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Digital control ushers in a new era of PFC design

Posted: 20 Aug 2014  Print Version  Bookmark and Share

Keywords:Digital control  power factor correction  THD  total harmonic injection  PFC 

Digital control is becoming ever more popular in high-end power supply designs. This is especially true in the area of servers and telecommunications where ever-growing performance specifications make an analogue solution unsuitable. Modern digital controllers provide advanced control algorithms that greatly improve power supply performance.

For power factor correction (PFC), its performance is usually assessed by these criteria: total harmonic injection (THD), individual harmonics magnitude, power factor (PF), efficiency, and total solution cost. Unlike the traditional way of tuning the PFC control loop, this article provides several novel and practical methods [2] to address each of them. A high-performance PFC can be achieved by employing these methods.

Harmonic injection
For a high-end PFC design, it not only requires the THD not exceed a certain percentage with a specific load, but also requires each of the individual harmonics not exceed a specific limit, as defined in IEC 61000-3-2. Sometimes a new PFC design can pass the THD test, but fail with individual harmonics distortion tests. How to reduce a specific order of harmonic distortion is a big challenge in PFC design.

A harmonics injection method is developed such that the harmonics with high magnitudes are suppressed actively.

The fundamental frequency of input AC voltage can be measured by the digital controller. Based on the fundamental frequency, a desired high-order sinusoidal signal is generated. For example, if the PFCs 3rd harmonic is too high and needs to be reduced, then a 3rd order sinusoidal signal is generated. Depending on the application, if more than one order of harmonics needs to be suppressed, then the same order of sinusoidal signals are generated and combined together. Finally, these sinusoidal signals are injected into PFC current loop. As a result, the specific order of harmonics are suppressed.

The magnitude of each sinusoidal signal also can be dynamically adjusted based on operating conditions, such as load and input voltage, to maximise the suppression effect.

This method was tested on a 360W single-phase PFC. Harmonic analysers show that the 3rd and 5th harmonics are high in this PFC. A 3rd and 5th order sinusoidal signals are then generated and injected into the current control loop. As can be seen in figure 1, the 3rd and 5th harmonics are significantly reduced with the harmonic injection.

Figure 1: Harmonic comparison with/without harmonic injection.

As the individual harmonics are suppressed, the THD also gets improved. Figure 2 shows that THD is improved significantly after harmonic injection.

Figure 2: THD comparison with and without harmonic injection.

EMI filter X-Cap reactive current compensation
A typical FPC converter has an electromagnetic interference (EMI) filter at the input end (figure 3). Capacitors C1, C2, C3 and C4 are called X-Cap and used to absorb the current ripples.

Figure 3: A typical EMI filter for PFC.


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