Full Control of a PWM DC AC Converter for AC Voltage Regulation

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I . INTRODUCTION

Full Control of a PWM DC-ACConverter for AC Voltage

Regulation

YING-W TZOU, Member, IEEE

SHIH-LUNG JUNG, Student, IEEENational Chiao Tung UniversityTaiwan

The design and implement action of a DSP-based fully

digital-controlled single-phase pulsewidth modulated (PWM)

dc-ac converter for ac voltage regulation is described. Theproposed multiloop digital controller (MDC) consists of a current

controller, a voltage controller, and a feedforward controller. This

MD C was realized using a single-chip digital signal processor(DSP). The PWM gating signals are determined at every sampling

instant by the proposed multiloop digital control scheme using

a set of detected feedback signals. A software current control

scheme has been developed to achieve fast current control of the

PWM inverter and decouple the inductor of the output filter.

Experimental results have been given to verify the proposed

digital control scheme. The constructed DSP-based PWM dc-ac

converter system can achieve fast dynamic response and with low

total harmonic distortion (THD) for rectifier type of loads.

Manuscript received March 22 , 1997; revised January 20, 1998.

IEEE Log NO. TAESl3414/07977.

This work was supported by National Science Council, Taipei,Taiwan, R.O.C. Project no. NSC84-0404-E-009-085.

Authors address: Dept. of Electrical and Control Engineering,National Chiao Tung University, No . 1001, Ta Hsueh Rd., Hsinchu300, Taiwan.

0018-9251/98/$10.00 @ 1998 IEEE

In recent years, closed-loop regulated pulsewidthmodulated (PWM) dc-ac converters have foundtheir wide applications in various types of ac powerconditioning systems, such as automatic voltageregulator (AVR) systems, uninterruptible powersupply (UPS) systems, and programmable ac source(PAS) systems. In these applications, the PWM dc-acconverters are required to maintain a sinusoidal outputwaveform under various types of loads and this canonly be achieved by employing feedback controltechnique.

To minimize the total harmonic distortion (THD)in the output voltage, selected harmonic eliminationPWM and programmed PWM techniques have beenused to regulate the fundamental amplitude andeliminate low-order harmonics [ 1, 21. However,these kinds of control techniques cannot meet thestringent requirements of modern high-performanceac power conditioning systems. Therefore, closed-loopregulation of PW M inverters becomes an importantissue in application of high performance ac powerconditioning systems.

control schemes for the PWM inverter withinstantaneous feedback by using analog techniqueshave been proposed to achieve both good dynamicresponse and low harmonic distortion [ 3 , 41. Theinstantaneous feedback contro l with adaptivehysteresis regulates the PWM inverter with directcurrent and voltage feedback [5 , 61. This controlscheme changes the hysteresis width as a function ofthe voltage reference, but its dynamic responses tolarge load change or rectifier types of load are left

unsolved. Instantaneous voltage feedback with aninner current loop was also developed for the controlof PWM inverters.

control schemes are predominantly used incompensator design of power converters, thereare several drawbacks that hinder the performanceof analog controllers, such as temperature drift,aging effect, complexity in component adjustment,and susceptibility to electromagnetic interference(EMI). With the rapid progress in microelectronicstechnology, digital control of power converters usingadvanced microcontroller and digital signal processor(DSP) becomes an active research area [7, 91.

Microprocessor-based deadbeat cont rol techniquehas been applied to the closed-loop regulationof PWM inverters for UPS applications [lo, 111.Deadbeat control scheme has the disadvantages ofhighly sensitive to parameter and load variations andrequiring large peak-to-average ratio of control signalsto achieve dead beat effect. Recently, discrete slidingmode control (DSMC) technique has been developedfor the regulation of PWM inverters [ 121. The main

During the past several years, various closed-loop

Although frequency-domain-based analog

1218 IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 34, NO. 4 OCTOBER 1998


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advantage of the DSMC scheme is its insensitivityto parameter variations and load disturbances, whichleads to invariant steady-state response in the idealcase, while its disadvantages are that it is not easy tofind an appropriate sliding surface and its performancewill be degraded with a limited sampling rate.

