Flicker-free dimming of LEDs

1. Introduction

Today, LED lighting is a mainstream technology. LED flashlights, traffic lights and car lights are everywhere, and countries are pushing to replace incandescent and fluorescent lamps in mains-powered residential, commercial and industrial applications with LED lights. The energy savings achieved by switching to energy-efficient LED lighting are staggering. In China alone, the government* estimates that if a third of the lighting market switched to LED products, they would save 100 million kilowatt-hours of electricity and reduce carbon dioxide emissions by 29 million tons each year. However, there is still one hurdle to overcome: dimming.

Incandescent lamps are easily dimmed using simple, low-cost, leading-edge thyristor dimmers. As a result, these dimmers are ubiquitous. For solid-state lighting replacement lamps to be truly successful, they must be dimmed using existing controllers and wiring.

Incandescent light bulbs are well suited to dimming. Ironically, it is their low efficiency and the resulting high input current that are the main reasons why dimmers work well. The thermal inertia of the filament in an incandescent bulb also helps to mask any instabilities or oscillations created by the dimmer. A number of problems have been encountered in attempts to dim LED lamps, often resulting in flickering and other unexpected behavior. To understand why, it is first necessary to understand how thyristor dimmers work, LED lamp technology, and how they relate to each other.

2. The principle of thyristor dimming

Figure 1 shows a typical leading edge thyristor dimmer and the voltage and current waveforms it produces.

Figure 1 Leading-edge thyristor dimmer

Potentiometer R2 adjusts the phase angle of the TRIAC, which conducts on each AC voltage leading edge when VC2 exceeds the breakdown voltage of the DIAC. When the TRIAC current drops below its holding current (IH), the TRIAC turns off and must wait until C2 recharges in the next half cycle before it can conduct again. The voltage and current in the bulb filament are closely related to the phase angle of the dimming signal, which can vary between 0 degrees (close to 0 degrees) and 180 degrees.

3. Problems with LED dimming

LED lamps used to replace standard incandescent lamps usually contain an array of LEDs to ensure uniform lighting. These LEDs are connected together in series. The brightness of each LED is determined by its current, and the forward voltage drop of the LED is about 3.4 V, usually between 2.8 V and 4.2 V. The LED string should be driven by a constant current power supply, and the current must be strictly controlled to ensure high matching between adjacent LED lamps.

To make LED lamps dimmable, the power supply must be able to analyze the variable phase angle output of the thyristor controller to unidirectionally adjust the constant current to the LED. Doing this while maintaining proper dimmer operation is difficult and often results in poor performance. Problems can manifest as slow startup, flickering, uneven lighting, or flickering when adjusting the light intensity. There are also issues such as component-to-component inconsistency and unwanted audible noise from the LED lamp. These negative conditions are usually caused by a combination of false triggering or premature shutdown of the thyristor and improper LED current control. The root cause of false triggering is the current oscillation that occurs when the thyristor is turned on. Figure 2 illustrates this effect in graphical form.

Figure 2. SCR current and voltage oscillations occurring at the input stage of an LED lamp power supply.

When the thyristor is conducting, the AC mains voltage is applied to the LC input filter of the LED lamp power supply almost simultaneously. The voltage step applied to the inductor causes oscillation. If the dimmer current falls below the thyristor current during the oscillation, the thyristor stops conducting. The thyristor trigger circuit charges and then turns the dimmer back on. This irregular multiple thyristor restart can cause unwanted audible noise and flicker in the LED lamp. A simpler EMI filter design can help reduce this unwanted oscillation. To achieve successful dimming, the input EMI filter inductance and capacitance must also be as small as possible.

The worst case for oscillation occurs at a 90-degree phase angle (when the input voltage reaches the peak of the sine wave and is suddenly applied to the input of the LED lamp) and at a high input voltage (when the dimmer forward current is at its minimum level). When deep dimming is required (e.g., the phase angle is close to 180 degrees) and the input voltage is low, premature shutdown occurs. To dim reliably, the thyristor must turn on monotonically and stay on until the AC voltage drops to almost zero volts. For the thyristor to remain on, the holding current required is typically between 8 mA and 40 mA. Incandescent lamps can easily maintain this current level, but for LED lamps that consume only 10% of the power of an equivalent incandescent lamp, this current can drop below the thyristor holding current, causing the thyristor to turn off prematurely. This can cause flickering and/or limit the dimming range.

There are many other issues that pose challenges when designing LED lighting power supplies. The Energy Star solid-state lighting specification requires a minimum power factor of 0.9 for commercial and industrial applications, lighting products must meet stringent requirements for efficiency, output current tolerance, and EMI, and the power supply must respond safely in the event of a short or open circuit in the LED load.

4. LED dimming practical solution

Recent technological advances by Power Integrations (PI) provide a reference example for solving the compatibility issues between LED drivers and thyristors. Figure 3 is the circuit diagram of a 14 W LED driver with thyristor dimming developed by PI.

Figure 3. Circuit diagram of an isolated thyristor dimming high power factor universal input 14 W LED driver

This design uses the LinkSwitch-PH series device LNK406EG (U1). The LinkSwitch-PH series LED driver IC integrates a 725 V power MOSFET and a continuous conduction mode primary-side PWM controller. The controller can achieve single-stage active power factor correction (PFC) and constant current output. The primary-side control technology used by the LinkSwitch-PH series devices can provide high-precision constant current control (performance is much better than traditional primary-side control technology), eliminating the optocoupler and auxiliary circuits (i.e., secondary-side control circuits) commonly used in isolated flyback power supplies, and the PFC part of the controller also eliminates large-capacity electrolytic capacitors.

LinkSwitch-PH devices can be set to dimming or non-dimming mode. For thyristor phase dimming applications, a programming resistor (R4) can be used on the REFERENCE pin and a 4 MΩ (R2+R3) resistor can be used on the VOLTAGE MONITOR pin to maintain a linear relationship between input voltage and output current, thereby extending the dimming range.

Continuous conduction mode has two major advantages: reduced conduction losses (thus improving efficiency) and reduced EMI signature. With a reduced EMI signature, a smaller input EMI filter can be used to meet EMI standards. An X capacitor can be eliminated and the common mode choke can be eliminated or reduced in size. The built-in high-voltage power MOSFET switching frequency jittering function in the LinkSwitch-PH device can further reduce filtering requirements. The reduced size of the input EMI filter means that the resistive impedance of the drive circuit is reduced, which has the important benefit of greatly reducing input current oscillation. Since LinkSwitch-PH is powered by its internal reference supply, stability can be further enhanced. For dimmable applications, the addition of active damping circuits and bleeder circuits can ensure that the LED lamp operates stably over a very wide dimming range without any flicker.

Constant current control allows a ±25% voltage swing, which eliminates the need to code the LED based on forward voltage drop, and a ±5% variance still ensures consistent LED brightness.

5. Conclusion

This 14 W LED design achieves the goals of compatibility with standard leading-edge thyristor AC dimmers, extremely 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 thyristor dimming of LED lamps can be overcome and even the driver design can be simplified, making dimmable LED lamps more cost-effective with consistent and reliable performance.