Taking The Liability Out Of Reliability

Designers who evaluate COTS products must take into account the intended application and the likely demands on the system.

Increasing use of COTS components and subsystems has allowed designers of high-reliability (hi-rel) applications to benefit from the short lead-times, extensive choice, rapid product evolution and lower cost of commercial electronic technologies where the demands of the application and environment will allow. By exploiting these opportunities, designers of space, military and civil aviation applications can configure systems for hi-rel applications under the functionality, performance cost and time-to-market pressures that prevail in the commercial sector.

Commercial power supply design, for example, has advanced rapidly in recent times, particularly in relation to the design of distributed power architectures. COTS offers designers the opportunity to use the latest converters, regulators and power semiconductors to achieve lighter, more efficient, lower-cost power supplies within tighter timescales.

However, COTS products cannot simply be designed-in on the designer's whim – there are caveats. The operating temperature of a given COTS product may be little or no wider than its standard commercial counterpart. The package may not be hermetically sealed, in which case the system cannot be exposed to moisture or particle ingress. Also, a COTS product may not be qualified for exposure to high or low levels of radiation, which may prevent its use in space or certain nuclear applications. For these reasons the engineers who evaluate and select COTS products must take into account the intended application and the likely demands on the system before sanctioning their use.

To expand the choices available to designers, companies are now offering products that are designed for hi-rel applications built using lower cost technologies, as well as dedicated, hermetically sealed hi-rel components. These include standard plastic packaged devices built on qualified manufacturing lines, with guaranteed performance over a wider temperature range than standard commercial products. Also, products designed for other severe-environment applications, such as automotive under-bonnet or cellular base station electronics, are now reaching a performance level that makes them a viable option for the hi-rel designer. For example, today's automotive power supplies are expected to operate under temperature extremes, moisture conditions and mechanical stress that are similar to those that some hi-rel systems are required to endure.

The result is that designers of power systems for hi-rel applications can now choose from a range of COTS and dedicated power semiconductor technologies that includes discrete devices such as power MOSFETs, voltage regulators, IGBTs and Schottky diodes, and integrated modular solutions such as DC-DC converters and motor control circuits.

DISCRETES FOR HI-REL DESIGN
Consider a power switching application that requires a through-hole N-channel power MOSFET rated at 30W and capable of withstanding up to 100V drain-to-source breakdown voltage (BVDSS), and capable of continuous operation at up to 8.0A at 25°C or 5A at 100°C.

Choosing, for example, from the IR product family, either the IRL1520N HEXFET power MOSFET in standard TO-220 FULLPAK plastic package, or the IRHF7130 HEXFET in a TO-39 package are viable options. Both of these devices have the same HEXFET integrated transistor/diode equivalent circuit and achieve fast switching speeds. The IRL1520 was originally developed for automotive applications, and will operate with a junction temperature between -55°C and +175°C. Therefore, despite the fact that this device is not supplied in a hermetically sealed package, it is still suitable for hi-rel applications where temperature extremes are among the prime design constraints. The IRHF7130, on the other hand, has a lower operating junction temperature range of -55°C to +150°C, but is supplied in a hermetically sealed package and is a radiation-hardened device compliant with MIL-STD and characterised for both Total Dose and Single Event Effects (SEE) radiation. As a result, the IRHF7130 is a natural choice for applications such as satellites or other spacecraft, or weapon systems designed to operate in environments subject to high levels of radiation. Figure 1 shows the two devices side by side: the same electrical parameters, with different environmental properties.

CONVERTER CHOICES
Designers of hi-rel systems are as dependent as any other group on lightweight, efficient DC-DC converters as part of distributed or conventional power architectures. To increase the choices for designers, IR has developed new families of high reliability converters suitable for use with the latest topologies. These are specifically targeted at military, aerospace and ruggedised industrial applications.

As an example, the AA28XX series offers products with a wide input voltage range from 15V to 50V, allowing the designer to optimise the primary bus voltage for overall power efficiency. Converters are available rated up to 25W output power, and there are single- and dual-output models with voltages of 3.3V, 5.0V, 5.2V, 12V, 15V, ±5V, ±12V and ±15V. The AA28XX converters are not delivered in hermetically sealed packages, but are designed to operate under extremes of temperature, mechanical shock, and vibration, making them suitable for military vehicle and flight applications where the increased ingress resistance of hermetic sealing is not required.

Radiation-hardened and radiation-tolerant converters are also available for space and weapon applications, such as IR's Z-series single output DC-DC converters (Figure 2). Outputs of 1.5V, 2.5V and 3.3V are available, to provide the very low supply voltages required by the electronics onboard space vehicles and satellites. Z-series devices are tolerant of low-level radiation such as that encountered by geo-synchronous and low earth orbit satellites. Low voltage design is one of the techniques used by satellite systems designers to minimise the vehicle power budget and avoid heavy, bulky power supplies that would otherwise impose far-reaching design constraints and also add to the launch costs. The Z-series converters use a combination of surface mount technology for the larger passive and power components and hybrid construction for control circuits, to simplify assembly and achieve a small footprint.

Another series of standard hi-rel DC-DC converters offers radiation tolerance for extended operation in moderate radiation environments. The AMA-series (5W), AMF-series (12W) and AMR-series (30W) converters are built using high-density chip and wire hybrid technology that complies with the class H requirements of MIL-PRF-38534. They are fabricated in a facility fully qualified to MIL-PRF-38534. The converters can be mounted directly to a heat conduction surface without signal leads penetrating the heat sink surface, allowing greater independence in mounting and greater mechanical security than traditional packages. The package, which is shown in Figure 3, uses ceramic seal pins to create a long-term hermetic seal.

The radiation performance of the AMA/F/R series supplements the higher radiation performance available in the ART2815T converter series, and is suitable for space vehicles operating in low earth orbits, including launch boosters or orbiting space stations.

HI-REL MOTOR CONTROL
In hi-rel applications, just as in any other market, electronic control of fans, pumps, actuators and other motor-driven subsystems deliver efficiency, noise, space, weight and cost benefits over mechanical solutions. But until recently, hi-rel designers have had to develop custom motion control designs because available off-the-shelf solutions have not been suitable for harsh environments. This is now changing as power electronic vendors continue to increase their expertise in applying COTS technologies in hi-rel applications. Highly integrated motion control modules that incorporate all of the electronics needed to accurately drive and control a motor are now entering the market.

The OM9375 3-phase brushless DC (BLDC) motor control subsystem is one example. As shown in the block diagram of Figure 4, the power stage, non-isolated driver stage, controller stage and MOSFET output stage are all integrated on-chip. Ideal for fans, pumps and actuator systems, the OM9375 is rated for 25A average phase current and a 160V maximum bus voltage, and can accurately control motor speed and direction. A soft start facility ensures safe motor starting.

Designers creating specialist solutions for hi-rel applications now have a wider choice of power electronic components and modules that will meet their requirements. COTS has been pivotal in encouraging this; in addition to streamlining design and development, it has also allowed vendors to gain a clearer understanding of the wide variety of environmental conditions under which hi-rel systems are required to operate.

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