Satellite mission requirements vary based upon orbit type, expected mission life, potential radiation hazards, type of payload, weight and cost. Ultimately the suitability of the component to meet the mission requirements can boil down to the function, reliability and performance of the component itself. DC-DC converters are universally required in satellite systems to efficiently convert energy from the solar array to usable power for either data acquisition and control or, to power systems such as propulsion, payload, altitude control and the on-board computer.

VPT offers space-qualified DC-DC converters with a range of input voltages and output powers to serve satellite applications. The diverse range of requirements associated with various space missions may pose complexity, but VPT's DC-DC converters are capable of fulfilling them.

A look at VPT’s space-grade DC-DC converters

Figure 1. SGRB, SVLFL, SVPL VSC space-grade converters. Source: VPT

VPT has an extensive portfolio of space-qualified DC-DC converters to support the wide variety of space missions:

• The SVR, SVL and SV series are all hybrid converters and therefore can be Class K-qualified.
• The SGRB series is built on GaN switching devices and consists of larger, high output power and highly efficient converters that ensure the end-application will have minimal wasted energy.
• The SVR, SVL, SV, VSC and SGRB follow Defense Logistics Agency approved (DLA-approved) RHA plans to ensure a level of radiation hardness and reliability that other commercial off-the-shelf (COTS) DC-DC converters may not.
• Finally, the VSC series is VPT’s COTS product designed for “NewSpace” applications that is available at a more affordable price point to fit the needs of LEO and NASA Class D missions.

Table 1: Specifications of VPT’s space-grade isolated DC-DC converters. Data source: VPT

The SVR, SVL and SV series offer many input voltage options as well as output power options within each product line (Table 2). Though the prefix is SV, the SVGA and SVPL family of buck converters are all 100krad(Si) and 85 MeV/mg/cm² parts.

Table 2: Input voltage, output power and weights. Data Source: VPT

Satellite missions and varying requirements

Radiation levels of different orbits

The characteristics of the radiation environment are dependent upon the type of mission, particularly its duration and orbit. The geostationary orbit (GEO), medium Earth orbit (MEO) and low Earth orbit (LEO) offer different radiation environments.

GEO- and MEO-based missions fall in the outer soft Van Allen radiation belt that houses protons and electrons of considerable energy. They must also remain active during geomagnetic storms with potentially damaging radiation.

LEO satellites pass through the inner Van Allen belt with protons and electrons of much higher energies. LEO satellites will also experience the auroral zone where low energy electrons of less than 200 keV will enter the atmosphere and produce these auroral displays.

All of these orbits may also be exposed to high energy protons from solar flares, galactic cosmic radiation and atmospheric neutrons. Most VPT converters are qualified according to MIL-PRF-38534 Class K.

The SVR, SVL and SV series are qualified according to DLA-approved RHA plans to ensure reliability in harsh radiation environments, making VPT converters a strong candidate for satellites in any orbit. The space-grade converters support common bus voltages for the secondary power distribution systems found in satellites (12 V, 28 V and 48 V), and all series include specific models that support varying ranges of output power. Within these models there are also converters that support dual outputs for even more design flexibility.

Figure 2. Varying radiation zones around the Earth. Source: Aerospace Nuclear Science & Technology Division

Differing missions at differing orbits

Oftentimes, GEO satellites are much larger-scale commercial communications satellites where the upfront cost of satellite development and launch is high, but the return on investment (ROI) comes with the longer mission duration. Performance of the electronics is therefore critical. For DC-DC converters, this often translates to efficiency where higher efficiencies will waste less energy in the satellite’s power distribution system. The SGRB 400 W DC-DC converters use GaN switching devices and advanced topologies to achieve high efficiencies of 96%, while a radiation-hardened design ensures long-term reliability.

MEO missions are often navigation-based with some satellites dedicated to communications and geodetic/space environmental science. These satellites have a similar lifetime to GEO satellites with operation beyond 10 years. The SV, SVR and SVL series of DC-DC converters are radiation-hardened and suited for use in LEO, MEO, GEO, deep space and launch vehicle programs. They offer a range of input voltages and output powers to suit a range of power buses for virtually all space missions.

As stated, higher altitude and higher inclination orbits in the LEO region are exposed to high levels of energetic protons from the inner Van Allen belt. Lower altitude LEO missions are inside the inner belt, and the total ionizing dose (TID) survival requirements are lower – this is the region where manned missions such as the International Space Station (ISS) are located for safety reasons. Satellites in LEO will have shorter mission durations on the order of five years largely due to gas from upper atmosphere presenting a drag on the spacecraft and must be countered with station-keeping thrusters. When the satellite is small, there are limited stores for fuel and thus cannot overcome the drag for long periods of time.

Many LEO-based satellites are meant to operate as part of a constellation where hundreds of satellites cooperate to provide a certain service such as weather analysis, Earth observation, reconnaissance or surveillance and more recently, communication with companies such as SpaceX and Amazon attempting to provide reliable broadband coverage globally.

Radiation considerations

Radiation accelerates the aging of electronics, leading to rapid performance degradation as well as transient phenomena. Any damage to the part level can lead to a functional failure of larger systems. Figure 3 illustrates a single event-effect (SEE) failure of a Schottky diode. These diodes are used in hybrid DC-DC converters and a failure of the component will cause a degradation and failure of the converter itself, potentially inducing a system-wide failure of the power distribution within a satellite.

