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10 DESIGN007 MAGAZINE I JANUARY 2022 "Don't let the smoke out." at's one of the PCB designer's unspoken objectives, albeit a sarcastic one. e idea is that there's a cloud of smoke inside each printed circuit board, and if you don't design the board correctly, you'll let all the smoke out. In high-voltage boards, you might say that there's a lot more smoke trapped inside, just waiting to get out and set your schedule back a few weeks. In recent surveys, readers mentioned high- voltage PCBs among the challenges that they're facing. Designers have been dealing with high-voltage components such as inverters and power supplies for decades, oen by segregating them from every- thing else because of their inability to "play nice" with the other kids. But now we're seeing more and more PCBs for electric vehicles, as an example, that don't just have a few high-voltage features here and there. ese are high-voltage boards. Much of the information about high-volt- age design is not exactly set in concrete. For instance, at what point does a PCB become a "high-voltage" board? Some engineers point to 100 volts as the beginning of high voltage. e only agreement seems to be that once you're in the kilovolt zone, you're flying high. Designing at higher voltages presents designers with a new set of challenges; design techniques that have always worked at 100 VDC don't work well at 4 kV. High-voltage designers learn to live by their material's comparative tracking index (CTI). is handy chart defines the point of no return—the voltage at which your substrate will start to break down. Typical FR-4 is too porous for most high-voltage appli- cations, and some of the most popular high-voltage lami- nates can only be used for single-sided PCBs. Criteria such as glass weave and resin content can make or break a board at high volt- ages. As the saying goes, with high-volt- age design, it's the same but different. In most cases, you can't run high-voltage traces in the internal layers of a multilayer board. EMI and thermal management can become giant pains. The Shaughnessy Report by Andy Shaughnessy, I-CONNECT007 High-Voltage PCB Design: Don't Let the Smoke Out

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