We generally assume that the output waveform of a power supply with active PFC is a sine wave. However, in reality, the input current waveform of a power supply with PFC is not a perfect sine wave. Instead, it is a "corrected sine wave" or "pulsating DC wave" that is "pieced together" by high-frequency switching and closely follows the voltage phase change. The reason it is not a smooth sine wave is determined by its working principle.
Do you know what is the function of the power supply PFC? One function is Making the phase of the input current consistent with the phase of the input voltage (eliminate phase difference and improve power factor);Another is Suppress harmonics in the input current to make its shape as close as possible to a sine wave in order to meet relevant electromagnetic compatibility standards (such as EN 61000).
The most important is PFC aims for "in-phase" and "low harmonic distortion," rather than a mathematically perfect sine wave. As long as the current waveform is sufficiently "sinusoidal" and the harmonic content is below the standard limit, it is qualified.
With the active PFC switching power supply Like a high-speed switching water pump. In order to obtain energy from AC mains power, it needs to rectify AC to DC first. The traditional solution of increasing the capacitor in the rectifier bridge will cause the current to only conduct near the voltage peak, forming sharp pulse current (high harmonics).
We know the power supply PFC,how about the waveform? What exactly does its waveform look like? From the low-frequency perspective: its envelope is a sine wave in phase with the input voltage. This is a result of PFC chip control;On a high-frequency scale: This sinusoidal envelope is composed of countless high-frequency sawtooth or triangular waves connected together. Each sawtooth corresponds to one PFC switching cycle. The current rises when the switch is on and falls when the switch is off, forming a high-frequency ripple.If the oscilloscope has a high enough bandwidth, these high-frequency switching ripples can be clearly seen superimposed on the 50Hz/60Hz fundamental frequency.Therefore, when you measure the input current waveform of a power supply with PFC and find that it is not a smooth sine wave, this is usually normal.But how to evulate the PFC circuit quality? There are two key indicators:one is whether the power supply power factor is more than 0.9 or not? Another is whether the THD is too less as soon as possible,such as less than 10%.
As long as these two indicators meet the requirements, the PFC circuit is successful and compliant even with high-frequency switching ripple in the current waveform. These ripples can be further smoothed using the input filter inductor (part of the EMI filter), but cannot and need not be completely eliminated.
So if the switching power supply output waveform is not the perfect sine wave,you don't think the dc power supply is not good,you need to check the power supply power factor value(PF value) and the THD.
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