Technical Application

Rectifiers with

Power Factor Correction

by Andreas Lindemann, IXYS Semiconductor GmbH

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2voltage Un thus have to be compen-sated in a further stage of powersection, if required.

Turning power on leads to a highmains inrush current peak In to chargethe capacitor in intermediate circuitpreviously discharged.

This may be overcome by replacingat least two of the diodes in theschematic figure 1 by thyristors, whichalso permits to control DC voltage.However this measure increasescontrol complexity and its use leadsto the additional generation of reactivepower.

The operational behaviour of three phaserectifiers basically corresponds to thesecharacteristics as discussed here forsingle phase rectifiers.It has become obvious that the use ofstandard rectifier circuits leads toproblems of electromagnetic compatibility(EMC) due to the harmonic distortion ofthe input current In. The recent standar-dization [1] [2] aims at their reduction. Thelimits specified may be met with astandard rectifier circuit, complementedby passive filter components towardsmains. These however are rather largeand expensive. Further, in EMC sensitiveapplications, such as power supplies fortelecommunications or computers, theoccurrence of harmonics in the rectifier,although filtered towards the grid, maydisturb the operation of the whole circuit.

1.2 Rectifiers with Power FactorCorrection

As an alternative, controlled rectifiers canbe used. They can be characterized asfollows:

The occurrence of harmonics in mainscurrent In is actively minimized.

In operation, the intermediate circuitis charged during the whole mainsperiod with sinusiodal current In inphase with the mains voltage Un; thisoptimizes the maximum availableactive power through a given mainsfuse.

The voltage of DC link UZ is controlledand thus independent of mainsvoltage Un over a wide range.This helps to overcome possibleproblems of unstable supply voltage.Additionally, the rectifier is suitable forwide input voltage range: This means,that the device may be connected toany mains voltage Un; it is notnecessary to preselect the voltagerange, because the controlled rectifierwill keep DC voltage UZ at the requiredlevel.

Only few and small passive compo-nents are required.

So this type of controlled rectifiers doesnot only help to meet the requirementsof the EMC standards, but it offerssignificant additional benefits.Different types of controlled rectifiers fora variety of applications are presented inthe following.

2. Single Phase PowerFactor Correction

2.1 Mode of Operation

The schematic of a single phase rectifierwith power factor correction in boosttopology is shown in figure 3. Itsoperation is discussed with reference tofigures 4, 5 and 6:

Fig. 3: Schematic of single phase rectifierwith power factor correction

Fig. 4: Typical input waveforms of singlephase rectifier with power factorcorrection (Pn = Un In = 3600 W)

Figure 4 depicts the waveforms of mainsvoltage un (t) (solid) and mains currentin (t) (dotted). Due to the - ideally - sinus-oidal shape of current in (t), there wouldbe no harmonic content; furthermore, thephase angle zero between mains voltageun (t) and current in (t) avoids theoccurrence of first harmonic reactivepower. Please note the significantly loweramplitude of mains input current of therectifier with power factor correction infigure 4 compared to the standard rectifieras in figure 2; both waveforms aredisplayed in the same scale and for thesame rectified power.

1. Introduction

1.1 Standard Rectifiers

The topology of a standard single phaserectifier is shown in figure 1. Theterminals L1 and N are connected to thegrid while L+ and L- supply the inter-mediate circuit. Usually the DC voltageUZ in the intermediate circuit is smoothedby a capacitor. To save cost, generallyno further reactive components are used;this means, that only mains inductanceand additional parasitic inductances haveany effect. Characteristic waveforms ofthis circuit are shown in figure 2.

Fig. 1:Schematic ofnon controlled,single phaserectifier

Fig. 2 : Typical input waveforms of noncontrolled, single phase rectifier(P

n = 3600 W)

This topology can be characterized asfollows:

It is simple - no control required - andrugged.

