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The Electronic Power Supply (EPS) is the system that delivers power to all other subsystems as regulated power on individual channels. These channels are also monitored and turned on/off by the EPS. The EPS is equipped with two batteries and a Maximum Power Point Tracking (MPPT) solar array battery charger. The EPS is based on a 8-bit AVR MCU (AT90CAN128). In operation, the EPS is responsible for handling the (de-)charging of the batteries, and for monitoring the general health of the satellite. Furthermore, the EPS is responsible for the deployment of the satellite, once released in orbit.


Overview of the EPS subsystem.

PV Cells

The satellite is equipped with photo-voltaic cells on five sides. These cells are the power source for the satellite. The type TJ Solar Cell 3G28C from Azurspace is used. Each cell is capable of delivering 1.15 W at maximum power point, at 28C and irradiation of 1367 W/m2. Two cells are connected in series on each side, giving a maximum power point voltage of 4.7 V. All five sides are connected through a blocking diode.

Battery

The battery pack consists of two Li-Ion batteries in series, giving a pack voltage in the range 6 V 8.2 V. The type used is CGR18650AF-1S1P from Panasonic. The cells have a capacity of 2 Ah each.

PCU

The Power Conversion Unit is responsible for converting the energy harvested by the photo-voltaic cells to energy that can be stored in the batteries. The main component in the PCU is a boost converter that can boost the 3 V 5 V of the photo-voltaic cells to the battery voltage of 6 V 8.2 V. Normally, the output voltage is used in a feedback in the boost circuit. However, since the output is directly connected to the batteries, the output voltage can be considered pseudo constant. Instead, the input voltage is used in feedback. Using a digital potentiometer this allows the EPS to control the voltage of the photo-voltaic cells. This is used to run a Maximum Power Point Tracking algorithm. The MPPT algorithm will ensure that the photo-voltaic cells are operated close to their optimum voltage, which is dependent on the environment. The PCU has both a hardware and software charge termination. The hardware termination is implemented such that the boost converted is disabled if the battery voltage reaches 8.2 V. Since the input voltage is always lower than the output voltage, this disconnects the photo-voltaic cells from the battery. The boost converter is re-enabled when the voltage has dropped a bit. The boost converter can also be disabled by the MCU. The software on the MCU will disable the charging, when the battery voltage reaches 8 V, in order to increase lifetime of the batteries. This limit is reconfigurable from ground.

PRU

The Power Regulation Unit is responsible for converting the battery voltage to 3.3 V and 5 V for the subsystems. The PRU consists of three buck converters. One converter for the 3.3 V and 5 V channels used by the subsystems, except the power amplifier in the UHF. The power amplifier has its own dedicated 3.3 V converter. Each converter is capable of delivering around 1.2 A. The main 3.3 V converter has a low voltage protection. The output of the this protection is fed to all three converters. When the battery voltage reaches around 7 V, the converters are shutdown, and the power consumption is reduced to a minimum. The PCU is still active, allowing the photo-voltaic cells to charge the batteries.

PDU

The Power Distribution Unit is responsible for controlling and monitoring the power channels used in the satellite. A total of 12 power channels is implemented. Each one is controlled individually, and the power consumption is monitored by measuring the voltage across the switch. A hardware latchoff protection is also implemented on each power channel. The MCU receives an interrupt if one of the latchoff protections is activated. The power channel will remain shutdown until reactivated from ground.