With the availability of 16-bit high-performance

DSP chips, most of its instructions can beaccomplished in one instruction cycle and complicatedcontrol algorithms can be executed with fast speed.This work describes the design and implementationof a DSP-based fully digital-controlled single-phasePWM inverter for ac voltage regulation. The proposeddigital controlled PWM inverter system employs asingle-chip DSP to realize a multiloop control schemewith sinusoidal reference. The PWM gating signalsare determined at every sampling instant by theproposed multiloop digital control scheme using a setof detected feedback signals.

There has been some research on the digitalcontrol of PWM inverters for ac voltage regulation,

but theoretical analysis and realization of thedigital controller still need further study. This workproposes a multiloop digital control scheme for theclosed-loop regulation of PWM inverters used forhigh-performance UPS and AVR systems. SectionI1 makes an analysis of the dynamic behavior of thedc-ac converter of an ac voltage regulator. Section I11introduces the proposed multiloop control schemefor the closed-loop regulation of a PWM inverter.Section IV describes the implementation of the digitalcontroller using a single-chip DSP. Section IV givessome simulation and experimental results to verify theproposed multiloop digital control strategy. Section Vis the conclusion.

II. DYNAMIC ANALYSIS OF DC-AC CONVERTERS

A. DSP Control of a PWM DC-AC Converter

The dc-ac converter used in an UPS/AVR systemusually consists of a pulsewidth modulated H-bridgeinverter and an LC filter. The block diagram of theproposed DSP-controlled dc-ac converter used for acvoltage regulation is shown in Fig. 1, where the dc-acconverter and the connected load is considered as theplant of a closed-loop digital feedback system. TheDSP controls the inverter switches so that the outputvoltage can track the sinusoidal reference at eachsampling instant. In the proposed system, the inductorcurrent and the output voltage are sensed as feedbackvariables, and the control algorithm computes therequired pulsewidth for the dc-ac converter.

There are many consideration factors in theselection. of a microprocessor in the design of a digitalcontrol system. After a thorough consideration ofperformance, price, simplicity in hardware design,and software support, we choose a single-chip DSP

DC-AC CONVERTER, _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - - - - - - - - - - - - . - - - . . -PWM INVERTER LCFILTER LOAD

L r , i i :.

I _ _ . _ _ _ _ _ _ _ . . _ _ _ _ . _ _ . - . - . . - - - - . - - - - _ _ - - -f to) t;.iti a

3206 14 D SP-Based DC-AC Converter Controller

Fig. 1. DSP-based digital control of a dc-ac converter for acvoltage regulation.

(TMS320C 14) from Texas Instruments to realize thedigital controller for ac voltage regulation [13]. TheTMS320C14 has many good features, which make ita good candidate to realize digital control for powerconverting systems, such as multiple independentprogrammable timers, 160 ns instruction cycle, 16-bit

parallel multiplier, and on-chip RAM and ROM, etc.

B. Modeling of DC-AC Converter

In this work, we present a control strategy basedon linear system theory. However, the dc-ac converterconcerned here is a nonlinear system by naturedue to the existence of the solid-state switches. Anonlinear system must be first linearized around itsoperating point before the linear controller can bedesigned. The performance of the controlled systemin the neighborhood of that operating point can beguaranteed if the designed controller is robust enough.The wider the bandwidth of the system, the larger the

neighborhood. Here, a nominal resistive load is set asthe operating point for linearization.

Fig. 2(a) shows the equivalent circuit of a dc-acconverter with connected load. The dc-ac convertersystem shown in Fig. 1 is a discrete nonlinear system.A linear sampled-data model was developed forthe analysis and synthesis of a discrete dead beatcontroller [ 141. Essentially, a digital-controlled powerconverting system is a multirate digital control systembecause there are two frequency components: one isthe sampling frequency of the digital controller, andthe other is the switching frequency of the powerconverter.

