Designing at Very High Voltages

Some people would say designing low voltage products pose minimal challenge to the makers. Only a few specifications and requirements need to be met. But when designing high voltage products, it is the other way around. Mouser Electronics are finding ways to address its clientele's need. 

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The Need for High Voltage

Given the challenges and risks, why do design engineers even consider using these voltages at all? It's either because the engineer has no choice, or because it's a really good and necessary idea. The applications fall into two broad groups:

In the realm of "the engineer has no choice," scientific, medical, and physics instrumentation need high voltages in specialized equipment such as X-ray machines, to develop high-intensity fields, ionize atoms, and accelerate electrons and other particles. The same applies to vacuum tubes that still need high-power broadcasting or even moderate-power microwave and mm-wave transmitters. In a more common application, even a commercial neon sign needs several kV to ionize the noble gases inside. Note that many of these applications require kilovolts and more, but at relatively modest currents of around 100mA.

In cases where using high voltages is a "really good and necessary idea," engineers are designing for power and efficiency. When a power supply or motor needs to produce large amounts of power, the source must deliver watts, which are the product of voltage and current. But at lower voltages the currents are obviously higher, so IR (current X resistance) losses in conductors, connectors, switches, and active devices cause inefficiency, losses, and I²R heating.

Start With Physical Dimensions

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:

1. Whether it is a potential shock hazard or only a functional-breakdown issue,
2. The region of the world: different zones have different standards,
3. The application: scientific, industrial, or medical, for example, or even a consumer product,
4. Maximum operating altitude and humidity (dry air at sea level has a flash-over rating of about 4kV /cm, or 10kV/inch),
5. Across PC boards and other surfaces: the degree of potential contamination that may be expected due to various kinds of pollution; the PCB material group; and the coating (if any).

Therefore, some serious research is needed to determine the required minimum creepage and clearance values, or engineers may need to call an experienced consultant, especially if the end product will need formal regulatory approval for manufacturing and sale.


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