small as possible.

The worst case for ringing is at a 90-degree phase angle (when the input voltage is at the peak of the sine wave, and is suddenly applied to the input of the LED lamp) and at high line mains voltage (when the dimmer forward current is at a minimum). When deep dimming is required (i.e., phase angle approaching 180 degrees) and at low line mains voltage, premature shutdown can occur. For reliable dimming down to low levels, the TRIAC must turn on monotonically and stay on almost to the point where the AC voltage falls to zero. For TRIACs, the holding current required to maintain conduction is typically in the range of 8 to 40 mA. For incandescent lamps, this current is easy to maintain, but with LED lamps consuming less than 10% of the power of an equivalent incandescent lamp, the current can fall below the TRIAC’s holding current causing the TRIAC to turn off prematurely. This can result in flickering and/or limit the dimmable range.

A number of other issues present challenges when designing an LED lighting power supply. ENERGY STAR® specifications for solid-state luminaries require a minimum power factor of 0.9 for commercial and industrial applications, tight requirements for efficiency, output current tolerance and EMI must be met and the power supply must respond safely in the event of a short or open circuit of the LED load.

A recent development by POWER INTEGRATION gives an example of how the challenges of driving LEDs and TRIAC compatibility can be addressed. Figure 3 is the schematic for a TRIAC-dimmable 14 W LED driver developed by PI.

The design is built around the LinkSwitch-PH™ family device LNK406EG (U1). The LinkSwitch-PH™ family of LED driver ICs incorporates a 725 V power MOSFET and a continuous conduction mode, primary-side PWM controller in a monolithic IC. The controller implements both active power factor correction (PFC) and constant current output in a single stage. The primary side control technique used by the LinkSwitch-PH™ family of devices provides highly accurate constant current control (well beyond that achieved with traditional primary side control techniques) eliminating the need for an optocoupler and supporting circuitry commonly used in isolated flyback power supplies implementing secondary side control circuitry, while the PFC part of the controller eliminates the electrolytic bulk capacitor.

The LinkSwitch-PH™ family of devices can be set to either dimming or non-dimming mode. For TRIAC phase-dimming applications, a programming resistor (R4) is used on the REFERENCE pin and 4 M? (R2+R3) on the VOLTAGE MONITOR pin to provide a linear relationship between input voltage and the output current, maximizing the dimming range.

Continuous conduction mode provides two key benefits: reduced conduction loss (hence higher efficiency) and a lower EMI signature. The lower EMI signature enables EMI standards to be met with a smaller input EMI filter. One X capacitor can be eliminated and the common mode choke reduced in size or eliminated. Built-in jitter of the high-voltage power MOSFET switching frequency in the LinkSwitch-PH™ family of devices further reduces the filtering requirement. The smaller input EMI filter presents lower reactive impedance to the driving circuit, delivering the key benefit of significantly reduced input current ringing. Stability is further enhanced with the LinkSwitch-PH being powered from its own internal reference supply. For dimmable applications, the addition of active damper and bleeder circuits ensures robust, flicker-free operation over an exceptionally wide dimming range.

The constant current control allows for ±25% voltage swing, eliminating the need to bin LEDs for forward voltage drop, and a variance of ±5% ensures consistent LED brightness.

The 14 W LED design achieves the goals of compatibility with standard leading edge TRIAC AC dimmers, very wide dimming range (1000:1, 500 mA:0.5 mA), high efficiency (> 85%), and high power factor (> 0.9). It demonstrates that the problems associated with TRIAC dimming of LED lamps can be overcome, even to the extent of making straightforward the design of drivers for cost-effective dimmable LED lamps with consistent and reliable performance.

About the Author

Bill Weiss is the business development manager for lighting applications at POWER INTEGRATIONS (PI). Prior to joining PI, Bill held the positions of marketing director at GCT Semiconductor, product line director at Advanced Analogic Technologies, marketing director at National Semiconductor, business development manager at Philips Semiconductors, and various engineering and marketing roles at Allegro MicroSystems. He holds an MBA from Clark University and a B.S. in Electrical Engineering, Mathematics, and Economics from Carnegie Mellon University.

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Figure 1. Leading-edge TRIAC dimmer
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Figure 2. TRIAC current and voltage ringing viewed at input to LED lamp power supply
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Figure 3. Schematic of an isolated, TRIAC-dimmable, high power factor, universal input, 14 W LED driver

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