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Why digital control technology in LED dimming more dominant?

Although LED lamps are now sold in large quantities, the performance of such solid state lighting (SSL) products, especially in areas such as dimming, will still be lower than traditional bulbs. Indeed, conventional drivers designed for LED replacement lamps have difficulty providing truly satisfactory dimming performance, energy efficiency and reliability at the right price. However, digital control in drive electronics now removes unnecessary bleed resistors, often used to enhance the operation of thyristor dimmers, while reducing flicker-free dimming to low levels of lighting.
The use of dimmers in home lighting is already widespread, saving energy, reducing electricity costs, and providing comfort and convenience. Using energy-efficient LED lighting, from the efficiency point of view, dimming is not very important. However, lighting can easily affect moods at home and in places such as restaurants, entertainment or conference halls. This requires a smooth, flicker-free dimming covering a wide range of light levels.
In many applications, the ideal scenario for deploying solid-state lighting is simply by replacing an incandescent lamp with an LED of the same form factor. This is because the user expects the new fixture to work perfectly with its existing dimmer, but the actual results may vary depending on the type and quality of the dimmer.
Dimmer operation
The standard triac based triac dimmer is designed to drive the resistive load provided by incandescent bulbs. Thyristor dimmers are also referred to as leading edge dimmers because dimming is achieved by changing the sinusoidal waveform by adjusting the conduction angle of each half-wave of the alternating current using a triac (FIG. 1). Delay the conduction angle of the triac to dim the lamp.
figure 1. Dimming through the leading edge of the thyristor dimmer is delayed until late in the cycle.
When a trigger pulse is applied, the triac turns on and stays on for the remaining cycles if the current remains above the device's specified holding current. If the load is an incandescent bulb, the current tends to remain above the hold current threshold, so the triac will remain on until the current decreases at the end of the cycle.
However, two factors can limit the dimming range, and even incandescent bulbs can have some impact. Dimming circuits typically include electromagnetic interference (EMI) filters that include inductor and capacitor components that can introduce pulsating noise into a current waveform. If the inductor is a low-quality component, this pulsating noise can be enough to turn the current from ON to immediately lower than the holding current of the triac, causing the device to turn off, resulting in a visible blinking of the lamp. Likewise, the dimmer will not be able to maintain the dimming level if a low cost triac with a relatively high hold current is used.
Figure 2 compares the minimum load current required for the five triac-based dimmers on the market today. The peak value indicates the minimum current that the LED driver needs to absorb from the triac to ensure proper operation.
figure 2. Five different dimmers hold current.
LED load current
Running dimmers at low load currents poses a lot of challenges, and the challenges increase when the load is an LED replacement bulb. Unlike incandescent lamps, LEDs are not purely resistive. The impedance is reactive, thus preventing the current from rising above the holding current threshold of the triac so that the device remains on after the trigger is removed. Low level dimming may also be difficult to achieve because LEDs typically consume less power than incandescent light bulbs. Therefore, the current developed through triacs can be very low at the phase angle required for maximum dimming. Its current can be reduced below the minimum holding current of the triac, causing the lamp to flash or abruptly stop working.
In practice, dimmers need to achieve more dimming than people perceive. As the human eye compensates for low light levels by expanding the iris, the relationship between the electronic dimmer settings and the perceived dimming effect is not linear. If you adjust the dimmer to reduce the light to 10% of its maximum measurement level, the human eye sees only about a 30% reduction. In order to achieve a perceived dimming level of 10%, the dimmer must be able to reduce the measured light level to only 1%. The Illuminating Engineering Society (IES) Tenth Edition "Lighting Handbook" records the relationship between perceived and measured light:
Perceived light = 100 × √ (Measured light ÷ 100)
Manufacturers of LED replacement lamps need to be able to interoperate with triac dimmers of varying quality and cost and to ensure that flicker-free lighting is lowered to low dimming levels to maximize customer satisfaction.
The bleeder circuit lets the current flow
A common solution is to add a bleeder circuit that keeps enough current in the triac at lower dimming levels. This may be a passive circuit using a resistor or a dynamic drain using a power transistor to prevent current in the bleeder resistor when not needed.
There are two disadvantages to using a passive bleeder. Because of the continuous current through the resistor, so through the use of LED technology to get the efficiency advantage will be greatly reduced. More important is the impact of heat on the working life of the lamp. Thermal management is crucial for LED lights. The LED emitter itself can work 25,000 to 50,000 hours at high temperatures. However, related components, such as high-capacity electrolytic capacitors, are more sensitive to operating temperature. For every 10 ° C increase in operating temperature, the service life of the electrolytic capacitor is reduced by 50%, which significantly reduces the life of the LED lamp. In order to achieve an acceptable lifetime, manufacturers may need to take steps to protect vulnerable components, such as by potting, which increases the cost of the final product.
The advantages of digital control
A more satisfying approach is to recirculate the bleed current instead of dissipating energy in the form of heat. This is not easy to achieve in the simulation domain. However, digital technology allows complex dynamic control schemes to use the energy needed more intelligently to keep traditional dimmers working.
In addition to being used with conventional triac dimmers to eliminate flicker or abrupt interruptions, suitable LED driver circuits must also be able to control the ac cycle inrush current to avoid momentary overloads, the grid cycle and internal magnetic Minimize audible noise due to interaction between components, and meet regulatory standards for power factor and electrical noise (EMI). They must also be compatible with the widest range of dimmer types.
Figure 3 shows a typical application circuit for a non-isolated LED driver circuit using the Dialog Semiconductor iW3688 controller. The device features digital cores that are compatible with a variety of triac dimmers and are a low cost solution.
image 3. Rectifier, current control and LED driver using iW3688 controller.
As shown, this circuit uses only one external MOSFET. This is possible because the driver ICs use the same switches to keep the triacs working and powering the control circuitry itself. This design does not require secondary windings on the main core. This approach allows designers to use low-cost, off-the-shelf inductors for non-isolated applications, or use standard flyback transformers if the application requires isolation. This architecture helps to reduce the number of components, save energy, and minimize heat dissipation, thereby simplifying thermal management.
Drive operation
The IC digital circuitry monitors the associated voltage and current and allows the controller to dynamically modulate the main power MOSFET to achieve the desired dimming level, allowing the triac to turn on even if the demand for LED load is very low. The extra current used to keep the triac operating is used internally rather than converted to heat.
IC also integrates intelligent functions, according to the characteristics of dimmers dynamic impedance adjustment. The design allows the device to be used with almost any standard triac dimmer to reduce LED brightness to 1% of maximum. The low dimming level gives you more dimming than earlier driver circuits.
When no dimming is applied, the main power converter, which delivers current to the LED load, operates in quasi-resonant mode to provide high power efficiency and low EMI. The power factor is also optimized to increase efficiency and minimize current harmonic distortion on the AC line and to meet regulatory requirements around the world.
End-users of energy-efficient lighting are looking to seamlessly transition from existing lighting technologies to LEDs. This requires lamp replacement to meet current industry standards and operation with existing triac-based dimmers to achieve low dimming and compliance with minimum flicker regulations. At the same time, new technologies must be offered at very competitive prices while maximizing energy efficiency and reliability. Digital driver control technology offers a better solution than traditional analog circuitry and resistor divider, enabling manufacturers to meet the high expectations of consumers.

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