Current flow from the grid to chargethe intermediate circuit Id > 0 is onlypossible in case the instantaneousvalue of the mains voltage is higherthan intermediate voltage un (t) > UZ.This leads to a short conduction periodof the rectifier with the consequencesthat mains current In has high peakvalues, high RMS values and isharmonically distorted - see figure 2.

Further harmonic distortion of themains current In is caused bycommutation effects of the diodesusing the mains inductance ascommutation inductance.

The DC voltage UZ depends on themains voltage Un. Variations in mains

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3Fig. 6: Typical boost chopper waveforms ofsingle phase rectifier with power factorcorrection (Pn = Un In = 3600 W)

Fig. 5: Typical rectified waveforms of singlephase rectifier with power factorcorrection (Pn = Un In = 3600 W)

On the secondary of the rectifier bridgeaccording to figure 3, the waveforms lookas shown in figure 5: The diodes haverectified primary current and voltage ashave been depicted in figure 4, thusfolding the previously negative half-waves of voltage and current to the firstquadrant, while their sinusoidal shapehas been maintained.

Finally figure 6 depicts current wave-forms taken at the chopper in a magnifiedtime interval: The solid line represents thecommand variable iw (t) for the boostchopper's input current id (t); the slightlyrising slope corresponds to a section ofthe sinusoidal half-wave of the rectifiedinput current id (t) according to figure 5.This desired waveform is approximatedby the boost chopper, composing thesinusoidal half-waves of id (t) accordingto id (t) = iT (t) + iZ (t). The boost chopper'spulse pattern is documented below thetime axis of figure 6: When the transistorT is turned on, it will carry a current iT (t)according to the broken line; currentrises, because the voltage ud (t) is appliedto the inductor L which will furthermagnetize. Having turned the transistorT off, the diode D11 will turn on and thuscause the inductor to demagnetize by adecreasing current iZ (t) (dotted) into theintermediate circuit, with the voltage of

power and high switching frequency,it generally stands the inrush currentat power on as mentioned above.

Several requirements refer to thepackage: The power circuit must beisolated from the heatsink for safetyreasons; thus the package shouldprovide an internal isolation. This, to-gether with the integration of severalpower semicondcutors in the samepackage, leads to low mounting effort.The integration as mentioned is furtherindispensable to achieve a goodoperational behaviour of the chopper,particularly regarding high frequencyfast switching.

Obviously, the whole rectifier with powerfactor correction should be considered asone system, the parts of which have tobe matched to each other and to theapplication.

2.2.2 Component Types, theirRatings and Characteristics

In this section, several combinations ofpower semiconductor components con-stituting power factor corrected singlephase rectifiers are discussed accordingto the approach, that the whole rectifiershould be considered as one system,Consisting of several components opera-ting together.

Different sets of power semiconductorcomponents are listed in table 1 togetherwith their major characteristics asexplained in section 2.2.1:

The left columns give IXYS' typedesignations: Either one type is men-tioned, integrating all components - ortwo types, the first incorporating therectifier bridge D1 to D4, the secondthe boost chopper T and D11 accordingto figure 3.

The next column names the packagetype. All packages are isolated. Theoutline of Isoplus I4-Pac is shown infigure 7; this new package combinesfeatures of discrete components - itlooks similar to - with features ofmodules - such as isolation andreliability, see [10]. Veridul modulepackage is depicted in figure 8. Eco-Pac is a similar module, however witha smaller footprint of 30.3 mm 47 mm.

Features of the chips - rectifier D1 toD4, boost chopper transistor T and freewheeling diode D11 are outlined in thethree columns on the right of table 1.

intermediate circuit being larger thanrectified mains voltage at any timeUZ > ud (t).This way, the sum id (t) = iT (t) + iZ (t)represents a waveform with an averagevalue according to the desired sinusoidalcurrent iw (t) and an additional triangularripple due to boost chopper operation.

