New power regulations bring power-factor correction to lower-power supplies

Until recently, power factor and PFC (power-factor-correction) circuits were of concernonly for utilities and manufacturersof motors. Utilities have for years specifiedthe power-factor performance oflarge inductive motors. The utilities alsogenerally can charge industrial customers forreactive-power consumption. However, residentialcustomers are starting to introducemore reactive-power loads into the mix as energy-efficient lights,such as CFLs (compact fluorescent lights) and LED-based lightscontaining their own ac/dc lighting ballasts, begin to emerge.As a result, power factor has moved from the realm of large-scaleindustrial motors down to that of consumer electronics.

Power factor is the ratio of real powerto apparent power. When both currentand voltage are sinusoidal and in-phase,the power factor is one. If they are sinusoidalbut not in-phase, then the powerfactor is the cosine of the phase angle.Purely sinusoidal current and voltagewaveforms occur when the load comprisesresistive, capacitive, and inductiveelements that are all linear.

In a purely resistive load, real poweris the same as apparent power, and thepower factor is one. When the load hasinductance or capacitance, however,the apparent power is greater than the realpower because the capacitance and inductanceintroduce a phase lag betweenthe current and the voltage. Althoughutilities currently charge residentialcustomers only for the real power theyconsume, the utilities must add powerto support the out-of-phase current andvoltage. The additional power is wastedin the form of resistive losses on thegrid’s transmission lines. Because theselosses increase as the square of the currentincreases, losses due to low powerfactor can quickly add up.

The ac line sees an SMPS (switched-modepower supply) as a nonsinusoidal,nonlinear impedance (Figure 1 ). Figure2 shows the voltage and current forthe circuit of Figure 1 . To more closelyfollow the input voltage and avoid sharp current spikes, the capacitor mustcharge over the entire positive portionof the cycle. In addition to upping thepower factor closer to one, this shapingof the current allows for the use ofa smaller capacitor and avoids the creationof harmonic noise, thus reducingTHD (total harmonic distortion).This compensating additional circuitryis the PFC circuit. Energy-efficiencyspecifications regulate both THD andpower factor as part of the power-factorspecification.

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Many topologies and approaches existfor enabling PFC (see sidebar “Utilitiesand PFC”). In general, PFC is eitherpassive or active. Government andindustry regulations specify only thepower factor and the THD, leaving the decision about whether to use a passiveor an active circuit to the design engineer(tables 1 through 3).

Passive PFC is a simple, relativelyinexpensive approach, but it has drawbacks.Chief among them is that it’s difficult,although not impossible, to geta power factor of more than 0.7, andthe trend in global regulations is towardpower factors of 0.9 and higher.Another difficulty with passive PFC isthat the capacitors go directly on theac line, necessitating capacitor ratingsof 400V or higher. This requirementmakes the use of electrolytic capacitorsthe most common approach. Electrolyticcapacitors’ lifetime drops withhigher temperatures, so you must deratethe capacitor if your system will needto work in a hot environment. To derateit, you must either choose a highertemperature, more-expensive capacitoror allow for a shorter capacitor lifetime.Another drawback with passive PFC is that its voltage output is unregulatedas it feeds into the dc/dc-conversionstage. For these reasons, the trend is towardactive PFC, usually in the formof a boost-converter circuit betweenthe bridge rectifier and the storagecapacitor.

Kishore Manghnani, vice presidentof Green Technology for Marvell, arguesthat single-stage power-converter/PFC ICs are the best design approachesfor LED lighting. “With two-stage youend up using two separate chips—onefor PFC and the other for the LEDdrivercircuit, which includes dimmingand the TRIAC [triode-alternatingcurrent-switch] interface,” he says. “Ina single-stage converter/PFC chip, youneed no additional components: Youput the LED driver and the PFC all inone chip.” You might wonder whetheryou can use cheaper passive PFC, giventhat cost is a pacing item in LED lighting,but Manghnani advises against thisapproach. “The biggest advantage ofactive PFC is that you can use a lowvoltagecapacitor,” he says. “In passivePFC, the capacitor must support 400V.In the active single-stage [approach],the capacitor is only 40V. Plus, the lifetime of the capacitor [for the same cost]could be four to five times longer.”

