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Building high-reliability power systems

Posted: 10 Sep 2014 ?? ?Print Version ?Bookmark and Share

Keywords:redundancy? thermal management? Power regulator? Input current limiting? LT3667?

In an ideal world, a high reliability system must 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.

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, is one solution. In each case, fault detection and management requires additional overhead circuitry 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.

High complexity systems increase power supply performance requirements and high conversion efficiency and good thermal management are critical as for every 10C 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 features
Output Current Limiting: This is not a new feature but its implementation has become more accurate and sophisticated and additional flexibility is provided as user programmable features are added. As an example, the LT3667 shown in figure 1 is a 40V 400mA step-down switching regulator with dual fault protected low dropout linear regulators. Internal protection circuitry includes reverse battery protection, current limiting, thermal limiting and reverse current protection. The switching regulator part of the IC contains both a switch current limit and catch diode current limit such that the output current is controlled during fault conditions such as a shorted output. The dual linear regulators also have individual user programmable current limits, which in the example application in figure 1 have been set to 100mA by R7 and R8.

These measures not only protect the device itself, but also the downstream electronics should a fault develop.

Figure 1: LT3667 fault protected, switching and linear regulator.

Input current limiting: This is commonly found in circuits such as those performing energy harvesting from photovoltaic cells where the high impedance source requires that the current be carefully controlled to prevent the source voltage collapsing. In addition to protecting the upstream electronics from overload, it can also be employed as a safety feature as shown in figure 2 for a backup supply where large capacitors must be protected and safely charged. The LTC3128 incorporates a programmable 2% accurate average input current limit. In this application, a 3A input current limit is set and the supercapacitor backup circuit will draw only the spare current that is not consumed by the main load through the buck-boost convertor.

Figure 2: LTC3128-based supercap backup circuit.

Thermal protection: It is implemented in the majority of power regulator ICs with internal power transistors. In the case of the LTC3128 featured above, thermal shutdown is triggered at approximately 165C and the device is disabled until the temperature drops to around 155C. However, the product also contains a thermal regulator to prevent it from going into thermal shutdown when charging very large capacitors at high current. It works by progressively lowering the average current limit when the die temperature exceeds 135C. Other products such as the 8-output buck regulator LT3375 feature a die temperature output and the ability for the user to set one of three die temperature thresholds.

Figure 3: LTC4370 dual redundant power source sharing.

Controlling multiple input sources
Power supply systems that contain a main supply and a redundant backup with perhaps an external auxiliary supply need a system to arbitrate which supply has priority and to monitor their status. Furthermore, it must protect the system from cross-conducting and back-feeding during source switching. Single chip ICs such as the LTC4417, provide one solution automatically selecting the source based on validation of user defined supply thresholds for each input.

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