When to use slots instead of barriers in high voltage design?

Question

I have read a lot of information about the clearance and creepage strategies, distances, materials, etc..

I am designing my first HV PCB, main LV circuit will be on TOP and HV (6KV) on the bottom. But I can' t avoid some low voltage points on the same side. Here I will be using insulation barriers to increase creepage and clearance at the same time. But I would like to use slots in order to separate and give some air gap between "hot" points. But when you put slots, the only factor that increase will be creepage. Clearance stills being the same with or without the slot.

So, here are my questions:

1. When to use slots instead of barriers?
2. When can be the clearence shortest than creepage?
3. Is it the same to put a 3cm slot between two points and to simply put a distance of 3cm without slot between 2 points?
4. How can it helps to avoid breakdown if clearance is the same at two both cases?

Answer 1

One of the risks with attaching barriers to a pcb is that they aren't consistently attached ("cemented" is the UL term) to the board with variation in effectiveness both along their length, and from part to part in production, leading to the possibility that there's a small gap or wormhole under the barrier - so you then have to look at the creepage along that path. For very high voltage systems, it's common to move to a molded insulator with integral ribs, and the conductors are stampings that can be insert molded as part of the molding - or attached by screws or rivets. Obviously, adding slots to a pcb is more cost effective than (carefully) adding a barrier, since it's just part of the milling of the board that is undergoes regardless.

The standards, like UL840, are often insufficient when it comes to very high voltage design, as other factors become important. There is a risk of corona discharge that is especially bad if there are sharp points e.g. through hole soldered lead ends or angles in the tracks, and the presence of the high voltages tend to attract dust that will over time compromise the tracking resistance of the surface, so air gaps tend to be better than barriers for this. If the device is used in locations where air pressure is reduced Paschen's law applies, this is relevant in aerospace applicaitons.

The HV circuits I worked on (10-15kV) were always encapsulated to avoid the risk of reduction of contamination reducing the tracking resistance, once this is done you then only have to worry about the breakdown of the materials involved. Asphalts were used for the large assemblies, since this was more cost-effective than the siicones or epoxies used on smaller units.

TLDR - airgaps are practically better than barriers, but once you get to multi-kV applications, encapsulation is preferred.

Answer 2

When to use slots instead of barriers?

Easier to produce.

When can be the clearence shortest than creepage?

Clearance is always at most as long as creepage path. That's the simple geometric triangle inequality.

Is it the same to put a 3cm slot between two points and to simply put a distance of 3cm without slot between 2 points?

No, as your pictures clearly illustrate.

How can it helps to avoid breakdown if clearance is the same at two both cases?

Not quite sure what you mean.

Answer 3

Not an answer, still an important thing to show (and comments don't show images):

When the bottom layer has the high voltage, there is another creepage way, e.g. the one like shown in purple in image below.

Answer 4

Please ignore people giving you fixed numbers. I work with high voltage design and it does things you dont expect, so make sure you get a creepage and clearance calculator which includes your material group, pollution degree and altitude your working at (the higher you go the more distance you need). The calculator will tell you the clearance you need. There are three distances, functional (bare minimum), basic and reinforced. If it is safety related then you must work to reinforced, however if none is going to handle this thing in operation use basic clearance. I would link you one but my company has developed it's own internal one.

You typically use slots where you can however if you have things like pcbs stacked or younphypically cant meet the clearance, you have to use barriers and these effectively reduce your clearance (still check the creapage). The standards you are working with will tell you what voltage to apply. If the material withstands then it means you dont have to worry about creepage. Some standards say you have to get a single layer to pass then double it. Physical barriers are usually more expensive as someone has to manually put it in but I've seen cases where a 'box's has slotted over things to reduce clearance so in places like these it's not too much more expensive.

Answer 5

Ionic contamination and sharp edges are the only reasons that nominal atmospheric BreakDown Voltage (BDV) reduce from ;

(Estimates with 50% tolerance due to humidity)

• 3kV/mm=Nominal parallel smooth curved surfaces.
• 1kV/mm = sharp edge or pin points.
• 500V/mm= lightly contaminated dust surface.
• 300V/mm= humidity + light dust.
  Lower for worse contamination & > 90% RH

So you can see 3:1 reduction on edges 10:1 on light dust and worse than 30:1 surface to air gap ratios. These and higher ratios apply to walls and HVAC insulation ribbed bushings and is the reason you see many different designs to increase surface path length to air gap.

Forced air cooling is generally worse due to accumulation of dust.

If enclosed, the surface ought to be cleaned with deionized (denatured) alcohol or other solvent as required, is the best prep. NASA uses 99.% pure for space electronics.

You can compute your surface path design and air gap design to estimate the worst case path lengths of both and then test it for breakdown using a <100uA current limiting resistor for non destructive validation with at least 300% voltage margin in a benign environment.

There are standards for various ionic contaminated environments, if you are inclined to learn.