Design and Implementation of On-board Electrical Power ... - OUFTI-1

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There is no data about the case to PBC thermal resistance in the datasheet of theTPS61085. Only the case to air thermal resistance is available but it is useless in vacuum.With no certitude about its capacity to dissipate the produced heat, it was decided to chooseanother IC, i.e. the TPS61087.The TPS61087 is equipped with a “Power PAD”, which allow a good thermal contactbetween the case and the PCB. The junction-air thermal resistance is 30 ◦ C/W when thePower PAD is soldered to the PCB. As the low thermal resistance is essentially due to thejunction-PCB resistance, the thermal resistance will be close to 30 ◦ C/W in vacuum.From “Absolute Maximum Ratings”, the maximum operating junction temperature is150 ◦ C. A margin of 25 ◦ C is taken. The converter is designed to work with a PCB temperatureof -40 ◦ C to 85 ◦ C. The maximum dissipated power isP D = T J − T A 125 − 85= = 1.33W,R ϑJA 30where T J is the junction temperature, T A the air (or PCB) temperature, and R ϑJA thethermal resistance between junction and air.Switching frequencyThe switching frequency f s on the TPS61087 can be chosen to be 650kHz or 1200kHz. Theefficiency can be improved by choosing a f s of 650kHz but the ripple will be more important.The breadboard prototype was designed with f s = 650kHz while the engineering model usesa f s of 1.2MHz. The tests show that the efficiency is practically the same with 1.2MHz as itis with 650kHz.Design of inductor and capacitorA worksheet was written to help with the design of the converters. The inputs are theminimum input voltage V in,Min , the maximum input voltage V in,Max , the output voltage V out ,the maximum output current I out,Max , the expected efficiency η, the switching frequency f s ,the maximum ratio between the current ripple and the average current in the inductor k(∆i L /I av max), and the maximum output voltage ripple ∆v max .There are two steps.In the step 1, the inductance is determined using Eqs. 5.26 and 5.28. The peak inductorcurrent is found with Eq. 5.31 (it is computed for several values of V in in a table and themaximum is kept). The desired series resistance and the output capacitor are also determined.Results for k = 0.3, ∆v max = 10mV, and η = 0.85 are presented in Fig. 5.7. On the basisof these results, an inductor and a capacitor are selected. The inductance and the capacitancewill not exactly correspond to the computed values (real components have standardizedvalues).In the second step, the worksheet is filled up with the values of real components (figure5.9). We obtain the real values of the maximum current in the inductor and of ∆v max .58
Figure 5.7: Worksheet for 7.2V converter, step 1.Inductor selectionThe chosen inductor is the EPCOS B82464Z4682M. A picture of the inductor is shownon Fig. 5.8. Its characteristics are given in table below.Figure 5.8: Inductor EPCOS B82464Z4682.CharacteristicsSize: 10.4 × 10.4 × 4.8(mm)L = 6.8µHMagnetically shieldedTemperature range: up to 125 ◦ CR max = 20mΩRated current: 4.3A59
Page 2 and 3:
AcknowledgementsI want to thank the
Page 4 and 5:
Contents1 Introduction 91.1 Cubesat
Page 6 and 7:
4.4.2 Mean case . . . . . . . . . .
Page 8 and 9: B Power budget worksheet 106C Pictu
Page 10 and 11: design and the tests are delegated
Page 12 and 13: Chapter 2Requirements of the EPS2.1
Page 14 and 15: • The Single-Event Upset (SEU)Thi
Page 16 and 17: the P-POD. RBF pins must fit within
Page 18 and 19: Figure 2.6: Top view of the PC104 c
Page 20 and 21: Chapter 3Design of EPS architecture
Page 22 and 23: • Voltage (4) and current (5) at
Page 24 and 25: Figure 3.6: The equivalent circuit
Page 26 and 27: of our Lithium-Polymer batteries va
Page 28 and 29: Figure 3.12: I-V curve of a solar p
Page 30 and 31: 3.3.3 CapacityA important value to
Page 32 and 33: Parameter SLPB723870H4 SLPB554374HN
Page 34 and 35: of the batteries is kept between -2
Page 36 and 37: Over Charge Prohibition 4.275 ± 0.
Page 38 and 39: supplied in 5V. The circuit will be
Page 40 and 41: Chapter 4The Power Budget4.1 Introd
Page 42 and 43: Figure 4.1: P-V curve of a solar pa
Page 44 and 45: 4.3.2 Efficiency of convertersTo at
Page 46 and 47: Figure 4.3: Consumptions in % in me
Page 48 and 49: Chapter 5Electrical Design of EPS5.
Page 50 and 51: V outV in= D. (5.1)Since D ≤ 1, t
Page 52 and 53: The power losses in the inductor ar
Page 54 and 55: ∆i L,1 + ∆i L,2 = 0, (5.16)V in
Page 56 and 57: Using the value of ∆i L given by
Page 60 and 61: Capacitor selectionFour 10µF ceram
Page 62 and 63: • Output voltage: 5V.• Maximum
Page 64 and 65: Figure 5.12: Burst mode operation (
Page 66 and 67: Figure 5.14: Simplified schematics
Page 68 and 69: Figure 5.15: Worksheet for 3.3V con
Page 70 and 71: sequently, the k was chosen above 0
Page 72 and 73: where G 1 is the initial control-to
Page 74 and 75: Figure 5.21: Measured Bode diagram
Page 76 and 77: Figure 5.26: Equivalence between th
Page 78 and 79: C f =12πf f R 0f,L f = R 2 0f C f
Page 80 and 81: Figure 5.37: Schematics of the firs
Page 82 and 83: R KR >1.45V100mA − 1.3A35= 23.07
Page 84 and 85: The schematics is shown on figure 5
Page 86 and 87: A commercial model meets all requir
Page 88 and 89: Figure 5.45: Schematics of the heat
Page 90 and 91: PrefixX7X5Y5Z5SuffixTemperature ran
Page 92 and 93: 6.2.1 The second dissipation system
Page 94 and 95: • The antenna deployment system.
Page 96 and 97: 6.3.3 TestsThe engineering model of
Page 98 and 99: 7.3 ActivitiesAs OUFTI-1 is designe
Page 100 and 101: 8.1.2 DesignA model of Li-Po batter
Page 102 and 103: [15] Fabien Jordan, Phase B Electri
Page 104 and 105: TaK = 273 + TaC;%Photo-current ther
Page 106 and 107: Appendix BPower budget worksheetIn
Page 108 and 109:
108
Page 110 and 111:
Appendix CPictures of the prototype
Page 112 and 113:
Appendix DSchematics of the enginee
Page 114 and 115:
876543213V3 CONVERTER AND INPUT FIL
Page 116:
87654321ANTENNA DEPLOYMENT CIRCUITB
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