In the past, because of the limited computation

speed of available microprocessors, the samplingfrequency is much lower than the switching frequency.This hinders and hesitates the application of digitalcontrol techniques to the power converting systems,especially in the area of the dc ando r ac powersupplies. However, the switching frequency of apower converting system is still limited due to theirreducible switching losses of the power devices andmagnetic cores. But in the mean time, there is a great

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- 1 S d S l - Iid

(b )

Fig. 2. Modeling of a dc-ac converter. (a) Equivalent circuit.(b) Block diagram representation.

improvement in the performance of microprocessorsand digital control integrated circuits (ICs) [151.With the recently developed high-performance DSP,realization of digital controllers with higher samplingrate becomes possible and this makes the samplingfrequency gradually approach the switching frequencyof a digital-controlled power converting system.

The dynamics of a dc-ac converter is mainlydetermined by the LC filter and connected load. Thecomplexity in the modeling and control of a dc-acconverter used for UPS applications comes from theconnected load. The load connected to an UPS systemmay be nonlinear, periodically switched, regenerative,

highly inductive, or time varying. Because of thediversity of these loads, it is not possible to formulatea general model to cover every kind of load. However,we can define a load as a nominal operating conditionpoint to derive its linear model and consider the loadvariations and model uncertainties as a specifiedload disturbance. The LC filter with a nominalresistive load can be modeled as a continuous timesecond-order system with state variables vc and ioutput load voltage vo and input voltage vu , whichtakes three values 0.5&,, 0.5&,, or zero. Considerthe inductor equivalent series resistance (ESR) rLand capacitor ESR rc , the state equation and outputequation become

1

+ [ ; ] v u

The transfer function of the inverter output voltage vuto the filter output voltage vo is

(3)O b ) - b,s+b,

V J S )a2s2 +

a ls+

a0G,(s) =

-where b, = rcRC, bo = R , a2 = ( R + r c ) L C , a , = L +(rL+ r,)RC + rLrcC, and a. = rL +R . The open-loopoutput impedance is defined as

vo(s) - rcLCs2 + (rcrLC + L) S + rLz0(s) L C S ~ (r , + rc )Cs + 1 Z0(S) =-

(4)

It can be observed from (3) that the capacitorESR rc will introduce a zero located at z , = --I/r,C.This left-half plane zero compared with the naturalresonant pole of the LC filter located at po = - 1 / mhas a ratio of

( 5 )

The capacitor ESR rc is usually very small and thiszero is high above the resonant frequency of the LCtank. If the ESRs of the LC filter are small enoughand can be neglected, (1) and (2) can be simplified as

[ -(7)

and (3) and (4) can also be reduced as

vo(s) - L s + 1Z O ( S ) = - _I_

I&) LCs2 + 1 (9)

The modeling of a digital-controlled PWM dc-acconverter can be represented by a block diagram asshown in Fig. 2(b). As shown in Fig. 2(b), there is adiscrete duty ratio d ( k ) applied to a PWM modulator.The d ( k )is determined at every sampling interval

To from a digital controller. The PWM modulatorgenerates the PWM gating signals according to amodulation strategy. A variety of PW M methodshave been developed to reduce both the switchinglosses and current ripples [16]. The digital unipolarPWM method as shown in Fig. 3 has characteristicsof minimum current ripple and simple switchingmechanism and is adopted here for the generation ofthe PWM switching patterns.

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(b)

Fig. 3. Unipolar pulsewidth modulation of a full-bridge converter.(a) Circuit diagram. (b) Timing diagram.

