Efficiency of electronic systems is a key design goal to save natural resources as well as reducing operating costs. Power factor correction (PFC) is an area where significant efficiency improvements are possible, says Mark Patrick.

 

Technology is rarely far from the news these days and, in particular, any impact it may have on the environment is big news – and often the subject of government policymaking. As we require more power, so the costs continue to increase. As a result, the efficiency of electronic systems is a key design goal, to save natural resources as well as reducing operating costs. The way we think about efficiency has changed and no longer relates to just the optimum performance at a specific power point, but rather across the entire operating range, from light load to full power. One area coming under scrutiny is power factor correction (PFC), where it is believed significant efficiency improvements are possible.

 

Energy is also big news, as governments get together to sign treaties and enact legislation, driving the world toward greater use of renewable energy to reduce the use of environmentally damaging fossil fuels. Governments are also legislating to avoid wasting energy, to save resources and put a cap on rising energy costs. As we rely on technology in our personal and business lives, so we are using more devices and consuming more power, which is leading to expensive energy bills for consumers and commercial users.As a result, consumers and commercial users are paying a lot of attention to the efficiency of a product during the purchasing process. However, the available data could be misleading, with product information referring to a “best possible” figure which was rarely achieved, leaving purchasers with higher running costs than expected.

 

Recognising that the unregulated system had flaws, governments, industry associations and standards bodies have developed and introduced standards that stipulate the minimum efficiency at all operating points from light load to full load. As engineers sought approaches that would allow them to meet these new stringent requirements they began to home in on the PFC stage. As this part of the circuit, along with the associated EMI filter, can consume 10 per cent of the system power, it seemed to be an area where gains could be made.

 

Why is PFC important?

It depends on the application – if the load is totally resistive, PFC is not an issue at all. In this scenario, the current and voltage are in phase, all of the available power can be passed to the load, and the power of the system is simply the product of the two waveforms. Very few loads are completely resistive and most contain a capacitive or inductive element – or both. These are often called “reactive” or “complex” loads, and they cause a shift in phase between the voltage and current that reduces the power that can be supplied to the load. This causes costs to rise, as the billing is based on the apparent power, which is higher than the power being delivered to the load. As the load becomes more reactive, so the phase shift increases and the real power to the load decreases, with respect to the apparent power.

 

The real power is related to the apparent power by the “displacement factor,” which is expressed as the cosine of the phase angle between the voltage and current waveforms. The real power is the product of the apparent power and the displacement factor. However, most AC-DC power supplies contain diodes in the input stage to rectify the mains. These devices operate in a non-linear way, and so further correction is required.

 

The diodes introduce spikes so the waveforms are no longer sinusoidal, and they now include harmonic elements and the associated harmonic currents. As these currents do not contribute to powering the load, they are a loss that reduces efficiency. Using the total harmonic distortion (THD) of the waveform, a “distortion factor” can be calculated – and the effects of this increase as the waveform becomes less sinusoidal. In a mathematical sense, the power factor (PF) of a system is calculated as the product of the displacement factor and the distortion factor. With resistive loads the current waveform is sinusoidal and in phase with the voltage, so the PF is unity and there is no effect on the power. However as reactive and non-linear elements are introduced there is a phase shift and distortion, and the real power is reduced.

 

PFC technology

The simplest way to achieve good PF is to only work with reactive loads. However, this is very limiting as it would preclude loads such as motors that must incorporate windings that will always be reactive. In order to address the effect of loads that are not purely resistive, most power systems will include an active PFC circuit on the input. As there is a huge variety of power topologies and power ratings available, multiple PFC approaches and control systems have been invented over the years. LED technology has become popular in lighting for its efficiency, so a lot of lighting now requires an SMPS to power it, as opposed to the mains voltages that were used with incandescent bulbs. One popular approach is to use a critical conduction mode (CrM) topology. This employs a variable frequency and zero-current switching to eliminate back EMFs as well as the need for a fast recovery diode. 

 

Continuous conduction mode (CCM) is another popular topology, especially in systems with power levels above 300W. CCM uses fixed frequency and does not use zero-current switching, meaning that a fast recovery diode is needed.There are several semiconductor companies that make PFC controllers for these two popular topologies, which make the task of designing PFC front ends faster and simpler. Also available are solutions that change the conduction mode depending on the load, ensuring the efficiency is as high as it can be.