The latter's switching frequenciestypically are in the range of50 kHz fT 100 kHz, which minimizessize and cost of the inductor L andpossible additional filter components. Thecontrol method for this kind of powerfactor corrected rectifiers is implementedin a variety of integrated circuits, whichsignificantly faciliates their design - seefor example [3], [4], [5], [6], [7], [8] or [9].The following section will deal withsuitable integrated power semicon-ductors.

2.2 Suitable Integrated PowerSemiconductors}

2.2.1 General

The following aspects should beconsidered in choosing power semicon-ductor components for a power factorcorrected single phase rectifier with atopology according to figure 3:

The rectifier diodes D1 to D4 mustbe able to stand the inrush currentpeak at power on as mentioned insection 1.1, however reduced by theinductor L. Further, fast switchingbehaviour is advantageous to reducethe emission of disturbances duringcommutation at zero transition ofmains current. Special mains rectifierdiodes with fast switching behaviourare referred to as semifast diodes inthe following.

The transistor in the boost chopperT should be a fast switching device -either a high voltage MOSFET or anIGBT with optimized switching speed-to operate at the high switchingfrequency as mentioned in section 2.1.The use of a component with low gatecharge QG is beneficial, because ithelps to minimize the required drivepower.

The free wheeling diode of theboost chopper D11 must be optimizedfor high switching speed, particularlyat turn off in switched mode operation.Fast recovery epitaxial diodes -FREDs - should be used; their perfor-mance can additionally be improvedusing a series connection of twodiodes. If the free wheeling diode iscorrectly sized for operation at nominal

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4According to the approach to consider thewhole rectifier as one system, detailedcalculations have been carried out todetermine the ratings of the power factorcorrected single phase rectifiers assuggested. The results are shown intable 2: Under the typical operating con-ditions listed in the caption, the rectifiersystems can take the indicated power outof mains and transfer it - reduced by thelosses - to the intermediate circuit. Twopower ratings are given, covering theinternational mains voltage range; thisway, the nominal power of a rectifiersystem can be determined either for afixed input voltage or for wide inputvoltage range.

The calculations, leading to the resultsas presented in table 2, use both - thecharacteristic values and maximumratings of the power semiconductor com-ponents, and the knowledge of powerfactor corrected rectifier's mode ofoperation as explained in section 2.1: At

Figure 9: Schematic of three phase rectifier with power factor correction - ''Vienna'' rectifier

Table 1: Features of components for single phase power factor correction

Type designation featuresrectifier chopper package(s) rectifier transistor diode

VUI9-06N7 Eco-Pac module semifast fast IGBT series FREDsFBO16-08N FID35-06C Isoplus I4-Pac standard fast IGBT series FREDsFBO16-08N FMD21-05QC Isoplus I4-Pac standard low QG MOSFET series FREDs

VUM24-05N Veridul module standard MOSFET FREDVUM33-05N Veridul module standard MOSFET FRED

Table 2: Typical nominal mains power Pn of components for single phase power factor correction;conditions: voltage of intermediate circuit UZ = 400 V, switching frequency fT = 75 kHz, casetemperature TC = 80 C

Type designation Pnrectifier chopper at Un = 110 V at Un = 240 V

VUI9-06N7 900 W 2100 WFBO16-08N FID35-06C 950 W 2600 WFBO16-08N FMD21-05QC 1400 W 3100 W

VUM24-05N 2200 W 2800 WVUM33-05N 3300 W 4200 W

given operating conditions - such asvoltage of intermediate circuit UZ,switching frequency fT and case tempe-rature TC - junction temperature of theseveral semiconductors D1 to D4, T andD11 is calculated with the parametersmains voltage Un and current In. Maxi-mum junction temperature of any semi-conductor may not be exceeded, whichdetermines the permitted mains voltageUn - mains current In operating range ofthe rectifier system. With these limits,nominal mains power can be calculatedby Pn = Un In.

