Electrical Power System (EPS) Role
Contents
1. Insolation period worst case 56 6 min with an altitude of 400 km Eclipse period 36 min Temperature worst case 10 C The last calculation result obtained by thermal subsystem for the ambient temperature was a range between 23 C and 10 C When the temperature grows the solar cells efficiency decrease So the worst case is 10 C 5 1 2 Power determination assumptions and approach The power that is obtained from solar cell is depended on the angle of incident light So we assume this generated power is approximated with a cosine of incident light To determine the power production two parameters have to be considered the limit angle of total reflection when the incident light ray is totally reflected by the cell and the cell efficiency which depends on several degradations factors First we can consider the best case Limit angle of total reflection 0 Cells efficiency 26 6 28 C Then we can consider the worst case Limit angle of total reflection 20 Cells efficiency 20 1 28 C Remark For the moment all the power simulations have been made with a temperature of 28 C In the next step Phase B it will be necessary to simulate with the exact temperature range If the temperature is low that s not a problem but an advantage because the solar cells efficiency is better so the power results obtained will be necessarily better Ref S3 A EPS 1 0 EPS doc Date 16 06 20062. Figure 28 LDO power losses results 6 2 5 Common Mode Filters The Common Mode Filter is a passive component In fact it is simply composed by a circular ferrite and two inductances as shown on figure 36 Figure 29 Schema of a Common Mode Filter When the output current passes through the first inductance it creates a magnetic field When the same current come back by the second inductance it generates also a magnetic field but inverse So the two fields cancel each other out It is important to place Common Mode Filters on each EPS Power output are This measure obliges the output current to come back by the same way This simple device assures that it doesn t have presence of loop currents between the subsystems Ref S3 A EPS 1 0 EPS doc c Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 36 of 43 6 2 6 Batteries lot of factors must be considered to find the right battery for the SwissCube But something is sure the technology choice will be either Li Ion either Li Po as shown on figure 33 A Lithium Polymer B Lithium Ion C Ni MH D Ni Cd E Lead R6 S Se oO Nh lt g EX e5 A gt o gt LL D ESA LL 5 Q E 2 O gt 0 50 400 Gravimetric Energy Density Wh kg Figure 30 Battery technology choice Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 37 of 43 1 Nominal Voltage V omn and Average Voltag
3. The floating Power Bus voltage level is 3 6 to 4 2 Volts The output voltage level for each subsystem is 3 3 5 Volts standard electronic voltage level Between these two levels we need a regulator to assure the output voltage specifications The LDO is an ideal component to do this regulation job The LDO can also be use by the EPS microcontroller as a switch to enable or disable the different subsystems outputs with a logic signal This function is very important in case of short circuit on one of the subsystems input For the moment we have principally considered the TEXAS INSTRUMENTS LDO s familiy TPS 770xx This family of LDO offers the benefits of low dropout voltage ultralow power operation and miniaturized packaging The TPS 770xx family series devices are ideal for micropower operations and where board space is limited These LDO also have an over current limitation a thermal protection and an operating junction temperature range of 40 C to 125 C ee DBV PACKAGE TOP VIEW IN rn A OUT Current Limit Thermal Protection FB Figure 24 Functional block diagram and DVB Package The dropout voltage is the minimum voltage that LDO needs to do its regulation between input voltage and output voltage The TPS 770xx family series devices need only 35 mV of dropout voltage at 50 mA The regulated output voltage is very accurate Each LDO will be chosen according to the different subsystems power needs
4. The mass of one TPS 77001 LDO of this family has been estimated at 2 grams and we need maximum 6 of them But actually the architecture is composed of 5 LDO The 6 may be used to supply the antenna deployment but this point must be defined For the moment we assume that the antenna deployment is supplied without regulator But anyway we will use an electronic component to switch on the deployment mechanism Ref S3 A EPS 1 0 EPS doc Be Swiss Cubs Remark Issue Date Page 1 Rev O 16 06 2006 33 of 43 The energy that the antenna deployment requires is very low It consumes only 2000 mW during 1 second which corresponds to 0 55 mWh only EPS micro controller Beacon Software Bus Transciever Service connector Beacon Switch Latch up protection 3 3 V Filter l LDO Battery LDO m Filter LDO m Filter LDO Filter LDO Filter 3 6 4 2 V Step up converter Solar Cells 3 3 V 3 3 V 3 3 V 33V Figure 25 Electrical block diagram LDO layout Here is an output voltage vs output current typical characteristic TPS 77033 3 284 3 282 3 280 3 278 3 276 Vo Output Voltage V 3 274 3 272 3 270 Gm D TT F T F a
