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Powering automated test equipment

22 Jan 2013  | Maurizio Salato

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Within the flexibility paradigm, re-use of a design has become a common choice to leverage standardisation within a diverse power system. Designers can "source and tailor" a multiplicity of regulators from a set of valid designs to electrically fit the specific loads and place them as close as possible to the load themselves to maximise dynamic performance. Clusters of regulators would be supplied by a single intermediate bus and sustained by a bus converter. While good design practice would call for matching the bus converter power throughput with the downstream regulator's needs, constraints normally arise in effectively distributing the intermediate bus "on board." At an intermediate bus voltage of 12V, distributing even just 300W means routing 25A, which can be challenging on high-density designs.

Reliability and redundancy
Objective: to achieve the highest degree of system availability through design for reliability and redundancy

The power distribution network is the "spinal cord" of complex equipment, powering measurement systems and DUTs. As an ATE lifespan is usually measured in decades, identifying and controlling critical components' mean time between failure (MTBF) is necessary. This includes:

Appropriate electrical and thermal de-rating – While de-rating is usually perceived as a means to reduce thermal stresses, current density also can contribute to failure mechanisms. One example is electro-migration on PCB traces.

Redundancy – Critical components whose failure jeopardizes machine availability should be made redundant and provisions made for field replacement.

Serviceability and diagnostic telemetry
Objective: to enable the highest level of modularity and provide the system with on-line diagnostic options over system lifecycle

The backplane distribution approach is inherently highly flexible, allowing hot-swap of non-critical loads (like, for example, fans) that can be field replaced while the equipment is operating. However, the same level of flexibility should be planned on the other end of the distribution network, enabling easy access to the bulk power system.

Diagnostics also becomes a critical aspect. It is necessary to identify and isolate components or sub-system issues quickly to protect the system from propagating failures. While almost impossible in the case of central power architecture, both distributed and factorized power architectures provide a means to automate the power distribution network diagnostic. Distributed power architecture traditionally utilises a "margining" technique, where the remote DC/DC converter is commanded a pre-set sequence and a dedicated feedback network verifies the obtained DC or AC dynamic. Modern power components and factorized power architecture provide a simpler, digital interface where supervisory controls can actually poll status information in real-time via a common serial interface bus, typically I2C or UART.

Figure 3: Point-of-Load standard solution example from 380V DC distribution: on the left, high voltage bus converters card; in the middle, a single 400V to 12V bus converter chip shown close to a paper clip; on top, a standard multi-phase buck converter.

Noise sources and immunity
Objective: to minimise EMC and EMI within the system, decoupling the power distribution network to minimise conducted and radiated interference

EMC and EMI are usually perceived as the "black magic" of power design. However, a few basic rules generally apply. First, identify the noise source; and second, contain or filter the noise as close as possible to the source (assuming the noise cannot be avoided altogether). Unfortunately, the power distribution network is a good candidate for distributing conducted noise within the system as well as radiating noise around the system rack. It is therefore extremely important to implement proper filtering if not at individual DC-DC component level, at least on each instrument card, to avoid cross-talk through the backplane.

A particularly insidious issue is the low-frequency noise generated on the backplane as well as intermediate buses. This is due to the input current interactions of various converters which then create low-frequency beats. Designers can try to avoid this issue by making sure that all the switching current components are sufficiently confined within each individual converter.

Conclusion
Power distribution network design is a challenge of state-of-the-art test systems. Several trade-offs exist and need to be carefully evaluated within the power system architecture of choice. While distributed power architecture from 48V backplanes is today's common choice, advanced architectures like factorized power from 400V DC distribution are becoming valuable, because they improve system density by offering higher efficiency and a higher level of granularity. Figure 3 shows an example of a standard two-stage design for point-of-load supply from 400V DC distribution: on the left, a front-end module consisting of two 400V bus converters (under the heat sinks, with a single module shown close to the paper clip); on top, a multi-phase Voltage Regulation Module (VRM) with classic 12V buck converter approach.

About the author
Maurizio Salato holds BS and MS in electrical engineering from Politecnico di Torino, Italy, and MS in systems engineering from Loyola Marymount University, Los Angeles, California. He is currently director of systems engineering with Vicor Corporation. Before joining Vicor, Maurizio worked for more than 10 years in the power semiconductor industry.

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