Microprocessor Controlled Digital Shunt Regulator . Introduction Microprocessor-Controlled Digital ShuntRegulator P.R.K. CHETTY W.M.POLIVKA,Member, IEEE R.D. MIDDLEBROOK California

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  • I. Introduction

    Microprocessor -ControlledDigital Shunt Regulator

    P.R.K. CHETTYW.M. POLIVKA, Member, IEEER.D. MIDDLEBROOKCalifornia Institute of Technology

    Abstract

    A new approach to the design of power systems is presented inwhich a microprocessor is used as a controller for a digital shuntregulator (DSR). This approach meets the demands of future spaceand ground missions, i.e., high efficiency, high reliability, lowweight, low volume, increased flexibility, and less developmenttime. This approach responds to future demands by permitting real-time modification of system parameters for system optimization.This feature is especially important in the event of an anomaly. Asthe microprocessor need not be dedicated to the DSR, it cansimultaneously be used for battery management and for chargeregulator/discharge regulator control. This approach also reducesthe component count, simplifies assembly and testing of the unit,results in significant time saving, and increases the reliability.

    Manuscript received October 12, 1979; revised November 22, 1979.Authors' address: Power Electronics Group, 116-81 California In-stitute of Technology, Pasadena, CA 91125.

    0018-9251/80/0300-0191 $00.75 ) 1980 IEEE

    In recent years use of microprocessors for powerprocessing systems has resulted in improved perfor-mance. To meet the predicted requirements ofspacecraft, it is necessary that new and improvedmehods of electrical power conditioning and controlbe developed. The power system should be modularand should not take any development time. Otherconstraints imposed are high reliability, minimumweight and volume, low cost, and flexibility. To meetthese demands an attempt has been made to use amicroprocessor to control the digital shunt regulator(DSR).

    Since the advent of the space age, photovoltaic cellarrays have been used as the main energy source forspacecraft power generation. In addition, solarenergy, which is abundant in space, may play an im-portant role in meeting the world energy requirementsby means of microwave transmission from space.Because the electrical output of photovoltaic cell ar-rays is unregulated, however, the power must be pro-cessed and regulated before it can be used by otherequipment. Thus these solar power systems requirebus regulators, of which the DSR is superior com-pared to other types of shunt regulators [1]. The mainadvantage of the DSR over other types of shuntregulators is that this can be employed for high-powersystems as its weight, size and volume do not increasein proportion to the power requirements as others do.Although the circuitry is somewhat more complexthan a simple analog shunt or sequential shunt, itdoes offer high reliability and is of low cost.

    The power systems described above, whether usedin space or on the earth must use batteries to meet thepeak and eclipse/shadow requirements of the load.Hence, the storage batteries have to be charged duringsunlit period/day time and have to be discharged dur-ing eclipse/nighttime or when there is a requirementfor peak power. In addition, these batteries have to beprotected from overcharge and undercharge. Attemptshave already been made to use microprocessors forbattery management [2,31. Microprocessors can alsobe used to control charge and discharge regulators(CR/DR).

    Thus the approach of using a microprocessor forthe control of the DSR is a very useful one, especiallysince the same microprocessor may be used for con-trolling DSR, CR/DR, and battery management. Thisapproach is expected to result in a single integratedsystem/unit for all the functions mentioned abovewith a bonus in system flexibility, high reliability,minimum weight and volume, and standardization.The processor can also be used to continuouslymonitor the status of its own system and the health ofthe overall power system as well.

    The purpose of this paper is to present the hard-ware and software details of a microprocessor con-trolled digital shunt regulator.

    IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-16, NO.2 MARCH 1980 191

  • DIGITAL SHUNT REGULATORII. Digital Shunt Regulator

    Fig. 1. Block schematic of power system using dsr.

    (A) Z \

    VOLTAGE

    z

    (B) \

    VOLTAGE

    Fig. 2. I-V characteristic of (A) typical solar cell array, (B) solararray simulator.

    Section II contains a brief descriptiQn of thedigital shunt regulator. Then follows the detaileddescription of solar array section simulators in SectionIII. The operation and design of the dissipative analogshunt is given in Section IV. Section V contains thedescription and design of shunt current comparators.The need for a special timing function and its designis discussed in Section VI. Section VII contains thedescription of the microprocessor controller, hardwareimplementation, software programs, and interfacing.The complete system is described in Section VIII. Theexperimental results of the model system constructedas described above is presented in Section IX. Somepossible extensions are suggested in Section X.