5. A Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 12 of 43 4 EPS DESIGN REQUIREMENTS This chapter will enumerate the different design requirements First the physical electrical and data flow requirements will be listed and then we will speak about some other important specifications 4 1 Physical structure requirements The physical structure requirements for the EPS are principally the mass allocation the PCB surface and the battery volume So here are these physical constraints Mass allocation 130 g given at the Team Meeting 16 03 06 PCB dimensions 88 x 98 mm given by Structure and Configuration subsystem Battery volume 45x 15x 90 mm given by Structure and Configuration subsystem Batteries black EPS PCB blue Payload Figure 6 Batteries and PCB position Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 13 of 43 4 2 Electrical requirements No electronics may be active during launch to prevent any electrical of RF interference with the launch vehicle and primary payloads CubeSats with rechargeable batteries must be fully deactivated during launch or launched with discharged batteries 4 3 Data Flow requirements ADCS need power information from each side covered by cells In fact ADCS will use these power datas to determine the satellite attitude This is an important constraint that we have to include in our design 4 4 Other Specifications 4 4 1
6. D A side E side B 35 side F camera 3 S25 g 2 A 1 5 1 0 5 0 0 50 100 150 200 250 300 350 pl Figure 13 Electrical power generated from the sun horizontal plane Ref S3 A EPS 1 0 EPS doc Date 16 06 2006 O SwissCuber Page 22 of 43 c Issue 1 Rev 0 We can now calculate the total power generated whichever the angle B or we choose Figure 14 Total power generated from the sun best case This matrix describe the electrical power that can be produce whatever the satellite s attitude in the best case limit angle 0 and efficiency 26 6 Maximum value obtained from Matlab is Pax 3 80 W This power peak happen when three sides covered by cells are equally exposed to the sun for 45 135 225 315 and B 125 or 235 9 Average value obtained from Matlab is P 2 47 W This value is valid under the assumption that all sides are equally exposed to the sun during the mission Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 a S UMSS Cubs Page 23 of 43 6 1 5 Power calculation Approach pessimistic Now we can do the same calculations for the worst case limit angle 20 and efficiency 20 1 3 5 side A side B 3 side C side D 2 5 2 D a 1 5 1 0 5 0 0 50 100 150 200 250 300 350 a Figure 15 Power generated from the insola
7. Fast charging can be achieved in a temperature range of 0 45 C Current of charging needs to be limited to the 1C rate max a 0 8 Ah battery can be charge with 0 8 A max The charging voltage has to be limited strictly with 4 2 V per battery cell in a narrow tolerance of 50 mV It recommended terminating the charging either after 3 hrs and or after the charging current falls below 0 02C The charging process is illustrated below showing current and voltage of a VARTA PoLiFlex cell using the 1C I V charging Charging Characteristics PLF 443441 600mAh Temp 20 C Discharge 1C 600mAh 2 75V Charge cc cv 1C 600 mA 4 2V 1 lt 0 02C 4 4 e a SE E A E edeeensenesnosenaenninesnnsoowsieshstnientscthantn 7 50 600 lt E gt aaa E Neate TOT 450 A hi Cell Voltage Q amp 8 2 E Charge Current 9 rs 7T 7 Charged Capacity lt EEE So S Gammmiamnan 300 se N E j i N 2 N 3 2 eisai aaa RES 150 K an aap wags _ S nan ion 2 9 Fi 0 0 2 0 4 0 6 0 8 1 1 2 1 4 1 6 1 8 2 Charge Time h Figure 32 Typical PoLiFlex Li Po Battery Charging Characteristics Due to their low impedance this batteries are also capable of providing very high levels of current Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 43 of 43 7 CONCLUSIONS lot of work and not enough time But very very interesting Re
8. Remove before flight pin A remove before flight RBF pin is required to deactivate the CubeSats during integration outside the P POD The pin will be removed once the CubeSats are placed inside the P POD This pin will be directly connected to EPS 4 4 2 Thermal requirements Operating temperature range is expected to be 40 C to 70 C 4 4 3 Deployment and Transmission delay To allow adequate separation of CubeSats antennas may be deployed 15 minutes after ejection from the P POD as detected by CubeSat deployment switches CubeSats may enter low power transmit mode LPTM 15 minutes after ejection from the P POD LPTM is defined as short periodic beacons from the CubeSat CubeSats may activate all primary transmitters or enter high power transmit mode HPTM 30 minutes after ejection from the P POD Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 14 of 43 5 DESIGN ASSUMPTIONS AND APPROACH This chapter will list the different design assumptions and describe the calculation approach First Space environment assumptions will be explained and then the power determination will be described 5 1 1 Space environment assumptions In a first approach we suppose that insolation and albedo are constant when the satellite is in the sunny area daylignt In the shady area eclipse we consider that radiations are null Solar constant out of atmosphere 1368 W m Albedo 410 W m
