Slots cut into power supply PCB.

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I was looking at an old PC power supply, and I notice in a couple of  
places slots have been cut into the PCB. I think it's done as part of  
the safe isolation between the mains and the low voltage outputs.

Anyone know the rationale for the slots?

Sylvia.

Re: Slots cut into power supply PCB.
On 12/12/2013 10:53 AM, Sylvia Else wrote:
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Never mind, although my initial searches producing nothing, further  
effort yielded this TI document.

http://www.ti.com/lit/ml/slup227/slup227.pdf

Sylvia.

Re: Slots cut into power supply PCB.
On Thu, 12 Dec 2013 10:53:21 +1100, Sylvia Else

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Creepage distance. A milled slot offers greater creepage distance vs
FR4.

Cheers

Re: Slots cut into power supply PCB.
On Thu, 12 Dec 2013 10:53:21 +1100, Sylvia Else

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  It does what you stated.  A surface can propagate carbon trails.  An
air gap cannot.  Examine the definition for the term "creepage".  The
electrical definition.

  Usually seen on HV supplies, but smart folks making power supplies
place them anywhere the voltage is getting up there.

Re: Slots cut into power supply PCB.
Hi Sylvia,

On 12/11/2013 4:53 PM, Sylvia Else wrote:
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Termite damage! ;)

Re: Slots cut into power supply PCB.
On Thu, 12 Dec 2013 10:53:21 +1100, Sylvia Else

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I can recall building a power supply to, IIRC, VDE specs... 3/4" long
opto-coupler straddling a slot in the PCB.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Slots cut into power supply PCB.
On Thu, 12 Dec 2013 12:42:11 -0700, Jim Thompson

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I remember those optocouplers. Didn't I feel like a bright spark
including them in a new layout......and didn't their hollow tubular
component turned out to be made of highly flammable plastic ....
...well, not self-extinguishing, anyways.

I'm sure that's all been fixed by now.

RL

Re: Slots cut into power supply PCB.
legg wrote:
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   Yes, those slots are exactly to increase isolation and safety.
   They seem to be more common in CFL circuits, especially at/near the  
xfmr outputs..


Re: Slots cut into power supply PCB.
On 12/12/2013 10:53 AM, Sylvia Else wrote:
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On a related note, and referring to the document whose link I posted  
previously,

http://www.ti.com/lit/ml/slup227/slup227.pdf

It seems odd to me that the functional (not safety) clearance  
requirements at mains voltages (see table 2, and note that I'm in  
Australia with 240V AC mains) are higher than the distance between the  
pins for a typical mains voltage triac. For example,

http://www.farnell.com/datasheets/1726515.pdf

Anyone have any thoughts on that?

Sylvia.

Re: Slots cut into power supply PCB.
On Sat, 14 Dec 2013 12:25:14 +1100, the renowned Sylvia Else

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I've seen the results when a numb-n*ts soldered a TO-220 triac with
acid-core solder (not pretty, but quite fragrant).  

I guess if you have to conform to that, you'll have to use something
like a TO-218 with > 4mm creepage.  

TO-220 will exceed the 0.6mm spacing for functional isolation < 150VAC
RMS and the "F" class.  

They could make triacs with an overmold on the middle lead like they
used to do on horizontal output transistors, but I can't recall having
seen such a thing.  


Best regards,  
Spehro Pefhany
--  
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: Slots cut into power supply PCB.
On Sat, 14 Dec 2013 12:25:14 +1100, Sylvia Else

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The creepage distance in a basic safety isolation barrier and a
functional creepage distance are handled in pretty much the same way.
If insufficient, the distance is shorted to see if an unsafe condition
is created.  

If it does not exhibit excessive operating leakage current, still
passes safety hipot, has no open traces resulting and no flame hazard
is created etc, then it's OK. This repeated for all single fault
abnormals.

Obviously, any short of an isolation barrier runs the risk of creating
excess leakage current or a hipot failure, even without the addition
of tracking due to destroyed internal components that may result. So
creepage shorctcomings in these areas are not acceptible.

Optocouplers can be used to couple across supplementary isolation -  
effectively 2xbasic isolation, only if internal and external physical
dimensions and materials are controlled sufficiently to do so, by
demonstration to the certifying agency.

Recent consolidation of safety standards has removed any advantage
that clearance distances had over creapage distances, in safety
isolation measurements, so slots are a waste of effort from a
regulatory point of view. Practically, however their function remains
unaltered.

RL  

Re: Slots cut into power supply PCB.
On Sun, 15 Dec 2013 12:06:03 -0500, the renowned legg

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Shorting seems an unduly optimistic test- a short will result in no
power dissipation at the short. Arcing and tracking could result in
significant heat, flames, toxic gases. But if that's how they do it..
I guess it would be best to make real sure about the flammability
ratings of everything in the vicinity.  


Best regards,  
Spehro Pefhany
--  
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
We've slightly trimmed the long signature. Click to see the full one.
Re: Slots cut into power supply PCB.
On 16/12/2013 4:06 AM, legg wrote:
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Thanks for the info.

As to my question about the triac pin separation, clearly, since any  
triac may fail shorted anyway, the circuitry has to handle that  
possibility, and will thus also handle a short across the pins.

Is there now a single place from which a definitive statement of the  
safety requirements can be obtained?

Sylvia.


Re: Slots cut into power supply PCB.
On Sun, 15 Dec 2013 17:05:05 -0700, Sylvia Else  

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join the IEEE discussion group [don't have to be a member to join the  
forum] for Product Safety and EMC
Website: http://www.ieee-pses.org/
Instructions: http://listserv.ieee.org/request/user-guide.html
List rules: http://www.ieee-pses.org/listrules.html
If you have any problems/questions email me directly and I'll provide  
email addresses for the group's moderators.

Post your question there and you'll get definitive, knowledgeable answers  
specific to any country you choose within hours usually. Including the  
EXACT wordings of requirements and penalties involved for non-compliance.  
 From memory, if your equipment has not been approved/certified and: if  
your equipment destroys a building in US; you're liable for the equipment  
cost. If your equipment destroys a building in Canada, you're liable for  
the equipment cost and the building cost and the cost of putting out the  
fire, or something along those lines. Not actually that different, but I  
was surprised to how much more Canadians hold people culpable compared to  
the US.

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