1, the challenges brought by LED lighting design
At present, the digital revolution in the global lighting industry is coming, and energy-efficient LED lights will replace incandescent, M16 halogen lamps and CFL bulbs. However, in recent days, the LED lighting designer's work face has a new challenge, that is , to meet the dimming function of the incandescent lamp and the M16 halogen lamp to achieve the dimming control function, and to achieve high power factor without any flicker. The new requirements for dimming control, especially to be compatible with existing infrastructures, include corner-angle dimming and electronic transformer dimming to support flicker-free dimming.
It should be said that dimming is a very common function of lighting systems. Compared with incandescent lamps or M16 halogen lamps, it can be easily realized at low cost, but it is difficult for dimming LED lamps, especially A dimming control without any flicker is achieved. Often for buildings and users to convert incandescent or M16 to LED lighting, the most lingering thing is to lose the advantage of dimming control applications, that is, there should be no flicker. Of course, the power factor is also a very important factor, because the high power factor can reduce the loss of the power distribution network and reduce waste. Therefore, regulators at home and abroad are further tightening their power factor specifications. For example, ENERGY STAR solid-state lighting energy efficiency regulations stipulate that the power factor of residential lighting products should be greater than 0.7, and the power factor of commercial lighting products should be greater than 0.9. LED bulb and luminaire manufacturers are responding to these requirements by requiring LED driver circuits to be suitable for a variety of dimming units with high efficiency and a power factor (PF) of 0.9, making them highly versatile.
How to deal with the challenges brought by LED lighting design to meet the new requirements has become an urgent concern for manufacturers and designers and consumers. Based on this, this paper will discuss the problems of high-power factor performance in the LED lighting application without any flickering dimming control technology and construction, and the LED driver and high-integrated MOSFET LED driver constructed with multiple controllers. The high power factor without any flicker dimming control is an important analysis.
2. The LED driver constructed with multiple controllers is a dedicated constant current LED load flyback power supply.
To this end, start with the basic concept of the dimming controller.
2.1 thyristor dimmers have problems in LED lighting applications
* SCR dimmer (see Figure 1 (a)). It is based on the principle of phase-cutting, reducing the VRMS to reduce the power of the normal load (resistive load), and the work efficiency is higher and the performance is stable.
Therefore, it is a common method of LED lighting, as shown in Figure 1 (b) shows the structure of the thyristor dimmer and LED driver.
The thyristor dimming in Figure 1(b) is to extract the conduction angle information when the phase-cut power supply is in the chip or in the application circuit, and control the driving current of the LED according to the signal control to obtain the dimming effect. . However, thyristor dimmers have flicker, uneven illumination, audible noise, and flicker in LED lighting applications. There are two reasons: the first is the thyristor dimming 100Hz frequency flash memory. Because the power frequency of the power grid is 50/60Hz, after the phase-cut waveform is rectified, a 100/120Hz pulse signal can be obtained. This can be used directly to generate a dimmed PWM signal. In practical applications, as long as there is any slight voltage fluctuation, the current changes will affect the duty cycle of the pulse, which is equivalent to a 100/120Hz flicker, and this problem is not for incandescent lamps? It is because the physical inertia of the tungsten filament of the bulb makes it difficult for people to feel it; the second is that the dimming control of the thyristor dimmer is achieved by changing the phase angle of each half cycle of the thyristor conduction, in order to maintain controllable The stable operation of silicon, one of its important parameters, the holding current (IH) is not zero (typically between 8mA and 40mA). When the incandescent lamp is driven, the IH sustain current is not a problem. However, when driving the high-efficiency LED, the sustain current cannot be maintained when the lamp is turned off. Especially in the case of oscillation, flicker, uneven illumination, audible noise and flicker are more likely to occur. If the oscillation causes the current to drop below IH (8mA~40mA), the thyristor will be turned off. As a result, the thyristor is restarted multiple times during the same input line cycle, and the LED lamp flashes frequently.
*Starting from improving the load capacity of LED drivers is the key to suppressing flicker
SCR dimming 100Hz strobe - Solution: You can't directly dim the phase-cut waveform directly, you need to shift the signal to a higher frequency band, so that people can't see the flicker. The drive controller chip adjusts the dimming signal to a higher frequency band of a few KHz.
Regarding the load problem of thyristor dimming, the thyristor dimmer should work stably, and a stable IH is needed to ensure the operation of the dimmer. In order to solve the overheating of the thyristor dimming load, it is necessary to apply signal filtering technology to shape and intercept the intercepted conduction angle signal, so as to improve the usability of the dimming signal; in order to solve the jitter in the case of insufficient load, it is necessary to add signal smoothing and The phase-locking technique smoothly locks the adjacent two conduction angle signals. It can greatly reduce the occurrence of flicker.
In order to avoid the problems associated with thyristor dimming, LED drivers must meet the various requirements of LED loads, while also being compatible with dimming circuits designed for incandescent lamps. LED lamps used to replace standard incandescent lamps typically contain multiple LEDs to ensure uniform illumination. These LEDs are connected together in series. The brightness of each LED is determined by its current magnitude. The forward voltage drop of the LED is approximately 3.4V, but is typically between 2.8V and 4.2V (±20%). Despite the large load variations, the LED string must still be driven by a constant current source, so the current must be tightly controlled to ensure a high degree of matching between adjacent LEDs. In order to achieve dimming, the LED lamp must detect the variable phase angle output of the thyristor controller and use this information to change the constant current drive of the LED.
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