Quickly and easily meet IEC61000-3-2 harmonic current limit standards with TI’s power factor correction (PFC) controllers. From low-power to high-power AC/DC applications, TI’s PFC controllers provide the highest power factor (PF), lowest current distortion (iTHD), highest efficiency and highest power density while optimizing cost.
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Power Factor Correction is a technique that promotes efficient energy consumption from the power grid. Power Factor correction is employed inside common electrical and electronic equipment that are powered from the AC outlet. Power factor correction enables the equipment to maximize the active power draw and minimize the reactive power draw from the AC outlet.
Consider an electrical circuit comprised of paralleled resistors, inductors and capacitors. When the electrical circuit is ‘powered’ up by applying an AC voltage, the resistors dissipate power, while the capacitors and inductors store the energy. The power dissipated in the resistor is known as ‘active power’. The power delivered to the inductor and capacitor that is stored as energy is called ‘reactive power’.
In an ideal power factor corrected equipment, 100% of the power drawn from the AC outlet is utilized to perform useful work. In doing so, the energy delivered by the source (the utility power generation plant) is most efficiently utilized by the equipment and losses due to distribution in wiring are minimized. Additionally the utility power plant has to generate less power for a given amount of useful work, and this means losses during generation of power are also reduced.
A power factor corrected equipment behaves like a resistive load to the AC outlet. Physically, the current drawn from the AC outlet appears to be a perfect replica of AC voltage i.e. in proportion to and in phase with the AC sine-wave voltage. Take a look at the picture below:
And how might AC input current waveform look like when there is no power factor correction inside the equipment?
Very distorted, as shown below:
Yes, Power Factor (PF) is a metric used to quantify how well power factor correction is achieved. An ideal power factor corrected equipment which behaves like a pure resistive load will exhibit a power factor equal to 1. A load that is drawing reactive power will exhibit a power factor less than 1. These measurements are easily made by inserting an equipment known as ‘AC power analyzer’ between the equipment’s input cords and a qualified AC outlet.
Yes. Power Factor is the ratio of the real power (measured in Watts) to apparent power (measured in Volt-Amperes) drawn by the equipment.
Power Factor correction is necessary wherever the local law mandates that the equipment be compliant with IEC61000-3-2 regulatory requirements on AC current harmonics. Typically, most electrical and electronic equipment (rated up to 16A input current) that draw 75W of higher from the AC mains continuously require this compliance. A special case is lighting equipment where compliance is required at 25W or higher. Note however that not all countries mandate compliance to these regulatory requirements.
So, laws which drive demand for power factor correction don’t actually measure the Power Factor?
Yes, for the most part. IEC61000-3-2 regulatory requirements require that the 1st 40 harmonics of the AC input current, when decomposed, be within certain levels. Equipment are categorized into 4 classes under this requirement and different limits apply for each class. These are shown in tables below. However, there are some other voluntary performance compliance programs which do monitor power factor such as 80PLUS (http://www.plugloadsolutions.com/80pluspowersupplies.aspx)
iTHD is the AC input current (i) Total Harmonic Distortion (THD). iTHD is a metric used to measure what proportion of the AC input current is not comprised of the 1st current harmonic. The 1st current harmonic is the current that would be drawn by a resistive load on the AC outlet. Lower the iTHD, better is the power factor correction. Mathematically iTHD is represented as shown below:
Compliance with IEC-61000-3-2 norms can be achieved to some degree by inserting capacitors and/or inductors close to the AC inputs of the electrical equipment. This passive approach to achieve power factor correction is called passive PFC. This approach usually demands heavy, bulky and expensive capacitors and/or inductors which is often disadvantageous.
Another approach to achieving power factor correction is by inserting active switching circuitry comprised of power switches such as MOSFETs and IGBTs, inductors and capacitors close to the input of the electrical equipment. This is an elegant approach which minimizes size, weight and cost but it needs a controller to manage the high-frequency switching sequence of the power switches. Such as controller is called a PFC controller.
That’s correct. Typically, the active PFC switching stage is a boost converter. Whether the input is 110VAC or 220VAC, the converter internally generates a DC bus voltage typically about 385V. If all other electronics in the equipment is designed to work from the 385V DC bus, then inserting this active PFC switching stage enables designers to build equipment that can be marketed around the world. This benefits in fast design cycles, increased marketability and easy inventory management.
Yes. Depending upon the power level of the equipment and needs such as efficiency, power density and cost, there are several different circuit architecture such as Transition Mode PFC (TM PFC), Continuous Conduction Mode PFC (CCM PFC), single phase PFC (1-ph PFC) and 2-ph Interleave PFC (2-ph IL PFC).
As mentioned earlier, the active switching circuit is typically a boost converter. If the PFC controller controls the inductor current in the boost converter allowing decays to 0A in every switching cycle, then that is known as Transition Mode PFC. Typically this is the desired architecture for power levels below 200W since it offers high-efficiency, small size and cost.
If the PFC controller does not allow the inductor current to decay to 0A in every switching cycle then this is known as Continuous conduction mode PFC. This is a versatile architecture that is commonly applied from anywhere between 200W through several kWatts.
Where high power density is desired, 2 stages of active PFC circuitry can be inserted in parallel, each handling 50% of the power, and the respective switches in each stage be made to operate 180degree out of phase with each other. This is known as interleaved PFC. It has many benefits such as smaller magnetics size for the boost inductor and EMI filter, smaller RMS current in the output capacitors and easier thermal design because of distribution of power lossess across more components.
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