IS.S Sumiimn of Key Points 691

Nonlinear load

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Harmonic corrector

Fig, 18.43 A harmunii; coircctof, based on the 30at-de CCM boost convertCE of Fig. 18,39,

the hartnotiic corrector schetne reijuires less total active silicon than the CCM boost-type rectifier of Fig, 18.39. But if the uncontrolled rectilier contains .small ac hne inductances, .such that it operates in the discontinuous conduction mode with large THD. then it is probably better to simply replace the uncontrolled recdlier widi the CCM boost-type rectifier of Fig. 18.39.

18.8 SUMMARY OF KEY POINTS

1. Thi; ideal rectifier presents ait effective resistive load, the emulated resistance Ji., to the ac power system. The power apparently consumed by is tr.iiisfcrred to the dc output port. In a three-phase ideal rectifier, input resistor emulation is obtained in each phase. In both the single-phase and three-phase cases, the output port follows a power source characteristic, dependent on the instantniicous ac input power. Ideal recuficrs can perform the funcdon of low-harmonic recdfication, without need for low-frequency reacdve elements.

2. The dc-dc boost converter, as well as other converters capable of increasing the voltage according to Eq. (18.12 ), can be adapted to the ideal rectifier application. Л control system causes the input current to be proportional to the input voltage. The converter may operate in CCM, DCM, or in both modes. The mode boundary can be cjLpresscd as я function of fi,. L/T and the instantaneous voltage ratio vtO/V A well-designed average curretit controller leads to resistor emulation regardless ofthe operating mode; however, other schemes in;ty lead to distorted current waveforms when the mode boundary is crossed.

3. In a single-phase system, the instantaneous ae input power is pulsating, while the dc load power is constant. Whenever the instantaneous input and output powers arc not equal, the ideal rectifier system must

contain energy storage. A large capacitor is commonly employed; tlic voltage of this capacitor must be allowed to vary independently, as ncecssary to store and release energy. A shw feedback loop regulates the dc component of the eapaeitor voltage, to ensure that the average ac input poHict and dc load powzt arc balanced.

4. RMS values of reetiftcrs waveforms can be computed by double integration. In the case of the boost converter, the rms transistor current can he as hv/ as З9й of the rms ac input current, when the dc output voltage I/is close in value to the peak ac input voltage Other converter topologies such as the buck-boost, SEPIC, and (Tuk conveners exhibit significantly higher nns transistor currents but are capable of limiting the converter inrush euirent.

5. In the three-phase case, a boost-type rectifier based on the PWM voltage-source invcncr also exhibits low rms transistor currents. This approach requires six active switching elements, and its dc output voltage must be greater than the peak input liiic-to-liiic vohage. Average current conffol can be used to obtain input rcsistorcmulation. An equivalent circuit can be derived by averaging the switch Hiaveforms. fhe converter operation can he understood by assuming that the SHiiteh duty cycles vary sinusoidally; expressions for the average convener waveforms can then be derived.

6. Convener losses and efficiency can be modeled using die steady-state equivalent circuit models of Chapter i, v/ith atime-vatying duty cycle. The output current is averaged over one ac line period, to determine its dc component. The converter losses and efficiency can then be computed. This approach is approximate, in that (0 it assumes that the converter dynamics arc ranch faster than tlic ac line frequency, and (( /) it neglects operation in the discontinuous conduction mode.

7. Average current control involves direct regulation of the low-frequency coraponcnts of the rectifier input current to follow the input voltage. 1-ccdforward can also be added, to cancel the influence of ac line voltage variations on the dc output voltage.

8. Current programmed control can also be adapted to attain input resistor emulation in rectifiers. The programmed current reference signal Д0 is made proportional to the ac input voltage. The difference between 1(0 and the average inductor curtcnt leads to distortion, owing to the inductot current ripple and the need for a stabilizing artificial ramp. Several approaches arc known for reducing the resulting harmonic distortion of the line current waveform,

9. Hysteretic comrol, particularly with eiirrcnt ripple, ha.s a simple controllct implementation. The disadvantages are variable switching ftcqucncy, and increased peak eurrcnts.

10. Nonlineat catrier control also leads to a simple conttoUer implementation, and has the advantage of CCM operation with small peak transistorcurrent.

11. The outer low-bandwidth control system, which regulates thedc output voltage to balance tlte rectifier and load powers, ean be modeled by averaging tlic rectifier waveforms over onc-lialf of the ae line period Г,. This causes the de-side system equations to become time-invariant. Л small-signal model is then obtained by perturbation and linearization.