C. Analysis of Current-Loop Control

Current mode control techniques are usuallyemployed in the design of power converters and motordrives. Fig. 4 illustrates the employment of an innercurrent loop within the outer voltage loop in theclosed-loop regulation of a PW M dc-ac converter. Ifthe dc-ac converter is closed-loop regulated using onlyvoltage feedback as shown in Fig. 4(a), the increase ofthe voltage loop gain K , has a tendency to unstabilizethe system and its root locus is as shown in Fig. 5(a).With the adding of a current control loop, as shown inFig. 4(b), the input-to-output transfer function of thecurrent control loop is

i (s)G,(s) = &K 2 [ s ( R+ rc)C + 11

It($) u2s2 + [K2(rc + R)C + a,]s + (K 2+a,)'(10)

It can be seen from (10) that the zero introducedby the load resistor and capacitor ESR will not bechanged by adding a current loop. If rL and rc canbe neglected, (10) can be reduced as

(1 1)K2(sRC + 1)

s2RLC + s(K2RC+ L ) + (K , + R ) ',(s> =

When the current loop gain K2 approaches toinfinity, one of its poles will approach to -1/RC,and it will be canceled out by the zero of (1 ) ,another pole will approach to -K,/L and becomes thedominant pole. From the above analysis, the currentloop has the effect of decoupling the resonant polesproduced by the LC filter and the dominant pole

n

(4Fig. 4. Control structures for a dc-ac converter. (a) Voltage loop

control. (b) Current loop control. (c) Voltage loop control withinner current loop control. (d) Simplified block diagram of (c).

s-plane AI resonantpen-loop mot loci

(c )

Fig. 5. Decoupling effect of analog current control, and root lociof s2RLC+ sL + R +K,(sRC + 1 +K ,R ) = 0.

will be limited by the filter inductor and permissiblecurrent-loop gain.

By adding a current loop to control the inductorcurrent as shown in Fig. 4(c), the output reflected

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(15), then (12) can be approximated by"O t

I b t

Fig. 6 . Effect of output voltage change on inductor current.

voltage vo can be viewed as a disturbance. Theinductor current can be expressed as

The current-loop gain K 2 has a unit of ohm. If v ,is viewed as a disturbance, it can be seen from (12)that a step change of A v o has an influence on i , withmagnitude of Av o / ( r L+K 2 )as shown in Fig. 6. Theusual condition is rL


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A. Digital Current-Loop Controller

If the sampling period of the CLC is much smallerthan the electrical time constant of the filter inductor,Le., To


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Fig. 8. Output voltage and current waveforms under rated step resistive load change. (a) Simulation. (b) Experimental results.

a lower loop gain to keep stability and prevent fromlimit-cycle ringing. The FC proposed here is designedempirically. It is hard to prove the stability of theconverter system analytically. In this paper, thestability is checked experimentally by applying theworst loading conditions.

IV. IMPLEMENTATION A N D EXPERIMENTALRESULTS

To verify the proposed control scheme, a 1 kWPWM dc-ac converter using the IGBT GT50J101from the Toshiba Co. [20] was constructed.A single-chip DSP from Texas Instruments(TMS320C14) was chosen to implement the proposeddigital control scheme. The TMS320C14 is essentiallya 16-bit microcontroller with DSP architecture; mostof its instructions can be executed in one instructioncycle of 160 ns. This single-chip DSP includes a

16-bit digital 110, 4 timers, 6 channels of PWM, 4capture inputs, a serial port with UART mode, and 15interrupts. With its high operation speed and on-chipUOs, this single-chip DSP is a good choice for thecontrol of power converters and motor drives.

The sampling frequency of the digital CLC isset at 15.36 kHz, therefore there are 256 samplesin one period of a 60 Hz sinusoidal waveform. Thesampling frequency of the digital VLC and FC is setat 7.68 kHz. The switching frequency of the PWMpower stage should be an integral multiple of thesampling frequency to reduce sideband harmonics;at the same time, it is also easier for softwareand hardware design. In the designed system, the

switching frequency of the PWM inverter is set at3 0.72 kHz.Fig. 8 shows the simulation and experimental

results of the output voltage and current waveformsunder a step load change from no load to ratedresistive load. The simulation of the PWM dc-acconverter with digital control algorithm is carried outby using the EMTP circuit simulation package [17].It can be found that the experimental results are very

close to the simulation results. Experimental resultsshow the output voltage transient due to a step loadchange can be reduced to 10% within 0.6 ms.

