Ground plane isolation question

[Jdo300]

Hello All,

I was talking to a friend about how to properly isolate a digital board from a high-powered output stage. The output stage in my application make square wave pulses that can be as high as 500V. So my first thought was to galvonically isolate the output stage circuitry from the digital board which contains a microcontroller to run it all.

Up to this point, I figured that a simple isolator (like the IL710) would work great. They are fast, and have 2500V of isolation between the input and output.

But my friend mentioned that there should be a 10 ohm resistor between the two ground planes of the digital and output stage board to keep a static voltage from building up and poping the isolator IC.

But I'm thinking that if you connect the ground planes at all, then it is kind of defeating the purpose of the isolation in the first place. But at the same time, I recognize the issue with the static voltage buildup. How do I deal with such a problem?

Thanks,
Jason O

 

[berkeman]

The value is just off. He probably meant more like 10 MegOhm, or 1MegOhm would work too. Bleeding off static charge doesn't take a low value resistor.

 

[Jdo300]

Ok, that makes sense. Is this what is commonly done in isolation situations? Will having the high value resistor still block fast transients from crossing into the digital side and blowing stuff up? I do have a lot of bypass caps and TVSs on the chips to prevent this from happening but maybe I'm just being paranoid...

 

[berkeman]

Depends on the isolation requirements (like, is it for UL approval of an AC Mains powered device, or isolation of analog and digital sections of a device for noise coupling issues, etc.).

A high-value bleed resistor is commonly used when you don't want static buildup between two things. Like between the shield and the twisted pair inside shielded twisted pair cable, for example.

Often, just using a star ground configuration, with the two grounds connected only at one point (where the power supply is, for example) is sufficient to prevent noise coupling between two parts of a circuit.

When you're following the UL rules for AC Mains isolation, there are spacing rules, and rules for the allowed resistance (big) and capacitance (very small) between primary and (SELV) secondary circuits.

 

[Jdo300]

Ahhh interesting Thanks for the info