Not all LED drivers provide ‘clean’ light
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 stabilized 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.
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 synchronized with the mains voltage and approximately sinusoidal (fig. 1).
Fig. 1: With active PFC, the current consumption is controlled by pulse width modulation to near-sinusoidal shape.
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.
While EN 61000-3-2 requires a power factor of >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 of120W, mains network operators would probably appreciate it if a proper power factor correction was applied. This is why RECOM Lighting offers products with active PFC from as low as 12W.
To meet the stringent non-interference requirements, RECOM has developed the RACT-20 with active PFC. Although the inclusion of power factor correction in a TRIAC dimmable LED driver is relatively costly, this 20W driver comes with integrated active control and a power factor of 0.95.
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 the powering of 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.
Figure 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 1 consumes a constant current of 0.45A from the 230V mains network. With an LED driver of power factor 1, 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.
Fig. 2: Comparison of incandescent bulb and LED driver with power factor <1
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 for dimming to 10 or 20% is, however, not satisfactory, as conventional incandescent lamps can be dimmed to much lower levels. In addition, the color temperature of a dimmed incandescent lamp is shifted to much warmer levels, while LEDs show no such shift. The brightness of an LED luminaire powered with a residual current of 10% is therefore perceived by the eye as much higher, equivalent to a brightness of about 35% of an incandescent lamp. Dimming to levels below 5% is thus even more crucial for LED lighting systems. The RACT20 from RECOM Lighting available since March 2012 offers flicker-free dimming to 0%.
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 the heat management across the entire LED array (fig. 4).
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 the reflective and optical devices.
This can be well illustrated by the 25W MegaZenigata from SHARP: a total of 168 LEDs arranged over an area of just below 2cm2 (fig. 4) are wired to form an array. This array can then be mounted on the MegaZenigata, using the specially devised RACD30 from RECOM. The RACD30 provides a constant current output of 700mA up to 42Vt, so that the MegaZenigata produces 2600lm at 4000°K, corresponding to the luminous flux of a 150W halogen spotlight.
Fig. 3: Thermal image shows relatively homogeneous heat distribution in MegaZenigata (photo courtesy of Sharp).
Of course, the quality of the light plays a major role. While daylight reaches a CRI (Color Rendering Index) of 100, the MegaZenigata from SHARP achieves a respectable 83. The MegaZenigata is thus not only efficient but also offers a light quality and a color temperature close to that of natural light.
The lifetime of the RACD series LED driver is in line with that of SHARP’s MegaZenigata. The driver has a service life of > 70,000 hours at +25°C. These specifications are tested and certified and therefore more relevant than a statistically calculated value such as the MTBF (Mean Time Between Failure), which would obviously be considerably higher. In practice, this corresponds to a lifetime of around 20 years, assuming daily operation of the device for about 10 hours. The driver’s service life is therefore in line with that of properly cooled LED arrays. RECOM therefore has no problems providing a 5-year manufacturer’s warranty for its drivers.
Fig. 4: The MegaZenigata from SHARP (left) contains 168 individual LEDs combined in an array (in series and parallel) powered by approx. 38VDC and a constant current of 700mA (photo courtesy of SHARP).
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, the 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 onto 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% should therefore be considered long-term guide values, even if they are not yet required by the relevant standardization organizations.
About the author:
Stephan Wegstein is VP Marketing & Sales for Recom Electronic GmbH.