Input Harmonics and Total Harmonic Distortion

The current an LED driver draws from the mains is never a perfect sine wave. Every nonlinear load distorts the current waveform, and that distortion is described by harmonics: sinusoidal components at integer multiples of the line frequency. A 230V, 50Hz supply sees harmonics at 100Hz, 150Hz, 250Hz, and so on, superimposed on the fundamental. Total harmonic distortion, or THD, is a single number that summarizes how far the current waveform departs from a pure sine wave.

The distortion originates in the front-end rectifier. A conventional driver rectifies the AC input and charges a large bus capacitor, which conducts current only near the peaks of the AC voltage, when the rectified voltage exceeds the capacitor voltage. The result is a train of narrow current spikes instead of a sinusoidal draw. Those spikes are rich in odd harmonics: the third, fifth, seventh, and higher. In a 230V network, the third harmonic is particularly problematic because it can add up on the shared neutral conductor of three-phase systems.

Because harmonics from many devices add on the same distribution network, regulators limit them. The reference is IEC 61000-3-2, which classifies lighting equipment as Class C and sets harmonic current limits for equipment drawing up to 16A per phase. The limits depend on power: above 25W, the higher harmonics are limited to a percentage of the fundamental, with the third harmonic limited to 30 times the power factor in percent; below 25W, a less stringent moderate limit applies; and below 5W, introduced in the 2018 edition, no limit applies at all.

THD and power factor are two views of the same waveform. Power factor is the ratio of real power to apparent power, and for a purely sinusoidal voltage, it equals the displacement factor multiplied by the distortion factor. A current waveform with high harmonic content has a distortion factor well below unity, which drags down power factor even when the current is perfectly in phase with the voltage. This is why a driver with no power factor correction can show a power factor around 0.6, while an active PFC stage can push it above 0.95 while also reducing THD.

Active power factor correction is the standard solution. A boost PFC stage shapes the input current to follow the input voltage, drawing a near-sinusoidal current that is nearly in phase with the voltage. The result is a power factor close to 1 and a THD low enough to meet the Class C limits. The PV100 specifies a power factor above 0.95, which indicates an active PFC front end; without it, a 100W constant-voltage driver would struggle to meet the harmonic limits for lighting in the European market.

The practical consequence of harmonics is that they are a network-level problem solved at the device level. For a single driver, the harmonics are small; for a building with thousands, they can overheat neutrals, stress transformers, and trip nuisance protection devices. The buyer's checks are straightforward: confirm that the driver's power factor and THD are measured and published, verify the values against the harmonics standard for the target market, and prefer active PFC for installations with many drivers. A driver with PF above 0.95 and low THD is not just a better citizen on the grid; it is the difference between passing and failing a building's power quality review.