9. Suiss Cuba Page 15 of 43 Issue 1 Rev 0 6 DESIGN TRADES ANALYSES 6 1 Energy captured by photovoltaic cells Due to increasing requirement for power mass and area traditional silicon solar cells will be more and more replaced by high efficiency multi junction solar cells on space solar generators R2 Swisscube will certainly be equipped with high efficiency triple junctions cells like this one Figure 7 High efficiency triple junction cell scale 1 1 6 1 1 Triple junctions cells characteristics GaAs based solar cells have the potential to reach efficiencies greater than 30 In the meantime cells with up to 28 efficiency are available R3 es Figure 8 Triple junction space solar cell grown on a ultra thin Ge substrate To carry out the electrical power calculation we have initially considered the following cell type RWE3G ID2 150 8040 which have an average efficiency of 26 6 begin of life N2 Ref S3 A EPS 1 0 EPS doc Date 16 06 2006 G SwissCuber Page 16 of 43 Issue 1 Rev 0 Here are some information about RWE3G ID2 150 8040 cell type Design and mechanical data Dimensions 40 x 80 mm Cell Area 30 18 cm Average Weight Electrical Data BOLX 28 C Average Open Circuit 2 554 Average Short Circuit x 0 498 I I P Corrent at max Power Tyas A 0 480 Ainte ia HW 1 088 _ 1 083 Average Efficiency n P 266 BOL
10. 6 2 Maximum Power Point Tracking MPPT All previous results are valid if we can extract the maximum power of each solar cell But one problem in connecting a solar cell to a load or to a battery is that the cell works at maximum efficiency for only one current and voltage value In order to use PV cells efficiently we need to know how they behave when connected to various electrical loads If a PV cell is connected to a variable electrical resistance R together with an ammeter to measure the current I in the circuit and a voltmeter to measure the voltage V developed across the cell terminals When the resistance is infinite the current in the circuit is at its minimum and the voltage across the cell is at its maximum known as the open circuit voltage Voc At the other extreme when the resistance is zeto the cell is in effect short circuited and the current in the circuit then reaches its maximum known as the short circuit current Isc If we vary the resistance between zero and infinity the current and voltage will be found to vary as shown in Figure 22 which is known as the I V characteristic of the cell It can be seen from the graph that the cell will deliver maximum power Le the maximum product of voltage and current when the external resistance is adjusted so that its value corresponds to the maximum power point MPP on the I V curve Pi HV max pmax pmax current I sun variable resistance R __
11. Begin Of Life Temperature Gradients I dT 0 272 BV yal OT 64 pmax inW G Ref S3 A EPS 1 0 EPS doc oie Issue 1 Rev 0 4 Swiss Cubs Date 16 06 2006 Page 17 of 43 Result of the temperature variations is plotted in Figure 10 The temperature range that we consider is 25 C to 10 C BOL 1E14 2E14 _ 4E14 3 5 T V T A SC P a TM 20 30 40 50 60 70 80 90 100 Temperature C Figure 9 Temperature effect Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 18 of 43 6 1 2 Triple junctions cells degradations Packaging and fabrication losses Mismatch amp fabrication 0 98 Wiring amp diode loss 0 96 Packing factor 1 Environment issues Temp loss factor 1 00 Shadowing losses 0 90 Life factors Ultraviolet degradation 0 98 Radiation degradation 0 95 Fatigue thermal cycling 0 98 Micrometeoroid loss 0 98 Additional margin 1 The product of all these factors gives the degradations factor If we multiply the BOL cell efficiency by the degradation factor we obtain the EOL cell efficiency Degradation factor 0 757 Cell effici
12. Flex model for our application is PLF 443450 C Nominal Voltage 3 7 V Typical Capacity 800 mAh Minimum Capacity 770 mAh Length 1 49 2 mm Width w 34 mm Height h 4 4 mm Pouch a 44 2 mm Weieth 14 g We have also considered some others battery manufacturers like KOKAM Li Po Sanyo Li Ion Danionics Li lon etc But the dimensions were not very appropriate Ref S3 A EPS 1 0 EPS doc ae ee Issue 1 Rev 0 Date 16 06 2006 Swiss Cubs Page 41 of 43 5 Redundancy In order to have a redundancy it is better to take two battery cells than one If we use two PLF 443450 C Batteries the total capacity is 1 6 Ah and the total mass is 28 g This is a good compromise 6 Mass Mparery The first EPS mass allocation was 130 g all inclusive But we rapidly realized that this allocation was not enough because the batteries are relatively heavy Here is the actual EPS mass budget EPS board re es g attery current sensor attery voltage sensor attery smart monitor ADCS PL COM CDMS Current sensors ADCS PL COM CDMS A Power sensors connector o prn fed Q D n O io O ep The batteries mass represent about a fifth of the total EPS mass budget Ref S3 A EPS 1 0 EPS doc ae Issue 1 Rev 0 Date 16 06 2006 Swiss Cubs Page 42 of 43 7 Charging Characteristics To do a correct charge it is necessary to respect a few parameters
