With the comprehensive improvement of light output, energy efficiency and cost of high-brightness light-emitting diodes (HB- LEDs ), combined with the advantages of compact, low-voltage operation and environmental protection, LED lighting (also known as solid-state lighting (SSL)) is setting off a field. Lighting revolution. In the trend of energy saving and environmental protection, LED lighting has naturally become the target of many regulatory agencies. For example, the 1.1 version of the US Department of Energy's "Energy Star" project has entered into force since February 2009. China's China National Institute of Standardization is also taking the lead in cooperating with relevant organizations to prepare for the release of the Chinese version of LED lighting energy efficiency standards in 2010.
As far as the new version of ENERGY STAR's solid-state lighting standards is concerned, an important feature of this standard is that it requires a minimum of 0.7 power factor for a variety of residential lighting products. Some of the typical products include portable desk lamps, cabinet lights and outdoor corridors. Lights, etc. This type of LED lighting application typically has a power range of 1 to 12 watts and is a low power application. The most suitable power topology for this type of low power application is the isolated flyback topology. Disadvantageously, the standard design techniques available for designing these power supplies typically result in power factor (PF) only in the range of 0.5 to 0.6. This article will analyze the reasons for the low power factor of the existing design, explore the technology and solutions to improve the power factor, introduce the relevant design process and share the test part data, showing how easily the reference design meets the "Energy Star" solid-state lighting specification for residential LEDs. The power factor requirements for lighting applications.
design background
A typical off-line flyback power converter uses a full-wave bridge rectifier and a large capacitor in front of the switching regulator. The reason for this configuration is that the line power is reduced every two line cycles up to zero and then rises to the next peak. The large capacitor acts as an energy storage component, filling the corresponding missing power, providing a more constant input to the switching regulator and maintaining the flow of electrical energy to the load. The power utilization of this 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, introducing interfering high frequency harmonics.
There are many solutions in the industry for Passive Power Factor Correction (PFC), which usually use more extra components, one of which is the valley-fill rectifier, which uses electrolytic capacitors. The combination with the diode increases the line frequency conduction angle, thereby improving the power factor. In effect, this process uses a high line voltage to charge the series capacitor with a low current, and then discharges the capacitor to the switching regulator with a larger current at a lower voltage. Typical applications use 2 capacitors and 3 diodes, and to further enhance power factor performance, 3 capacitors and 6 diodes are used.
Figure 1: Typical valley filling circuit
Although the valley-filled rectifier improves line current utilization, it does not provide a constant input to the switching regulator. The power supplied to the load will have a large ripple, which is twice the frequency of the line supply. It should be noted that four diodes are still needed to rectify the line supply, making the number of diodes used in this solution up to seven or ten. These diodes and multiple electrolytic capacitors increase solution cost, reduce reliability, and take up considerable board area.
Optical time-domain reflectometer (English name: optical time-domain reflectometer, OTDR) is an instrument that understands the uniformity, defect, fracture, and coupling of the optical fiber through the analysis of the measurement curve. It is made according to the principle of light backscattering and Fresnel reverse. It uses the backscattered light generated when light propagates in the optical fiber to obtain attenuation information. It can be used to measure optical fiber attenuation, connector loss, fiber fault location and understanding The loss distribution of the optical fiber along the length, etc., is an indispensable tool in the construction, maintenance and monitoring of the optical cable.
The optical time domain reflectometer will inject a series of optical bursts into the optical fiber for inspection. The method of inspection is to receive the optical signal from the same side of the incident wave, because the incident signal will be scattered and reflected back when it encounters a medium with a different refractive index. The intensity of the reflected light signal will be measured and is a function of time, so it can be converted into the length of the optical fiber.
The optical time domain reflectometer can be used to measure the length and attenuation of the optical fiber, including the fusion splice and transition of the optical fiber. It can also be used to measure the interruption point when the fiber is broken.
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