Incandescent bulbs are essentially ohmic resistors and consume sinusoidal current from the mains grid. The power factor of these devices is essentially 1. With LED lights, things are somewhat more complex. LEDs are semiconductors operated with direct current. Their characteristic shows a marked kink at approximately 3V. When the maximum value is exceeded, the LED might be destroyed. LEDs therefore require special drivers that convert the mains voltage to a constant direct current. This constant current ensures that all LEDs in a chain are lit at equal brightness – irrespective of the threshold voltage. Such drivers are, however, not ohmic resistors, but instead consumers with a power factor that tends to be far below 1. This leads to the reflection of harmonics back to the mains, resulting in undesired reactive currents.

Pulsed direct current causes problems
The above problem results from the need to convert alternating current into constant direct current. To do this, the current must be rectified and stabilised by a capacitor with sufficient capacitance. The capacitor is charged through the half wave to its peak value and supplies energy until the next half wave reaches the capacitor value. If the voltage at the rectifier is greater than that from the capacitor, a brief high-amplitude current is generated during the respective half wave. This current peak is much higher than would be expected based on the power rating. The resulting current is no longer sinusoidal and includes a large share of harmonics (the steeper the edge, the higher the harmonic share). This problem arises from the fact that the alternating current needs to be rectified at the input and smoothed before it can be used further down the line. If a converter is installed to generate the required constant current from the high direct voltage, the situation becomes even worse.

 

Figure 1: With active PFC, the current consumption is controlled by pulse width modulation to near-sinusoidal shape.

 

Pulse width modulation corrects power factor
Since it is expected that LED lighting systems will replace other lighting solutions across the board, corrective measures must be taken in order to ensure that the mains quality does not deteriorate too much. EN 61000-3-2 therefore demands that LED drivers 25W and higher come with power factor correction (PFC). "EnergyStar" is even more explicit, prescribing a power factor of 0.9 or better for commercial drivers. Without active PFC, it is, however, only possible to reach values that are significantly lower – around 0.5 or even less, depending on the power rating. AC/DC LED drivers therefore need to be equipped with special PFC circuits. Their principle is straight-forward: instead of connecting the charging capacitor directly to the rectifier, a pulse width modulator is installed between the two components. This modulator ensures that the capacitor is charged by several small current pulses during the half wave. The current consumption is therefore more or less synchronised with the mains voltage and approximately sinusoidal (figure 1).

A well-designed PFC circuit such as the one in the RACD series from RECOM Lighting increases the power factor to a value of around 0.95, thus exceeding the stringent "EnergyStar" requirements as well as the EN 61000 specifications. Although it is technically possible to achieve even better values, the associated costs outweigh the benefits.