Power in space: what is the most critical system in a spacecraft?
Satellites are a part of our everyday existence. They help us receive hundreds of TV channels, they allow us to place fast efficient telephone calls, and they allow us to use pagers to keep in contact with our business and personal contacts.
By Steve Strickler
Satellites are a part of our everyday existence. They help us receive hundreds of TV channels, they allow us to place fast efficient telephone calls, and they allow us to use pagers to keep in contact with our business and personal contacts. They can tell us our position anywhere on the Earth right down to a street address, help with an airplane landing approach, or assist in a rescue at sea by identifying the position and identity of the vessel in distress. These are just a few of the many ways that satellites serve us. They are becoming not just useful, but absolutely essential tools for modern life.
Space and the orbital area surrounding the Earth is a harsh environment. Satellites orbit in an area of temperature extremes and radiation. When a satellite malfunctions while in orbit, one rarely can go up and repair it. Any malfunction can seriously degrade the performance of a satellite or cause it to fail completely. Everything in a satellite, therefore, has to be reliable and extremely robust.
In 1999, several satellites failed in orbit. The year, in fact, was somewhat of a lesson for some in the satellite industry. As a result of these problems, satellite purchasers and operators tightened their quality requirements and are insisting on more oversight during the construction and testing of their contracted spacecraft. It is definitely not business as usual.
All subsystems on a satellite are critical, but some carry a higher priority than others do. Modern communication satellites for instance, have redundant transmitters and receivers (transponders) to send and receive information. If one transponder in the payload fails, it may mean some loss of revenue for the operator, but the satellite will still perform its basic function. The spacecraft bus keeps the satellite in orbit and supports the payload. If the spacecraft bus itself fails in some way, a redundant system usually can take over for the failing system. The satellite power system, however, is different. Although the satellite power system also has a backup, it is critical to both the payload and the bus. The main power system simply cannot fail.
The power system has two basic parts — the central power-generation system, and the distributed power system that provides electricity for the individual subsystems. The central portion consists of the primary power-storage subsystems such as the solar cell arrays and the primary power-regulation system. The distributed portion consists of all the major and minor components spread throughout the systems on the satellite that enable it to function as designed.
Each subsystem has its own power requirements. These requirements are for different voltages, load currents, and specifications on regulation, parametric variation and many other specifications.
All the components of the power system have one thing in common: they cannot fail. If the central power system fails the system goes down and nothing works. If a major subsystem power component fails that subsystem will go down or the redundant power component must cut in. Nothing works without power. Because these power subsystems are so crucial, they should have the most robust and reliable components. They need to be able to withstand the extremes of heat and cold and the radiation environment that exists in orbit.
Until recently there were essentially two kinds of power system components: commercial parts that were upscreened, and subsystems fabricated from discrete components because no fully rad-hard and designed-for-space parts were available. In the last year that has changed.
Intersil Corp — formally Harris Semiconductor — has been building space-qualified devices for more than 40 years. During that time Intersil has participated in programs from the first satellites and missiles up through and including the International Space Station. The experience gained has been and is being used to support the satellite industry with products designed for the harsh environment found in space.
Intersil with its expertise in integrated power solutions for the commercial market and its radiation-hardened expertise has focused its resources on the space power needs of the satellite industry. There were many deficiencies in the components that were being used. Extra circuitry had to be included in subsystems in order to provide protection from the hazards of radiation, both total ionizing dose and single event phenomena. Commercial parts were upscreened and systems were built from many discrete components. Each of these components has its own failure rate. When the reliability of the large number of components in a discrete system is considered, it is much less reliable than an integrated device or a few power system building block integrated circuits that will do the same job as the multiple discrete components.
Intersil's Space & Defense Business Unit has dedicated significant engineering resources to developing a family of extremely reliable radiation-hardened power devices. This series of building block products has been defined in cooperation with Intersil's major customers. The first of these devices has been released and is available for design into satellite power systems. These building block products consist of basic power circuits like regulators, references, comparators, and MOSFET drivers. A series of more complex devices has also been designed and a release begun.
All of these devices are fabricated on processes designed for the space environment. The best way to fabricate any specific family of devices that are targeted for a particular application is to develop a targeted fabrication process that meets the specific needs of that application. When defining devices for the harsh space environment, processes must be used that will meet the reliability requirements and radiation resistance needed in the space environment.
These processes are much different from commercial processes. They are much more robust and the parts fabricated on them do not have to have the performance tested; in upscreened commercial devices, it is designed in. The parts by virtue of being built on a more robust process will be radiation hard and ultra reliable. They will have substantially improved performance and much reduced risk of failure in the space environment because they are built on a process that has the inherent characteristics needed for meeting or exceeding satellite longevity expectations. Intersil develops processes that meet the criteria needed for the space environment for both the power management ICs and the MOSFETs.
Intersil has developed a process for radiation-hardened power management ICs called RSG — a BiCMOS process with characteristics tailored to produce parts that will meet or exceed the requirements of the space environment. Most of the devices built on this process will be total-dose hard to 300 kilorads and will resist single-event upset. They will also be latchup free.
With both the radiation-hardened power building block ICs and the space-qualified MOSFETs in the Intersil portfolio it was natural next step to design and build a family of more highly integrated power devices in hybrid form. The first of these products is a high-power DC-DC converter. Originally developed to address the needs of the phased array antenna market, this hybrid converter offers the best efficiency and most robust construction of any converter now available. It is feature rich with multiple input voltages and output voltages. The first converter released has a built-in 461 compliant EMI filter, in rush current limiting, soft start capability, and other requested features. The converter was designed from the ground up to support the specific needs of the satellite industry.
Although Intersil has had experience with smaller multichip modules and hybrids, the DC-DC converter dictated that a partner be found that had the experience and the reputation to assemble and test this family of devices. The firm of MS Kennedy in Cicero, N.Y., was chosen. MS Kennedy is MIL-PRF 38534 Class K certified and has a very strong reputation for technical expertise and manufacturing performance in the space market.
With the availability of space qualified, radiation-hardened power management circuits and radiation-hardened low Rds(on) MOSFETs, many other higher-level integration products are targeted for development. These products are being defined and engineered.
Satellites are almost a utility in our environment. They are necessary for all manners of tasks and functions that we take for granted in our daily lives. Reliability and performance to specification for these satellites is an absolute necessity. The power system in a satellite is at the top of the list of systems that have to function at the specified performance level for the life of the satellite.
Steve Strickler is the lead marketing manager of the Intersil Space & Defense Star*Power team. He has been with Intersil and previously Harris Semiconductor for 15 years, during which he has been a program and marketing manager with Intersil's military and space business.