13. GN RBODIRENTEN RS aa EEE ie 12 4 1 PHYSICAL STRUCTURE REQUIREMENTS 12 4 2 ELECTRICAL REQUIREMENTS 13 4 3 DATA FLOW REQUIREMENTS 13 4 4 OTHER SPECIFICATIONS 13 4 4 1 Remove before flight pin 13 4 4 2 Thermal requirements 13 4 4 5 Deployment and Transmission delay 13 5 DESIGN ASSUMPTIONS AND APPROACH sesseeccessosesesessssocsocccccceessecccocssosoococceeecsseesossssosocccessseeseeesssss 14 5 1 1 Space environment assumptions 14 5 1 2 Power determination assumptions and approach 14 6 DESIGN TRADES ANALYSES tocessce teccscscect recede ce cedacaedoedccsvecncacdeececaceseeadadanicntecsveeseaddeeeeces eases 15 6 1 ENERGY CAPTURED BY PHOTOVOLTAIC CELLS 15 6 1 1 Triple junctions cells characteristics 15 6 1 2 Triple junctions cells degradations 18 6 1 3 Cells layout 19 6 1 4 Power calculation Approach optimistic 20 6 1 5 Power calculation Approach pessimistic 23 6 1 6 Albedo s contribution 25 6 1 7 Total Energy generated by the solar cells 25 6 2 MAXIMUM POWER POINT TRACKING MPPT 26 EPS ARCHITECTURE 28 6 2 1 One step up converter per side 28 6 2 2 One step up converter for all sides 29 6 2 3 Step up converter 30 6 2 4 Low Dropout Linear regulator LDO 52 6 2 5 Common Mode Filters 3J 6 2 6 Batteries 36 7 OTN USING arc E E E E E E E E E E 43 Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 Swisseub Page 4 of 43 FOREWORD SwissCube project aims at designing a picosatellite according to the c
14. Maximum power point short circuit current Isc current __ open circuit voltage Voc voltage Figure 18 I V characteristic of a typical solar cell Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 a S UMSS Cube Page 27 of 43 At lower levels of solar radiation than the maximum the general shape of the I V characteristic stays the same but the area under the curve decreases and the maximum power point moves to the left The short circuit current is directly proportional to the intensity of solar radiation on the cell while the open circuit voltage is only weakly dependent on the solar radiation intensity The open circuit voltage also decreases linearly as cell temperature increases as shown in Figure 10 Power can also be tracked on several parallel cells even if the intensity of solar radiation differs on each two cells there are two cells per side because the MPP of the different cells are approximately on a straight line as shown on Figure 24 But these different MPP can move if the temperature is very different between two sides So the voltage level is the same for each branch and then several solar cells branches can work together With this method of global power tracking we can t assure extracting the maximum power of each solar cell but almost So we will certainly use this method of power tracking for the SwissCube i Figure 19 Parallel cells with protection diodes l
15. OUTPUT VOLTAGE Vs OUTPUT CURRENT a 10 20 30 COLLE tt 40 l Output Current mA 50 Figure 26 Typical LDO load regulation characteristic COM CDMS PL ADCS Ref S3 A EPS 1 0 EPS doc aCe Issue 1 Rev 0 Date 16 06 2006 Swiss Page 34 of 43 Here is a ripple rejection vs frequency typical characteristic TPS 77033 RIPPLE REJECTION vs FREQUENCY Ripple Rejection dB 10 100 1k 10k 100k 1M 10M f Frequency Hz Figure 27 Typical LDO ripple rejection characteristic The ripple frequency on the input voltage will certainly be included between 50 kHz and 100 kHz At this frequency the ripple rejection is at worst 27 dB So if we have an input ripple of 50 mV the output ripple will be at worst about 2 5 mV LDO power losses calculation method LDO power losses depend on the output current and levels difference between input and output voltage This voltage difference has been estimated at 0 7 Volt on average The output current has been calculated with the average power consummation of each subsystem lane AVE A V Vs Von I subsystem out V Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 35 of 43 Here are the results for each subsystem Subsystems ACDS EPS Beacon CDMS A RE transmit receiver and control Pause MW 550 100 f 4 20 5 aAa at o e e 1 Pia 200 mW
