Selecting the correct level of DC-DC converter for your space applications
High reliability in DC-DC converters for space must meet certain standards for electrical and environmental performance as well as defined quality requirements. The differences between these reliability grades can be subtle and can be concealed with clever marketing. The product literature must be studied carefully, and questions must be asked, to determine exactly what the product is, and, equally important, what it is not. Basic elements to investigate are: temperature rating, hermeticity, military specification compliance and a rigorous environmental qualification. The ultimate test of reliability beyond that is official qualification and certification by the US Department of Defense.
Space level hybrid DC-DC Converters, radiation tolerant or radiation hardened, are also governed by MIL-PRF-38534. The manufacturer should have a radiation hardness assurance plan certified by DLA to MIL-PRF-38534 Appendix G. Space level DC-DC converters are available on SMDs and are typically procured to Class K.
Typical characteristics of space grade DC-DC converters include:
Total Ionizing Dose (TID) radiation
For low earth orbits or where the DC-DC converter is adequately shielded, a 30 krad(Si) guarantee is often sufficient. For higher orbits or longer missions, a 100 krad(Si) guarantee may be required. TID performance should be verified by the manufacturer with component test data or guarantees, worst case analysis, and test data on the complete converter. Additional test margin can sometimes be substituted for analysis.
Enhanced Low Dose Rate Sensitivity (ELDRS)
TID testing is normally performed at high dose rates to shorten test time and reduce test cost. Testing at lower dose rates, closer to those seen in actual space environments, has shown increased sensitivity to radiation in some components, especially bipolar technologies. Modern space programs will almost certainly have an ELDRS requirement. Older DC-DC converter designs may not have an ELDRS guarantee, so be sure to inquire about this. ELDRS performance is proven through testing and analysis.
Single Event Effects (SEE)
Single event effects are caused by energetic particles which interact with the semiconductors internal to the DC-DC converter. SEE cannot be shielded and must be dealt with in the DC-DC converter design itself. SEE can cause simple transients on the output, dropout, shutdowns and restarts, latch offs or hard failures. Hard failures in a DC-DC converter are often cause by failure of the power MOSFET. SEE performance is verified primarily with testing of the complete DC-DC converter. Testing should include high temperature latch up testing.
Worst Case and Radiation Analysis
A guarantee of end-of-life post-radiation performance of the DC-DC converter is usually required. The manufacturer will have completed a detailed worst case analysis for circuit performance including both end-of-life and radiation effects. Radiation degradation of components is fed into analytical and simulation models to predict post radiation performance. Extreme value, root sum square, and Monte Carlo analysis methods are used.
MIL-PRF-38534 Class K
Space grade DC-DC converters are typically procured to MIL-PRF-38534 class K. Class K includes additional element evaluation and additional screening beyond Class H. Most space level DC-DC converters are procured to an SMD. Procuring to a Class K SMD is less costly than procuring to a custom source control drawing (SCD).
No Optocouplers
Although isolation of the feedback control in a DC-DC converter can be accomplished with an optocoupler operating in the linear region, the LED within an optocoupler is sensitive to displacement damage from proton radiation. A reliable space grade DC-DC converter will not use optocouplers.
Aerospace TOR
Some space programs are governed by The Aerospace Corporation report, “Technical Requirements for Electronic Parts, Materials, and Processes Used in Space and Launch Vehicles,” commonly referred to as the “TOR.” The TOR specifies additional quality requirements above and beyond MIL-PRF-38534 Class K. These requirements have historically been met on a custom basis with a modified or modified flow Class K hybrid DC-DC converter. However there are options available that have been developed to meet the requirements of TOR as a standard offering.
Space level DC-DC converters are specially designed for radiation tolerance. Upscreening by test or even substituting a few radiation hardened components into an existing design will not meet the stringent analysis and testing requirements of modern space programs.
Anyone looking for DC-DC converters and EMI filters for a space programme should ensure they are backed up by full worst case analysis and radiation testing and have the proven design and program heritage that are such critical factors in successful space programs.
About the author
Steve Butler is Director of Advanced Product Development for VPT, Inc., in Blacksburg VA. He received his B.S. and M.S. from Virginia Tech, both in electrical engineering where he studied power conversion and satellite power systems. Since joining VPT in 1996, Steve has led the development of the company’s custom and standard product line and holds a patent in power conversion.
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