Design of an Electrical Power Subsystem Using the CubeSat Stackable

Transcription

1 Design of an Electrical Power Subsystem Using the CubeSat Stackable Interface Bao Jun Lei Department of Electrical Engineering University of Hawai`i at Manoa Honolulu, HI Abstract The design of an Electrical Power Subsystem using the CubeSat Stackable Interface (CSI) is presented, focusing on integrating the DC-DC converters and the voltage-monitoring system on the CSI bus. This modular design can be used with different payloads, fulfilling NASA educational outcomes by providing a fast learning curve for students to develop new projects reduced cost, time, and risk. Introduction The University of Hawaii has been a part of the small-satellite community since Throughout this period, numerous CubeSats and nanosats have been fabricated, each designed with a different bus. Presented here is a modular Electrical Power Subsystem bus design that eliminates the need to reinvent the wheel, reducing development time, risk, and cost. The Electrical Power Subsystem is responsible for providing, storing, distributing, and controlling the entire satellite s electrical power. A basic block diagram of the Electrical Power Subsystem used on a nanosatellite platform is shown in Figure 1. Figure 1 Basic Block Diagram of the Electrical Power Subsystem

2 Error! Reference source not found. 2 shows UH s CubeSat Stackable Interface (CSI) pin assignments 1. Figure 2 CubeSat Stackable Interface The CubeSat Stackable Interface is a standardized 120-pin configuration designed around the Electrical Power Subsystem. The ground pins are placed throughout the adapted PCI-104 pin configuration to help shield against electrical interference. With 8 assigned connectors each, the 3.3V and 5V pins can provide a total of up to 8A of continuous current. For voltages that are specific to different components or payloads on a satellite, the raw battery voltage pins allow for further conversion on separate boards. The solar cell inputs provide power through the boards, which also helps eliminate the possibility of single-point failure since multiple boards could be included for redundancy and to allow operations even if an individual power board fails. NASA Educational Outcome 2, to attract and retain students in STEM through a progression of educational opportunities for students, will be met with the goal of designing a modular electrical power subsystem using CSI. This would allow future students to use this standard for a simple plug-and-play bus to conserve time and cost on designing and fabricating new small satellites with different payloads. Description of Electrical Power Subsystem Using CSI To step-down to the appropriate +3.3V and +5V voltages, the component chosen is the LM2576 step-down switching regulator. This regulator also has a wide 7 40V input range while able to

3 output a fixed 3.3V, 5V, and 12V. This component is used for CSI fixed output voltages of 3.3V and 5V. The increased 3A output current capability allows the CSI to be managed across more payloads that may consist of higher power loads. Multiple LM2576 components can be placed in parallel to provide a higher current output load. The fabricated board of the LM2576 is shown in Figure 3. Figure 3 LM , +5V Buck Voltage Regulator PCB While the LM2576 regulator was able to output appropriate +3.3V and +5V, power efficiency tests were conducted to measuree the amount of power loss with this component. The measured values were then evaluated using: % Due to a 30W limitation on the power supply, the LM2576 regulator was tested to provide a 1.8A load capacity. Figure 4 shows the LM2576 regulator with an approximate 80% efficiency. Power Efficiency (%) LM2576 5V Power Efficiency Load Current (A) Figure 4 LM V Buck Regulator Power Efficiency To monitor the voltage lines on CSI, the component chosen is the MAX6652 temperature sensor and system monitor. The MAX6652 is capable of monitoring 2.5V, 3.3V, 5V and 12V while also monitoring local temperatures 2. This component monitors the 3.3V and 5V lines on the CSI bus and also the temperature of the voltage lines on the CSI bus. The I 2 C Serial Clock Line

