Power Factor and Total Harmonic Distortion: What They Mean for LED Driver Performance

Two input-side parameters are easy to overlook on an LED driver datasheet, yet they determine how politely a driver behaves toward the electricity grid and whether a commercial installation passes acceptance testing. Power factor (PF) and total harmonic distortion (THD) describe the quality of the current the driver draws, and they deserve the same attention as efficiency and output specifications.

Power factor is the ratio of real power, the power actually doing useful work, to apparent power, the product of RMS voltage and RMS current. An ideal load has a PF of 1.0. A simple switch-mode power supply without correction typically draws current only in narrow pulses near the peaks of the AC voltage waveform, which lowers the power factor to roughly 0.5 to 0.6. The missing energy is not consumed; it circulates as reactive and harmonic content that still heats wiring and transformers.

THD quantifies the distortion of the input current waveform. A nonlinear load such as an uncorrected power supply generates odd-order harmonic currents, predominantly the 3rd, 5th, and 7th harmonics, in addition to the 50 or 60 Hz fundamental. THD expresses the total magnitude of these harmonic components as a percentage of the fundamental. High THD means a polluted current waveform that can overload neutral conductors, overheat distribution transformers, and interfere with other equipment.

The engineering solution is a power factor correction (PFC) circuit. Active PFC, implemented as a boost converter between the input rectifier and the main converter, actively shapes the input current to follow the sinusoidal input voltage. A driver with active PFC typically achieves a power factor above 0.9 at full load, often 0.95 or higher, and a THD below 20 percent, with well-designed products under 10 percent. Passive PFC, based on a large inductor or valley-fill circuit, is cheaper but delivers a lower power factor and higher THD.

Harmonic emissions from equipment drawing up to 16 A per phase are regulated by IEC 61000-3-2. Lighting equipment, including dimming devices, falls under Class C of the standard, and drivers intended for the EU market must demonstrate compliance. Drivers with active PFC typically meet the Class C limits with comfortable margin.

The practical consequences of ignoring PF and THD show up on site. Low power factor increases the RMS current on the feeder for a given real power, which means higher I²R losses in cables and a greater burden on upstream transformers and generators. High harmonic content can cause the shared neutral conductor in three-phase installations to carry more current than any single phase, a known fire risk in buildings with many uncorrected LED drivers.

Dimming adds a complication worth understanding. The power factor of a driver tends to fall as the output is dimmed, because the input power drops while the control circuitry and power factor correction stage continue to draw a baseline current. A driver that achieves PF 0.95 at full load may drop below 0.8 at 10 percent output. For projects with strict requirements, the datasheet PF curve across the dimming range is more useful than the single full-load figure.

When specifying a constant-voltage DALI-2 driver, check whether the datasheet states both PF and THD, whether the values are measured at rated load, and whether active PFC is used. Drivers sold for commercial projects in Europe should meet IEC 61000-3-2 without external filtering. Combined with high efficiency, a PF above 0.9 and THD below 20 percent at full load are reasonable minimum expectations for a professional product.