Common Requirement, Extraordinary Environment

When the Dawn spacecraft launches in 2006, it will take several special dc-dc converters on an eight-year plus mission into deep space. The spacecraft in NASA’s Dawn project will travel to the two oldest and most massive asteroids in our solar system, Vesta and Ceres. A comparison of the measurements from instruments on the craft will provide information about the formation and evolution of the early solar system.

After its planned launch in June 2006, it will take Dawn four years to reach Vesta and four more years to get to Ceres. The surfaces of these minor planets are believed to contain a rich source of data from the first 10 million years of our solar system providing a timeline from the beginning to the present. Instrumentation on the spacecraft will measure the asteroids mass, shape, volume, and spin state. In addition, it will record detailed elemental and mineral composition as well as determine tectonic and thermal history, magnetism, and core size. Since the two asteroids are quite different, one cool and wet, the other hot and dry, comparison measurements of the internal structure could reveal other valuable information. NASA’s Jet Propulsion Laboratory (JPL) is the program manager for the Dawn project. Orbital Sciences will build the spacecraft, but JPL will provide the ion engines. Both of these assemblies require several dc-dc converters. “From our account, we have six different dc-dc converters with multiple versions of each used on the spacecraft,” says Bryan Rogers, Vice President of Sales for International Rectifier’s (IR’s) Hi-Rel Products Sector Business Unit. The dc-dc converters power various electrical subsystems taking voltage from the 28-V bus for payloads and the propulsion systems of the craft. The output voltages range from 3.3V, 5V, 12V, to 15V, essentially standard voltages used by electronic components. With both positive and negative output voltages available from the converters, other voltages are also available. “They tie the ±15V together to get 30V and typically they are driving motors,” says Mike Shakar a regional sales manager for IR’s Hi-Rel products unit. JPL is one of his customers. On any spacecraft, there are typically a number of motors to position the antennas or to move sensors or instruments. The 3.3V output powers the digital controls that are part of the satellite control architecture. The 5V output supplies sensors and older ASICs, which engineers prefer to use on spacecraft because they provide flight-proven technology. For higher voltages, such as the ion thruster, boosting the ±15V output provides from 2000 to 3000V. The low voltage dc-dc converters are typically duals or triples addressing multiple voltage outputs, but single converters are used as well. For example, one of the dc-dc converters on the Dawn is IR’s ART2815T, a triple from a series of converters designed using a single-ended forward switched mode converter topology. One of the advantages of this topology in radiation-hardened (rad-hard) designs is the elimination of the possibility of simultaneous turn on of both switching elements during a radiation-induced upset. Compared to double-ended topologies, single-ended versions are not subject to transformer saturation problems.

The Dawn spacecraft will take eight years to reach two of the oldest and largest asteroids in our solar system. The data from these two minor planets should provide details regarding the beginning of the solar system.

Based on the length of time in space, the radiation requirements for the dc-dc converters are 100 Krad minimum. Among the tradeoffs that can occur when products, such as the power MOSFETs in the converter are radiation hardened is an increase in the gate charge capacitance. As much as a 20% increase can occur to the capacitance based on the changes made to the gate oxide to meet rad hard requirements. The changes involve the structure, thickness, and materials. However, the on-resistances in rad hard products are in line with commercial technology says Shakar. As the technology has transitioned from planar to trench in commercial products, the performance of the rad hard units is comparable, although the availability could be a couple of years behind. The hermetically sealed packaging for the converters has a parallel seam welded lid and ceramic pin-to-package seals to withstand the rigors of deep space. The hybrid packaging allows the units to be 20 to 35% smaller in size and up to 50% lighter in weight than an equivalent through hole or surface mount supply. “The hybridization of the dc-dc converters really lends itself well to this program and missions like this,” notes Shakar. The single forward converters that IR supplies for the Dawn project are all the same power output, 30 watts. They use magnetically coupled feedback, which is important since optocouplers do not perform very well in space applications. “We find on scientific missions a lot of usage of lower wattage converters because of the smaller loads for all of the instruments that they use,” says Rogers.

The core of the spacecraft includes components of the ion propulsion system developed by JPL and instruments that will make numerous measurements.

The spacecraft and all of its instruments can be viewed as a distributed architecture, very similar to other more well known distributed power systems. “For automotive you generate your power from the alternator or the battery and for a spacecraft you get it either from the batteries or the solar panel, one or the other,” says Shakar. In a car the power bus is 12V and in this particular spacecraft it is 28V. What JPL calls their power control unit (PCU) is part of a distributed power architecture, or point of load (POL) converter in other terra firma applications. On the Dawn spacecraft, the converter is located very close to or even on the equipment it is powering. For all the similarities the dc-dc converters have to those in other applications, there are striking differences. “Elements that a spacecraft will see that are not seen by the typical system here on earth are: one is the radiation environment, two is the wide temperature range, and third you see a very high shock and vibration profile during the launch of the vehicle,” notes Rogers. “And you can’t send a repair technician up there to repair it when it goes.”
 
For more information on NASA’s Dawn mission go to:
http://discovery.nasa.gov/dawn.html

For more information on International Rectifier’s ART2815T rad hard dc-dc converters go to:
http://ec.irf.com/v6/en/US/adirect/ir?cmd=catProductDetailFrame&productID=ART2815T