(Editor's note: There are links to other PFC-related articles at the end, below “About the Author”)
What is power-factor correction (PFC) good for? PFC avoids apparent power being drawn from the power grid. This apparent power becomes larger as the phase shift between current and voltage gets larger, due to increasingly large capacitive or inductive loads. Such load behavior can be observed with LED light stripes or very often with fluorescent lamps.
This topic is generally neglected, since the apparent power is not paid for by private utility customers whereas large enterprises already pay for it. This becomes increasingly important with the trend towards new lighting sources which require electronic circuitry. Without this PFC circuitry, the power grid would be heavily loaded with apparent power. Also, it is important to note that this power has to be generated in power plants as well. Upon a closer look, this makes such energy-saving light sources less effective.
For this reason, governmental regulations in some countries have been in place for a couple of years now, but not world wide, e.g. the EN61000-3-2. The common, standard incandescent lamps (with a power factor of one) will disappear from the European Union (EU) within two years. Instead, there will be billions of LED and luminescent lamps. They are much more energy efficient, but they require electronic circuitry to operate with a 110 Vac or 220 Vac power supply.
This circuitry is typically located within the housing of the light sources, and so is not visible or noticeable to the user. All of these electronic circuits have in common a rectifier and reservoir, or smoothing capacitor, on the power supply side. The capacitor is charged to the peak voltage every half-cycle period. The charging current depends on how much the capacitor has been discharged by the load. This causes the current to not follow the voltage and, in turn, to cause a power factor of less than one.
Following the rectification step is the circuitry for the special lighting source. With LEDs, it is a DC/DC converter. With fluorescent lamps, it is an electronic ballast circuit and a resonant tank circuit to ignite and run the lamp. This circuit, as well as the DC/DC converter, runs at a higher frequency and additionally affects the power factor.
For power LEDs, the power factor drops to 0.5 and for fluorescent lights, it drops to 0.6. The federal regulations require PFC for lighting applications above power levels of 25 watts for a single source. These light sources typically have far less wattage when considered alone (3 W for an LED/7 W fluorescent light bulb), but who in the world has only one of these in their chandeliers? Now if the whole European Union will replace all lights one can imagine what that means. None of the currently available energy saving light bulbs have power factor correction implemented. So it is to be expected that the federal regulations will have to account for these disharmonies (EN61000-3-2, class C).
Active PFC is based on the principle of making the input current follow the input voltage exactly, in such a way that very few harmonics and phase shifts can be observed. This can be done by charging a capacitor with a voltage above the peak voltage of the input voltage (AC line voltage multiplied by the square root of 2). This capacitor has enough energy so that it can be used to adapt the nonlinear load to draw linear current from the power line.
ICs such as the PE4301 from PEGmbH with Continuous Conduction Mode (CCM) PFC have been designed for larger loads. The PE4201 critical conduction mode/discontinuous conduction mode (CRM/DCM) PFC is better suited for lighting applications up to 30 watts. A miniaturized application has been developed at PE's application lab to fit into an E27 fluorescent lamp, right next to the already-installed electronic ballast.
The transformer is optimized for a 22 W fluorescent lamp that achieves an efficiency of more than 90%, along with a very small shunt resistor, a special feature of the chip family. The power factor is higher than 0.95. By optimizing the transformer, it can easily be adapted to fit smaller loads while achieving the same performance, even at 7 W loads. The form factor can be reduced even more by using a planar transformer.
The output voltage of the PFC circuit has been set at 330-335 V. This is just a few volts above the peak voltage of the rectifier smoothing capacitor and the electronic ballast can be used unchanged. The PFC circuitry is limited to the transformer, a small FET, the PFC chip, one fast diode and a few passive devices (0805 sizes).
With power LEDs, the problem is different. The high DC voltage after power factor correction has to be converted via a DC/DC converter into a lower voltage or, even better, a constant current. For physical reasons, PFC and voltage transformation cannot be done with just one transformer. The output voltage of such a circuit would have a strong AC-induced 100/120-Hz fluctuation (potentially even visible) and the power factor would go down to 0.7 or below, which puts this principle in question.
As a result, the commonly used DC/DC conversion method seems a more feasible approach. The PFC circuitry can be used almost unchanged from the previously cited fluorescent-light solution. But the application engineers at PEs lab went one step further, by analyzing the requirements.
