Required Creepage Distances for 12.5kV AC-DC Bridge Rectifier With Series Avalanche Diodes

Hello everyone,So I have been given a project at work here to try and design a 12.5Kv AC-DC rectifier for a new customer. This is somewhat out of our typical range as we normally only work with low voltage assemblies rather than medium voltage, but it's not entirely beyond our capacity to design.What I'm hoping someone can help me with is the required creepage distances needed. Attached is a screenshot taken of a preliminary design I have in my head. Each diode is of an avalanche type with a PIV of 5Kv, anode to stud. With 6 in series between each AC and DC point this should give us a total rating of 30Kv which falls in line with our standard practice of doubling the required PIV of the rectifier, minimum.Now I understand the need for the large creepage distance between each 'limb' of the rectifier as this is where we will see the largest difference in voltage potential, but am I right to assume that because the voltage potential between each heatsink in a single limb is ~0.7v, can I design this assembly to have a creepage and clearance between each heatsink to suit that 0.7v? Or will I need to design the full creepage and clearance between ALL heatsinks to suit 12.5Kv?Normally I wouldn't even question my logic here, but another of our design engineers isn't so sure and has planted a seed of doubt in my head.For reference purposes, the material these heatsinks are being mounted to is FR4, which according to our supplier has a CTI of 300, placing it in insulating material group IIIa.With regards to the attached image, forgive my MS Paint skills but I've tried to show where the diodes will connect to each heatsink and relevant connection points for the AC and DC.If you need more information to help me out, let me know and I'll try to help.

Hi Andrew-The drawing is very helpful. Before we get to the creepage distance part- how much current do you need?  Will the load have a capacitive component? How will you protect the rectifiers in case of a failure?The way I see it the creepage distance between heatsinks in a row should be calculated for the maximum PIV of each rectifier.  Just because they are avalanche type there's no guarantee that they will all divide equally.The creepage distance between rows should be wide enough to accommodate 17.6KV (12.5KV X 1.414), plus a generous fudge factor. At this voltage I presume you're coming directly off the utility company's distribution lines.  What happens when lightning strikes? (and it will, sooner or later).-Rick

Andrew,The diodes will only have a voltage between them of 0.7V during one half of the cycle when they are conducting. On the other half of the cycle the string will see the full AC voltage. As rick said, you should size the creepage between diodes for the maximum PIV of the diodes.

Hi Rick,Thanks for clearing that up for me. Creepage between the rows has already been sorted out, it was just the distance between each diode in the row that was the area of confusion.As for your questions, it's only got 50A on the DC side. Protection of the assembly will all be handled by the customer. We've only been tasked with designing this single component.-Andrew

Hi Elizabeth,Thanks for your reply. I admit I'm a little ashamed I didn't even consider the situation of the reverse cycle on the diodes. It's the first time I've worked with these voltages, so it didn't even cross my mind. Normally I only work with low voltage single diode/thyristors at a time.
-Andrew

The reverse-avalanche voltage of each rectifier will be higher than the PIV rating of that rectifier, to ensure the rectifier will not break down at the PIV.

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