Harmonic pollution is principally due to low power electronic loads installed in large numbers. In the public mains supply, domestic TV sets and office equipment account for about 80% of the problem. Other types of load which also take significant harmonic currents are not widely enough distributed to cause a serious problem, or are dealt with individually at the point of installation as in the case of industrial plant. Nevertheless, harmonic emission limits now apply to many classes of electronic products.

Non-linear loads

A plain resistive load – a heater or a filament lamp - across the mains draws current only at the fundamental frequency, but electronic circuits are anything but resistive. The universal rectifier-capacitor input draws a high current - typically between 5 and 10 times the average current - at the peak of the voltage waveform and zero current at other times, since the rectifier only conducts when the input voltage is greater than the DC link voltage, to charge the DC link capacitor. The triac phase control method for power control begins to draw current only partway through each half cycle. These current waveforms can be represented through the Fourier transform as a series of harmonics in the frequency domain, and it is the harmonic amplitudes that are subject to regulation.

The standard which covers mains harmonics up to 16A is IEC 61000-3-2, and its big sister IEC 61000-3-12 applies to higher currents. 61000-3-2’s requirements are either fixed limits for the harmonic content up to 2kHz (40th harmonic) or variable limits depending on the power drawn by the equipment; choice of limits depends on the class of product.

To quote the foreword of one manufacturer's application handbook (referenced below),

...harmonic reduction requirements as mandated by IEC 61000-3-2 stands out as the biggest inflection point in power supply architectures in recent years.

Series impedance

One way to reduce the harmonic content of the input current is to increase the series impedance in front of the rectifier. This slows the inrush of current into the DC link at the peak of the waveform and spreads out its duration over the half cycle. The impedance could be implemented either as resistance or as inductance.

Increasing input series resistance to meet the harmonic limits is expensive in terms of power dissipation except at very low powers. Deliberately dissipating between 10 and 20% of the input power rapidly becomes unreasonable above levels of 50–100W and is in opposition to the need for energy efficiency. In any case, the requirements of IEC 61000-3-2 do not apply to equipment having an active input power below 75W (except for lighting equipment). Alternatives are to include a series input choke, which since it must operate down to 150Hz at the full input current is expensive in size and weight, but will not lose much power; or to include electronic power factor correction (PFC), which converts the current waveform to a near-sinusoid but represents an increase in complexity.

Power factor correction

Power Factor is defined as the ratio of real power (in watts, average over a cycle of the instantaneous product of voltage and current) to apparent power (in VA, the product of the RMS value of current and the RMS value of voltage). If both voltage and current are in phase and sinusoidal, the PF is 1. If they are sinusoidal but out of phase, the PF is the cosine of the phase angle; this is the definition known by power engineers, but it only applies for linear systems where the loads are purely resistive or reactive. For non-linear, distorted current inputs, the power factor can be related to the total harmonic distortion (THD) expressed as a ratio (root-mean-square sum of harmonic components versus the fundamental):

PF          =          (1/(1 + THD²))^0.5

For electronic circuits, a Power Factor Correction (PFC) circuit is essentially a switched mode converter on the front-end of the supply. It fits well with contemporary design requirements such as the need for a “universal” (90–260V) input voltage range. Such power supplies are standard off-the-shelf, or you can design a PFC SMPS yourself. The wide availability of special-purpose control ICs makes the task easier. A useful source for an overview of PFC techniques is ON Semiconductor's Power Factor Correction Handbook.

The diagram shows the basis of operation of a widely-used power factor correction circuit. Instead of an input rectifier/reservoir combination, the rectified input feeds a switched mode boost pre-converter circuit directly whose operational input voltage range extends from near-zero to the peak supply voltage, and which provides the necessary DC link voltage. The pulse width and frequency of the switching circuit is regulated to give an average input current which approximates to the required sinusoidal waveshape. The effective distortion is very low, and so is the harmonic content. The switching inductor is working at high frequency and is therefore much smaller than an inductor that would directly attenuate the 50/60Hz harmonics. The disadvantage, of course, is that this will contribute extra RF switching noise at the same time. The input supply filter therefore needs to be designed with this constraint in mind.

Phase control

Power control circuits which vary the switch-on point with the phase of the mains waveform are another major source of harmonic distortion on the input current. Lighting controllers are the leading example of these.  The diagram above shows the harmonic content of such a waveform switched at 90° (the peak of the cycle, corresponding to half power). The maximum harmonic content occurs at this point, decreasing as the phase is varied either side of 90°. Lighting dimmers without input filtering or PFC of greater than about 5A rating are outlawed by IEC 61000-3-2, since the limits are set at an absolute value.