Fig. 9 shows the experimental steady-stateresponses of output voltage and current waveforms.The corresponding power spectrum of each voltagewaveform are also depicted in the same figure.The THD of voltage waveform was measured by adynamic signal analyzer (HP3562A). Since only theharmonics of 60 Hz are concerned here, a frequencyspan of 5 kHz was set during measurement. Theharmonics of sampling and switching frequencieswere ignored in our application due to their littlerelevance to the quality of output voltage. Thedefinition of THD is defined as

(33)

where v i epresents the ith harmonics inside themeasured frequency span. The crest factor is definedas the ratio of the peak value to the rms value of aperiodic waveform. For UPS applications, the outputvoltage is required to have a THD below 5% underrated rectifier load with a current crest factor of 3. TheTHD in Fig. 9(c) is -30 dB which is about 3.16%.These results show that the proposed digital controlscheme can also satisfy the required performancespecifications which conventionally use analog controltechnique.

V. CONCLUSION

In this paper, a single-chip DSP-based(TMS320C 14) fully digitally controlled PWMdc-to-ac converter has been implemented to verifythe proposed multiloop digital control scheme. Theoutput voltage error for a step-rated load change canbe reduced below 10% within 5 sampling intervalswhen operating at 1 OV, 15A, and 60 Hz. THD below5 % of a rated rectifier load at crest factor of 3 can beachieved. The constructed DSP-based PWM dc-to-ac



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d0

200, 1 ria I I

HZ

( c l ) (c2)

Fig. 9. Experimental steady-state output voltage and current waveforms and output voltage spectrum under (a) no-load, (b) ratedresistive load, and (c) rated rectifier load with crest factor of 3.

converter system can achieve fast dynamic responseand with low THD for rectifier-type loads.

REFERENCES

[l ]Patel, H. S ., and Hoft, R. G. (1973)

Generalized techniques of harmonic elimination andvoltage control in thyristor inverters-Part I: Harmonicelimination, Part 11: Voltage control techniques.IEEE Transactions on Industry Applications, IA-9 (1973),310-317.

[3] Kemick, A., Stechschulte, D. L., and Shireman, D. W.(1977)

Static inverter with synchronous output waveformsynthesized by time-optimal response feedback.IEEE Transactions on Industrial Electronics and ControlInstrumentation, IECI-24, 4 (1977), 197-305.

[4] Sekino, Y., Shibata, M., and Hotaka, N. (1983)Inverter output voltage waveform closed-loop controltechnique.In Proceedings of INTELEC Tokyo, Oct. 1983, 205-212.

[2] Pitel, L. J., Talukdar, S . N., and Wood, P. (1980) [5] Kawamura, A,, and Hoft, R. (1984)Characterization of programmed-waveform pulsewidthmodulation. adaptive hysteresis.IEEE Transactions on Industry Applications, IA-16(Sept.-Oct. 1980), 707-715.

Instantaneous feedback controlled PWM inverter with

IEEE Transactions on Industry Applications, IA-20, 4(1984), 769-775.

TZOU & JUNG: FULL CONTROL OF A PWM DC-AC CONVERTER FOR AC VOLTAGE REGULATION 1225


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[6] Bose, B. K. (1990)An adaptive hysteresis-band current control technique ofa voltage-fed PWM inverter for machine drive system.IEEE Transactions on Industrial Electronics, 37, 5 (Oct.1990), 402-408.

Microcomputer Control of Power Electronics and Drives.New York: IEEE Press, 1987.

