High voltage diode string - Page 1
Forum section: electronics / projects

author: JohnnyP
author_rank: Regular Contributor
timestamp: July 09, 2022, 10:42:23 am
content: Say I need a 60kV high voltage diode, so I put two 30kV diodes in series, then encapsulate the assembly using epoxy rated for 450v per mil.Assume the encapsulated assembly is sitting on a ground plane, the cathode end is grounded, and the anode is getting hit with minus 60kV pulses from an ignition coil.In theory, it should survive with a minimum thickness of 0.133 epoxy (OD of 266 mils, neglecting the diameter of the leads).What is the minimum thickness of epoxy required at the junction of the two diodes?The question is, does every point along the string need to be covered with 133 mils of epoxy? Or does the requirement lessen as you go toward the low voltage end?With the cathode grounded, the junction should never exceed 30kV, so why would it be necessary to protect against 60kV at that point?

author: Gyro
author_rank: Super Contributor
timestamp: July 09, 2022, 10:56:00 am
content: Be careful that you don't add too much capacitance to the junction (ie. across the bottom diode), especially with a sharp edge from something like an ignition coil. You need the diodes to share the reverse voltage nicely.

author: jonpaul
author_rank: Super Contributor
timestamp: July 09, 2022, 11:36:57 am
content: Bonjour1/ AC and DC balance of reverse voltage across multiple diodes...use balancing caps and resistance2/ use fast recovery diodes3/ easier to just get the correct diode Vrev, 60...100 kV available from various manufacturers4/ potting HV assembly required 100% solids epoxy with vacuum oven cure and annealing.Any bubble or void will ionize and eventually fail.Bon chanceJon

author: JohnnyP
author_rank: Regular Contributor
timestamp: July 09, 2022, 11:45:19 am
content: There is no room for higher rated diodes. I am folding the string as it is.The circuit works fast enough.I'm using Stycast 2651 which is like tar when mixed with Cat-9. It thins when heated but then the pot life is about 5 minutes. Not enough time to degas in a vacuum chamber. I have a pump and chamber and I've tried it.Why would you need to protect against 60kV very near the grounded end?With 30kV diodes, the junction should never exceed 30kV. So insulate for 30kV at that point, plus some safety factor. Say 40kV.It would seem the required thickness of the epoxy would grow as you reach the hot end of the string.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 09, 2022, 12:54:59 pm
content: Are you orienting in air it with the anode vertical on top of the cathode which is at ground plane?If so the air clearance to ground plane will be higher on top so the voltage per junction vertically is even.In that case, the anode end is partially air insulated.But if it is with both cathode and anode horizontal to the ground plane and encapsulation in contact all along, then there will be a voltage gradient through the epoxy alone ,with higher volt/mm on anode end.

author: JohnnyP
author_rank: Regular Contributor
timestamp: July 09, 2022, 03:40:46 pm
content: Assume the encapsulated assembly is in full contact with the ground plane.I believe you are correct there is a voltage gradient along the axis of the string. In that case, the epoxy could get thinner along the length. Not that I am trying to skimp on epoxy, but it’s a layout issue, as the string is folded and the junction is near other components. How near is the question.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 09, 2022, 09:29:43 pm
content: Adding polymer coatings or barrier sheets etc needs care that it does not have an effect opposite to the desired increase in insulation breakdown voltage.This is due to the relative permittivity of organic polymer, also silicon rubber and ceramics.It is easier to do a hand sketch with the approximate math to describe the effect.https://app.box.com/s/p18hh5cudb9nrzy6wicgn34lue6v4vysThis shows, for example if the bare metal conductor had 10 kV, and the spacing to ground of 100mm,the "Eta" is 10/(0.1) = 100kV/metre shown as case AWhen the bare metal conductor is encased in 50mm of epoxy, had 10 kV, and the spacing to ground is reduced to 50mm (from epoxy surface)the "Eta" is 10*0.75/(0.5*0.1) = 150kV/metre shown as case B

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 10, 2022, 12:18:31 am
content: The problem is not as easy as setting up a distance.- the “rating” of your epoxy is probably for DC and does not apply to AC and of course not for pulses- for mixed “barriers” with various materials as magtherm says you have to consider how the electric field distributes and how much the field strength is in each one.- there are low level discharges called partial discharges , that are important in the long term as they will destroy the insulation, this is where the voids are important. Since you can’t control them over dimensioning is a solution- yes, the requirements for insulation lowers as you approach the cathode- I would not trust just one insulating material. Covering the diodes with a sheath of silicone tubing or kapton besides epoxy should do it.- the distances you are talking about(0.2..0.3 inches ) seem low. I would add a resistor (or series of resistors) in series to limit the current thru the diodes

author: JohnnyP
author_rank: Regular Contributor
timestamp: July 10, 2022, 02:53:21 am
content: Thanks everyone, for all your replies. I appreciate it very much.

author: JohnnyP
author_rank: Regular Contributor
timestamp: July 10, 2022, 03:15:31 am
content: mag_therm, thanks for the drawing but I’m confused. The drawing shows a sleeve adds resistance but your opening statement sounds like it could detract.The diodes are 0.12” OD. Would adding 1/8” shrink tubing be a plus?

author: JohnnyP
author_rank: Regular Contributor
timestamp: July 10, 2022, 03:18:06 am
content: More V/mm is not a good thing I take it? More R/mm is better.

author: CaptDon
author_rank: Super Contributor
timestamp: July 11, 2022, 01:30:04 pm
content: Two diodes in series rarely share PRV equally do to unequal leakage resistance, stray capacitance and turn-off time. That is why equalizing resistors and capacitors are often used in high reliability applications.

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 11, 2022, 02:12:08 pm
content: Quote from: JohnnyP on July 10, 2022, 03:15:31 amHi Johnny,The sketch I put up was an over simplification to show how adding polymer to an airgap can actually increase the V/mm stress in the air.I think if you can put up sketches of your intended layout along with voltages, frequencies at various points and distances,you might get more suggestions, particularly from member PartialDischarge.Regards

author: JohnnyP
author_rank: Regular Contributor
timestamp: July 11, 2022, 03:02:07 pm
content: Ignition voltage is typically less than 30kV.60kV might happen if a spark plug wire popped off and the arc had no where to go.