Space Power Supply for Traditional and New Space Missions

­Space satellites use solar energy as the main source to power its loads. A simplified power architecture is shown below as shown in Figure 1. with Radiation Hardened (Rad-Hard) standard SA50-120 series DC-DC 50 W power converters from Microchip.

Some of the commonly used bus voltages are 28V, 50V, 72V, 100V and 120 V.. A DC/DC converter converts these voltages to secondary voltages such as 3.3V, 5V, 12V, 15V and 28V. Secondary bus voltages are further converted into usable voltages such as 0.8, 1.2, 1.5V with the help of Point of Loads (LDOs /Voltage regulators) to feed to the MCUs and FPGAs that drive the spacecraft loads.

Configurability for Customization for Various Mission Specification

Voltage levels in the electrical power bus are generally standardized to certain values, however voltage of the solar array is not always standardized. This calls for redesign of all the converters in the power subsystems depending on the nature of the mission. This increases costs and development time. Thus, it is inherently important to provide DC-DC converters and LDOs across the power architecture that have standard specifications, while  having the flexibility for customization depending on the system and load voltages. Functions such as Paralleling, synchronization, and series connection are of paramount importance for space power supplies when considering the specifications of different space missions.

Size, Weight, Power, and Costs (SWaP-C)

Due to limited volume available and resource intensive task of sending the objects in the space against the pull of gravity, it is imperative to have smaller footprints, smaller size (Volume) and lower weight while packing more power (kW) in the given volume. This calls for higher power density for space optimization and higher efficiency (> 80%) to get maximum out of the resources available in the power system. The load regulations need to be optimal to make sure that the output of the DC-DC converter feeds the next stage (LDOs, direct loads) matching the regulation requirements. Additionally, the tolerances of regulation against temperature variations are key in providing ruggedness and durability.

Reliability and Durability Deigned for Harsh Environments

Now, we will look at the most critical challenges for DC-DC converters to survive in space.

The space environment consists of effects such as solar plasma, protons, electrons, galactic cosmic rays, and solar flare ions. This harsh environment causes environmental effects such as Displacement Damage (DD), Total Ionizing Dose (TID) and Single Event Effects (SEE) that in turn results in device level effects. These effects, if not mitigated properly can cause failure of electronics, and impact the reliability and durability of the space missions.

Total Ionizing Dose (TID) Effects

TID is caused mainly by protons and electrons and measured in rads. TID accounts for cumulative damage of the semiconductor lattice caused by ionizing radiation over time. This results in parametric degradation of device parameters that can lead to functional failure.

Displacement Damage (DD) Effects

DD is caused primarily by protons and electrons and measured in kEV-cm2/g Or 1 MeV neutron fluence in n/cm2. DD inflicts cumulative damage that causes atoms to be removed from the semiconductor lattice site. This results in parametric degradation of device parameters that can lead to functional failure.

Single Event Effects (SEE)

SEE is caused mainly by protons and heavy ions and measured in LET in MeV-cm2/mg. SEE inflicts disturbance to normal operation of device or circuit caused by a single particle. This results in transient effects that can be either destructive or non-destructive.

Mitigation measures such as shielding, ground floating, Rad-Hard part selection, radiation lot acceptance test, SEE testing and circuit design modifications need to be employed. Figure 2 below depicts the details from harsh environments to the mitigation technique used for space power supply manufacturing.

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Figure 2: Space environment, effects and mitigation techniques

All the electronics components used inside the converter need to be individually Rad-Hard, enhancing the durability (product life) of the power system to survive in space for many years of the mission. Rad-Hard components along with specific circuit design techniques guarantee Single Event Error (SEE) immunity. Components are derated accordingly and the circuit design is such that the end-of-life parameters and performance are predictable. Lot-specific TID/ELDRS testing can be performed if requested. This also calls for qualification and testing of the DC-DC power converters to withstand the harsh environment as per the following figures. Microchip’s standard Rad-Hard isolated SA50 series converters meet these stringent requirements as given in figures 3 and 4.

