Creepage - Does a surface mount isolator body negate a PCB slot?

Nick_EMC

Starting to get Involved
#1
Hi,
I am trying to replace a bulky potted isolator with a surface mount digital isolator. For a standard FR4 PCB we have been working to a creepage of 16mm.
There is a high creepage isolator from TI with a 14.5mm creepage distance and a body CTI of >600.
This part should be very suitable for the stress so I am hoping to use it with a PCB slot to achieve the 16mm. However the body of the isolator may sit just 0.1mm above the PCB, so it effectively bridges the PCB slot (<1mm distance).
I was wondering if there was a way around this? A wide slot would be bridged by the high CTI isolator body, maybe this can be taken into account?
Hoping someone has some experience with this.
Nick
 
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#2
If your starting point is 14.5mm -
a 2mm wide slot would count as would the 1.6mm (x2) thickness of the PCB.

That would give a total of 19.7mm.
 

Nick_EMC

Starting to get Involved
#3
Hi,
What I mean is the creapage distance is along the top of the PCB, as is is surface mount, then the gap to the body of the isolator can be as low as 0.1mm, so this will be ignored. So the component body will bridge the PCB gap. There is no easy way to stand a surface mount part away from the PCB to eliminate this.
I was hoping that the component body, being of a higher tracking index could be taken into account in determining the requirement.
Thanks
Nick
 
#4
Hi Nick,
OK - I see what you are saying.
One 'trick' I have seen using high speed digital isolators is to use 2, one after the other. Each provides 1 MOPP, so the Cr and Cl requirements are halved.
 

Nick_EMC

Starting to get Involved
#5
Unfortunately for this you need a power supply in the centre.
It may be that the creepage can be looked at again and does not need to be quite so large.
 

Peter Selvey

Leader
Super Moderator
#6
16mm seems very large, what is the Vp, Vrms used as the working voltage? Also it 2MOOP or 2MOPP?

You might be able to justify a lower limit by referencing IEC 60664-1 which is the "bible" for creepage/clearance. Sometimes the values in IEC 60601-1 start to get away from the fundamentals especially for unusual voltages.
 

Nick_EMC

Starting to get Involved
#7
Hi Peter,
Thanks for your interest. I agree with you about 16mm being very large - it seems to be a historical thing and very conservative.
It is actually a 1 MOPP barrier between an intermediate and a patient (applied part). There is another MOPP in the power supply. The intermediate is approx. 180V DC and the applied part 340V RMS. Looking at previous test house reports this is assessed at more like 6mm.
I have just decided that 8mm is a more realistic figure for this barrier, and it gives a much easier design and saves a lot of space.

I think the high figure came from taking the mains input to patient output voltages and adding these together, disregarding the fact that the output was floating, and applying 2 x MOPP to this figure, and dividing the resultant creepage between the barriers, giving most of the creepage to the intermediate/patient one.
To me it seems more sensible to consider the barriers separately.
Best regards
 
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