Power Factor and THD: Why These Two Numbers Belong on Every Driver Datasheet

Two numbers show up on the datasheet of every modern LED driver: power factor and total harmonic distortion. Buyers often treat them as compliance boxes. They are not. They describe how cleanly the driver draws current from the wall, and they have real consequences for the electrical infrastructure that feeds the lighting.

Power factor is the ratio of real power, measured in kilowatts, to apparent power, measured in kilovolt-amperes. For a perfectly resistive load such as a heater, the current and voltage are in phase and the power factor is 1.0. Switching power supplies without correction draw current in short pulses near the peaks of the AC sine wave. The current is not in phase with the voltage, and it contains multiple frequencies. The result is a power factor around 0.5 to 0.6, which means the wiring carries 60 percent more current than the actual power being delivered to the LEDs.

Total harmonic distortion measures how much of that current is riding at frequencies other than the 50 or 60 Hz fundamental. A high-THD driver injects harmonics into the building's neutral conductor, where odd-numbered harmonics add rather than cancel. This overheats neutral cables, stresses transformers, and interferes with sensitive loads on the same distribution circuit. Utilities in many regions bill customers for the reactive power that low power factor creates, and commercial rebate programs such as ENERGY STAR and the DesignLights Consortium specify minimum power factor and maximum THD as conditions of approval.

The regulatory framework in the EU and much of Asia is IEC 61000-3-2, which sets harmonic current limits by equipment class. Lighting equipment above 25 W falls into Class C. The limits are expressed as a percentage of the fundamental input current: the third harmonic is capped at 30 times the power factor, the fifth at 10 percent, the seventh at 7 percent, the ninth at 5 percent, and the eleventh at 3 percent. Meeting these limits for a 100 W driver without active power factor correction is effectively impossible, which is why all contemporary drivers above a certain size include a PFC stage.

Active PFC uses a boost converter between the rectifier and the main DC rail. It shapes the input current so that it follows the voltage waveform closely, raising power factor above 0.9 and pushing THD below 20 percent, often into the single digits. The cost is a slightly more complex circuit and a small efficiency penalty at very light loads. At full load, a good driver still reaches 90 percent or better efficiency, which means less heat to get rid of in a sealed enclosure.

Specifiers should read the numbers at the load they actually intend to run. Power factor and THD are load-dependent; a driver rated at 0.95 PF at full load may drop to 0.85 at 20 percent load. If the installation dims to a low level for much of the day, the part-load PF matters more than the nameplate full-load number. The curves on the datasheet, not the headline value, tell the real story.

There is an unavoidable engineering tension between efficiency and power factor at light loads. The active PFC stage draws a small quiescent current to keep its control loop alive, and at very low output that quiescent current is a larger fraction of the total input draw, which pulls the power factor down. A driver that dims to 1 percent for most of the night therefore has a worse PF at midnight than at midday. Some manufacturers respond by disabling PFC below a threshold, which improves efficiency but worsens harmonics at that operating point. The datasheet's PF-versus-load curve shows exactly where that trade-off lands, and a project that runs dimmed most of the time should size the neutral conductor based on the worst-case part-load THD rather than the full-load nameplate.