Abstract: Whether it is to protect the safety of power systems or to protect the safety of electrical equipment and personal safety, it has become a consensus to strictly control and limit the current harmonic content to reduce the harm caused by harmonic pollution.
1. The relationship between total harmonic distortion THD and power factor PF
There are many LED driving power supplies on the market, and the input circuit uses a simple bridge rectifier and a rectifying and filtering 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:
For each current harmonic, there is usually a sine or cosine period, and the nth harmonic current RMS value In can be calculated by:
Input total current RMS
In the root of the above formula, I1 is the effective value of the fundamental current, and the remaining I2, 3 represent the effective values ​​of the 2, 3, ... n harmonic currents, respectively. The total harmonic content of the current expressed as a percentage of the fundamental current is called total harmonic distortion (THD). The total harmonic content reflects the distortion characteristics of the waveform, so it is also called the total harmonic distortion rate. defined as
According to the definition of the power factor PF, the power factor PF is the ratio of the active power P of the AC input to the input apparent power S, ie
Where is the input power supply voltage; U cosΦ1 is called the phase shift factor, which reflects the phase relationship between the fundamental current i1 and the voltage u, Φ1 is the fundamental phase shift angle; the input fundamental current rms I1 and the input total current rms Irs The percentage of K=I1 / Irms is called the input current distortion factor. The above equation shows that in a nonlinear switching power supply circuit such as an LED driving power supply, the power factor PF is not only related to the phase between the fundamental current i1 and the voltage u, but also related to the input current distortion coefficient K. Substituting equation (6) into equation (7), the power factor PF has the following relationship with the total harmonic distortion THD:
The above formula shows that when the phase shift factor cosΦ1 is constant, reducing the total harmonic distortion THD can improve the power factor PF; vice versa, the higher the PF, the smaller the THD. For example, by calculation, when the phase shift angle Φ1=0, THD=30% @ PF=0.9578; THD=10% @ PF=0.9950.
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