When it comes to power supplies, two important metrics that are often discussed are power factor and total harmonic distortion (THD). In this article, we will discuss what power factor and THD are, why they are important, and how they affect the performance of a power supply.
What is Power Factor?
Power factor (PF) is a measure of how efficiently a power supply converts the incoming AC voltage into the DC voltage that is needed to power electronic devices. It is defined as the ratio of the real power (in watts) to the apparent power (in volt-amps). A power factor of 1.0 means that all of the power is being used to perform useful work, while a power factor of less than 1.0 means that some of the power is being wasted.
The efficiency of a power supply is affected by the way in which it draws power from the AC source. If the power supply draws power in a non-linear manner, it can create harmonics that can cause power quality issues in the electrical distribution network. This is where total harmonic distortion (THD) comes into play.
What is Total Harmonic Distortion (THD)?
THD is a measure of the distortion in an AC voltage or current waveform caused by the presence of harmonics. Harmonics are multiples of the fundamental frequency of the AC waveform, and they can cause distortion in the waveform by creating peaks and troughs that are not present in the pure sine wave. THD is expressed as a percentage and is calculated by dividing the root-mean-square (RMS) value of the harmonic content by the RMS value of the fundamental waveform.
Why are Power Factor and THD Important?
Power factor and THD are important metrics for power supplies for several reasons:
Energy Efficiency: A power supply with a low power factor will draw more current than is necessary to deliver the required power to the load. This results in wasted energy and reduces the overall energy efficiency of the system. Power factor correction (PFC) techniques can be used to improve the power factor and reduce energy waste.
Power Quality: THD can cause distortion in the AC waveform, which can create issues with power quality in the electrical distribution network. High THD levels can cause problems with voltage stability, electromagnetic interference (EMI), and equipment damage.
Compliance: Many countries have regulations on power factor and THD for electronic equipment. Compliance with these regulations is necessary to ensure that electronic equipment operates safely and does not cause power quality issues in the electrical distribution network.
How to Measure Power Factor and THD
Power factor and THD can be measured using a power analyzer, which is a specialized instrument that is used to measure various electrical parameters such as voltage, current, power, power factor, and THD. The power analyzer typically measures the AC waveform at the input of the power supply and calculates the power factor and THD values.
Improving Power Factor and THD
Power factor can be improved through the use of power factor correction (PFC) techniques. PFC circuits can be designed to reduce the amount of current that is drawn from the AC source, which can improve the power factor and reduce energy waste. There are two main types of PFC circuits: passive PFC and active PFC. Passive PFC uses a passive component, such as a capacitor, to correct the power factor, while active PFC uses a circuit that actively corrects the power factor by controlling the input current.
THD can be reduced by designing the power supply to have a more linear load characteristic, which reduces the amount of harmonic content in the AC waveform. This can be achieved through the use of filter circuits, such as LC filters or active filters.
Conclusion
Power factor and total harmonic distortion are two important parameters of a power supply that can affect the performance and efficiency of electrical devices. A high power factor and low THD are desirable for ensuring the safe and reliable operation of electrical systems. By using power factor correction capacitors, harmonic filters, high-efficiency electrical devices, and good power management practices, it is possible to improve power factor and reduce THD, resulting in improved efficiency, reduced energy costs, and increased reliability of electrical devices.
Today’s commercial, industrial, retail and even domestic premises are increasingly populated by electronic devices such as PCs, monitors, servers and photocopiers which are usually powered by switched mode power supplies (SMPS). If not properly designed, these can present non-linear loads which impose harmonic currents and possibly voltages onto the mains power network. Harmonics can damage cabling and equipment within this network, as well as other equipment connected to it. Problems include overheating and fire risk, high voltages and circulating currents, equipment malfunctions and component failures, and other possible consequences. A non-linear load is liable to generate these harmonics if it has a poor power factor. Other loads can present poor power factors without creating harmonics. This post looks at these issues, the circumstances that can lead to damaging harmonic generation, and practical approaches to reducing it.
The two causes of poor power factor
At the simplest level, we could say that an electrical or electronic device’s power factor is the ratio of the power that it draws from the mains supply and the power that it actually consumes. An ‘ideal’ device has a power factor of 1.0 and consumes all the power that it draws. It would present a load that is linear and entirely resistive: that is, one that remains constant irrespective of input voltage, and has no significant inductance or capacitance. 1 shows the input waveforms that such a device would exhibit. Firstly, the current waveform is in phase with the voltage, and secondly both waveforms are sinusoidal.
In practice, some devices do have unity power factors, but many others do not. A device has a poor power factor for one of two reasons; either it draws current out of phase with the supply voltage, or it draws current in a non-sinusoidal waveform. The out of phase case, known as ‘displacement’ power factor, is typically associated with electric motors inside industrial equipment, while the non-sinusoidal case, known as ‘distortion’ power factor, is typically seen with electronic devices such as PCs, copiers and battery chargers driven by switched-mode power supplies (SMPS). We shall look briefly at the displacement power factor before moving on to the distortion case, which is of more immediate concern to electronic power system designers. However it is important to be aware of both cases. For example, some engineering courses discuss the power factor issue only in terms of motors, which causes confusion when their students later encounter poor power factor as exhibited by an SMPS.
Electric motors and displacement power factor problems
Electric motors create powerful magnetic fields which produce a voltage, or back electromotive force, in opposition to the applied voltage. This causes the supply current to lag the applied voltage. The resulting out of phase current component cannot deliver usable power, yet it adds to the facility’s required supply capacity and electricity costs. Fitting capacitors across motors reduces the phase lag and improves their power factor.
SMPS and distortion power factor problems
Whereas displacement power factor loads do not cause harmonics and their associated problems, distortion power factor loads such as SMPS will do so unless their power factor is improved.
An SMPS’s AC front end typically comprises a bridge rectifier followed by a large filter capacitor. This circuit only draws current from the mains when the line voltage exceeds that across the capacitor. This causes current to flow discontinuously, resulting in the non-sinusoidal current waveform shown in 2.
It is possible to use Fourier transforms, a mathematical process, to analyze this waveform and break it down into a set of sinusoidal components. These comprise the fundamental frequency – 50Hz in Europe, 60Hz in America – and a set of predominantly odd multiples of the fundamental, known as harmonics. The third harmonic is 150Hz (or 180Hz), the fifth, 250Hz (300Hz) and so on. 3 shows a typical harmonic spectrum for an electronic SMPS load. The fundamental component is usefully consumed by the SMPS, while the harmonics are reactive and create the problems described above. The ratio of the fundamental amplitude to the sum of all the harmonic amplitudes gives the device’s power factor.
International standard
An international standard exists to describe and set acceptable limits for a product’s mains harmonics generation. Within the EU, its reference is IEC -3-2, covering equipment power levels from 75W to 600W. The standard assigns equipment into four Classes – A, B, C and D. Class D covers personal computers, personal computer monitors and television receivers.
Established and innovative PFC solutions
Although passive power factor solutions exist, the general industry view is that active designs offer the best power factor improvements. These are typically based on boost converter technology, as in the example shown in 4.
