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Thomas F

Tue Mar 26 2019, 11:42AM

Registered Member #503 Joined: Sun Nov 19 2006, 06:37PM
Location:
Posts: 59

Hello HV Gurus,

I need some advice on PCB design for a DC HV Power supply PCB I am making. I have already made a 5kV one which works pretty well. The 10kV is an extension of the 5kV with a doubler circuit. It works fine till about 8kV and then arcs along the PCB (creepage). I have made cutouts to increase the creepage,but it arcs around this.
I dont want to go the potting route. There are dimension for HV break down in air(clearance) , but I could not get any good information for creepage.Most of the creepage calculators max out at 1kV.
Can anyone give a link or empirical advice for dimensioning PCBs for these voltages ?

1) Does removing the copper islands between tracks help for creepage ? They are masked and no exposed copper traces.
2) Does the solder mask help or exacerbate creepage problems as compared to a bare PCB ?

TIA
Thomas

DerAlbi

Tue Mar 26 2019, 01:52PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

1) Yes.
2) Yes.

Also, use a 4 layer PCB. Use internal layers. They cant suffer from pollution that increases creepage currents while aging. However solder joints will always be a weak point.

FR4 is also not the best material. google "Very high voltage pcb material" lead to

there are huuge differences among the substrates.

Maybe post a photo...

Thomas F

Tue Mar 26 2019, 05:59PM

Registered Member #503 Joined: Sun Nov 19 2006, 06:37PM
Location:
Posts: 59

DerAlbi,
Thanks for replying.
1) So I will remove all copper islands and leave only traces.
2)Actually my question was will a solder mask,help or make worse the creepage problem.So are you telling me to remove the masking and leave the PCB naked with FR4 material ?

As far as PCB materials go I do'nt have a choice,I am forced to use FR4 because that is the only material my PCB guy has.
I will post a photo ,tomorrow when the PCB is with me.
Thanks again!
Thomas

PS: Any guidance on creepage distances ? (mm/kV types)

johnf

Tue Mar 26 2019, 06:02PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

If you look at high voltage decks like glassman etc you see huge amounts of solder used to make each copper area near a ball in shape this stops corona production which is an ever rising problem with voltages over 15kV however a sharp edge somewhere will produce corona at lower voltages. Once you have corona your standoff voltages are reduced dramatically as the ionised air in the corona is conductive FR4 is fine to 80kV with care Glassman uses it to these voltages

Thomas F

Tue Mar 26 2019, 06:11PM

Registered Member #503 Joined: Sun Nov 19 2006, 06:37PM
Location:
Posts: 59

Thanks JohnF,
Actually the arcing I am facing happens on both sides the component side as well as the solder side. I will put some ceramic stand-offs on the HV components and "lift" them from the PCB surface to see if it helps! And of course your suggestion of using excessive solder to form globes.

DerAlbi

Tue Mar 26 2019, 06:41PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Sorry, i misunderstood 2). Of course solder mask will help. Leaving it naked is not good. My whole idea was to be even better than solder mask by using a 4 layer PCB for traces.

Maybe you could use even 70Âµm copper - a thicker copper layer should have smoother edges, but i honestly dont know.

Sulaiman

Tue Mar 26 2019, 07:28PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If for professional use then epoxy conformal coating or potting or silicone potting is reliable.

For hobby use - keep eht away from PCBs.

The use of a ceramic standoff/pillar is good,
but provided that the components do not generate too much heat, nylon pillars are adequate.

Patrick

Wed Mar 27 2019, 04:59AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

well the real answer is relieved areas as seen here :

our pictures arnt working but ill see on the way back machine. --- nope.

You can also solder ball bearings or copper rings to otherwise sharp edges to reduce field strain. FEMM 4.2 may help with geometry and dimensions.

EDIT go here:

And here, i made porcelain insulators.

Thomas F

Wed Mar 27 2019, 06:55AM

Registered Member #503 Joined: Sun Nov 19 2006, 06:37PM
Location:
Posts: 59

Thanks for answering,Sulaiman and Patrick!
I tried using this conformal coating :https://www.duzzit.in/aerosols.php/coatings
the Duzzit 2000 thing.It had a slight improvement,but nothing significant as compared to no conformal coating.The ceramic standoffs are very cheap,nylon is rather hygroscopic,I dont know if that would affect performance.
Nice website,Patrick. Is is possible to solder ball bearings to a PCB ? Do you drill a hole for the component leads into the bearing ? I was planning to use excessive solder as suggested by JohnF.

Thomas F

Wed Mar 27 2019, 11:57AM

Registered Member #503 Joined: Sun Nov 19 2006, 06:37PM
Location:
Posts: 59

Hi,
Was just going through this,maybe it might be interesting :

Apparently removing solder mask seems to be very beneficial to improving creepage.
Thomas

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