**7. Solar array sizing; impact of the power conditioning and electromagnetic constraints**

The definition of the solar array, conceived as a set of solar cells connected in series to form a string and strings connected in parallel cannot be made without considering the power conditioning device placed at its output in order to have the electrical power delivered within a certain voltage range. This is not the suitable seat for a complete examination of all the possible power conditioning and power architecture solutions, what can be said is that there are two main concepts: the Direct Energy Transfer (DET) and the Maximum Peak Power Tracking (MPPT). These two methods of regulation have an important impact on the solar array design not only from the sizing point of view, but also from the electromagnetic compatibility (EMC) one. The following section will detail the impact of the adopted power

without endangering the whole spacecraft design. This is particularly true for scientific missions, where many times the development of the instruments may reveal so challenging

At this point harness distribution losses are introduced, 2% of the power demand defined with all margins at unit and system level may be a good compromise between losses

The Power Control and Distribution Unit (PCDU) is the electronic unit devoted for the solar array and battery power conditioning and regulation, power distribution and protection, execution of received telecommands (i.e. switch on/off of the loads) and telemetry generation. Its power consumption without considering the efficiencies of primary bus power converters depends on the management of the digital interfaces with the on-board computers, the control loop and protection electronics, the value of such consumption is not immediate to calculate but it can be said that a PCDU capable to manage 1kW can consume about 30W. However it consumption strongly depend on the number of implemented

Now its time to add the power needed for the recharge of the battery, this power strongly depend on the mission profile, and many times the maximum discharge of the battery occurs at launch, from lift-off up to the successful sun acquisition by the satellite with optimal sun pointing of the solar panels. Some times due to the complexity of the satellite design and mission profile it is not possible to have a full recharge of the battery in one orbit before the next eclipse, then the power allocated for such incumbency has to assure a

The power delivered by the solar array is conditioned by suitable power converters in order to provide it to the loads with a regulated voltage, or at list with the voltage varying between a maximum and minimum value. These converters may have an efficiency between 98.5% and 95% and the choice of their topology is made according to several criteria and constraint dictated by the overall satellite system design. Such efficiencies are taken into

The harness losses between solar array and PCDU may be calculated having as objective 1V voltage drop at the maximum required power; again, considerations about the harness mass

Finally, in case of the European ECSS standard (ECSS-E-ST-20C) is considered as applicable, an additional 5% margin on power availability shall be assured at the satellite acceptance

**7. Solar array sizing; impact of the power conditioning and electromagnetic** 

The definition of the solar array, conceived as a set of solar cells connected in series to form a string and strings connected in parallel cannot be made without considering the power conditioning device placed at its output in order to have the electrical power delivered within a certain voltage range. This is not the suitable seat for a complete examination of all the possible power conditioning and power architecture solutions, what can be said is that there are two main concepts: the Direct Energy Transfer (DET) and the Maximum Peak Power Tracking (MPPT). These two methods of regulation have an important impact on the solar array design not only from the sizing point of view, but also from the electromagnetic compatibility (EMC) one. The following section will detail the impact of the adopted power

that an excess of power demand cannot be excluded a priori.

distribution lines, and relevant electronic protections.

positive battery recharge trend throughout a limited number of orbits.

account adding up to an additional 5% to the budget defined so far.

review End of Life (EOL) conditions and one solar array string failed.

can provoke the change of such objective.

**constraints** 

containment and harness mass.

conditioning concept, and some sizing constraints mainly raised by the space environment such as electrostatic discharges and earth magnetic field.

#### **7.1 Regulation based on Sequential Switching Shunt Regulator (S3 R)**

The first concept is based on the use of a shunt regulator; the figure below shows the electric schematic of a cell of a Sequential Switching Shunt Regulator (S3R), several solar array strings can be connected in parallel to the input of the regulator's cell; the voltage at the terminals of the output capacitor (Main Bus capacitor) is regulated by the switching of the MOSFET contained in the blue oval.

Fig. 10. Electrical Section of a Sequential Switching Shunt Regulator (S3R)

Fig. 11. Solar array working points as function of required power

The operating voltage of the solar array is constant and equal to main bus nominal voltage plus the voltage drops due the two diodes in series along the line, the solar array harness, and the blocking diode placed at the string positive output. In case of a fully regulated power bus, this operating voltage remains fixed during both sunlight and eclipse periods throughout the orbit; if the power bus is instead a battery regulated one it implies that the bus voltage decreases during eclipse periods, when the battery discharges, provoking a migration of the operating point of the solar array towards the short circuit one.

