Designing at very high voltages: Everything changes, especially your way of thinking

By Mouser Electronics, www.mouser.com

For engineers who spend their design time in the single-digit, low-voltage world, the phrase “high voltage” may conjure up voltages in the double digits, perhaps as high as 24 or 48 Vdc, or even 120/240 Vac. Yet there's a huge and essential world of engineering design that must be done at 1,000 V, 1,500 V, and even higher voltages.

Designing products for this region calls for very different thinking and component selection. These concerns apply to passives, connectors, cables, MOSFETs/IGBTs, layout, and of course, safety and regulatory issues. It's a difficult, unforgiving world when your voltage potentials are that high. Trivial oversights can suddenly become major equipment- and life-threatening events. 

Dealing with high voltage begins with conductor spacing and associated dimensions. The critical terms for spacing conductors at higher voltages are “creepage” and “clearance.”

• Creepage is the distance an arc may travel measured over a surface, such as between two traces on a printed-wiring board or across the surface of a connector or IC.
• Clearance is the shortest distance an arc may travel through air, such as from the pin-to-pin of a connector or IC.

The creepage and clearance requirements are a function of the peak voltage; for a sine-wave ac signal, the peak value is 1.4 times the RMS value, plus a substantial safety factor. While it would be nice to be able to call out specific creepage and clearance dimension requirements at any given voltage, it is not possible to do so because their dimensions depend on many factors. Serious research may be needed to determine the required minimum creepage and clearance values especially if the end product will need formal regulatory approval for manufacturing and sale.

Designers who work at lower voltages rarely need to look at the voltage ratings of their basic passive components; yet each of these does have a maximum working voltage rating specification. Above this voltage, the component may not work to specification, may “gracefully” degrade, may fail prematurely, or may suffer catastrophic failure. Most designers prefer to work with a safety factor of two to three times their expected maximum voltage, so a 1-kVdc circuit would need passives rated for 2 to 3 kV.

Although they are often not considered along with “passive” components, connectors and cables are also a critical link in the high-voltage chain. As with layout and wiring, creepage and clearance are primary factors. Nearly all high-voltage connectors target specific industries, rather than addressing general purpose high-voltage applications.

High-voltage designs require more than just routing current at high potential. The design also involves controlling and switching current at high voltages. IGBTs and MOSFETs are the most common devices used here, including those made from newer wide-bandgap materials such as silicon carbide (SiC) or gallium nitride (GaN). Regardless of which discrete power device is chosen, packaging is determined by three related factors: voltage, with issues of creepage and clearance, again; current, with larger lead dimensions to reduce IR (current x resistance) drop; and power dissipation, including low-thermal impedance from die to case to maximize internally generated heat, whether due to on-resistance R_DS(on) in MOSFETs or diode drops in IGBTs, out of the die and package.

Despite the many challenges of design directly with — or even just around — these high voltages, they are an unavoidable, essential aspect of many products. That's why it is important for engineers to be familiar with basic high-voltage-related issues, as well as safety and regulatory concerns, to develop a proper respect for what high voltages can do and why they are needed.  Learn more about high-voltage design techniques and available products at www.mouser.com/application/high-voltage-design.