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The Hubble telescope, the Cassini-Huygens mission, and other exploratory spacecraft utilize high-voltage DC power supplies for everything from vidicon camera tubes and mass spectrometers to radar and laser technologies. NASA has experienced performance problems with the 1.5 kV supplies because—as a 2006 report stated—“designers did not take the high-voltage problems seriously in the initial design.” The report cited very narrow parts parameters, electrical insulation problems in dielectrics, ceramics, bad geometries, small spacing, the use of the wrong insulating materials, and thermal expansion as causes for the power supply failures.
Designing a circuit that includes high-voltages requires a different—and much more rigorous—approach than seen with other PCB designs. And the need for more attention increases for high-density designs. Along with that approach, design teams also must become familiar with terminology that covers insulation, board materials, clearance, creepage, and altitude. Designers also should have an overall knowledge of regulations that can impact the circuit.
High-Voltage Design Problem-Solving Begins With the PCB Layout
All of us know that proper trace spacing in a PCB design maintains signal integrity and helps with preventing the propagation of electromagnetic interference. In high-voltage PCB design, trace spacing becomes even more important. If we rightfully consider the board as a series of conductive elements, the possibility of differences in potential—creating high-voltage flashover with narrow trace spacing—becomes a certainty.
Along with the IPC-2221 Generic Standard on Printed Board Design that establishes the design principles for interconnections on PCBs, the International Electrotechnical Commission (IEC) and the Underwriters Laboratories (UL) also produced IEC/UL 60950-1, the “Safety of Information Technology Equipment” standard, that describes safety requirements for products and details minimum allowed PCB spacing requirements. As a combination, the standards also set guidelines for PCB layouts that include two important parameters called clearance and creepage.
Using the IEC 60950 definition, clearance equals the shortest distance between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured through air. A small clearance value between two conductors establishes the environment for a high-voltage flashover or arc. Clearance values vary according to the type of PCB material used for the circuit, the voltages, and operating environment conditions such as humidity and dust. Those environmental factors—and others—decrease the breakdown voltage of air and increase the opportunities for a high-voltage flashover and a short circuit.
We can address clearance issues through ECAD/MCAD design principles. Since the bounding surface described in the IEC definition is the outer surface of an electrical enclosure, we can use 3D design tools and design rules to establish the clearance between enclosures and components for rigid and rigid-flex circuits. We can also apply good PCB design principles by isolating high-voltage circuits from low-voltage circuits. Fabricators often recommend placing the high-voltage components on the top side of a multilayer board and the low voltage circuits on the bottom side of the PCB. Other methods involve placing the appropriate insulating materials between high-voltage nodes and over any exposed high-voltage leads.
To read this entire article, which appeared in the January 2022 issue of Design007 Magazine, click here.