creepage, clearance and voltage rating.

[KX36]

Hi all,

I have a question about creepage and clearance ratings and how they compare to the volage ratings of components.

As an example, I'm looking at high voltage chip resistors. https://www.welwyn-tt.com/pdf/datasheet/HVC.pdf

The 2512 package has a limiting element voltage of 3000V, but I can't see how you could get sufficient creepage or clearance between the exposed metal on each side of the component. It does talk of derating this LEV for creepage distance. Would you have to pot around the component with epoxy resin to get sufficient creepage/clearance to use at the specified LEV? Would the potting compond get under the SMD component or would you have to add a slot in the board to help the compound surround it?

This leads to another question. Often you get isolation slots cut in a PCB between pads to increase creepage distance, but the leads of a component rated for a high voltage often come together on the package close together anyway, so surely the closest distance for creepage will be across the insulator of the package and not across the PCB? In which case does it help to cut these slots? I'm thinking of 1000V rated TO-220 MOSFETs in this example.

Cheers,
Matt

 

[FvM]

If pad creepage and clearance of resistors become a problem, most people will use multiple series connected resistors instead of sealing or potting circuit parts. Inhomogene encapsulation can even promote partial discharge and endanger the insulation strength.

Pad to pad clearance is often lower than pin distance for TO220 parts, in so far slots may serve a purpose.

 

[KX36]

Thanks for the reply.

I know you can use multiple resistors in series to increase the voltage across them, I was just curious about any parts with a voltage rating which seems to be difficult to practically achieve given the package, and if one were to need to get the full voltage specification out of the device, how one would go about doing so.

 

[D.A.(Tony)Stewart]

B mm Overvoltage (2s)
1.9 1500 790 V/mm
3.9 3000 770 V/mm
5.0 4000 800 V/mm

Air can breakdown with sharp edges around 1kV/mm but may withstand 3kV/mm with smooth conductors and contaminate gap with sharp points below 500V/mm

Some Ceramic or epoxy coated packages "can" be rated perhaps 5x higher, if clean.

I hope this puts things into perspective.

These parts are not intended to put on 120V/240 AC lines which might carry 6kV power line transients.

 

[chuckey]

I was going to add, some devices like DIL opto-isolators, have their pins configured, input on one side and output on the other side of the body, so the PCB can be slotted with a width right up to the width of the IC body (.2 ins?)
Frank