The EN 61000-3-2 standard establishes four classes of equipment, each with its own harmonic emission limits:
Class D: televisions, personal computers, and monitors that consume power
; Class C: lighting equipment;
Class B: portable tools;
Class A: all others
. Class A and B equipment have absolute harmonic limits regardless of input power. Class C equipment has limits expressed as a percentage of the 50 Hz current consumed. Class D equipment has limits that are proportional to the mains power consumed. Equipment classified in classes C and D will normally require a power supply that incorporates active power factor correction.
Rectification Process and Storage Capacitor Operation:
In the diagram below, the incoming AC voltage waveform is identified as VLINE. The dashed line represents the rectified AC voltage after the rectifier bridge.
The storage capacitor charges during the conduction angle and slowly discharges through the power supply stage (VCAP). As soon as the incoming sine wave voltage drops below the storage capacitor voltage, the bridge rectifier diode is reverse-biased, and no current flows until the incoming rectified sine wave is again above the storage capacitor voltage. The conduction angle is typically 2–3 ms.
The complex incoming current waveform generates the harmonics that are of concern to the power generator. Harmonics contribute to the apparent power (real power and apparent power are discussed in more detail later). The current waveform shown will result in a power factor of around 0.5–0.6.
Why is harmonic distortion a problem?
The utility company must supply the voltage and all the current, even though some of it isn't converted into useful output power. The company has no way to charge for the extra current, since energy is billed in kWh.
The combined effect of millions of power supplies is to clip the AC voltage, as all the current is consumed at the peak of the sine wave. Power conductors must be sized to handle the extra current caused by the low power factor. Neutral conductors can overheat because they are not typically sized to carry all the harmonic currents, which don't exist in loads with a high power factor.
What is passive power factor correction?
To comply with harmonic distortion legislation, there are two main solutions available for power supplies. We will first discuss passive power factor correction, which typically involves adding a line-frequency inductor or resistor to the AC line. The inductor's effect is to flatten the current waveform, as it is a reactive component that resists changes in current. The resistor's effect is to reduce the peak current. The smoother the current waveform, the lower the harmonic distortion.
When is passive power factor correction appropriate?
Passive power factor correction is a very simple solution, but its advantages and disadvantages must be considered. It is not practical for power supplies over 300 W due to the size of the components required to provide adequate inductance at 50/60 Hz and keep resistive losses sufficiently low. This solution is not suitable for lighting, personal computing, or color television applications, but it is a viable solution for Class A equipment.
Figure 4 shows a real-time measurement of passive power factor correction and harmonic current levels.
Active Power Factor Correction
The second solution to consider is active power factor correction. This uses a high-frequency boost converter to electronically control the waveform of the incoming current. The incoming AC voltage is monitored and used as a reference to determine the width of each pulse of the high-frequency switched current.
The current is drawn in a series of pulses at approximately 100 kHz, which is equivalent to 2000 pulses per cycle of the mains voltage.
The low-pass EMC filter captures and eliminates the high-frequency component, so the current seen by the electrical grid is sinusoidal. The system regulates the DC output to approximately 400 VDC. The diagram below shows the real-time measurement of the active power factor correction.
Comparison between Passive and Active Power Factor Correction:
Passive power factor correction offers simplicity and reliability, while also helping to reduce noise (EMI) and aiding in filtering. However, it has disadvantages such as the need for heavy and bulky components, AC range switching, and a tendency toward a lower power factor. These and other factors can limit the use of multiple power supplies within a system.
On the other hand, active power factor correction provides a high power factor (>0.9), low inrush current, and compatibility with universal inputs. It also ensures a regulated high-voltage bus, maintenance time that is less dependent on the input voltage, and the ability to use multiple power supplies. However, it entails higher costs, greater complexity, a larger number of components, and a potentially lower calculated mean time between failures (MTBF).
Abstract:
Managing harmonic currents in power supplies is essential to ensure regulatory compliance, maintain power quality, and improve energy efficiency. Furthermore, it allows for the reduction of electromagnetic interference (EMI) problems, extends equipment lifespan, and controls operating costs.
Author: Kent Smith, XP Power - https://www.xppower.com
