COVER STORY FEATURE


Some products such as the fault


tolerant LT3007 linear regulator are also available with so called FMEA (Failure Mode and Effects Analysis) compliant pinout where the output stays at or below the regulation voltage if adjacent pins are shorted together or if a pin is left floating. 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. 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 they are each 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 2 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.


TRANSIENT AND CIRCUIT PROTECTION Military and aircraft electronics must conform 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 such at LT4364. There is also a wide variety of circuit protection functions available including products such as the LTC4368, a 100V Bidirectional Circuit Breaker that includes protection from power supply voltages that may be too high, too low, or even negative and from overcurrent faults in both forward and reverse directions. In these examples we can observe how new products with increasingly sophisticated protection and safety feature sets can simplify the application circuit design and reduce solution size. New products are combining the advantages of analogue power regulation


/ ELECTRONICS


Figure 2:


Figure 2


with digital control over an I C-based 2


PMBus interface to enable remote management of power supply systems. Telemetry and diagnostics data can be used to monitor load conditions, read die temperature and provide access for trimming and margining to very high accuracy, maximising system stability, efficiency and reliability. One concern with digital power supply


management is the complexity of software, however, the LTC3815 implements a simplified PMBus “Lite” command set, with no on-chip non- volatile memory or microcontroller, it simplifies design while providing the benefits of digital control and monitoring. As previously mentioned good thermal


control is essential for reliability and the LTC3815 has two levels of thermal thresholds and two levels of responses.


Figure 2:


LTC4370 dual redundant power source sharing


When the internal die temperature exceeds 150°C, the overtemperature condition is flagged to the PMBus and the ALERT pin pulls low to alert the PMBus master. If the temperature continues to rise and exceeds 170°C, the LTC3815 shuts down all circuitry, including output regulation until the overtemperature condition has cleared. Such systems that can report their status 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.


Figure 3:


LTC4368 bidirectional circuit breaker with protection


Figure 4:


LTM9100 Isolated switch controller with telemetry


ISOLATED SYSTEMS High reliability power supply systems often include an isolation barrier to protect the power buses from faults in downstream line replaceable units. Increasing numbers of sensors and actuators are also driving demand for smaller, locally isolated power supplies and data interfaces to reduce noise induced problems from ground loops and common mode interference. There are now complete galvanic isolated BGA module solutions to simplify design and increase reliability. The LTM9100 isolated switch controller is an all-in-one solution for controlling, protecting, and monitoring high voltage power supplies up to 1000VDC. A 5kVRMS galvanic isolation barrier separates the digital interface from the switch controller, driving an external N- channel MOSFET or IGBT switch. Isolated digital measurements of load current, bus voltage, and temperature are accessed via the I2


C/SMBus interface, Figure 3


enabling power and energy monitoring of the high voltage bus. Most of this article has been dedicated to new functions that simplify designing high reliability power supplies or product features that protect the device from fault conditions or mistreatment. However, it is critical not to overlook the importance of component quality and of selecting the correct grade of component for the anticipated environmental conditions. For example Analog Devices military plastic grade provides 100% tested and guaranteed performance over -55°C to 125°C, avoiding the need for costly rescreening or characterisation of the component in the application circuit where very harsh conditions are anticipated.


Figure 4


Linear Technology (UK) Ltd. Now part of Analog Devices www.linear.com 01628 477 066


ELECTRONICS | JULY/AUGUST 2017 13


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