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The Shaughnessy Report: High-Voltage PCB Design—Don’t Let the Smoke Out

“Don’t let the smoke out.”

That’s one of the PCB designer’s unspoken objectives, albeit a sarcastic one. The 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, often by segregating them from everything 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. These are high-voltage boards.

Much of the information about high-voltage 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. The only agreement seems to be that once you’re in the kilo-volt 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). This 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 applications, and some of the most popular high-voltage laminates can only be used for single-sided PCBs.

Criteria such as glass weave and resin content can make or break a board at high voltages. As the saying goes, with high-voltage 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 segment is clearly on a growth pattern. To explore this topic, we asked our contributors to share their thoughts on designing high-voltage PCBs.

Be sure to download your copy of the January 2022 issue of Design007 Magazine: "High Voltage PCB Deisgn".