FAQ

1. What is the EPS? What is it for?

The Electrical Power System is a subsystem of the satellite responsible for harvesting the energy received from six solar panels, storing it in two lithium-ion batteries connected in series and distributing it to the other modules through separated integrated DC-DC converters for each module. This is important because each one needs different voltages and currents.

2. How much energy is harvested with the solar panels?

The maximum power is approximately 2.4 W, with V_mpp = 4800 mV and I_mpp = 500 mA, at Spectrum: AMO WRC = 1367 W/m².

3. How does the EPS measure currents?

To measure currents, the EPS uses shunt resistors and High Precision, Low Voltage, Current-Sense Amplifiers, along with MCU internal ADCs.

4. How are the voltage measurements done?

To measure voltages, the MCU internal ADCs are also used. If the voltage is within the 2.5 V reference range, they can be measured directly. If not, voltage adaptation circuits composed of a voltage divider and a buffer are used.

5. Is there a MPPT algorithm for the solar panels? How is it implemented?

A Perturb & Observe algorithm is used. It runs in software in the MCU, which generates PWM signals fed to the switch of the boost converters that connect the solar panels to the batteries.

6. How much energy is stored in batteries?

The batteries have a nominal voltage of 7.4 V (3.7 V each battery) and electrical charge capacity of 3 Ah.

7. How does it control the batteries charging?

The board uses a BQ2057wsn chip that controls the batteries charging procedure.

8. How are the batteries measurements made? Is there a protection circuit for the batteries?

A dedicated fuel gauge (DS2775) is used to make the measurements. They are sent to the MCU via one-wire protocol. There is also a protection circuit, composed mainly of two N-channel MOSFETs, which are controlled by the fuel gauge.

9. How are the kill-switches implemented?

They are implemented by two P-channel MOSFETs in parallel, as redundancy. The gates of the MOSFETs are connected to the batteries voltage or to a pulldown resistor depending on the position of the Remove Before Flight and Kill switches.

10. How much power can be sourced to the satellite?

The Power Consumption Dynamic maximum is approximately 5.4 W. However, each module can source a different value for voltage and current:

• Main power bus: batteries voltage/3A;
• EPS/Beacon: 3V3/2A;
• PA Transceiver Radio: 3V3/5A;
• PA Beacon: 3V3/5A;
• Antenna Module: 3V3/2A;
• OBDH/Transceiver radio/solar panel sensors: 3V3/1A;
• RUSH: 5V/3A.

11. What if there are no batteries?

The consumption of all systems will come directly from the solar panels, so it will depend on positioning as well.

12. Why does the module use Crystals?

Because it is necessary to generate PWM signals in high frequency to control the solar panels boost converters. A fraction of the crystals output is also used as clock by the MCU.

13. Why use a MSP430 MCU?

Because it has Ultra-low-power design, a robust and flexible system (I²C interfaces, SPI, UART, internal flash memory, timers, PWMs, ADC ...) and a step-by-step debugging interface through JTAG.

14. Why are there temperature measurements?

The batteries don't work at low temperatures, so it is essential to heat them. To control the heating, measurements are needed.

15. What is the function of the 555 timer?

To control the solar panels boost converters, in case the MCU fails.