16. cut Swiss Prepared by Fabien Jordan Checked by Approved by heig vd Yverdon Switzerland 16 06 2006 Date 16 06 2006 Issue 1 Rev 0 Page 1 of 43 Phase A Electrical Power System EPS Final report Haute Ecole d Ing nierie et de Gestion du Canton de Vaud Institut d Automatisation r Industrielle Hes sola ECOLE POLYTECHNIQUE Ha ole san aa DS era 96 EDERALE DE LAUSANNE Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 2 of 43 RECORD OF REVISIONS ISS REV Created modifed by 14 06 2006 Fabien Jordan heig vd Ref S3 A EPS 1 0 EPS doc a Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 3 of 43 RECORD OF RE VISTOND annissa a nas taste tisse tels aaa ea 2 PORN ORD ae cena sn eee teinte usb asie die rest sale 4 INTRODUCTION ssadnssescstuasencassacertsdassensesatsebuatuaseasadnadestubucaeatadadcebentasseasedsaseitubues a aar eaa aa aaa doiana rabais 5 CUBESAT CONCEPT 5 P POD INTERFACE 6 LAUNCH VEHICULE 6 SCIENCE MISSION OBJECTIVE 7 ELECTRICAL POWER SYSTEM EPS ROLE ji 1 REFERENCE oi E T E EE E T O 8 1 1 NORMATIVE REFERENCES 8 1 2 NFORMATIVE REFERENCES 8 2 TERMS DEFINITIONS AND ABBREVIATED TERMS sscccccccccsssssssssssssssscccccsssssssssssssssssssssssees 9 2 1 ABBREVIATED TERMS 9 2 1 1 Subsystems abbreviations 9 2 1 2 Technical abbreviations 9 2 1 3 Definitions 10 3 SEMESTER PROJECT OBJECTIVES irrisoria RERE EES AEE es itins 11 4 EPS DESI
17. e V The output nominal voltage of two solar cells in series will be about 3 9 V with a fall voltage of 0 5 V on the protection diode The output voltage of LDO is 3 3 V So we decided to use a battery with a typical voltage of 3 6 or 3 7 V depends on technology Li Ion or Li Po With this choice we don t unnecessary loose energy to step up the voltage to a higher level like 5 V or more and then to step it down to 3 3 V with the LDO Actually we are very interested by a Li Po technology made by VARTA Microbattery which has a nominal Voltage of 3 7 V But the average Voltage on the battery terminals will not be 3 7 V because when we charge the battery its terminals voltage increases to 4 2 V But for the moment we suppose that this level of 4 2 V will never be reached and we assume that the Average Voltage on the battery terminals is 4 0 V Assumptions Vion 3 7 V V 40V av Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 38 of 43 2 Accumulated Energy Ecc During the daylight the solar cells generate some electrical energy For the most part this energy is directly transmitted to the subsystems But a smaller part of this power production must be kept in the battery in order to return it to the subsystems during the eclipse Of course the energy accumulated in the batteries must be greater than the amount of energy needed during an eclipse So we need to know 1 the amount of energ
18. ency BOL 26 6 Cell efficiency EOL 20 1 Ref S3 A EPS 1 0 EPS doc Date 16 06 2006 SwissCuber Page 19 of 43 c Issue 1 Rev 0 6 1 3 Cells layout As it has been decided with the other subsystems SwissCube will have five sides covered by triple junction solar cells Each of these five sides is covered by two solar cells Figure 10 Two cells per side on five of the six sides One side must be free for the hole of the camera s objective and for the antenna s fixings Figure 11 The camera and antenna side Now the power generated by the solar cells is calculated with a Matlab Model Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 a S UMSS Cube Page 20 of 43 6 1 4 Power calculation Approach optimistic To do this calculation we have supposed that the SwissCube is fixed and a light source is turning around For the first step of the calculation we consider the vertical plane with an angle alpha 4 5 side A 4 side B side C 3 5 side D 3 S25 g 2 Oo 1 5 1 0 5 0 0 50 100 150 200 250 300 350 a Figure 12 Power generated from the insolation vertical plane Ref S3 A EPS 1 0 EPS doc Date 16 06 2006 Suiss Cuba Page 21 of 43 c Issue 1 Rev 0 Here is the second step plane of 8 4 5 side
19. f S3 A EPS 1 0 EPS doc
20. ld choose a DOD of 30 for this number of cycles So we need to know 4 the Accumulated Energy EX Wh 5 the Depth Of Discharge DOD 6 the average Voltage on the battery terminals V V 7 the ambient temperature Tae Poi Assumptions for the worst case Ec 0 67 Wh DOD 30 see the SMAD Va 40 V Calculation C E V 1 DOD 0 67 4 0 1 0 30 0 56 Ah If a Li Po technology is used it is important to consider the ambient temperature For example the KOKAM Li Po manufacturer declares that we can consider the capacity about half reduced if the temperature decreases to 30 C Given the fact that the temperature can decrease until 23 C we decide to double the total capacity Ref S3 A EPS 1 0 EPS doc Date 16 06 2006 Swiss Cuber Page 40 of 43 Issue 1 Rev 0 4 Dimensions The structure and configuration group give us the available battery volume 45 x 15 x 90 mm The batteries have been chosen according to these dimensions and according to the last constraints The Li Po battery technology made by VARTA Microbattery is an interesting solution They are specialized in Microbattery and their products dimensions and weight are really appropriated for the SwissCube constraints The PoLiFlex product range contains an integrated thermal and over current protection Figure 31 Typical VARTA Li Po battery with thermal and over current protection The most interesting PoLi
