Power Op-Amp based Battery and Solar Array …Power Op-Amp based Battery and Solar Array Simulator for Integrated Spacecraft Tests Gladys Priyadarshini R.1, Ankur Sharma2, Tushar Golani3,

Power Op-Amp based Battery and SolarArray Simulator for Integrated

Spacecraft Tests

Gladys Priyadarshini R.1, Ankur Sharma2,Tushar Golani3, Mallikarjuna G.M. 4

and Gowrishankara C. K5

[email protected], [email protected],[email protected], [email protected],

[email protected]

1 2 3 4 5Spacecraft Checkout Group,ISRO Satellite Centre,

HAL Airport Road,Bengaluru-560017, India

January 5, 2018

Abstract

A Spacecraft needs to be extensively tested on ground forvarious scenarios which occur in the orbit. In order to testit, spacecraft needs to be Powered On. As it is prohibitiveto use on-board batteries due to battery life constraints andsolar panels due to sun simulation issues, Power Simulatorsare thought of to facilitate spacecraft testing on ground.

Broadly, Power Simulators can be classified as:

• Battery Simulator (BAS) which simulates the charac-teristics of on-board battery which can source as wellas sink current.

• Solar Array Simulators (SAS) which simulates charac-teristics of on-board Solar Array Strings.

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International Journal of Pure and Applied MathematicsVolume 118 No. 16 2018, 1417-1432ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

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This paper deals with the configuration of battery Sim-ulator, which is a bi-quadrant supply and can source aswell as sink. It employs a combination of Power Op-Ampsin Master-Slave configuration. In this design, Master Op-Amp maintains the output voltage and ensures equal dis-tribution of current through the slave Op-Amps whereas,Slave Op-Amps equally shares the source and sink current.This particular design of BAS has the capability of 5A ofsource and sink each which, if required can be augmentedto a higher number.

Along with BAS, this paper also discusses a method toarrive at Solar Array Simulator with Power Op-Amps. Itis a special power supply which simulates the static anddynamic V-I characteristics of solar arrays. Here, Power Op-Amp does the function of amplification of the reference V-Icurve which is generated using signal op-amps and discretedevices.

Both designs are complemented with protection featuressuch as over current protection and over voltage protection.Moreover, Analog Devices microcontroller (ADC 7026) fa-cilitates the remote operation. Both type of simulators arerealised and deployed for spacecraft checkouts.

Key Words :Spacecraft Checkout System, Power Op-Amp, Power Simulators

1 Introduction

Solar energy is the ultimate renewable energy source and is widelyused across the world as an alternate source of energy for most ofthe terrestrial as well as space applications. Almost all of the EarthObservation as well as communication spacecraft power system isdriven by solar energy. All satellites require solar cells to gener-ate power and performance of power system, from the generation,through conditioning till distribution to the destination system de-cides the overall performance of spacecraft and its working life. Inorder to ensure the performance of spacecraft, it is required to as-sess its performance on ground in conditions which spacecraft willface in on-orbit. Here, Spacecraft Checkout System plays a majorrole, which is involved in the assessing the flightworthiness of space-

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craft from the beginning of integration phase up to the launch. Tosimulate on-orbit conditions, Power Simulators are required whichhave the response time and performance close to on-board powergeneration and storage system.

Spacecraft Power System falls in two category viz.:

1. Battery Simulator (BAS): which simulates on-board batterycharacteristics and can source as well as sink current and

2. Solar Array Simulator (SAS): which simulates characteristics ofon-board solar array strings.

1.1 Battery Simulator (BAS) Design

In order to perform exhaustive testing on the spacecraft, it is re-quired to have Battery Simulator (BAS) performance and specifica-tions as close as possible to on-board battery. A chemical batterysuch as Li-ion battery can source the current whenever demandfrom load is high and sink the current when generation from solarpanels is more than load demand. It was required to build a bi-quadrant power supply which cater to source and sink requirements.The critical design parameters for BAS were:

i. Output Voltage: 15-21V (Programmable)

ii. Current:

a. Source Mode: 5 Amp (Max)

b. Sink Mode: 5 Amp (Max)

iii. Power: 100 W (Max)

iv. Protection Features: Over Voltage, Over Current and UnderVoltage

a. Response Time: < 30 msec

v. Isolation: By means of mechanical relay

vi. Zero offset between source and sink modes of operation

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The key aspect of the design includes use of Power Op-Amps(PA) as voltage sources and their parallel operation to ensure cur-rent sharing as well as reducing power dissipation in single element.

Fig-1 shows the power-amplifier (PA) based BAS. Here PA isconfigured as an inverting amplifier. For simplicity only one stage ofPA circuit is shown. For higher current requirements and for betterthermal design number of PAs are paralleled. The PA chosen hereis OPA549, due to its ability to work with single power supply.