 

PFC controllers 

The AL1771/AL1772 from Diodes Inc are offline high-PFC controllers that are specifically intended for lighting applications. These highly integrated devices also incorporate single- and dual-channel LED drivers. They provide a two-stage topology for a variety of connected lighting designs including single-channel dimmable and two-channel tuneable white LED solutions. Supplied in small TSSOP-16 packages and built on primary-side regulation (PSR) technology, there is no secondary feedback in these simplified devices, saving the cost (and space) needed for an optocoupler. The efficiency is good, as they operate in quasi-resonant mode where the MOSFET is turned on at the lowest point of the drain voltage.

 

Power Integrations’ HiperPFS-4 family of PFC controllers are focused on higher-power applications. Each of the devices in the family contains a boost PFC controller running in CCM, a driver for the gate and a power MOSFET rated at 600V. These elements are all within a single highly integrated package that can be mounted vertically or horizontally to suit the space available in the application. All devices in the HiperPFS-4 family have a novel control architecture that changes the main switching frequency as the load, input voltage and input frequency change. This removes the requirement to have output sense resistors, and improves efficiency by removing the loss associated with these devices.

 

The control architecture ensures the best possible efficiency over the entire load range. It also performs well in terms of EMI due to its wide-bandwidth spread spectrum effect that reduces any filtering requirements. The primary function of these devices is analog, but peripheral features including line monitoring, feed-forward scaling and power factor enhancement are digital, which reduces the power consumption at no-load. As the efficiency curve is flat over the full range of possible loads, compliance with current efficiency requirements, including EN61000-3-2, is vastly simplified. As the PF is good (>0.95) at loads below 20%, the devices are ideal for use in applications such as LCD TVs, laptops, motors, fans and LED lighting.

 

The NCP1622 from ON Semiconductor is an enhanced PFC controller intended to drive PFC boost stages using a novel valley synchronized frequency foldback (VSFF) technique. When a user-programmable control voltage surpasses a threshold, the devices operate in CrM. If the voltage is below the threshold, the frequency is slowed via a skip function until the voltage gets to the threshold. 

 

VSFF is particularly good at improving efficiency at low load levels, and is also excellent in reducing standby losses. The NCP1622 is able to deliver near-unity PF in all situations, including at the lower operating frequencies. As is essential for power applications, the NCP incorporates a number of safety features including thermal shutdown, over-voltage protection (OVP), over-current limitation and brownout detection. Additional features enable the device to identify and address issues such as a disconnected feedback pin, open ground pin or shorted bypass diode. The NCP1622 is intended for use in offline applications and can also be used in PC power supplies and lighting ballasts (both LED and fluorescent).

 

Available from Texas Instruments, the UCC28064A is an interleaved PFC controller that enables higher power ratings than ever before. Based on TI’s own Natural Interleaving technique where two channels function as masters at the same frequency, the device offers faster response times, better phase-to-phase on-time matching and transition mode operation for all of the interleaved channels. High efficiencies at light loads are made possible by a burst mode function with an adjustable threshold. This allows compliance with modern power requirements including EUP Lot6 Tier II, CoC Tier II and DOE Level VI. Suitable for use in a wide variety of TVs, all-in-one PCs, home hi-fi systems and power adapters, the UCC28064A is supported via TI’s WEBENCH software for power designers.

 

Conclusion

Anyone who designs, specifies, purchases or operates a power system is keenly interested in efficiency – if not for the environmental 

impact, then for the rising costs associated with wasting energy. As the world becomes more efficiency-conscious, so standards have evolved to focus on low-load efficiency and standby power to give a more realistic estimate of running costs. Adding to this the need for good levels of PFC means that designing power supplies is a complex and specialist task. The best approach is to make use of one of the many advanced PFC controllers that include novel techniques, enabling designers to meet current energy efficiency requirements with relative ease.

 

About the author:

Mark Patrick is the Supplier & Technical Marketing Manager EMEA of Mouser Electronics. Mark joined Mouser Electronics in July 2014 having previously held senior marketing roles at RS Components. Prior to RS, Mark spent 8 years at Texas Instruments in Applications Support and Technical Sales roles and holds a first class Honours Degree in Electronic Engineering from Coventry University. For details, contact Helen Chung, Asia PR Specialist of Publitek, on email: helen.chung@publitek.com