1. The relationship between total harmonic distortion THD and power factor PF
There are many LED driver power supplies on the market, and the input circuit uses a simple bridge rectifier and a rectification filter circuit of the electrolytic capacitor, as shown in Figure 1.
figure 1
The circuit only turns on the rectifier diode near the peak of the input AC voltage, so its conduction angle θ is relatively small, about 60°, so that the input current waveform is a sharp pulse, and the pulse width is about 3ms, which is half. One-third of the cycle (10ms). The input voltage and current waveform are shown in Figure 2. It can be seen that the root cause of the input current distortion of the LED power supply is caused by the capacitive load of the DC filter electrolytic capacitor.
figure 2
Non-sinusoidal waves that are distorted by the input current of the LED driver must be described in Fourier series. According to the Fourier transform principle, the instantaneous input current can be expressed as:
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