Fig-1: Power Op-Amp as Voltage Regulator

In above shown figure, Power Op-Amp does the function of loadvoltage regulation. Here, load end voltage is decided by the gainof the op-amp and the reference voltage set (VRef ). In this design,source module need not to be a precise regulated power supply,since PA does the real regulation. In the source mode the currentflow is from +VCC to the load and back to the circuit ground.

Whenever the solar array strings are switched on, the load endvoltage (VL) tends to increase which in-turn reduces the currentsupplied by PA and reaches the point where the o/p voltage matchesto the gain equation. If the current generation by Solar ArrayStrings is more than load current, the excess current has to besunk by the PA, so as to maintain the load end voltage constant.The resistors Rs have been added for closing the loop and to avoidOp-Amp saturation in case the sense terminals +S and S open.

The merits of a PA based battery simulators design over discretedevice design are:

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i. Power and signal circuits are integrated, thus reducing thecircuitry delays associated with the harness and hardware.

ii. Source to sink and sink to source transition duration is less.This is due to the fact that power stage is the part of op-ampitself. Which helps in achieving lesser transition times andbetter transient response.

iii. A common error amplifier for source and sink reduces muchof circuitry.

Circuit Analysis

LetI1: Current through feedback resistor at inverting input of op-

ampVx: Voltage drop on one side of cableV0: Voltage at the output of BASVL: Voltage at the load endV1, V2: Voltages at inverting and non-inverting inputs of op-amprespectively

Vx: Voltage drop in return line at the Load end

I1 = (V0−V1)

Rf+R′cb

Where, R′cb = Rcb || Rs ≈ Rcb

Vo = V1 + I1(Rf + Rcb)Since,(Rf � Rcb so, Rf + Rcb ≈ Rf )

Vo = V1 + I1Rf

I1 =(V1+Vref )

Ri

V1 = V2(by virtual ground)

V2 = VxRi

Ri+Rf

From Fig. 3: VL = Vo − Vx

Substituting (2), (3) and (4) in (1)...

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Vo = VxRi

Ri+Rf

From (5) and (6)

VL = VrefRf

Ri

Paralleling of Power Op-Amps and Current Sharing

Paralleling of op-amps is practiced to bring down power dissipa-tion and thereby, taking care of thermal design. Current sharing inop-amps is achieved by closely monitoring the voltage across each ofthe Current Sense Resistors (CSRs). One end of each of the CSRsis tied together, whereas, the other end is connected to the outputof PA. One of the PA is assigned as the Master op-amp which main-tains constant voltage across the load. The other PAs are referredas slave op-amps. Effectively, all slave Op-Amps behave as voltagefollowers with reference voltage being derived from the output ofthe Master PA (the other end of the CSR).

Thus, the voltage across each of the CSR is maintained samewhich, in turn, ensures equal current flow through each of it. Thefollowing figures depict the paralleling of op-amps.

Fig-2: Paralleling of op-amps with one assuming the role of themaster, while other being the slave.

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Fig-3: Configuration with five PAs paralleled together

1.2 Solar Array Simulator

To carryout exhaustive testing of spacecraft, along with BatterySimulator it is required to have Solar Array Simulator (SAS) whichcan simulate characteristics of on-board solar array strings. A SolarArray Simulator is a special power supply which electrically simu-lates the V-I characteristics of solar arrays.

It design incorporates OPA549, which is a power Op-Amp asconstant current source circuit with programmable Open CircuitVoltage (Voc) and Short Circuited Current (ISC). The circuit isconfigured to get the V-I characteristics of the string of a solarpanel.

Fig.5 shows the block schematic of power op-amp based currentsource. An isolated converter of 40V, 1A is utilized by the PowerOp-Amp to generate a constant current source. The output currentof power Op-Amp is sensed using a small value of current sense re-sistor and the voltage drop across it is fed to a differential amplifier.The differential OP-Amp output is fed back to the power op-amphence closing the loop to maintain the current regulation.

In order to simulate the V-I characteristics the above basic cir-cuit is modified by adding programmability to Isc and knee pointof V-I characteristics

For simplicity only one string diagram is shown here.

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Fig-4: Power Op-Amp as CurrentSource Fig-5: Circuit Schematic of SAS

Circuit Analysis

The power Op-Amp at its +ve terminal is fed with a network(Three stage signal op-Amp network) which generates voltage ac-cording to the required current. The output of power op-amp is fedto the load (Spacecraft) through a current sense resistor (CSR).The voltage across the CSR is corresponding to the current flowingthrough it. This voltage is fed to a high CMRR differential OP-Amp to convert to a single ended voltage. The output of this stageis fed to the non-inverting terminal of PA. As like any other Op-Amp, the voltages at inverting and non-inverting terminals haveto be same. Hence, PA adjusts the output to maintain the samevoltage and it behaves as a constant current source, which followsthe reference.

In the Iref generation circuit, the signal Op-Amps are used andit is a three Op-Amp stage circuit.

