# Minimum space between tracks

#### Marsino

Joined May 17, 2019
2
Hello,
I am designing a planar transformer for a dc dc converter. But I have a doubt about the trace spacing for the primary side. I have a Vin max of 150V, 14 turns and the insulator material is FR4.
What is the minimum space between two tracks (the “s” parameter in the image below), in order to prevent arches or breakdowns?

#### oz93666

Joined Sep 7, 2010
730
The voltage difference between two adjacent tracks will not be the whole 150V , lets say 30V at most ..

Air breaks down at 3kv/mm so a hundredth of a mm should be breakdown point , make gap a tenth mm to be safe ....

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#### Marsino

Joined May 17, 2019
2
The voltage difference between two adjacent tracks will not be the whole 150V , lets say 30V at most ..

Air breaks down at 3kv/mm so a hundredth of a mm should be breakdown point , make gap a tenth mm to be safe ....
Thank you for your information, but there is a précisé formula which can i use to determine the space trace to trace?

#### SLK001

Joined Nov 29, 2011
1,543
Thank you for your information, but there is a précisé formula which can i use to determine the space trace to trace?
Well, the PCB house capabilities are what reign supreme here. It doesn't matter what value you come up with if the PCB house can't fabricate it.

I can't imagine that your efficiency with this type of transformer will be very high. Just how much power do you want to transform? There is a reason why most transformers use iron in their cores.

Since E fields will concentrate at sharp points, I recommend that you round your corners. The optimum coil form would be a true circular spiral.

The dielectric strength of air is ~26kV per inch. Put humidity and dirt in the equation and it starts dropping rapidly. If you are putting in 150V, you need to design for 300V minimum. However, since you didn't put in your schematic or layout in your post, that's about all we can say about your question.

#### MrChips

Joined Oct 2, 2009
19,931
Electrical field strength is inversely related to the radius of curvature. In other words, electrical breakdown will occur at sharp corners.

Remove those 90-degree bends.

#### AnalogKid

Joined Aug 1, 2013
8,259
Assuming the voltage gradient is constant throughout the inductor, then the peak voltage across it divided by the overall length gives you the gradient. With this, you can calculate the voltage between any two points in the winding based on the trace distance between them. Now that you have the voltage difference at any adjacent points, refer to UL1950 (60950) rules for creepage and clearance distances. The reason for those rules does not apply to your application, but they are well documented as guidelines for conductor spacing.

A real switching power magnetics guy probably has a better method, but this is should be a good starting point. Also, Signal Transformer has app notes on planar transformer design that might get into this.

ak