So the calculations as described havetwo uses: The indications of nominalpower for the whole power factor con-trolled rectifier system permit to easilyselect power semiconductor componentsfor a given rectifier rating in a variety ofapplications. Thus rectifier design issignificantly faciliated. Further the systemapproach helps to match the differentpower semiconductors to an optimum,leading to optimized components: Themost economic solution will match the

ratings of the single semiconductors D1to D4, T and D11 in a way, that the Un Inoperating ranges of all parts are ascongruent as possible.

3. Three Phase PowerFactor Correction

There are several topologies and controlmethods to implement power factor corr-ection as described in section 1.2 forthree phase systems; a survey of tech-niques is given in [11].Different types of three phase powerfactor corrected rectifiers with continuousmains current will be discussed in thefollowing sections.

3.1 Combination of Three SinglePhase Rectifiers

It is possible to connect one single phasepower factor corrected rectifier as shownin figure 3 and as explained in section 2between each of the three mains phases

Figure 8: Outline of Veridul package:dimensions 31.6 mm 63 mm

Figure 7: Outline of Isoplus I4-Pacpackage: dimensions ~ 20 mm 21 mm

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5and the neutral conductor. However thissolution is hardly used because of itsdrawbacks: Often no neutral conductoris available. Furthermore the rectifiedpower is transferred to three DC links -one per phase; additional DC-DC conver-ters with galvanic isolation would beneeded to make the rectifier a single DCvoltage source as commonly required.

True three phase rectifier systems asoutlined in the next sections prove to bebetter solutions.

3.2 Three Phase ''Vienna''Rectifier

The topology of ''Vienna'' rectifier isshown in figure 9; it can be characterizedas follows:

On the mains side, there is one inductorfor each phase L1, L2, L3. There is noneed for a neutral conductor. The circuitwill operate with wide input voltage range.

The output of the rectifier is an inter-mediate circuit with controlled DC voltagebetween L+ and L- with center point MP.

There is one controllable switch perphase - MOSFETs are depicted. Togetherwith the surrounding four diodes bridges,they operate as bidirectional switches:When turned on, they connect the res-pective mains phase to the DC centerpoint via two diodes and the inductor,which makes the latter magnetize. Whenturned off, the inductor demagnetizes intothe DC link via the free wheeling diodesconnected to L+ or L- respectively.

It is obvious that this operational principleis similar to the one described for thesingle phase power factor correctedrectifier in section 2.1. Further detailsabout operation and control of the circuitcan be found in [12], [13], [14].

In particular, the method explained in [12]permits the calculation of the powerratings of the ''Vienna'' rectifier analogousto the approach for the power factor

Table 3: Typical nominal three phase mains power Pn of components for three phase powerfactor correction; conditions: mains voltage UDn = 400 V, case temperature TC = 80 C

Type designation Pn package options

VUM25-05 10 kW V1-PackVUM85-05A 30 kW V2-Pack soft start thyristor

corrected single phase rectifier in section2.2.2. Basic ratings and characteristicsof ''Vienna'' rectifiers built with IXYSmodules are listed in table 3.

A ''Vienna'' rectifier will use one of theindicated modules per phase. As couldbe expected, its range of rectified poweris higher, compared to single phaserectifiers as rated in table 2. Bothcomponents in table 3 are isolatedmodules, where V1-Pack has the samefootprint as Veridul package - see figure8 - while V2-Pack is bigger with a footprintof 40.4 mm 93 mm according to thehigher nominal power. The VUM85module additionally provides a soft startthyristor to give the capability to limit theinrush current at power on, as alreadydiscussed in sections 1.1 and 2.2.1.

3.3 Three Phase Full Bridge

The last circuit to be presented is the selfcommutated three phase full bridgeshown in figure 10. Mains would be con-nected via inductors to the phase outputsL1, L2, L3, while L+ and L- represent theconstant voltage DC link. The self com-mutated three phase full bridge can beused as rectifier and inverter; thus itpermits bidirectional energy transfer,which is useful for applications withenergy recovery. However, the circuitcontains twice the amount of controllableswitches - six IGBTs in figure 10 - com-pared to the ''Vienna'' rectifier asdescribed in section 3.2; consequently

Figure 10: Schematic of three phase fullbridge

driving effort is somewhat higher.Furthermore, semiconductors with higherblocking voltages are needed. In the end,the particular requirements of the actualapplication will decide which solution toprefer.