21. loping picosatellites containing scientific private and government payloads The primary mission of this program is to provide access to space for small payloads The CubeSat program gives students the opportunity to work in the space technologies field This opportunity is possible by reducing the cost and development time generally associated with satellite design To reduce these cost and time constraints it is necessary to drastically reduce the dimensions of the satellite It is the reason why the fundamental defining feature of the CubeSat standard is its dimensions of 10 x 10 x 10 cm and its mass of no more than 1 kg 8 5 MIN 8 5 MIN E Deployment Switches Separation Springs Figure 2 CubeSat standard dimensions N1 Ref S3 A EPS 1 0 EPS doc A Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 6 of 43 P POD Interface The Poly Picosatellite Orbital Deployer P POD is Cal Poly s standardized CubeSat deployment system It is capable of carrying three standard CubeSats and serves as the interface between the CubeSats and LV Launch Vehicule The P POD is an aluminum rectangular box with a door and a spring mechanism CubeSats slide along a series of rails during ejection into orbit CubeSats must be compatible with the P POD to ensure safety and success of the mission N1 Figure 3 Poly Picosatellite Orbital Deployer P POD Launch Vehicule The SwissCube will be launched by a Vega LV M
22. onference on Photovoltaic Energy Conversion Osaka Japan May 2003 ECSS E 10 04A Space environment January 2000 M Correvon Alimentations d coupage a inductance simple cours de syst mes lectroniques heig vd Yverdon Suisse http iai heio vd ch cours ph VARTA Microbattery Sales Program and technical handbook PoliFlex Superior Polymer Technology Ref S3 A EPS 1 0 EPS doc Swiss a a Issue 1 Rev 0 7 WG Date 16 06 2006 Page 9 of 43 2 TERMS DEFINITIONS AND ABBREVIATED TERMS 2 1 Abbreviated terms 2 1 1 Subsystems abbreviations ADCS CDMS COM EPS MECH P E 2 12 BOL CAN DOD DnTnS DTS EOL EnTnS ETS GaAs LDO Li Ion Li Po LV MPPT LC PCB PV SEL TBD Attitude Determination and Control System Control and Data Management System Communication System Electrical Power System Mechanisms Payload Technical abbreviations Beginning Of Life Controller Area Network Depth Of Discharge Daylight without transmission and without science Daylight with Transmission and with science End Of Life Eclipse without Transmission and without science Eclipse with Transmission and with science Gallium Arsenide Low Dropout Linear regulator Lithium Ion Lithium Polymer Launch Vehicle Maximum Power Point Tracking Inter Integrated Circuit Printed Circuit Board Photovoltaic Single Event Latch up To be defined Ref S3 A EPS 1 0 EPS doc ae ee Issue 1 Rev 0 Da
23. onventional standard provided by CubeSat The design and the development of the SwissCube picosatellite will be carried out by students of Swiss Federal Institute of Technology in Lausanne EPFL and other Swiss engineer schools and universities This report was written at the request of the SwissCube project steering committee Its purpose is to examine and evaluate the different possibilities and to make recommendations concerning the SwissCube Electrical Power System EPS The introduction gives a description of the CubeSat concept the science mission objective and of course the electrical power system role Chapter 3 enumerates the semester project objectives Then chapter 4 describes which requirements are imposed to the EPS design The assumptions and approach are described in the 5 chapter and finally chapter 6 explains the different trades off and gives some analysis recommendations Figure 1 Example AAUSAT 1 CubeSat Ref S3 A EPS 1 0 EPS doc Date 16 06 2006 Swiss Cuber Page 5 of 43 Issue 1 Rev 0 INTRODUCTION The SwissCube project is directed by the Space Center at the Swiss Federal Institute of Technology in Lausanne EPFL This project is based on the CubeSat program started by Stanford University and Polytechnic State University Cal Poly in 1999 CubeSat concept The CubeSat project developed by Cal Poly is an international collaboration of over 40 universities high schools and private firms deve
24. ore information about the Vega LV is given in the VEGA launch vehicule User s Manual R1 Figure 4 Vega LV Ref S3 A EPS 1 0 EPS doc a Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 7 of 43 Science Mission Objective The science Payload objective is taking comprehensive measurements of the NightGlow Phenomenon This luminous phenomenon appears on the earth atmosphere and can be observed by satellite during eclipse because it generates a green luminous glow as shown in Figure 5 The SwissCube will be able to take optical measurements and characterize the Nightglow phenomena over all latitudes and longitudes for at least a period of 3 months Figure 5 NiethGlow Phenomena Electrical Power System EPS Role The primary EPS function is capturing solar energy from the sun and albedo with solar cells during the daylight and then to transmit it to the subsystems Given the fact that NightGlow phenomenon can only take measurements during the eclipse SwissCube will have a helios synchronous orbit in such a way that it will be in eclipse during about 30 of the orbital period So a part of the captured solar energy must be kept in a battery in order to return it to the subsystems during the eclipse during this time the solar cells are of course quasi inefficient The 3 main SwissCube electrical consumers ate Electrical Power System converters and linear regulators Attitude Control and Determination System magnet