Supposing a power need of 280W, Figure 11 shows that a solar array composed of 20 strings of 18 cells (18s – 20p), at the eclipse exit (Varray= 27V) cannot provide the required power. In this condition the battery keeps discharging, lowering further down the operating voltage. This power bus lock-up has to be avoided increasing the number of strings in parallel. Adding 5 more strings (i.e. 25% more) the solar array can deliver 320W at 27V when cold;

Architectural Design Criteria for Spacecraft Solar Arrays 175

Figure 13 clearly shows that the original array composed of 20 strings is now capable to deliver the needed power in both hot and cold conditions, providing power to the loads

Clearly from the sizing point of view of the array, the MPPT provides unquestionable benefits, but the price to be paid consist in additional mass (inductances and capacitances, as it can be seen in figure 12), and higher complexity because of the presence of three control

The design of a spacecraft solar array and its power conditioner has to satisfy several requirements, not only in terms of mass, dimensions and power output, but also in terms of electromagnetic compatibility. This is particularly true for scientific mission, when instruments highly sensitive to electromagnetic fields may be boarded. In these cases it becomes crucial for the success of the mission to know which electromagnetic fields are generated at solar array level due to the circulating current and its frequency content, once this is connected to the power conditioning unit. The wires connecting the solar array to the PCDU, via the Solar Array Driving Mechanism (SADM) when necessary, are always twisted pairs (positive and return), but the return connections of the strings are routed on the rear side of the panel, they are not twisted of course, hence the solar array can behave as a transmitting antenna at frequencies which may result incompatible with some of the

Power Bus

VBUS

IS.A.

+ - + - + -

+ -

Section #n

String #1 String #2 String #m

These issues are strongly dependent on the power conditioning approach adopted.

In the case of the S3R, with reference to figure 10, it can be seen that within the blue oval there is the shunt switch (MOSFET) together with a linear regulator in order to limit the current spikes at the regulator input when the MOSFET switches ON/OFF. Such spikes are strongly dependent on the total output capacitance of the strings connected in parallel and hence from the capacitance of the single triple junction solar cell. Fewer cells are in a string, or more strings in parallel, higher is this capacitance. The linear regulator can reduce the amplitude of the spikes by a suitable sizing of the dump resistor. For sake of completeness, the inductances present in the circuit diagram are the parasitic ones. Figure 15 shows the frequency spectrum of the current circulating in the harness between solar array and power regulator for different values of the dump resistor. The next figure 16 instead shows the

+ - + - + -

+ - + - + -

+ -

+ -

(280W) and the additional 40W for the recharge of the battery.

Section #1

String #1 String #2 String #m

+ - + - + -

+ -

**7.3 Electromagnetic Compatibility (EMC)** 

loops.

equipments on board.

Fig. 14. Solar array electrical scheme

+ -

+ - + - + -

+ - + - + -

+ -

therefore 40W become available to assure the battery charge. However, this increase might not be enough for assuring a full recharge of the battery in one orbit, or a positive recharge trend through several orbits; and an assessment of the energy budget by numerical simulation becomes necessary, taking into account orbital and attitude constraints.

#### **7.2 Regulation based on Maximum Peak Power Point Tracker (MPPT)**

The MPPT concept is based on the use of a switching dc-dc converter; usually it has a buck topology, where the primary voltage at solar array side is always higher of the secondary one on the distribution bus. Figure 12 shows an example of this type of converter. There are three control loops; a conductance control of the output current, an output voltage controller, and the Maximum Peak Power Tracker which regulates the output voltage of the solar array around the maximum power point in case of maximum power demand. In all the cases the required power is lower than the maximum available one the operating voltage of the solar array is kept between the maximum power voltage and the open circuit one.

Fig. 12. Low ripple Buck converter topology

When this power conditioning concept is applied the solar array operating voltage is always independent from the bus one. Hence the phenomenon of the lock-up mentioned for the S3R is not present and the solar array does not need to be sized in order to cope with such issue.

Voltage [V]

Fig. 13. Solar array P-V curves and required power, MPPT power conditioning

therefore 40W become available to assure the battery charge. However, this increase might not be enough for assuring a full recharge of the battery in one orbit, or a positive recharge trend through several orbits; and an assessment of the energy budget by numerical

The MPPT concept is based on the use of a switching dc-dc converter; usually it has a buck topology, where the primary voltage at solar array side is always higher of the secondary one on the distribution bus. Figure 12 shows an example of this type of converter. There are three control loops; a conductance control of the output current, an output voltage controller, and the Maximum Peak Power Tracker which regulates the output voltage of the solar array around the maximum power point in case of maximum power demand. In all the cases the required power is lower than the maximum available one the operating voltage of the solar array is kept between the maximum power voltage and the open circuit one.

When this power conditioning concept is applied the solar array operating voltage is always independent from the bus one. Hence the phenomenon of the lock-up mentioned for the S3R is not present and the solar array does not need to be sized in order to cope with such issue.

**Solar Array performances with MPPT**

solar array Cold 18s-20p solar array Hot 18s-20p

**required power at eclipse exit**

Fig. 13. Solar array P-V curves and required power, MPPT power conditioning

<sup>0</sup> <sup>10</sup> <sup>20</sup> <sup>30</sup> <sup>40</sup> <sup>50</sup> <sup>60</sup> <sup>70</sup> <sup>80</sup> <sup>0</sup>

Voltage [V]

simulation becomes necessary, taking into account orbital and attitude constraints.

**7.2 Regulation based on Maximum Peak Power Point Tracker (MPPT)** 

Fig. 12. Low ripple Buck converter topology

Power [W]

Figure 13 clearly shows that the original array composed of 20 strings is now capable to deliver the needed power in both hot and cold conditions, providing power to the loads (280W) and the additional 40W for the recharge of the battery.

Clearly from the sizing point of view of the array, the MPPT provides unquestionable benefits, but the price to be paid consist in additional mass (inductances and capacitances, as it can be seen in figure 12), and higher complexity because of the presence of three control loops.