12. The inner high-bandwidth control system, which regulates the ac input current waveform to attain resistor emulation, is in general highly nonlinear. However, in tlic case of tlic boost rectifier, a valid small-signal model can be derived. This approach is unsuccessful in die case of other converters; one must then resort to other approaches such as the quasi-static approximation or simulation.

References

[1] D. Chambers and D. Wang, Dynamic Power Factor Correction in Capacitor Input Off-Line Convcit-ers, Prifceeditigs Sixth Natiomt SoM-Staie Fiywer Cimverskm Cimferetice (Powercon 6), pp. B3-1 to B3-6, May im.

P] R. Erickson. M. Madigan. and S. Singer, Design of a Simple High Power Facior Reclifier Bused on

Ihe Flybiick Converler, IEEE Applied Power Electronics Conference, 1990 Record, pp. 792-SOl.

[3] S. Singer and R.W. ERickson, Power Source Elemeni and Its Properlies, lEE Proceedings-Circuits

Devices Systems, Vol. 141, No. 3, pp. 220-226, June 1994.

[4] S. Singer, Realkalion of Loss-Free Rcsislive Elements, IEEE Transactions on Circuits and Systems.

Vol. CAS-36, No. 12, January 1990.

[5] W. E. RiPPEL, Oplimizing Boosl Chopper Charger Design. Proceedings Sixth National Solid-State

Power ConversionCottference (Powercon 6), 1979, pp. DI-1 - DI-20.

[6] M. F. ScHLECUT and B. A. MiWA, Aclive Power Facior Correclioii for Swilching Power Supplies, IEEE

TransactionsouPowerElectronics, Vol. 2. No. 4. October 19Я7, pp. 273-2Я1.

[7] J. Sebastian, J. Uceda. J. A. Cobos, J. Arau. and F. Aldana, improving Power Facior Correclion in

Dislribuled Power Supply Systems Using PWM and ZCS-QR SEPIC Ttiptjlogies, IEEE Power Electronics Specialists Conference, 1991 Record, pp. 780-791.

[8] E. Yang, Y. JIang, G. Hua, and F. C. Lee, Isolated Btjost Circuit for Power Factor Correction, IEEE Applied Power Electronics Conference, 1993 Record, pp. 196-203.

[9] C. A. Cakesik and 1 Barbi, A Unity Power Facior Multiple Isolated Outputs Switching Mode Power

Supply Using и Single Switch, IEEE Applied Power Electronics Conference, 1991 Record, pp. 430-436.

[10] S. Freeland, 1. A Unified Analysis of Converters wUh Resonant Switches, 11. Input-Current Shaping for Smgle-Phase Ac-dc Power Converters, Ph.D. Thesis, California Institute tif Techntjlogy, 1988.

[11] M. J. ScttUTTEN, R. L. Steicerwald, and M. H. Kiieraluwala, Characteristics of Load-Resonant Ctmverters Operated in a High Power Factor IVIode, IEEE Applied Power Electronics Conference, 1991 Record, pp. 5-16.

[12] J. Hong, E. Ismail, R. Erickson, and 1. Kuan, Design of the Parallel Resonant Converter as a Low Harmonic Recdfier, IEEE Applied Power Electronics Conference, 1993 Rectird, pp. 833-840.

[13] I. Barbi and S. A. O. Da Silva, Sinusoidal Line Current Rectification at Unity Power Facior wilh Boost Quasr-Resonanl Ct>nver1ers, IEEE Applied Power Electronics Conference, 1990 Rectird, pp. 553-562.

[14] R. Redl and L. Balogii, RMS, DC. Peak, and Harmonic Currents in High-Frequency Power-Factor Correctors wilh Capacitive Energy Storage, IEEE Applied Power Electronics Conference, 1992 Record, pp. 533-540.

[151 J. Sebastian. J. A. Cobos, P. Gil, and J. Uceda, The Determination of the Boundaries Between Continuous and Di.sconlinuous Conduction Modes in PWM Dc-lo-Dc Converters Used as Power Facior Preregii-lators, IEEE Power Electronics Specialists Conference, 1992 Record, pp. 1061-1070.

(Ifi) M. nalba14t, Design of a 1 kW Power Factor Corrector, Powr Conver.t/oH, October 1989 Prtreeedings, pp. 121-135.

[17] R. Mammano and R. Neidorff, Improving Input Power Factor-A New Active Ctmtroller Simplifies the Ta.sk, Power Conversion, October 1989 Proceedings, pp. 100-109.