COVER STORY MILITARY & DEFENCE SUPPLEMENT HIGH RELIABILITY OPERATIONS
Steve Munns, Mil-Aero Marketing Manager at Linear Technology Corp. explores some of the key new features that are currently available for simplifying high reliability power supply design
I
n 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 and report diagnostic information to the management system.
IMPLEMENTING REDUNDANT SYSTEMS One common approach is to provide built- in redundancy, either in the form of parallel circuits that share the load actively or that wait in a standby until a failure occurs. Fault detection and management requires additional overhead circuitry to arbitrate which supply has priority and to monitor the status of each. Furthermore, it must protect the system from cross- conducting and back-feeding during source switching. Products such as the LTC4417 from
Linear Technology Corporation provide a one-chip solution with these features; automatically selecting the source based on validation of user defined supply thresholds for each input. An alternative approach is to share the
load between two input sources that operate simultaneously, increasing reliability by reducing the burden on each source and at the same time providing protection against failure of one source if each is suitably sized to support the full load requirement. In the past, a simple but inefficient diode-OR arrangement might have been adopted, but that required each supply to have active control to balance the loading. Figure 1 shows how this can now be accomplished with a single chip solution. The LTC4370 is a current sharing controller with reverse blocking that prevents a fault in one supply bringing down the power system. Power supplies for high-reliability
applications often specify a minimum hold-up time for the power rails such that the system can ‘ride-though’ interruptions and continue to function normally. The design of backup systems has been greatly simplified with the advent of new products such as LTC3355 that provides a complete 1A DC/DC
S6 SEPTEMBER 2014 | ELECTRONICS Figure 2:
Transient Protection & Output Hold Up
Figure 1: Other common safety features include
internal protection circuitry for reverse battery protection, current limiting and reverse current protection. New products such as the LT3667, Fault Protected Switching and Linear Regulator incorporate these features and add an accurate user programmable current limit for each output to further simplify design. Military and aircraft electronics must
confirm to transient protection specifications such as MIL-STD-1275 (vehicles) and MIL-STD-704 / DO-160 (aircraft). However, protection from voltage surges, spikes and ripple is desirable in any high reliability system and there are products that are dedicated to that function. While advances in silicon process
Figure 1. LTC4370 Dual Redundant Power Source Sharing
technology now allow regulator ICs to operate with input voltages of 100V or more, the dedicated transient protection ICs provide more functionality and control. In Figure 2, the LTC4364 provides a clamped output at 27V (user programmable) to protect the downstream regulators from transients and also hold up the output during input short circuit or disconnect/reset conditions.
DIGITAL POWER MANAGEMENT New products are combining the advantages of analogue power regulation with digital control over a 2-wire PMBus I2
C-based digital interface protocol to Figure 2:
switching regulator and backup system in a single chip.
THERMAL MANAGEMENT CHALLENGES Thermal management is becoming increasingly challenging as system densities and power continue to increase, this has a direct influence on IC reliability as for every 10°C rise in junction temperature the IC lifetime is approximately halved. New feature rich power supply ICs and dedicated power management functions now provide increased protection to the IC itself and the surrounding system. Thermal protection is implemented in
the majority of power regulator ICs with internal power transistors. Typically thermal shutdown is triggered at approximately 155-165°C and the device is disabled until the temperature drops by around 10°C. Some new products feature a die temperature output and the ability for the user to set one of three die temperature thresholds.
enable remote management of power supply systems. Telemetry and diagnostics data can be used to monitor load conditions, read fault logs and provide access for trimming and margining to ±0.25% accuracy, maximising system efficiency and reliability. Such systems offer the opportunity to move from time based maintenance schedules to condition based maintenance and can potentially highlight performance degradation prior to system fault conditions taking hold. The design of high reliability power supplies has been simplified by user-programmable features, more sophisticated on-chip protection mechanisms and system level ICs that integrate the required functions into a single package. Digital Power System Management provides the means to remotely monitor and control power systems and to further improve efficiency and reliability.
Linear Technology Corporation
www.linear.com
01628477066 Enter 212 / ELECTRONICS
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