- The LED DC/DC converter has to fulfill the following requirements:
- PFC output-voltage ripple has to be smoothened
- a very constant, input-voltage independent of the output current has to be provided
- galvanic separation has to be achieved
- high efficiency, so that the character of an energy-saving power LED will not be compromised by other losses
- output over-voltage protection, if the load is being turned off
- potentially dimming of the LEDs
The PE engineers came to the conclusion that a simple solution also makes use of a second PE4201 chip. The IC has very low power consumption by itself. Most of the current of the chip goes into the driven FET gate. The voltage regulation is very precise, and the current shunt can have very low resistance which, again, causes very little power loss. Additionally, the current regulation input can be used for dimming the LEDs without additional PWM noise.
The flyback converter principle can be used for galvanic separation. A very low RDSon FET is being used to preserve energy and keep self-heating low, which is especially important within the housing of such a lamp. For the LED application shown below (10 W), a continuous-current FET rated below 500 mA can be used. Due to the flyback converter connected to the output of the PFC, a 600 V device is required. Since the input voltage is almost constantly at 330 volts due to the PFC circuitry, only the current for the LEDs has to be controlled.
Thus, the secondary side of the flyback converter does not have to be controlled by an optocoupler, which reduces the bill of materials (BOM), Figure 1 . The transformer is optimized such that the LED chain voltage is set at maximum current for the number of LEDs in the chain, which results in maximum brightness. The secondary-rectifier diode has to be chosen for low voltage and fast recovery. This performance directly impacts the efficiency.
Figure 1: Circuit diagram for the PFC-LED power supply
(Click on image to enlarge)
The PE4201 has three control-loop inputs that can be used for a DC/DC converter application. Pin 8 (ZC) serves to observe the energy flow of the transformer core and optimizes the on-time of the FET (comparable to the PFC application). Pin 4 (SH) serves to control the current through the FET, and here it indicates the current through the LEDs as well. Pin 2 (IN) is used for output-voltage regulation in PFC applications, but serves an over-voltage output protection in case of no load. Without a load, the output voltage will increase and so the induced voltage on the turned-off drain of the FET will rise. Pin 2 provides the information into the chip (Vovlp ) that controls the converter in burst mode.
The efficiency of the DC/DC converter is about 80%. The efficiency of the PFC is about 90%. The total efficiency that can be achieved is 72% for both circuits in this application. Since the PFC is placed before the DC/DC converter, filtering towards the power supply can be minimum compared to a direct connectION of a typical DC/DC converter for LEDs.
If desired, the current-control loop of the converter can be used for dimming the LED brightness down to 50%. By using an optocoupler, this dimming can also be with galvanic separation, depending on the application and location of the LED light source.
The whole circuit can be adapted to any input voltage, 80-120 V or 190-240 V, and can achieve the measured efficiency and power factor under both conditions. It would also be possible to have one solution running from 80-240 V, though with compromises for power factor and efficiency, so is not recommended for this reason.
The power factor without input filter is measured at 0.99 (Figures 2, 3, and 4 ).
Figure 2: Power-factor-corrected current and voltage diagram [horizontal axis: time (seconds); left vertical axis: voltage (microvolts); right vertical axis: current (amps)]
(Click on image to enlarge)
Figure 3: Voltage amplitude spectrum [horizontal axis: frequency (Hz); vertical axis: voltage (microvolts)]
(Click on image to enlarge)
Figure 4: Current-amplitude spectrum [horizontal axis: frequency (Hz); vertical axis: current (amps)]
(Click on image to enlarge)
The use of an input filter causes a phase shift again and so reduces the power factor, so an optimum balance for EMI requirements and power factor has to be applied here. Additional application notes for the these PFC ICs can be found here.
About the author
Gert Bannert is Application Engineer, Smart Power product line at Productivity Engineering (PE) IC Design GmbH, Dresden, Germany, where he is responsible for customer application development, with a focus on PFC ICs. He is also interested in applications for PE's SmartRFID standard products. He has a diploma degree and looks back on his many years in the industry for companies such as Philips and others.
Other PFC articles of interest:
- “LED ballast design delivers 200W with PFC”
- “Design tips for compact, high-efficiency AC/DC supplies”
- “Power Tip #7: Efficiently Driving LED’s Offline”
- “An update on international power-line immunity and emission standards”
- “Implementing power electronics in fluorescent lighting”
- “Powering high-brightness LEDs”
- “Will the IEC 61000 power-factor spec work in the US?”