25. otorquers motors Communication System RF transmitter The payload will take photographs during the eclipse but this work doesn t consume a lot of energy The secondary EPS function is protecting the different subsystems against a phenomenon called Single Event Latch up SEL This phenomenon happens when a high energy particle hits the device If the impact on the device is of a serious nature the high energy particle can directly cause damage to the devise This phenomenon happens very quickly and must be detected and corrected in hardware In case of the power not being turned off at a latch up a burn out can occur and destroy the chip Ref S3 A EPS 1 0 EPS doc ae Issue 1 Rev 0 Date 16 06 2006 Swiss Cuber Page 8 of 43 1 REFERENCES 1 1 Normative references NI N2 1 2 R1 R2 R3 R4 R5 R6 CubeSat Design Specification CDS June 2004 http littonlab atl calpolv edu pages documents developers ph RWE Space Solar Power GmbH Cell Type RWE3G ID2 150 8040 Informative references VEGA launch vehicule User s Manual March 2006 http www atianespace com site documents vega man index html M Meusel Development status of European multi junction space solar cells with high radiation hardness 20 European Photovoltaic Solar Energy Conference Spain Barcelona June 2005 G Strobl Advanced GalnP Ga In As Ge triple junction space solar cells 3 World C
26. rge control duty cycle adjustment Advantages less heavy less voluminous Disadvantage we can t assure extracting the maximum power of each solar cell but almost ADCS need power information from each cells covered side This is an important constraint that we have to include in our design The two solutions make it possible Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 a S UMSS Cube Page 30 of 43 6 2 3 Step up converter The Step up converter or Boost converter uses a parallel static contactor which must be supply by a current source and to output in a voltage source The receiver itself consists of a resistance R and the filtering capacitor C assembled in parallel to its terminals This receiver has the behaviour of a voltage source R3 Current source Voltage source Figure 21 Step up converter Figure 22 Q ON and Q OFF Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 4 Date 16 06 2006 Suiss Cuba Page 31 of 43 Figure 23 Step up current and voltage characteristics Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 32 of 43 6 2 4 Low Dropout Linear regulator LDO Generally power supplies give a regulated voltage It means that the voltage amplitude must vary as little as possible when the output current and the input energy source vary The voltage amplitude should also not be altered by the temperature variations
27. t Pioa P2ma P rotal 3 E a Z 6 5 5 S Foi Vj 2 Output voltage of the PV module V Figure 20 The different MPP are on a straight line Ref S3 A EPS 1 0 EPS doc a Issue 1 Rev 0 Date 16 06 2006 Swiss Page 28 of 43 EPS Architecture 6 2 1 One step up converter per side Step up Floating Power Bus converter 3 7 4 2V Batteries Step up converter CLELELELELELEL EE EEE Stepup 1 converter LDO Filter CDMS ceoscosooooosoooososoe sesesoooososocoooosooe Step up converter Step up converter EPS Power tracking on each side duty cycles adjustment Step up output current control duty cycles adjustment Battery charge control duty cycles adjustment Advantages we can extract the maximum power of each solar cell Disadvantage too much heavy converter s inductances require important calculation power voluminous converter s inductances Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 ae S UMSS Cube Page 29 of 43 6 2 2 One step up converter for all sides Floating Power Bus 3 6 4 2 V Step up Batteries converter eesoososeocososooooooo ceosoosooooocoooososoe sesesocoososoooooosose eesoososeooososooooooo s O O 0 Oo EPS Global power tracking duty cycle adjustment Step up output current control duty cycle adjustment Battery cha
