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 LED 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.
Fig. 1: With active PFC, the current consumption is controlled by pulse-width modulation to near-sinusoidal shape
Fig. 2: Comparison of incandescent bulb and
LED driver with power factor of less than one
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.
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 standard therefore demands that LED drivers rated 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 (refer Fig. 1).
A well-designed PFC circuit in some LED drivers increases the power factor to a value of around 0.95, thus exceeding the stringent EnergyStar requirements as well as EN 61000 specifications. Although it is technically possible to achieve even better values, the associated costs outweigh the benefits.
While EN 61000-3-2 requires a power factor of greater than 0.9 only from 25W upwards, active PFC also makes sense at lower power rates. This becomes obvious if one considers that many circuits include a large number of small- or medium-power LED luminaires or consist of small luminaire clusters with separate drivers. Since ten 12W loads consume a total of 120W, mains network operators would probably appreciate it if a proper power factor correction was applied. This is why some manufacturers offer products with active PFC from as low as 12W.
Relationship between PFC and efficiency
Many people incorrectly believe that a driver with a low power factor offers poor efficiency. While such drivers consume considerably more energy from the mains than is required for powering the LEDs, a large share of this power is actually fed back to the mains network. This share is thus not lost, as would be the case with a low-efficiency device. It is simply fed from the ‘wrong’ side. This is probably the reason why many people confuse the power factor value with efficiency.
Fig. 2 compares the current consumption of a 100W incandescent lamp (red curve) with that of a 25W LED. Both devices produce about the same amount of light. The incandescent bulb with a power factor of one consumes a constant current of 0.45A from the 230V mains network. With an LED driver of power factor one, the current consumption would be around 0.11A. At a power factor of 0.95, it would be slightly higher. At a power factor of 0.25, the current consumption would amount to 0.45A, which corresponds to that of the incandescent lamp—the actual LED output would, however, only be 25W. The remaining 75W is returned through the ‘wrong’ phase back to the mains. The energy is thus not lost and the reactive current is not metered by the power meter.
For AC/DC drivers, active power factor correction is, however, as important as high efficiency, especially if one takes into account that billions of such drivers will be connected to the mains over the next few years. Power factor correction is therefore not so much geared towards keeping electricity costs down, but helps maintain the quality of the mains power by eliminating harmonic interference.
Rapid pace of development in the field of power LEDs and drivers
For the foreseeable future, the industry is focusing on solutions for the existing infrastructure in residential and office buildings. For LED luminaires, this means that they must thus be dimmable with conventional TRIACs. This creates a number of technical problems, since the leading or trailing edge control of dimmers and PFC circuits of drivers interfere with each other. Conventional drivers can therefore not be dimmed down to zero.
A function that allows dimming to 10 or 20 per cent is, however, not satisfactory, as conventional incandescent lamps can be dimmed to much lower levels. In addition, the colour temperature of a dimmed incandescent lamp is shifted to much warmer levels, while LEDs show no such shift. The 10 per cent brightness of an LED luminaire powered with a residual current is therefore perceived by the eye as much higher, equivalent to about 35 per cent brightness of an incandescent lamp. Dimming to levels below 5 per cent is thus even more crucial for LED lighting systems.
Fig. 3: Thermal image shows relatively homogeneous heat distribution in MegaZenigata (Image courtesy: Sharp)
In the recent past, LEDs have also been developed further at a rapid pace. Initially, several individual 2W or 3W LEDs with separate housings were combined on a PCB. Today, the trend is clearly towards multi-chip solutions. For this purpose, a number of small LED dies are mounted on ceramic chips. The ceramic substrate improves heat management across the entire LED array (Fig. 3).
In addition, such LED arrays require much less space, and the entire luminous surface is covered by a phosphorus coating so that the multi-chip LEDs appear as one single light source. This facilitates the design of reflective and optical devices.
Fig. 4: The new MegaZenigata from Sharp (left) contains 168 individual LEDs combined in an array (in series and parallel) powered by approx. 38V DC and a constant current of 700 mA (Image courtesy: Sharp)
This can be well illustrated by the 25W MegaZenigata from Sharp: a total of 168 LEDs arranged over an area of just below 2 cm2 (Fig. 4) are wired to form an array. This array can then be mounted on the MegaZenigata, using a specially devised LED driver.
The LED driver provides a constant-current output of 700 mA up to 42V, so that the MegaZenigata produces 2600 lumens at 4000°K, corresponding to the luminous flux of a 150W halogen spotlight. Of course, the quality of the light plays a major role. While daylight reaches a colour rendering index of 100, the MegaZenigata achieves a respectable 83. The MegaZenigata is thus not only efficient but also offers a light quality and a colour temperature close to that of natural light.
In the future, manufacturers of LED drivers will cooperate even more closely with LED chip producers in order to take full advantage of the possibilities of new lighting technology. While energy efficiency and long service lives remain the main concerns, quality of the light is also a major issue, since it determines how we perceive the light.
The trend towards LED lighting systems will bring billions of new drivers into the global market over the next few years, which all need to be connected to mains networks. Because they will all produce harmonics and reactive currents, drivers need to have not only a high efficiency rating but also a good power factor. Values around 95 per cent should therefore be considered long-term guide values, even if they are not yet required by the relevant standardisation organisations.
The author is vice president-marketing & sales, RECOM Lighting