    Fig. 1 shows the block diagram of a power systemusing a digital shunt regulator. As mentioned above,the energy source is a photovoltaic cell array. Thedigital shunt regulator regulates the output of theenergy source to the needs of the load. The solar cellarray is divided into N sections, one section of whichis permanently connected to the bus and all other sec-tions are connected through switches. The DSR con-tains a small dissipative analog shunt which is design-ed to regulate one section of the array. The currentthrough this dissipative shunt is monitored. Wheneverthis current exceeds approximately the current of asingle section, Imax, the digital processing part of theDSR switches off one section. Whenever this currentreduces to a minimum, Imin, then the digital part ofthe DSR switches on one section. Thus coarse regula-tion is achieved by the digital part of the DSR andfine regulation is achieved by the dissipative shunt.This DSR will be described in detail in the followingsections.

    Because the solar cell array is not available to testthe DSR, solar cell array section simulators have beenconstructed and used instead. First vafious buildingblocks of the microprocessor controlled DSR will beexplained and then the overall description of the com-plete system will be given.

    The demonstration system has been designed tothe following criteria: Regulated bus voltage 28 V,Maximum power to be handled 30 W, Number ofsolar cell array sections 4.

    III. Solar Cell Array Simulator

    Fig. 2(A) shows the I-V characteristic of a typicalsolar cell array section. For the model system beingused here, the simulator need not exhibit an I-Vcharacteristic identical to that of a real solar ar-ray--it need only be similar. The nearest simulationwith the least complexity is achieved by using a cur-rent source (as a solar cell is also a current source)whose I-V characteristic is shown in Fig. 2(B). ThisI-V characteristic is enough to simulate the solar cellarray for testing of the DSR.

    Each solar cell array section simulator has beendesigned to give about 300 mA at 28 V. Fig. 3 showsthe circuit diagram of one simulator. Four units ofthis type have been constructed. The I-Vcharacteristics of all four simulators are given in Fig.4. The portion of the circuit within the dotted line inFig. 3 is used to switch the simulator on and off. Anlight-emitting diode (LED) has been included in eachcircuit as shown to indicate visually whether the sec-tion is switched on or off.

    IV. Dissipative Analog Shunt RegulatorAs mentioned above, the dissipative analog shunt

    is used to achieve fine regulation of the bus voltage.This type of shunt has been chosen over the alter-

    IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-16, NO 2 MARCH 1980192

  • Fig. 3. One section of solar array simulator.

    native pulsewidth modulated shunt because the analogcircuit has a much higher bandwidth and therefore ismore desirable for achieving finer regulation. Fig. 5shows the circuit diagram of the dissipative analogshunt. Divided-down bus voltage is compared to thereference voltage and the amplified error voltage is us-ed to control the current through the shunt. A resistorof suitable value (depending on the maximum designcurrent of the shunt) is used in the shunt current pathto monitor the current that is flowing through theshunt. This signal is used for further digital process-ing.

    V. Imax and Imji Comparators

    The shunt current, measured automatically using aresistor in the shunt path (Fig. 5), is compared againsttwo reference voltages to determine whether it isabove Im. or below min. Fig. 6(A) shows the circuitdiagram of the Im. and Imi. comparators. Fig. 6(B)shows the waveforms of the shunt current and theoutputs of both the comparators.

    The reference voltages for the Im. and Imi. currentcomparators are derived from the bus voltage by us-ing a zener diode and resistor dividers. It would alsobe possible to generate the references from digital-to-analog converters. In this way the reference voltagescould be changed when necessary. Occasionally thismust be done to compensate for degradations due toageing. In a space system these changes can betelecommanded from ground. In ground-basedsystems, such maintenance commands can be givenfrom remote locations.

    An alternative approach to the one presentedabove is to use an analog-to-digital converter to put

    a.n300

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    200-28 V

    1505 10 15 20 25 30 35 40

    VOLTS

    Fig. 4. I-V characteristics of all four solar array simulator sec-tions.

    +28 ADJUST

    +28V BUS

    +28V 5 KSENSE 47 K 47K

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    MONITOR

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    Fig. 5.