28. te 16 06 2006 Suiss Cuba Page 10 of 43 2 1 3 Definitions Albedo Albedo is a measure of reflectivity of a surface or body It is the ratio of total electromagnetic radiation reflected to the total amount incident upon it The average albedo of Earth is about 30 Dropout voltage The dropout voltage is the minimum difference between input and output voltage that a LDO needs to do the output voltage regulation Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 11 of43 3 SEMESTER PROJECT OBJECTIVES The mains objectives of this semester project are collecting information about the space power supply methods and making some preliminary calculations about the power production the power needs the dimensions and the mass of the SwissCube Electrical Power System Finally the goal is giving several design trade off with a favorite solution To fulfill these objectives the following tasks have to be done Read the project documentation specifications infrastructure Read about EPS other CubeSat papers Build a Functional Block Diagram of EPS with the other subsystems interfaces Build a Hardware Block Diagram Build an Architecture Design and Technology option trees These different tasks have to be used as tools to correctly develop each subsystem This working method will help us to think about of a maximum of solutions and configurations Ref S3 A EPS 1 0 EPS doc
29. tion vertical plane 3 5 side D side E 3 side B side F camera 2 5 2 a 1 9 1 0 5 0 0 50 100 150 200 250 300 350 BE Figure 16 Power generated from the sun horizontal plane Ref S3 A EPS 1 0 EPS doc Date 16 06 2006 O Swiss Cuber Page 24 of 43 Issue 1 Rev 0 Figure 17 Total power generated from the sun worst case This matrix describe the electrical power that can be produce whatever the satellite s attitude in the worst case limit angle 20 and efficiency 20 1 Maximum value obtained from Matlab is Pax 2 86 W This power peak happen when three sides covered by cells are equally exposed to the sun for 45 1357 225 3157 and B 1257 OF 235 Average value obtained from Matlab is P 1 75 W This value is valid under the assumption that all sides are equally exposed to the sun during the mission But the satellite s attitude is quite favourable because the side without cells would hardly never be exposed With the two matrix represented at figure 15 and figure 18 we have calculated an average value with the probable satellite s attitude for the best and worst cases Assumption 107 lt B lt 119 Inclination given by P L subsystem 17 to 29 Best case P 3 26 W Worst case P 2 30 W For the next developments we will consider the worst case P 2 30 W av_s
30. un Ref S3 A EPS 1 0 EPS doc Issue 1 Rev 0 Date 16 06 2006 Swiss Cube Page 25 of43 6 1 6 Albedo s contribution We have used exactly the same approach to determine the albedo s contribution In fact albedo can be considered like insolation because waves length are approximately the sames A value of 0 3 and the same spectrum as the Sun were specified as standard for the Earth albedo On a short time scale albedo can be very variable and range from about 0 05 to 0 6 The albedo spectrum can change depending on properties of the surface and atmosphere Ground vegetation and atmospheric water and dust can lead to absorption in certain wavelength bands and result in a highly variable albedo spectrum R4 So Albedo is not constant but we can consider that average albedo is about 30 of insolation Albedo 1368 30 410 W m Assumption 61 lt B lt 73 Inclination given by P L subsystem 17 to 29 Best case P 0 72 W Worst case P 0 55 W 0 55 W For the next developments we will consider the worst case P av_albedo 6 1 7 Total Energy generated by the solar cells Now we can easily determine the energy potentially captured by photovoltaic cells during one revolution insolation 5 6 6 min See Ease sa P eado To 60 2 69 Wh Insolation period T Ref S3 A EPS 1 0 EPS doc a i Issue 1 Rev 0 Date 16 06 2006 Suiss Cuba Page 26 of 43
31. y generated by the cells ee Wh 2 the amount of energy needed during the daylight pe Wh 3 the amount of energy needed during an eclipse Plad Wh Assumptions for the worst case a 2 69 Wh see chapter 6 1 7 Figg needs 2 02 Wh see Power Budget created by System Engineering gt DTS Esa needs 1 19 Wh see Power Budget created by System Engineering gt ETS Calculation Ex Eptoa Euy needs 2 69 2 02 0 67 Wh But E must be greater than Esa 4 SO at least it is necessary to accumulate an amount of energy greater than 1 19 Wh In these conditions the satellite need at least two more daylights with a minimum consumption DnTnS and EnTnS to recharge the batteries Ref S3 A EPS 1 0 EPS doc a a Issue 1 Rev 0 7 WG Date 16 06 2006 Swiss Page 39 of 43 3 Capacity C tot The DOD is the percentage of the full capacity of the battery that is used during each charge discharge cycle The satellite lifetime depends on the batteries lifetime and the batteries lifetime depends on the chosen DOD So it is necessary to choose a correct DOD The DOD can be chosen according to the number of charge discharge cycles As it is required the mission will last at least 3 months but we would like to be operational during one year On a helio synchronous orbit the satellite will turn 15 times around the Earth in one day For ones year it will do 5475 revolutions According to the literature we shou
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