Applications of this topology are widespread in power electronics. Many controlmethods are known and implemented inintegrated circuits. A variety of integratedpower semiconductors for a wide powerrange is available. Without claimingcompleteness, table 4 lists some moduletypes of IXYS with their most importantratings.

Table 4: Self commutated full bridges for three phase power factor correction; breakdown voltageU(Br)CEs and DC ratings at case temperature TC = 80 C of IGBTs (IC80) and diodes (IF80)

Type designation U(Br)CEs IC80 IF80V A A

MWI30-06A7 600 30 24MWI50-06A7 600 50 45MWI75-06A7 600 60 85MWI100-06A8 600 85 85MWI150-06A8 600 125 125MWI200-06A8 600 165 170MWI25-12A7 1200 35 33MWI35-12A7 1200 44 33MWI50-12A7 1200 60 70MWI75-12A8 1200 100 100MWI100-12A8 1200 120 130

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4. Conclusion References

Power factor correction for mainsrectifiers is an upcoming issue. Operatingprinciples of single and three phasepower factor corrected rectifiers havebeen explained. Suitable integratedpower semiconductors have beenpresented. Power factor correctedrectifier systems using thesecomponentshave been rated as a result of detailedcalculations. This paper has shown thatsingle and three phase power factorcorrected rectifiers are feasible and howthey can be designed.

[10] A. Lindemann: Combining theFeatures of Modules andDiscretes in a New PowerSemiconductor Package; PCIMConference, Nrnberg, 2000

[11] J. W. Kolar, H. Ertl: Stand derTechniknetzrckwirkungsarmerdreiphasiger Gleichrichter-schaltungen; VDE-ETG-Tage,Essen, 1995

[12] J. W. Kolar, H. Ertl, F. C. Zach:IXYS-VUM25-E - A New IsolatedPower Module for Low-Cost/Weight/Volume HighPerformance Three-PhaseSinusoidal Input Current PowerConditioning; Power QualityConference, Nrnberg, 1995

[13] J. W. Kolar, U. Drofenik, F. C.Zach: DC Link VoltageBalancing of a Three-Phase/Switch/Level PWM (Vienna)Rectifier by Modified HysteresisInput Current Control; 29thPCIM Conference, Nrnberg,June 20-22, 1995

[14] J. W. Kolar, U. Drofenik, F. C.Zach: Space Vector BasedAnalysis of the Variation andControl of the Neutral PointPotential of HysteresisCurrent Controlled Three-Phase/Switch/Level PWMRectifier Systems;International Conference onPower Electronics and DriveSystems, Singapore,Feb. 21-24, 1995

[1] IEC61000-3-2: Grenzwerte frOberschwingungsstrme(Gerte-Eingangsstrom 16 Aje Leiter)

[3] TDA4817 IC for High PowerFactor and Active HarmonicFiltering; Siemens, 1995

[4] TDA4862 Power FactorController(PFC) IC for High Power Factorand Active Harmonic Filter;Siemens, 1998

[5] M. Herfurth: Power FactorController TDA4862Applications;Siemens HL application noteAT 2 9402 E

[6] B. Andreycak: UC3854A andUC3854B Advanced PowerFactor Correction Control ICs;Unitrode Corporation,applicationnote DN-44, 1994

[7] P. C. Todd: Boost Power FactorCorrector Design with theUC3853; Unitrode Corporation,application note U-159, 1999

[8] UC1854, UC2854, UC3854High Power FactorPreregulator;Unitrode Corporation, 1999

[9] UC1858, UC2858, UC3858High Efficiency, High PowerFactor Preregulator; UnitrodeCorporation, 1999

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