In the first stage Knee voltage reference is fed and it is comparedto the actual O/P voltage of string / PA is compared. If the PAvoltage is less than the reference value, the first OP-amp stage willbe in the negative saturation.

The second stage is a current reference stage. The required ISCwill be set at the inverting terminal of signal Op-Amp. The outputof this stage will be inverted and fed to the PA as Iref . The thirdstage is an inverting stage.

When PA output touches the Knee voltage the first signal Op-Amp comes out ofve saturation and moves towards +ve saturation.The output of first stage is fed to the second stage through twodiodes. The diodes take voltage of 2Vd, to get forward biased. Thediodes leakage current develops voltage at non-inverting terminal

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of second stage op amp and brings down the Iref set at PA, andsimulating V-I characteristics of solar array simulators.

1.3 Over Voltage and Over Current ProtectionCircuit

In order to safeguard the system under test form the spurious sig-nals and failures the circuit employs over voltage protection andover current protection circuit. These OCP/OVP circuits are pro-vided essentially to protect the spacecraft from Overvoltage andovercurrent conditions arising out of failures within and external tothe power simulator units.

An OCP/OVP circuit has been integrated with the both BAS& SAS, to ensure current and voltage levels to be within the setsafe limits.

This circuit is shown in Fig-4. LM339 is a quad op-amp com-parator and it is used for sensing the over voltage and over current.Two of the four comparators are referenced using an external bias,while the voltages across the CSR and load are suitably scaled andfed to the other inputs of the comparators. A latch is realized usingthe remaining two comparators of the LM339 that actually latchesthe faulty condition and triggers further circuitry. The triggeringsignal switches on the PNP-transistor which sends a gate pulse forthe thyristor, thereby, posing a short across output and also opensthe series MOSFET switch thus isolating the load from the unit.

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Fig. 6: OVP/OCP Circuit

1.4 Digital Interface

The digital interface of both units is by ADC 7026 microcontrollerbased card. Each unit houses a dedicated digital interface. Thiscard receives the command from the remote computer and sends thestatus of the unit and analog parameters back on RS-485 interface.

Analog parameters are monitored using the on-chip ADC. Thevoltage reference required by the unit is set through the on-chipDAC.

All protection parameters are monitored by controller and sta-tus is updated. Relay operations are also controlled by the samecontroller.

Embedded software is written in C using Keil Vision4.

1.5 Test Results

Test Results for BAS

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Fig-7: Load Transition from 0.5A Source to 1.2A Sink

Fig-8: Load Transition from 1.2A Sink to 0.5A Source

Fig-9: Load transient 1A to 4A

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Fig-10: Load transient 4A to 1A

Test Results for SAS

Fig-11: V-I Characteristics

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Fig-12: Switching at 100Hz

Fig-13: Switching at 100Hz expanded (MOSFET Off to On)

Fig-16: Switching at 100Hz expanded (MOSFET On to Off)

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1.6 Conclusion

The characteristics of a two quadrant power op-amp have been high-lighted in this paper. The design criteria for meeting the operatingrequirements have been presented in detail. Based on the resultsand operating requirements, the two quadrant power Op-Amp isshown to be a viable solution for low to medium power spacecraftapplications. The transition times with this design is best in itsclass compared to earlier generation of BAS. The proposed batterysimulator methodology for spacecraft test system can be extendedto any applications demanding two quadrant battery unit.

The characteristics of a SAS based on power op-amp have beengiven in this report. The design criteria for meeting the operatingrequirements have been presented in detail. The adjustability of VKnee and ISC has been provided in the design.

The protection circuits incorporated in the design derives her-itage from existing SAS design and the circuit is well proven.

The digital interface to Rover BAS & SAS is achieved usingADuC 7026 microcontroller. The OTP is sensed using thermostat.The voltage and current parameters of the unit are set and moni-tored through serial (RS485) using microcontroller.

1.7 Acknowledgment

The authors are very thankful to Group Director Spacecraft Check-out Group, ISAC, Bangalore, for his valuable support, technicaldiscussions and encouragement for this work.

References

[1] OP-AMPS and Linear integrated circuits Ramakant A. Gayak-wad. 3rd Edition,2002 ,Published by Printice hall of India

[2] Application notes No: AN 936,AN 937,AN 941 Published byInternational Rectifiers

[3] Thermal design application note No: AN 185, March 2002 Ver2.1 Published By Altera.

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[4] Solar Array Simulators for INSAT-2E/3B IETE Technical re-view ISSN 0256-4602.

[5] Two quadrants battery simulator a new approach, Journal ofSpacecraft technology, vol.20, NO:2, pp-31-35, july-2010.

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Documents

Power Op-Amp based Battery and Solar Array …Power Op-Amp based Battery and Solar Array Simulator for Integrated Spacecraft Tests Gladys Priyadarshini R.1, Ankur Sharma2, Tushar Golani3,