Electronic Circuit Design And Analysis For Space Applications

Electronic circuit design and analysis in commercial applications takes into account component parameter variations due to initial tolerance, temperature, and aging. For space applications, the additional component parameter variation due to radiation needs to be taken into account. The charged particles in space radiation environment consist primarily of high-energy electrons, protons, alpha particles and heavy ions (cosmic rays). The radiation effects of these charged particles are dominated by ionization in electronic devices, and the resulting total ionizing dose (TID), single-event effects (SEE), and enhanced low dose rate sensitivity (ELDRS) issues are briefly discussed. Details of circuit design and worst case analysis are presented for two simple circuits: a three-terminal linear regulator, and a relay coil driver. Approaches to the detailed worst case circuit analysis, and an awareness on the need of the analysis to be performed before the design is released to manufacturing should be part of a well-rounded electronics design curriculum. Introduction Electronic circuit design starts with the customer electrical specifications. Environmental specifications such as temperature and life quickly become important design inputs. And for space applications, commercial as well as military, radiation environment becomes a exceedingly important factor in electronics design. Case/junction temperature and TID level for the parts are known only as a best estimate at the start of the electrical design process since mechanical packaging, and thermal and radiation analyses are all carried out in parallel. This situation typically forces the circuit designer to be somewhat conservative in part selection and design approach. In this paper, the space radiation environment is first reviewed briefly including the total ionizing dose, single event effects, and enhanced low dose rate sensitivity of parts. Then the circuit design and analysis is presented in detail for a three-terminal linear regulator circuit and a electromechanical relay driver circuit. These examples are chosen based on their simplicity, however, the design approach and the necessity of detailed circuit analysis is clearly presented. Failure to analyze a circuit design in a timely fashion can cost dearly in terms of cost and schedule due to last minute design changes. The importance of detailed worst-case circuit analysis should be conveyed to the students as part of a well-rounded electronics design curriculum. Space Radiation Environment for Electronic Devices The space radiation environment poses a risk to all earth orbiting satellites and missions to other planets. Charged particles in this environment consist primarily of high-energy electrons, P ge 611.1 Proceedings of the 2001 American Society for Engineering Education Annual Conference & Exposition Copyright © 2001, American Society for Engineering Education protons, alpha particles, and heavy ions (cosmic rays). The radiation effects of these charged particles are dominated by ionization in electronic devices and materials. Energy deposited in a material by ionizing radiation is expressed in “rads”, with one rad equal to 100 ergs/gram. However, energy loss per unit mass differs from one material to another because of the atomic differences in various materials. For semiconductor devices, the unit of absorbed dose is rads (Si). There are two types of radiation damage induced by charged particle ionization in the natural space environment. These are total ionizing dose 1,2 (TID) and single-event effects 3,4 (SEE). The TID effects are cumulative ionization damage caused by the charged particles passing through a semiconductor device . For MOS devices, this ionization traps positive charges in the gate oxide and produces interface states in silicon at the Si-SiO2 interfaces. These effects cause threshold voltage shifts and decreased channel carrier mobility, resulting in increased leakage current and power supply current, and possible loss of device functionality. For bipolar devices, ionization adversely affects current gain and junction leakage currents, causing significant degradation in device performance. This leads to increased offset voltage and bias current in op-amps and comparators, and loss of accuracy and functionality in analog-todigital and digital-to-analog converters. Single event effects 3,4 (SEE) are caused by a high energy single ion (heavy ion or energetic proton) passing through a device. SEE include single event upsets (SEU), single event latchup (SEL), single event burnout (SEB), and single event gate rupture (SEGR). While SEU are nondestructive and do not cause permanent damage to the device, the other single event effects can be destructive. SEU occur due to either the deposition or depletion of charge by a single ion at a circuit node, causing a change of state in the memory cell (bit upset). In very sensitive devices, a single ion hit can cause multiple bit upsets in adjacent memory cells. However, these SEU cause no permanent damage, and sometimes power recycling is all that is needed. Additionally, autocorrecting circuits can be implemented in many designs. SEL can occur in any semiconductor device that has a parasitic n-p-n-p path. A single heavy ion or high-energy proton can initiate regenerative action. This leads to excessive power supply current and loss of device functionality. Device burnout may occur unless the current is limited or the power to the device is recycled. SEL is of most concern in bulk CMOS devices. SEB and SEGR may occur in MOSFETs, however, they are avoidable by design as long as the applied drain and gate voltages are properly derated. A radiation risk assessment for any electronic device includes the determination of TID and SEE susceptibility of the device caused by the projected radiation environment of the spacecraft. It should be noted that the TID on a device can vary significantly with the amount of shielding interposed between the device and the outside environment, however, the SEE susceptibility do not change significantly with shielding . TID testing of devices is generally performed by exposing devices to gamma rays from a Co-60 source with a dose rate of typically 50-300 Rads/sec, per MIL-STD-883. However, dose rates in the natural space environment are very low (0.1 to 10 mRads/sec). It is not feasible to simulate the low dose rate of space environment during ground testing. However, more and more bipolar linear devices such as op-amps, comparators, and linear regulators are tested at low dose rates since the recent discovery of enhance low dose rate sensitivity 5,6 (ELDRS). As a designer, P ge 611.2 Proceedings of the 2001 American Society for Engineering Education Annual Conference & Exposition Copyright © 2001, American Society for Engineering Education either we select ELDRS tested parts or design very conservatively especially with input offset voltage and bias currents for linear bipolar ICs. Design Examples The following two simple examples illustrate the approach to design and analysis as well as the importance of detailed analysis prior to releasing the engineering drawings to manufacturing. Three terminal linear regulator Consider the three-terminal linear regulator circuit shown in Fig. 1, commonly used to postregulate the low power outputs of a switching power converter. LT1086 , a low dropout linear regulator from Linear Technology is chosen as an example since radiation data on this part is readily available. Figure 1 Three-terminal linear regulator circuit. Design Requirements: 10.5 V ≥ Vin ≥ 7.5 V, Vout = 5.0 V ± 5%, and 500 mA ≥ Iload ≥ 100 mA. Circuit Design: Basic design equation can be derived from Fig. 1 as: Iadj R R R R R Vref Vout • + +       + + • = ) 3 2 ( 1 3 2 1 (1) Per the LT1086 datasheet 7 and design requirements, Vref = 1.25 V, Vout = 5.0 V, and Iadj = 55 μA. Selecting R1 as 249 Ω, the required value for (R2 + R3) can be computed using Equation 1 to be 738.9 Ω. The following resistor values will work for this design: R2 = 365 Ω, and R3 = 374 Ω. The power dissipation in the resistors can be computed as PR1= 6.25 mW, PR2= 9.4 mW, and PR3= 9.6 mW. We could easily use 100 mW, 1% thick-film chip resistors of R0805 size with a temperature coefficient of 100 ppm/C. It is possible to get by with 50 mW resistors, however, it all depends on the operating case temperature of the resistors as well as the customer and/or internal parts derating guidelines. Worst case circuit analysis: Let’s calculate the worst case steady state dc regulation of the output voltage to see if the design meets the regulation requirement of ± 5%. Variation of component parameters due to initial tolerance, temperature, and space radiation environment needs to be taken into account in the analysis. We are assuming the maximum case/junction temperature to be 85C, a design life of 18 years, and a total dose radiation level of 40 kRads at the part level. U1 (LT1086) IN