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Figure 3: Qualification tests

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Figure 4: ATP screen tests

It is also imperative to provide full documentation such as below to support the DC-DC converter design:

• Structural analysis

• Stress analysis

• Thermal analysis

• Radiation analysis

• Worst case analysis (WCA)

• Reliability analysis

• Failure mode and effects analysis (FMEA)

• EMI report

• Qualification report

Mechanical Construction

One of the biggest challenges in designing space grade Rad-Hard power supplies is reliability and ruggedness of construction.  Hybrid converters consist of fragile substrates with raw die and bonding processes, which adds to the complexity of the power converter and drives risk in the production process that can jeopardize the lot and  program schedule.

Microchip’s standard Rad-Hard SA50 DC-DC converter series solves these hybrid construction–related concerns by utilizing a printed circuit board and standard space-grade SMT components. The design of the SA50 series allows for quick and easy customization in single or triple outputs. Two 1 mm (0.04-inch) diameter vent holes are designed into the housing to allow the pressure inside the module to equalize to the external atmosphere. The standard offering employs a mix of Grade 1 and Grade 2 parts as defined in NASA EEE-INST-002 to meet space flight requirements including derating, reliability and radiation. An option to use only Grade 1 parts can be offered.

Figure 5 Shows the typical external and internal construction of the modules.

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Figures 5a & 5b: External and internal view of atypical SA50 converter module

Orbits, Applications and Influence on Power Supply Design

Satellites in Geostationary Orbit (GEO) orbit around the Earth in approximately 24-hours at the speed of around 3 Km/sec at an altitude of about 35786 Km to cover a large range of Earth. There are only three main satellites that can cover the full globe, as these satellites are far away from the Earth.

Satellites in a Low Earth Orbit (LEO) orbit around the Earth at the speed of 7.8 km/sec at an altitude of less than 1000 km but could be as low as 160 km above Earth. This is lower as compared to GEO but still > 10 time higher than commercial plate altitude at14km.

Individual satellites can be used for higher resolution imaging being very near to the Earth, however, the key application to form constellations of large number of exact or similar types of relatively smaller satellites forming a web or net around the earth to give uninterrupted coverage. By working in tandem these constellations provide simultaneous coverage and are used in applications such as internet service and telecommunication. This alternate approach is called “New Space” which has enabled the launch of multiple smaller satellites with lighter payloads for commercial purpose. Satellite internet services are slowly and steadily competing with traditional broadband, and are providing more reliable connectivity for remote areas, passenger vehicles and even for aerospace.

Figure 6 shows the source of harsh environments and its implications on different orbits.

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Figure 6. LEO constellation

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Figure 7. Environment faced by space crafts in different orbits

GEO orbit faces harsher environment due to plasma, trapped electrons, solar particles, and cosmic rays the environmental effect is higher in magnitude as compared to LEO-Low Inclination, LEO-Polar and ISS (International Space Station) orbits.

The satellites placed in GEO orbits face hasher conditions due to radiation as compared to LEO orbit satellites. This is the primary reason the power supplies used in these satellites need to comply with stringent MIL standards related with design, manufacturability, and quality. IS and SEE effects are some of the key aspects that need to be addressed by power supply in space. GEO orbit being farther from Earth is more susceptible to radiations than LEO orbit and hence the components used GEO satellite power supply needs to be Rad-Hard by design, which means all the components must comply with TID and SEE, as high as 100 Krad and 82 MeV•cm2/mg. In comparison, the LEO satellite components need to be Radiation-Tolerant with relatively lower level of requirement of TIDs and SEE. However, using no shielding from these harsh conditions could result in failure.

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Table 1. The comparison of GEO Vs LEO

As the new Space market is volume based, the mission time is far lesser than traditional space, there is a tendency to use automotive qualified components in the power supplies to reduce the cost. However, sacrificing quality just to reduce the cost has dire consequences in terms of failure.

Conclusion

While the requirements with regards to minimization of space, weight, power and costs are key, the power supplies in GEO orbit faces severe harsh space environment compared to those in LEO orbits. The traditional space satellites need to withstand higher level of TID and SEE and should be comprised of Rad-Hard components and relevant mitigation measures to last for decades in space environment.

On the contrary, the challenges faced by power supplies in LEO are somewhat less challenging but still need to have optimum level of TID and SEE while being Radiation-Tolerant. Using commercial grade and automotive grade power supplies may pose dangerous mission failures. Thus, even though the mission time of LEO and New Space is far shorter than the traditional mission, it is important to make sure that power supplies are Radiation-Tolerant with optimized level of TID, SEE, testing and screening to be commercially viable without compromising mission safety.

Microchip