GreenPoint LED reference design for the "Energy Star" solid state lighting standard

With high brightness light-emitting diode (HB-LED) overall improvement in light output, energy efficiency and cost, combined with small, low voltage operation and environmental protection and many other advantages, LED lighting (also known as solid-state lighting (SSL)) is set off a revolution in lighting. In the trend of energy saving and environmental protection, LED lighting naturally become the target of many regulatory agencies aim. Such as the United States Department of energy Energy Star program version 1.1 standard solid-state lighting have been in effect since February 2009, China Institute of standardization Chinese also to the relevant agencies in the lead, ready to release the Chinese version of LED lighting energy efficiency standards in 2010.

"Version of solid-state lighting standard energy star", one of the most important features of this standard is the power factor requirements of a variety of residential lighting products to reach the minimum 0.7, some typical products are portable lamps, cabinet lights and outdoor corridor lamp etc.. The power of this kind of LED lighting applications is generally between 1 and 12 W, which belongs to low power applications. The most suitable power supply topology for low power applications is the isolated flyback topology. The disadvantage is that the existing standard design techniques for the design of these power supplies typically make the power factor (PF) only in the range of 0.5 to 0.6. This paper will analyze the reasons of the existing design of low power factor, to improve the power factor of technology and solutions, introduces the design process and test part of the data, display the reference design how to easily meet energy star specification for solid state lighting LED lighting for residential power factor requirements.

Design background typical off-line flyback power converter adopts full wave bridge rectifier and capacitor switching regulator in front, the reason to choose this configuration is every 2 cycle line power line decreased until zero, then rose to a peak. As the energy storage element, the large capacitor is filled with the corresponding power, which provides a more constant input for the switching regulator and maintains the power flow to the load. The power factor of the configuration or the power factor of the input line waveform is low. The line current is consumed at a large narrow pulse near the peak of the voltage waveform.

Industry (Passive) related to passive power factor correction (PFC) scheme is numerous, these additional components usually use more scheme, a scheme which is the bottom of the valley fill (valley-fill) rectifier, which electrolytic capacitor and diode combination with line frequency increases the conduction angle, thereby improving the power factor. In fact, this process uses a high line voltage to charge a series capacitor at a low current, and then discharges the capacitor to the switching regulator at a lower voltage. Typical applications use 2 capacitors and 3 diodes, and to further enhance the power factor performance, the use of 3 capacitors and 6 diodes. Figure 1: typical valley fill circuit.

Although the valley line current rectifier can improve the utilization rate, but did not provide a constant input to the switching regulator. Power supply to the load will have a greater ripple, up to 2 times the line power supply frequency. It should be noted that 4 diodes are still required to rectify the line power supply, so that the number of diodes used in this scheme is up to 7 or 10. The diodes and a plurality of electrolytic capacitors increase the cost of the program, reduce the reliability and occupy a considerable area of the circuit board.

Another program is using active in flyback converter (Active) before PFC, such as NCP1607B. This scheme provides excellent power factor above 0.98, but increases the number of components, reduces the efficiency and increases the complexity. The most suitable power Pingyuan is higher than the power level of the application.

Solutions high power factor sinusoidal line current and usually needs, requirements for phase line current and voltage difference is minimal. The first step is to modify the design of the switch segment to obtain very low capacitance, so as to get closer to the sinusoidal waveform of the input current. This causes the rectified voltage to follow the line voltage and produce a better sinusoidal input current. In this way, the input voltage of the flyback converter is 2 times the line frequency. If the input current is kept in the same waveform, the power factor is high. The energy supplied to the load is the product of voltage and current, which is a sine squared (sine? Squared) waveform. Due to the energy transfer of the sine square waveform, the load will encounter 2 times the line frequency ripple, essentially similar to the ripple in the valley fill circuit.

As mentioned above, the input current must be kept in the almost sinusoidal waveform, thereby realizing high power factor. The key to the high power factor is to maintain the feedback input at a constant level associated with the line frequency. One option is to substantially increase the output capacitance, thereby reducing the ripple of 120 Hz, and some applications may require the use of such a scheme. If the frequency is higher than the visible light range, the LED for general lighting applications can tolerate ripple. A more compact and inexpensive solution is to filter back to the PWM converter feedback signal to establish a nearly constant level. This level fixes the maximum current in the power switch. The current of the power switch is determined by the applied transient input voltage divided by the transformer primary inductance multiplied by the power switch on time.

NCP1014LEDGTGEVB evaluation A Morimi semiconductor plate after optimization, can drive 1 to 8 high-power high brightness LED, such as CREE XLAMP?? XR? E/XP? E, Luxeon RebEL, Seoul Semiconduct??