FEATURE COVER STORY
Designing out system failure
Steve Munns, Mil/Aero marketing manager at Linear Technology Corporation (now part of Analog Devices) investigates how the latest integrated circuits are offering enhanced protection & improved safety features for high reliability power supplies typically found in military and aerospace applications
he design of high reliability systems encompasses the use of fault tolerant design techniques, the selection of suitable components to meet the anticipated environmental conditions and compliance to standards. This article focuses on semiconductor solutions for the implementation of high reliability power supplies including redundancy, circuit protection and remote systems management. It will highlight how improvements in semiconductor technology and new safety features can simplify design and enhance component reliability.
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REQUIREMENTS OF HIGH RELIABILITY POWER SYSTEMS In a perfect world a high reliability system should be designed to avoid single point failures and provide a means of isolating faults in such a way that operation may continue perhaps at a reduced performance level. It should also be able to contain faults to avoid propagation to downstream or upstream electronics.
Increasing system complexity places a greater burden on the power supply performance so high conversion efficiency and good thermal management are critical as for every 10°C rise in junction temperature the IC lifetime is approximately halved. As we shall see, new feature rich power supply ICs and dedicated power management functions now provide increased protection to the IC itself and the surrounding system.
POWER REGULATOR SAFETY Voltage regulators have seen increasingly more accurate and sophisticated forms of current limiting to avoid excessive output currents damaging the device itself or downstream components. It is also fairly common to find internal protection circuitry including reverse battery protection, current limiting, thermal limiting and reverse current protection. One product that provides an example
LTC7801 Figure 1: Built-in redundancy, either in the form
of parallel circuits that share the load actively or that wait in standby until a failure occurs, is one solution. In each case, fault detection and management requires additional overhead circuitry
12 JULY/AUGUST 2017 | ELECTRONICS
contributing to the overall complexity and cost. Some systems also create dissimilar parallel circuits to add diversity and avoid the risk of a common failure mechanism; this is the case for some aircraft flight control systems.
Figure 1:
LTC7801 high voltage step down DC/DC controller
of improvements in both process technology and in safety features is the LTC7801 DC/DC Switching Controller, it can safely sustain input voltages up to 150V and implements a protection feature that inhibits switching when the input voltage rises above a programmable operating range. This functionality simplifies the input supply transient protection circuitry reducing component count and solution size. The output is also well protected with an overvoltage comparator that guards against voltage overshoots while a foldback current limiter controls power dissipation during overcurrent and short- circuit fault conditions. The physical packaging aspects of safety are also addressed by offering package options with widely spaced pins to avoid the danger of arcing between adjacent high voltage and low voltage pins. The breakdown voltage reduces with lower air pressure so unpressurised aircraft applications can select the LTC3895 that offers the same functions and performance as the LTC7801 but with a 0.68mm double pin spacing package option.
/ ELECTRONICS
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