Integrated Circuit Provides Enhanced Protection and Improved Safety Features for High-Reliability Power Supplies
“The design of a high-reliability system involves the use of fault-tolerant design techniques, selection of appropriate components to meet expected environmental conditions, and compliance with standards. This article focuses on semiconductor solutions for implementing high-reliability power supplies, including redundancy, circuit protection, and remote system management. This article will highlight how improvements in semiconductor technology and new safety features can simplify designs and enhance device reliability.
By: Steve Munns, Military/Aviation Marketing Manager, Analog Devices
The design of a high-reliability system involves the use of fault-tolerant design techniques, selection of appropriate components to meet expected environmental conditions, and compliance with standards. This article focuses on semiconductor solutions for implementing high-reliability power supplies, including redundancy, circuit protection, and remote system management. This article will highlight how improvements in semiconductor technology and new safety features can simplify designs and enhance device reliability.
Requirements for high reliability power systems
In an ideal world, high-reliability systems should be designed to avoid single points of failure and provide a means of isolating failures so that operations can continue (perhaps at reduced performance levels). It should also be able to contain faults before they propagate to downstream or upstream electronics.
One solution is built-in redundancy—either parallel circuits that actively share the load, or circuits that wait on standby until a failure occurs. In each case, additional circuit overhead is required for fault detection and management, increasing overall complexity and cost. Some systems also create different parallel circuits to add variety and avoid the risk of the same failure mechanism; this is the case with some aircraft flight control systems.
Increasing system complexity places a greater burden on power supply performance, so high conversion efficiency and good thermal management are critical, knowing that IC lifetime is approximately halved for every 10°C rise in junction temperature. We will see that new feature-rich power ICs and dedicated power management functions now provide better protection for the IC itself and surrounding systems.
Power Regulator Safety Features
Voltage regulators are increasingly limited in precision and sophistication to prevent excessive output current from damaging the device itself or downstream devices. Internal protection circuits are also common, including reverse battery protection, current limiting, thermal limiting, and reverse current protection.
An example of improvements in both process technology and safety features is the LTC7801 DC/DC switch controller, which can safely withstand input voltages up to 150V and implement a protection feature that increases when the input voltage rises to a programmable Above the working range, it inhibits switching action. This feature simplifies the input power transient protection circuit, reducing component count and solution size. The output is also well protected by an overvoltage comparator, which prevents voltage overshoot, and a foldback current limiter, which controls power dissipation during overcurrent and short-circuit faults.
Safe physical packaging aspects are also addressed by offering package options with a wide lead pitch, avoiding the risk of arcing between adjacent high-voltage and low-voltage pins. The breakdown voltage decreases with decreasing air pressure, so unpressurized aircraft applications can choose the LTC3895, which has the same functionality and performance as the LTC7801, but in a 0.68mm dual-pin pitch package option.
Some products (such as the fault tolerant linear regulator LT3007) also offer so called FMEA (Failure Mode and Effects Analysis) compatible pinout, if adjacent pins are shorted or the pins are left floating, the output remains at or below regulation value.
Figure 1: LTC7801 High Voltage Step-Down DC/DC Controller
Control multiple input sources
Power systems that include primary and redundant backup power sources, and possibly even external auxiliary power sources, require a system to decide which power source takes priority and monitor its status. Additionally, it must prevent system cross-conduction and feedback during power switching. Single-chip ICs such as the LTC4417 provide a solution that automatically selects the power supply by validating the user-defined power supply thresholds for each input.
Another approach is to share the load between two simultaneously operating input sources, increasing reliability by reducing the burden on each supply, while providing protection from failure of one supply without being affected – if each supply fails is sized appropriately enough to support full load requirements. A simple but inefficient diode “OR” arrangement may have been used in the past, but it required each supply to be actively controlled to balance the load. Figure 2 shows how a single-chip solution can now be used to achieve this goal. The LTC4370 is a current sharing controller with reverse blocking to prevent a single supply failure from overwhelming the entire power system.
Figure 2: LTC4370 dual redundant power supply current sharing
Transient and Circuit Protection
Military and avionics must meet transient protection specifications such as MIL-STD-1275 (vehicles) and MIL-STD-704/DO-160 (aircraft). However, any high reliability system needs protection against voltage surges, spikes, and ripples, and some products, such as the LT4364, provide this feature exclusively.
Figure 3: The LTC4368 bidirectional circuit breaker with protection
There is also a wide variety of circuit protection features to choose from, including products like the LTC4368. The LTC4368 is a 100V bidirectional circuit breaker that provides protection against over-voltage, under-voltage, or even negative supply voltages, as well as forward and reverse overcurrent faults.
Through these examples, we can see how new products with increasingly complex protection and safety features can simplify application circuit design and reduce solution size.
Digital Power System Management
The new product combines the advantages of analog power regulation and digital control via an I2C-based PMBus interface, enabling remote management of power systems. Telemetry and diagnostic data can be used to monitor load conditions, read die temperature, and perform trimming and margining with very high accuracy to maximize system stability, efficiency, and reliability. One concern with digital power management is that the software is too complex, but the LTC3815 implements a simplified PMBus “Lite” command set with no on-chip non-volatile memory or microcontroller to provide digital control and monitoring while simplifying the design The advantages.
Figure 4: Isolated switch controller LTM9100 with telemetry
As mentioned before, good thermal control is critical for reliability, and the LTC3815 has a two-level thermal threshold and two-level response. When the internal chip temperature exceeds 150°C, the PMBus will receive an overheat status flag, and the ALERT pin will be pulled low to alert the PMBus host. If the temperature continues to rise and exceeds 170°C, the LTC3815 will shut down all circuits, including output regulation, until the overtemperature condition disappears.
This status-reporting system provides an opportunity to move from time-based maintenance planning to condition-based maintenance, and can highlight performance degradation before a system failure state is complete.
High reliability power systems often include isolation barriers to protect the power bus from failures in downstream line replaceable units. The growing number of sensors and actuators is also driving an increased need for smaller locally isolated power and data interfaces to reduce noise-related issues caused by ground loops and common-mode interference. Complete galvanically isolated BGA module solutions are now available to simplify design and improve 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 electrical 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 through the I2C/SMBus interface, enabling power and energy monitoring of high-voltage buses.
Much of this article is devoted to discussing new features that simplify the design of high-reliability power supplies, or product features that protect devices from failure or improper handling. However, it is also critical not to overlook device quality and the importance of selecting the correct grade for the expected environmental conditions. For example, Analog Devices military plastic grade is 100% tested and guaranteed performance over a temperature range of -55°C to +125°C; no costly device rescreening or characterization is required for application circuits expected to be very harsh environments .
User-programmable features, more complex on-chip protection mechanisms, and higher levels of integration simplify high-reliability power designs and reduce overall solution size. Digital power system management provides the means to remotely monitor and control power systems and further improve efficiency and reliability. Finally, choosing the correct grade of device from a reputable supplier will reduce the chance of quality and reliability issues.