4 (SCL) and a Serial Data Line (SDA) data lines are used within CSI s PM_SDA and PM_SCL lines that are dedicated for use of power management. Figure 5 shows the typical application circuit where Vcc is tied to the 5V line. Figure 5 MAX6652 Typical Application Circuit While the MAX6652 monitors the 3.3V and 5V line voltages and temperature, the MAX4377 was chosen to monitor the current on these lines. Current monitoring is important to ensure that the loads are not drawing an increased amount of current. If the current draw exceeds the allocated amount, then the components turns off to ensure proper balance of the electrical power system. The typical operating circuit is shown in Figure 6. Figure 6 MAX4377 Typical Operating Circuit The MAX4377 is a dual, high-side current-sense amplifier with three versions of internal gain 3. This component uses an external sense resistor to measure a typical full-scale accuracy of 0.5%. The MAX4377 is designed to reduce power losses and increase current-sense accuracy. While the current from the source flows through the R sense resistor, the full-scale output range is given by: 3 The MAX4377 has three gain versions of 20, 50, and 100. As shown in Table 1, the gain and the current sense resistor are chosen to reduce power losses and increase current-sense accuracy.

5 Table 1 MAX4377 Recommended component Values 3 To accommodate for large full-scale current load, the R sense resistor is chosen to be 10mΩ and the gain is set at 100. The gain was chosen to increase the resolution of the output data. To increase the resolution more significantly, the R sense resistor can be chosen to a higher value if the CubeSat operates at a smaller full-scale current load. The MAX4377 requires an ADC at the output to read the current sense data. Typically, an ADC is available on a PIC, but to implement the conversion, a reference voltage is needed. Instead, to utilize the CSI I 2 C bus and eliminate the need of providing a reference voltage, the MAX6652 is used as an ADC to communicate the current sensed through the I 2 C bus. Figure 7 shows the designed circuit of the MAX43777 interfacing with the MAX6652. Figure 7 MAXIM 3.3V and 5V Monitoring System The MAX4377 package was chosen because it is capable of dual-current monitoring, in which the 3.3V and the 5V lines with be covered with this component. To ensure simplicity, the MAX6652 is used as an ADC to communicate the current sense data with the unused pins 12V in and 2.5V in. The MAX4377 outputs an analog voltage that is read through the input pins of the MAX6652.

6 Table 2 MAX6652 Voltage Data Format From Table 2, the 12V in and 2.5V in pins have resolution readings of 62mV and 13mV, respectively. The design allows the 5V and 3.3V lines to be monitored by the 12V in and 2.5V in, respectively. On a CubeSat, delicate components (such as a PIC in the Command and Data Handling Subsystem) operate at 3.3V that requires a higher resolution of current monitoring to ensure proper operation. These components are essential to control and maintain the satellite s operations. Other components such as a radio typically have a larger scope in which a 68mV resolution is adequate for proper operation. The MAX6652 reads the voltage outputs from the MAX4377 and provide an 8-bit reading that is communicated through the I2C bus. The actual operating current of on each voltage line on the CSI bus is calculated as shown in Table 1. The board layout is shown in Figure 8. The board has been etched and is ready to be populated. The Maxim components have been ordered but have not yet arrived. Upon arrival of these components, the board will be fabricated and fully tested by the Electrical Power Subsystem team. Figure 8 3.3V & 5V Maxim Monitoring Layout

7 Conclusion The design of Electrical Power Subsystem components DC-DC convertors and voltage and current monitors have been presented for use with the CubeSat Stackable Interface. The LM2576 DC-DC converter provides appropriate +3.3V and +5V with maximum current load of 3A. The MAX6652 and MAX4377 communicate through the I 2 C power management pins to monitor the voltage and current on the CSI bus. Through the development of this modular design, future students will save time in the design of the Electrical Power Subsystem. Acknowledgements The author would like to acknowledge the assistance from his Electrical Power Subsystem members, Malcolm Menor, Kelson Lau, and James Ah Heong. He would also like to thank the valuable guidance from his advisor Dr. Wayne Shiroma. Finally the author would like to thank the University of Hawaii College of Engineering and the Hawaii Space Grant Consortium for providing the opportunity to conduct this research. References 1 Lim, Toy. CubeSat Stackable Interface, Spring 2008 EE496 Report. 2 MAXIM Temperature Sensor and System Monitor in a 10-pin µmax 3 MAXIM High-Side Current-Sense Amplifiers with Internal Gain