Power System Design
The power board took the most time to design and layout as power signals are the most delicate to deal with when ensuring that loss is not generated, and that power is consistently output. The board is split into two subsystems, the battery management system (BMS) and the buck converter. The flow of power begins with the BMS (Fig. 2) with the supply of the 4S6P LiPo Battery provided by BlueRobotics. This battery supplies 14.8V with a maximum 60A current draw. To protect this battery and the rest of the circuity, two chips are used for the charging anddischarging side. The discharging side uses a TPS259480 3.5V to 23V, 8A eFuse chip to protect the main 14.8V being used by other circuitry and the thruster power. This chip replaced a previous back- to-back MOSFET design that had errors in implementation. With a lack of expertise in MOSFETs from team members, this chip contains a working bi-directional MOSFET protection. Other faults like over and under voltage, short and open circuits, and overtemperature are handled by the eFuse. The charging side uses a BQ77207 Voltage and Temperature Protection for 3-series to 7-series Cell Li-Ion Batteries chip. This IC takes in the protected main voltage and the battery balance inputs that charge the battery and provide constant charging of the battery cells to prevent over draining. The same aforementioned electrical faults are also monitored with this chip.
stepped down to 5V with the buck converter (Fig. 3) circuit. This subsystem uses the MPQ4480 6A, 36V, Step-Down Switch-Mode Converter chip. This IC uses a switching frequency of 440kHz, voltage divider, and feedback to step down the voltage. Ceramic capacitors are connected to the input to reduce ripple current and at the output to reduce ripple voltage. The enable line for this chip is tied to the BMS’s discharging chip to validate the protection of the voltage and not enable any component early. A ground short MOSFET is also tied to the buck converter for added short protection.
