As an engineer with TI’s high-performance isolated power team, I work with server and telecom companies which typically need a high-performance power supply. Developing a high-end power factor correction (PFC) design requires not only the total harmonic distortion (THD) to be below a certain percentage with a specific load, but also requires each of the individual harmonics to not exceed a specific limit, as defined in the IEC 61000-3-2 compliance standard. In this two part blog, I will introduce a harmonic injection method to reduce PFC harmonics and improve THD.
Table 1, IEC 61000-3-2 harmonics limit
Traditionally, loop tuning is an effective way to reduce harmonic distortion and improve THD. However, I have seen a PFC design pass the THD test, but fail the individual harmonics distortion test, no matter how hard the engineer tried to tune the control loop. Traditional loop tuning doesn’t help much in this case. A new method is needed to deal with individual harmonics.
In single-switch three-phase rectifier designs, I’ve seen engineers inject a third order current signal to reduce THD. This made me think of a similar method where the harmonics with high magnitudes can be suppressed (compensated). The method should be easy to use, involve no extra hardware cost and be flexible so that any order of harmonics can be suppressed.
With the help of a digital power controller such as TI’s UCD3138, I developed a simple harmonic injection method in which any order of harmonics can be suppressed (compensated) actively. As a result, the specific order of harmonic magnitude is reduced and the THD is improved.
The basic idea is to generate a specific order of sinusoidal signal and inject this signal into the PFC current control loop. The magnitude of the sinusoidal signal can be dynamically adjusted based on operating conditions, such as load and input voltage, to maximize the suppression effect.
More specifically, in digital controlled PFC, the firmware is divided into two major loops: a background loop for low priority and low speed tasks, such as housekeeping; and an interrupt loop for high priority and time critical tasks, such as PFC state machine.
Follow these steps to implement this method, in background loop:
This process is demonstrated below:
Figure 1: Flow chart of harmonic injection in background loop
Next week, I will cover how to inject this sinusoidal signal into PFC control loop along with an example of this method in practice. Stay tuned by email subscribing to this blog in the top right-hand corner of this page. If you have any questions about this new method, please leave them in the comments below.
PFC Harmonic Injection
where is the 2nd part of this article???
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