Power Factor Correction Techniques in LED Lighting
High power LED based light fixtures are replacing fluorescent and HID light sources in many general lighting applications. Since LED lighting represents a green technology, the issue of power factor is very important. Power factor is defined as the ratio of real power consumed by a load (expressed in Watts) to apparent power (expressed in VA), which produces a figure from zero to unity that indicates the degree of distortion and phase shift in the current waveform. The overall power factor is the product of the distortion power factor and the displacement power factor, where displacement power factor considers only the fundamental of the current and distortion factors in the harmonics.
Real power is defined as the apparent power multiplied by the power factor, where the apparent power is the product of (rms) voltage and current. This relationship indicates that more current is required to provide the same amount of real power for lower power factors.
Low power factors negatively impact the environment because transmission lines lose more power in the form of heat proportional to the square of the current. Higher current results in wasted energy in transmission lines as well as generators and transformers consuming more fossil fuels and generating more pollution and higher costs. Clearly, high power factor is very beneficial in all electrical products that operate from the AC power grid!
Although individual lights consume relatively small quantities of power (typically 10 W to 100 W), lighting is nevertheless very significant since somewhere around 20 percent of the worlds electricity produced is consumed for this purpose.
Figure 1 illustrates AC line current waveforms for high and low power factors, the green traces are the sinusoidal line voltage, and the blue traces are the line current. The trace on the left shows a highly distorted line current with power factor 0.55 typical of many CFLs. The trace on the right shows a line current with a power factor close to unity.
The US Department of Energy (DOE) Energy Star program mandates minimum acceptable power factors or 0.7 and 0.9 respectively for domestic and commercial LED lights. In addition, the European standard IEC 1000-3-2 (class C) stipulates power factor and total harmonic dostortion (THD) requirements for all lighting products above 25 W. Power factor correction was not an issue in the days when all lighting used purely resistive incandescent lamps since they naturally have a power factor of one; although they are extremely inefficient in terms of light output for the amount of power consumed. Fluorescent lighting has been in use for many decades; but until the 1980s simple magnetic ballast coils were used to supply them with large capacitors added to provide some measure of power factor improvement. These have now been largely replaced by electronic ballasts, most of which include active power factor correcting circuitry.
Compact fluorescent lamps however generally do not include power factor correction and have got away with this on the grounds that they represent only low power loads. This way of thinking doesn't really make a lot of sense because CFLs have been widely adopted and they combine to produce a large cumulative low power factor load on the electricity grid. This situation was largely driven by price pressure.
Since the advent of LED lighting in recent years, there has been more effort to introduce standards of performance in order to ensure that this new lighting technology is as efficient as possible (Lumens per Watt and power factor) while minimizing waste by maximizing operating life. The drive is towards replacing the old low cost lighting technologies with greener alternatives. Standards need to be adopted to overcome the natural desire for consumers to choose products where the initial outlay is cheap without considering the long term drawbacks and rising energy costs.
LED lights are driven by various different types of electronic circuits. These generally consist of a full wave rectifier stage followed by a DC to DC switching converter that provides a regulated constant output current. LED driver circuits range from very simple and crude circuits consisting of a few diodes, capacitors and resistors, to advanced multi-stage converters. Between these extremes lie a number of different converter circuit topologies which cover the different power levels and safety and performance requirements. The following list breaks this down into the commonly used converter types:
1. Basic converter: A very simple passive circuit. This also includes so called AC LEDs which consist of back to back LEDs with some passive current limiting element.
2. Buck converter: A simple switching regulator which does not provide electrical isolation. This is suitable only in in applications where the LEDs not accessible.
3. Flyback converter: A slightly more complicated single stage switching regulator which provides electrical isolation. The LEDs can be accessible without posing any risk of electric shock.
4. Multistage converter: Consists of a Boost regulator front end which provides a high power factor and a back end to provide isolation and current regulation. This could be a Flyback or resonant stage.
There are several other circuits; however they are not widely used in general lighting and will not be considered here.
The basic converter (1) cannot provide a good power factor and cannot be dimmed using a triac based dimmer. This type of LED driver was introduced by a number of manufacturers aiming to enter the lighting market with low cost LED products that would replace CFLs. This type of design fails miserably to meet the requirements of new green energy standards that are in the process of or will be introduced to cover LED lighting. These were never intended to be green but are simply the cheapest possible LED light that could be produced. AC LED approaches find a niche in the marketplace; however these tend to include very little electronics and do not lend themselves easily to power factor correction.
Passive LED driver circuits consist of a number of low current devices connected in series. Such circuits can be used with large numbers of small LEDs working at low current but would dissipate too much power if high current LEDs were used, which precludes the possibility of using the most efficient state of the art LED devices. Power factor is necessarily poor since the combined voltage drop of the LEDs is high enough that current can only flow for a portion of the AC line cycle. This also explains why triac-based dimmers don't work with these products.
The Buck converter (2) is very widely used in LED drivers due to its simplicity and low cost. In applications where isolation is not needed, it is ideal. Power factor correction can be added to the circuit by means of the simple "Passive Valley Fill" circuit, which consists of only three diodes and two capacitors as shown in Figure 2.
The Flyback converter (3) is also very widely used and provides the added advantage of electrical isolation. This is a safety requirement in many LED lamps, especially where there is an exposed heat sink which is connected to the LEDs. The majority of LED light bulb replacement products incorporate large heat
sinks in order to reduce the operating temperature of the LEDs thereby optimizing light output and operational life.
The Flyback converter is a single stage relatively simple and low cost approach although it is a little more complicated than the Buck. The Flyback can realize power factor correction in a couple of different ways; using the Passive Valley Fill circuit as the Buck does, or alternatively operating directly from a full wave rectified DC bus without any smoothing capacitor as shown in Figure 3.
The simple Flyback converter however is suitable only for power levels below about 50 W. Above this level, it becomes bulky and less inefficient; although more complex Flyback converter designs including interleaved circuits have been used.
The multi-stage LED converter (4) consists of a PFC Boost regulator front end very similar to that used in many electronic fluorescent or high intensity discharge lamp ballasts.
Several different back end stages can be used in a multi stage LED driver; however this has little bearing on the power factor as long as they provide a substantially uniform resistive load to the Boost stage output. A back end stage with a high speed current regulation loop attempts to regulate-out the DC bus ripple, thereby providing a load to the Boost stage which changes over the AC line cycle. This is likely to cause distortion of the AC line input current, which reduces the effectiveness of the power factor correction circuit and can also cause the Boost stage feedback loop to become unstable. It is therefore important to consider the integration of the front and back end stages.
Multi stage LED driver designs are relatively expensive and are generally used at higher power levels. LED street lights for example typically operate between 100 W and 200 W where the high power factor, wide input range, higher efficiency, controllability and reliability of the multi stage driver provide obvious advantages.
The LED driver circuit shown in Figure 4 consists of a Boost PFC front end pre-regulator followed by an LLC resonant half bridge that provides a regulated current output at low voltage. In this implementation, both stages are integrated and controlled by a single ASIC. The front end stage can operate and provide a power factor >0.95 over an AC supply voltage range of 90 V to 305 Vrms, creating a 450 VDC bus from which the back end stage can operate. In this example, the back end uses an opto isolator to relay the feedback information from the secondary output circuitry to the half bridge driver IC, which controls the current through frequency modulation.
This method provides a platform for high end LED lighting applications without requiring excessive complexity, component count and cost.