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

Figure 2.7: OUFTI-1 subsystems and power buses.The requirements for the power conditioning unit are based on the above assumptions.The maximum current of the 3.3V and 5V outputs is higher than computed above, to letsome flexibility.Requirements for the power conditioning unit:• One output at 3.3V with un maximum current of 200mA.• One output at 5V with un maximum current of 200mA.• One output at 7.2V with un maximum current of 420mA.These requirements should be reviewed each time changes are made to the architecture ofthe cubesat.2.3.4 Housekeeping parametersThere are important values on the EPS, like solar panels currents, batteries voltages, batteriestemperature, and power bus voltage, that must be measured and transmitted to the ground,to know the state of the CubeSat and to verify that everything works fine. The EPS willinclude the measurement circuits, sensors, and ADC’s. There must be an interface with theOBC to command the measurement circuits and to transmit data. An I 2 C bus has beenchosen as an interface.A current-limiting protection circuit must also be included on the EPS for the measurementcircuits.2.3.5 Antennas Deployment SystemDuring the launch, antennas are wound. They are held back by a string. To release theantennas, the string is cut by a thermal cutter. It is a resistor crossed by an importantcurrent (the exact current value is still unknown but its order of magnitude is around 1A).The antenna deployment is controlled by the OBC, but the OBC cannot deliver such acurrent. A power switch will be used and it will be located on the EPS card. The switch willbe designed to withstand a current of 3A.19