With practical threaded into the type of LED to replace incandescent bulbs may need several years of time, and in building lighting in the use of leds is increasing, it has higher reliability and energy saving potential. Like most electronics, it requires a power supply to convert input power into an LED available form. In street lighting applications, a feasible configuration is to create 80 tandem leds with a 300V / 0.35 amperage load. When selecting the power topological structure, the requirement of isolation and power factor correction (PFC) is required. Isolation requires a great deal of security trade-offs, including the trade-off between providing shock protection requirements and complex power design. In this application, the LED is high, generally thought that isolation is a must, but PFC is necessary, because in Europe more than 25 watt lighting requirements with PFC function, and the launch of this product for the European market.
In terms of this application, there are three optional power topologies: pressure topologies, reverse topological and transfer modes (TM) single end low-level inductors (SEPIC) topologies. When the LED voltage is about 80 volts, the decompression topology can be effectively used to meet the harmonic current requirements. In this case, the higher load voltage will no longer be able to continue using the descending topology. Then, a more eclectic approach is to use a reverse topology and a SEPIC topology. The advantage of SEPIC is that it can clamp the switch waveform of power semiconductor device and agree to use lower voltage to make the device more efficient. In this application, you can get an efficiency improvement of about 2%. In addition, there are fewer ringing bells in SEPIC, making EMI filtering easier.
The circuit USES a boost TM PFC controller to control the input current waveform. The circuit starts with an offline C6 charge. Once the work is started, the controller's power supply is provided by an auxiliary winding on the SEPIC inductance. A relatively large output capacitor limits the LED ripple current to 20% of DC current. In addition, the AC current and current in TM SEPIC are very high and need to be enamelled and low dissipated inner core plates to reduce the inductance loss.
Figures 2 and 3 show the experimental results of the prototype circuit matching the schematic diagram in figure 1. Compared with the European route, the efficiency is very high, up to 92%. This efficiency is achieved by limiting the ringing of the power device. In addition, as we can see from the current waveform, the power factor is very good at 96% efficiency. It is interesting to note that the waveform is not pure sine curve, but the rising or falling presents some slope, it was no measurement circuit into current and only for measurement of current switch. However, the waveform is sufficient to pass the European harmonic current.
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