完美的PFC舞台是否必须提供统一的功率因数？

半导体

假设我们设计了一个完美塑造输入电流的PFC级。
我们会有几乎统一的功率因数（PF）吗？
不一定，EMI滤波器电容器不幸地会降低PF。
如果您不相信，请运行这个简单的模拟。
如图1所示，用一个具有完美PF的电阻代替PFC级，并添加一个由电容器和电感器组成的前端EMI滤波器。
您会注意到线电流是正弦的，但它与线电压不同相。
该电流实际上是电阻器Rin吸收的电流与流过C1和C2的无功电流之和。
该无功电流是正弦的，但相位提前90度。
实际上，我们可以定义给定输入电容可实现的最大功率因数。
简单计算的结果给出以下表达式：
要么：
根据（1）或（2），我们可以注意到：
输入电容和线路频率越大，输入电容产生的无功电流越高，因此PFmax越低。
功率越高或线路均方根电压越低，PFmax越高，因为两种条件都会导致PFC级吸收更高的电流。
因此，无功分量在总线电流中的相对贡献减小。
出于同样的原因，Rin越低，PFmax越高。
换句话说，该公式表明，与PFC级1相比，当无功电流幅度增加时PF变差。
图2显示了输入电容的影响可以通过播放线路频率来说明。
在左侧，您可以观察到安森美半导体的NCP1653 PFC控制器评估板（[1]）在300 W和230 V rms下使用50 Hz线电压工作时获得的线电流（蓝色）。
在这种情况下，THD低（7％），PF高（0.986）。
在图2的右侧，完成了相同的操作，但线路电压为400 Hz。
线电流仍然接近正弦曲线，但不再与输入电压（黑色迹线）同相。
THD没有变化，但功率因数更差（仅0.730！）。
此示例说明EMI滤波器设计在高频线路频率下尤其重要。
它进一步指示THD比PF更好地测量PFC级性能，因为功率因数可能受到EMI滤波器的影响。
还请注意，输入电容通常也放置在二极管桥上，以滤除整流后的线电压。
这种电容器与刚才讨论的EMI滤波器的输入电容器起着相同的作用。
然而，当PFC电流不足以在线过零点附近完全放电时，它会进一步降低THD。
因此，存在一段时间，在此期间PFC级仅由输入电容器馈电，并且没有电流从电源吸收。
因此，电流在零线交叉附近变形。
（[1]）NCP1653是一种连续导通模式PFC控制器。
凭借其后继产品（NCP1654），它为300 W以上的应用提供了非常紧凑的解决方案。

以上来自于谷歌翻译

以下为原文

Assume we have designed a PFC stage that perfectly shapes the input current. Will we have a nearly unity power factor (PF)? Not necessarily as the EMI filter capacitor unfortunately degrades the PF.
If you are not convinced, run this simple simulation.
As shown by Figure 1, replace the PFC stage by a resistor which exhibits a perfect PF and add a front-end EMI filter made of capacitors and an inductor. You will note that the line current is sinusoidal but that it is not in phase with the line voltage. This current is actually the sum of the wished current drawn by the resistor Rin with the reactive current flowing through C1 and C2. This reactive current is sinusoidal but 90-degree ahead of phase.

Actually, we can define the maximum power factor achievable with given input capacitors. The result of a simple computation gives the following expression:

or:

In light of (1) or (2), we can note that:

The larger the input capacitors and line frequency, the higher the reactive current generated by the input capacitors and hence the lower PFmax.
The higher the power or the lower the line rms voltage, the higher PFmax since both conditions cause a higher current to be drawn by the PFC stage. Hence, the relative contribution of the reactive component in the total line current reduces. For the same reason, the lower Rin, the higher PFmax.

In other words, this formula expresses that the PF gets worse when the reactive current magnitude increases compared to the PFC stage one.

Figure 2 shows that the input capacitor’s influence can be illustrated by playing with the line frequency. On the left side, you can observe the line current (in blue) obtained with ON Semiconductor’s NCP1653 PFC controller evaluation board ([1]) at 300 W and 230 V rms, operated with a 50-Hz line voltage. In this case, the THD is low (7%) and PF is high (0.986).
In Figure 2 right side, the same is done but with a 400-Hz line voltage. The line current is still nearly sinusoidal but it is no longer in phase with the input voltage (black trace). The THD is unchanged but the power factor is much worse (only 0.730!).
This example illustrates that the EMI filter design is particularly critical at high-frequency line frequencies. It further instructs that the THD gives a better measure of the PFC stage performance than PF since the power factor can be affected by the EMI filter.
Please also note that an input capacitor is also generally placed across the diodes bridge to filter the rectified line voltage. Such a capacitor plays the same role as the input capacitors of the EMI filter just discussed.
However, it further degrades the THD when the PFC current is not large enough to fully discharge it near the line zero crossing. There is hence a period of time during which the PFC stage is only fed by the input capacitor and no current is absorbed from the mains.
The current is hence distorted near the line zero crossing.

([1]) The NCP1653 is a continuous conduction mode PFC controller. With its successor (NCP1654), it provides a very compact solution for applications above 300 W.

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完美的PFC舞台是否必须提供统一的功率因数？

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