MEASUREMENT DEVELOPMENT
DURING THE DEVELOPMENT of Keithley’s Model 2657A High Power SourceMeter Source Measurement Unit (SMU), the design team encountered a range of challenges related to the high voltage levels involved. The last high voltage SMU Keithley had developed was an 1100V/20W unit introduced in the 1990s. The Model 2657A, which is equipped with a ±3000V, 200W power source, is the highest voltage instrument we’ve ever designed. In order to characterize and test the next generation of high power semiconductors, however, this instrument also requires the precision to measure down to hundreds of femtoamps.
This combination of requirements forced us to re-evaluate all safety practices, from the design labs to the manufacturing, calibration and repair facilities. It also demanded exceptional discipline and ingenuity in terms of design.
We encountered challenges related to issues designers can usually take for granted, such as the components and equipment used, debugging techniques, and especially the project schedule: Just about everything was more difficult, more costly, and more time-consuming than with a lower voltage project. In addition, from a broader, system-level perspective, we had to factor in how customers would use the product and how sales and applications personnel would demonstrate it. Ensuring the system was simple to use and demonstrate was a major consideration. Special test fixtures were also necessary to ensure the safety of the user and prevent damage to other equipment integrated into the system.
Safety First Our first step was preparing people and the work environment for the new safety requirements. That involved in-house personal safety training for working with high voltages and CPR training for our design and manufacturing teams. Later, we developed safety training for the product’s end users. We also created new high voltage lab areas for engineering, manufacturing, calibration, and repair in both our U.S. and international locations, equipping them with emergency power shutdown and interlock safety switches.
With our focus on designing a safe product for our customers, we involved our safety team at the early stages of our design effort, working with them to develop a system insulation block diagram (IBD) that defined the voltage circuit groups and determined the required spacing, creepage, and clearance distances between each group. These voltage spacing rules are defined by international standards. These requirements posed many mechanical and electrical layout
challenges, with some circuit groups requiring more than two inches of spacing between them. The ability to enter these voltage spacing rules into PCB design CAD software helped us enforce the IBD spacing rules during the PCB layout; we also used 3D mechanical CAD software to view board-to-board or board-to-chassis spacing before the boards were built.
We came up with a variety of innovative solutions to minimize the impact of these spacing requirements. In one case, the chassis-based screws used to mount the main analog PCB would have forced us to keep analog components or traces a minimum of 17 millimeters away from each screw head. The board’s large size required 12 mounting locations, which would have forced us to leave a lot of space on the PCB unoccupied. To minimize this loss of PCB real estate, we developed a mechanical mounting device that isolated the screw into the PCB from the chassis, providing the necessary creepage distance from the screw to the chassis. By “floating” the screw head, we freed up that PCB real estate for component placement. In other cases, the only way to achieve the required spacing was to separate the circuit groups physically onto multiple smaller PCBs.
Another challenge was high speed digital communications from the chassis-based microprocessor board to the floating analog circuitry. We can usually use low cost off-the-shelf multi-channel digital isolators to meet our voltage spacing requirements. In this case, because there are no single-component solutions available, we ended up using optical fiber with discrete transmitters and receivers to meet the high voltage spacing. This is an expensive solution with large components, so to minimize the impact on space and cost, we redesigned our communications scheme, which originally involved eight signal paths, to be just a three-signal solution.
One advantage of using fiber with separate transmitters and receivers was that it allowed us to move the chassis-based communication signals off the main floating analog board, freeing up space on the PCB that was reserved for the creepage from the analog circuits to the chassis- based communication signals. Even with all the information provided by the advanced CAD tools we used, we were still occasionally surprised by the zap of an electrical arc while powering up new revisions, a not- so-subtle way of telling us we hadn’t allowed sufficient spacing somewhere. As the design project progressed, we began holding regular safety reviews to address new problems as they emerged. As we refined the product, we had to
Issue I 2013
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