Figure 3. Scanning electron microscope image of a destructive heavy ion induced SEE failure in a Schottky diode. Source: NASA

Radiation hardness assurance (RHA) is a process used by organizations including NASA, JPL and Boeing to ensure that the materials and electronics used within a space system do not compromise system success when exposed to various levels of space radiation. The methodology describes system requirements, environmental definitions, part selection, part testing, shielding and radiation tolerant design.

Characterizing part performance with radiation threats such as TID and SEE are a necessary part of RHA for DC-DC converters. TID refers to the ionization of a target material caused by interaction of high-energy photons or charged particles such as protons and electrons with the part, and generally deals with longer term radiation effects. Ionization accumulates slowly over time where some charges are trapped, causing threshold shifts, leakage currents and timing changes that can lead to functional failures. Radiation-hardened parts are, in fact, very frequently TID-hardened.

SEE are generally caused by a single, high-energy particle such as a cosmic ray or proton that deposits itself in the semiconductor material, leading to various errors, part degradation or failures. These can be broken down into soft errors with single event upsets (SEUs) and single event transients (SETs), or hard errors with single event latchup (SEL), single event burnout (SEB) and single event gate rupture (SEGR). SEUs might appear as transient pulses in logic or as bitflips in memory cells while SEL will cause higher operating currents above device specifications. SEB can realize itself as burnout of power MOSFETs, which must be avoided in DC-DC converter designs by selecting MOSFETs that are immune to this behavior and by proper voltage derating.

VPT follows the DLA-approved RHA process plan per MIL-PRF-38534 for the SVR, SVL, SV, VSC and SGRB DC-DC converters. This process guarantees performance under various environmental conditions in space including SEE and TID. Additionally, enhanced low dose-rate sensitivity (ELDRS) effects are considered for all bipolar ICs used in hybrids (Figure 4).

Figure 4. RHA plan details for the SVSA2800D (SV series) DC-DC converter. Source: VPT

VPT partners with sister company, VPT Rad, a full-service, DLA-approved radiation testing facility that specializes in TID Co-60 gamma irradiation, High Dose Rate (HDR) TID GC-220, Low Dose Rate (LDR) broad beam TID irradiation and Single Event Effect (SEE) testing and analysis among a host of other services. This partnership guarantees VPT's space-grade products for the harsh environment of space and enables shorter lead times.

MIL-PRF-38534 qualifications

MIL-PRF-38534 specifies the performance requirements of hybrid microcircuits, multi-chip modules (MCMs) and similar devices as well as their verification and validation requirements. This is the top-level specification used for nearly all hybridized parts where the DLA is the logistics center used to perform the stringent audits necessary to include a manufacturer on the Qualified Manufacturer List (QML). Vendors listed on the QML have displayed consistent manufacturing processes, a highly defined and rigorous environmental test flow and well-defined quality systems to ensure the parts produced are qualified.

For space-qualified hybrid DC-DC converters, MIL-PRF-38534 Class K is the go-to standard to ensure the entire end-product is reliable. This class includes additional pre-build inspections of components within the part, post-build inspections and testing on top of meeting all the requirements of Class H to address the special needs of space applications. Space-grade DC-DC converters always correspond to standard microcircuit drawings (SMDs). These are component specifications published by the DLA that include manufacturer datasheet information for a unique part with parameters listed in the DLA-specific format, imposing the MIL-PRF-38534 requirements and class level.

For many space customers, Class K qualified components are non-negotiable to avoid manufacturers that may market their converters as “space-grade” but may still not meet mission reliability requirements. VPT’s SV, SVL and SVR series are all Class K qualified. The SGRB series cannot be tested according to these standards as it is not a hybrid microcircuit. It is, however, tested according to a DLA-approved RHA plan.

Bus voltages and satellite power capability

Bus voltages and satellite power capabilities will vary. Common satellite bus voltages can be 28, 50, 70 and 100 V with an associated power output that typically increases with these bus voltages (Table 3). DC-DC converters used within these satellites must easily integrate within these standard power trees. The wide range of voltages and output power handled by VPT’s space-qualified converters enable these devices to be used within these applications regardless of their bus voltage and power handling capabilities.

Table 3: Common bus voltages and powers of various satellites and spacecrafts. Data source: VPT

Performance and cost considerations

After the DC-DC converter baseline reliability and functional requirements are considered, the performance and cost can be analyzed. While these are generally not primary concerns for space equipment, they can be assessed after initial requirements are met. A highly efficient converter will ensure that the bulk of the power input into the converter will reach the output and come out as a clean, regulated signal. This ensures that every ounce of equipment launched into space (a costly endeavor) is not wasted. However, wide bandgap (WBG) technologies such as GaN and silicon carbide (SiC) have enabled power electronics designers to easily reach efficiencies of 95% and higher with high switching speeds and lower losses. Converters such as the SGRB rely on GaN to establish a high efficiency of 96% (Figure 5).

Figure 5. The SGRB 400 W GaN DC-DC converter is suitable for use in telecommunication systems. Source: VPT

VPT: Ensuring mission success

Radiation tolerance is a critical aspect for the performance of DC-DC converters in space applications. Poor performance can lead to erratic system behavior and part damage, potentially shortening the expected life of the mission. Other factors to consider are input voltage, output power, size, weight and efficiency. VPT DC-DC converters not only meet the stringent requirements of space applications, but also offer the range of products necessary to be easily integrated in virtually any space mission. To learn more about VPT and their product offerings, visit their website or contact their sales team.