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2016August

High efficiency drive LED off line lighting

From: www.szbako.com Data:10/08/2016

in street lighting applications, a feasible configuration is to create 80 series of 300V/0.35 amp load LED. Isolation and power factor correction (PFC) requirements are required when selecting a power supply topology. Isolation requires a large number of security trade-offs, including a comparison between the protection requirements and the complexity of power supply design.
with practical screw thread screw type led to replace incandescent lighting may also need a few years time, and in architectural lighting in the use of LED is continuously increasing, which has higher reliability and energy saving potential. Like most electronic products, it requires a power supply that converts the input power to the LED's available form.

in street lighting applications, a feasible configuration is to create 80 series of 300V/0.35 amp load LED. Isolation and power factor correction (PFC) requirements are required when selecting a power supply topology. Isolation requires a large number of security trade-offs, including a comparison between the protection requirements and the complexity of power supply design. In this application, led to the existence of high pressured, that is non essential isolation and PFC is necessary, because in Europe for more than 25 watts of lighting are required to have PFC function, and this product is for the launch of the European market.

for this application, there are three optional power supply topology: buck topology, transfer mode reverse topology and transfer mode (TM) single ended primary inductor (SEPIC) topology. When the LED voltage is about 80 volts, the buck topology can be used to meet the requirements of harmonic currents. In this case, the higher load voltage will not be able to continue using the buck topology. Then, a more eclectic approach is to use the reverse topology and SEPIC topology. SEPIC has the advantages is that the clamp power semiconductor device switching waveforms and allows the use of low voltage, thereby the device is more efficient. In this application, the efficiency improvement of about 2% can be obtained. In addition, ringing less SEPIC, which makes EMI easier to filter. Figure 1 shows the schematic diagram of the power supply.

Figure 1 transfer mode SEPIC played a role in a simple LED driver
the circuit uses a boost PFC TM controller to control the input current waveform. The circuit to offline for C6 charging as a start. Once the work is started, the power supply of the controller is supplied by an auxiliary winding on a SEPIC inductor. A relatively large output capacitor will limit the LED ripple current to 20% of the DC current. Note, TM SEPIC AC flux and current is very high and need to paint package strand and low loss inner core plate to reduce the loss of inductance.

Figure 2 and figure 3 show the experimental results of the prototype circuit matching the schematic in figure 1. Compared with the European line, its efficiency is very high, up to 92%. This is a high efficiency by limiting the power device based on ringing. In addition, as we see from the current waveform, the power factor is very good at 96% efficiency. Interestingly, the waveform is not pure sine curve, but in the rising edge and falling along with a number of gradient, which is the circuit does not measure the input current and only for the measurement of the switching current. However, the waveform is sufficient to pass the requirements of the European harmonic currents.

Figure 2 SEPIC
with good efficiency and high PFC efficiency TM Figure 3 line current easily through the Class C EN61000-3-2 standard

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