Small SMD components: are they really capable of withstanding high voltages?

Question

You can easy find some SMD components which are pretty small for such a high voltage (500V @ 0805 package for example - crazy?!). 0805 case would have 1.3 mm distance between the contacts, so I doubt that this voltage would be normal under real world circumstances.

So my questions are:

1. Would you pick such component for your circuit? I'd better place two or even three components in series to make the voltage across 0805 case not more than 100-150 Volts or 150-200 for 1206. But this estimation based on my sensation (I never experienced any failures or never researched the limits)

2. Probably such components should be used inside sealed cases or under a layer of a good insulating varnish (which is rare for me)? This could explain their existence.

3. Do any of you guys have any experience with these components? What is the real-world V/mm limitation?

Answer 1

When deciding the voltage rating of a 'component on a board' system, you need to define what sort of rating you mean. Safety critical is much more stringent than simply working.

For instance, the withstand voltage on a clean dry board will be much higher than the rated personal safety voltage on a dirty board in a damp salt-laden atmosphere.

If the component is rated at 500v, then you can assume that the component itself, under conditions that the manufacturer specifies, will withstand 500v.

Would an 0805 footprint meet the creepage clearance regulations for separation between mains carrying and low voltage tracks, for personal safety? No. However, 'mains' is expected to have transients of at least 1500v on it, which is why capacitors for mains connection are specifically rated for such, see X and Y rated capacitors. For a regulated 500v, in a dry environment, when personal safety is not an issue, then there's no problem supporting 500v across an 0805 footprint.

If you are intending your PCB+components system to be used in a dirty damp environment, then you may well need a conformal coating to mitigate board surface contamination problems, even if you don't have to meet saftey rated creepage distances.