Currently, Energy Star’s draft proposalfor energy-efficient luminairescalls for a power factor of 0.7 in residentiallighting and 0.9 in commercial,but Marvell argues that the power factorfor LED lighting should be 0.9 forall lights with wattage higher than 5W,which is currently the specification inEurope and Korea. “The United Statesis a bit behind in this area,” Manghnanisays. “Europe and Korea require lightingpower to be more than 0.9. It doesn’tcost anything extra, so why not add it?”

The mandate for minimum power-factorrequirements comes at the sametime as the industry is imposing increasinglytight efficiency standards on powersupplies, causing a double whammyon designers, who must strive for morestringent efficiencies as power factorcreeps up. Thus, research is ongoing onthe most efficient ac/dc-converter/PFCcircuits. It’s important to understandthe general categories for control andwhich power-supply types these methodscan work with. A brief overview ofactive-PFC methods follows. For moreinformation and circuit diagrams, seereferences 1 and 2.

The main control methods for active-PFC circuits are DCM (discontinuous-conduction mode), CCM (continuous-conduction mode, and CRM(critical-conduction mode). Variouschip manufacturers have their own versionsof CRM control, such as BCM(boundary-conduction mode) and TM(transition mode). “Current conduction”in these terms refers to the inductorcurrent.

Low-power supplies typically useDCM. CCM works for all power levelsbut involves a hard reverse recoveryof the output diode when the MOSFETswitch turns on. This recovery can cause high losses in a standard inexpensivediode. For high efficiency, therefore,you must use a more expensivediode, such as one made from siliconcarbide.

In contrast, BCM circuits switch onthe MOSFET with no current in thediode, allowing you to use inexpensiveoutput silicon rectifiers. The trade-offsare that BCM uses a more complexvariable-switching-frequency scheme,and its higher peak currents result inhigher losses at higher power, limitingmost BCM designs to less than 300W.At higher power levels, the CCM boostis more effective due to its lower ripplecurrents that result in lower peak currentsand lower differential-mode EMI(electromagnetic interference).

However, recent innovations in converter/PFC design interleave multiple-phaseBCM-controller ICs, such asthose from Fairchild, On Semiconductor,and Texas Instruments. Interleaveddesigns parallel two or more BCM powerstages, allowing your design to reach1 kW or more and reducing the ripplecurrent in the output, which allows forsmaller inductors. “If you look at thecosts of the controller in comparisonto the costs of the PFC, which includesthe inductor and all the power components,the controller is not a significantcost,” says Jim Aliberti, product-marketingengineer at Texas Instruments.“It’s the magnetics. People are lookingfor a way to reduce the costs, and interleaved design has enabled lower systemcosts because you don’t have to processas much ripple current, allowing you touse smaller magnetics.”

As with most power-conversion applications,digital power is making inroadsinto PFC, as well. For example,Cirrus Logic offers an IC for digitallycontrolled PFC. The active-PFCCS1500 and CS1600 DCM ICs targetuse in power supplies requiring asmuch as 300W. The CS1500 addressespower supplies for applications such aslaptops, digital TVs, and PCs, and theCS1600 targets electronic-lighting ballasts.At approximately 30 cents (highvolumes), the chips compare in pricewith analog PFC ICs but use 30% feweradditional components and fewer partsfor EMI filtering. The power factor,which varies with the input line voltageand the load, is greater than 0.95.The iW3620 LED driver from iWatt isalso a digital single-stage, active-PFCdevice. Texas Instruments offers a developmentkit for high-voltage PFC,which includes hardware and softwareto implement two-phase interleaveddigital PFC for regulation compliance.The kit can work with the company’sapplication-development kits for theC2000 Piccolo microcontrollers, suchas the C2000 ac/dc developer’s kit, aswell as end-product kits, such as motorcontroland LED-lighting-control kits.

Do you remember the advertising pushseveral years ago for the smart refrigeratorthat would track when you were lowon milk and autonomously order more?That idea didn’t catch on. Perhaps themore likely intelligence will be a DSPthat controls all of the power-efficientfunction of home appliances.

You can reachTechnical EditorMargery Connerat 1-805-461-8242and .