Servo Motor and Motion Control Using Digital SignalProcessors.Englewood Cliffs, NJ: Prentice-Hall, 1990.

Real-time digital control of PWM inverter with PIcompensator for uninter ruptible power supply.Int. Conj I d . Electronics Contl: a d nst., 2 (1990),

[7] Bose, B. K. (Ed.) (1987)

.

[8] Dote, Y. (1990)

[9] Cha, H.-J., et al. (1990)

1125-1128.[lo] Kawabata, T., Shikano, Y., and Higashino, S. (1986)

Chargeless UPS using multi-functional BIMOSinverter-sinusoidal voltage waveform inverter withcurrent minor loop.In Proceedings of the IEEE IAS Annual Meeting, 1986,513-520.

[ I l l Hua, C., and Hoft, R. G. (1992)High performance deadbeat controlled PWM inverterusing a current source compensator for nonlinear loads.In IEEE Power Electronics Specialists Conference Record,1992, 44345 0.

Jung, S. L., and Tzou, Y. Y. (1993)Sliding mode control of a closed-loop regulated PWMinverter under large load variations.In IEEE Power Electronics Special ists Conference Record,June 1993.

TMS32OCIx Users Guide (1991)Texas Instruments, July 1991.

Kawamura, A . (1987)Sampled-data model of power electronics system drivenby a PWM inverter.In Conference Record of JIEE (in Japanese), 1987,636-637.

Le-Huy, H. (1994)Microprocessors and digital ICs fo r motion control.Proceedings of the IEEE, 82, 8 (Aug. 1994), 1140-1163.

Design and analysis of pulsewidth-modulated amplifiersfor dc servo system.IEEE Transactions on Indust rial Electronics and ControlInstrumentation, IECI-23, 1 (Feb. 1976), 47-55.

Tal, J. (1976)

The EMTP Users Guide (1993).Ljung, L. (1987)

System Identification: Theory o r the User.Englewood Cliffs, NJ: Prentice-Hall, 1987.

The Mathworks, Inc., 1993.

Toshiba, 1992.

The Optimization Toolbox o r Use with PC-MATLAB (1993)

Catalog of the GTR Module (IGBT) (1992)

Ying-Yu Tzou (S81-M88) was born in Taiwan, R.O.C., on February 13, 1956.He received the B.S. and M.S. degree in control engineering from the NationalChiao Tung University, Hsinchu, Taiwan, and the Ph.D. degree in electricalengineering from the Institute of Electronics Engineering of National Chiao TungUniversity in 1978, 1983, and 1987, respectively.

During 1980-1981 he was with the Electronic Research and ServiceOrganization (ERSO) of Industry Technology Research Institute (ITRI) as adesign engineer in the control system department. During 1983-1986 he waswith Microtek Automation, Inc., as a project manager for the development of

a computer numerical controller (CNC) for machine tools. He is currently aProfessor of the Department of Electrical and Control Engineering of NationalChiao Tung University, and also serves as an industrial consultant for manylocal power electronics and automation companies. He was the Director of theInstitute of Control Engineering during 1992-1994. His special interests now aresensorless ac drives, intelligent UPS, FPGA-based control ICs for motor drivesand power converters, and DSP applications in power electronics and motioncontrol.

Shih-Liang Jung (S93) was born in Hsinchu, Taiwan, R.O.C., on September15, 1969. He received the B.S. and Ph.D. degree in control engineering from theDepartment of Control Engineering of National Chiao Tung University, Hsinchu,Taiwan, in 1991 and 1996, respectively.

university. He joined the army for military duty and has served as an instructor inthe training center of Military Integral Communication Command (MICC) sinceJuly 1997. His research interests include variable structure systems, DSP-baseddigital control techniques, control of power electronic systems, and microwavecommunication systems.

From 1996 to 1997, he was a postdoctoral research fellow at the same


Full Control of a PWM DC AC Converter for AC Voltage Regulation

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