0805 cap across rectified mains?

[Faringdon]

Just keep asking until you find someone who tells you it's ok.

Thanks, what would be  great , would be someone to proove the statement wrong....that an 0805 1kv cap, across two 450V rated 'lytics is bad....

Pollution degree 1, and FR4 with solder resist, its going to take >10kV to flash over between 0805  pads 1mm apart.

[Gyro]

Why don't you put your neck on the line? If you're so confident, then why even ask here? A professional design engineer doesn't need the approval or otherwise of the members of an electronics forum - I never did when I was at the workface.

Take an proper engineering decision and be prepared to defend it - in court if necessary as it is safety related. The buck stops with you and only you!

[tggzzz]

Why don't you put your neck on the line? If you're so confident, then why even ask here? A professional design engineer doesn't need the approval or otherwise of the members of an electronics forum - I never did when I was at the workface.

Take an proper engineering decision and be prepared to defend it - in court if necessary as it is safety related. The buck stops with you and only you!

Exactly.

Group responsibility and democratic decisions are not relevant in such cases.

Understanding and personal responsibility are what matters.

But then quite a few people wonder whether the OP's questions are realistic or aspirational.

[ebastler]

Group responsibility and democratic decisions are not relevant in such cases.
Understanding and personal responsibility are what matters.

Well, but how about "Some anonymous guy on the internet told me"? Does that count? 

[temperance]

I looked at what's available in 0805 1KV. The largest one I could find is 4.7nF. The remaining capacitance at 320V is probably less than 60% of that and more like 70%.

Placing a 4n7 - 70% in front of the switcher you mentioned isn't doing anything except creating high Q circuits which resonate with every bit of stray inductance around.

There are two ways to solve an EMC problem: the wrong way is trying to stop things from radiating caused by ringing by adding more low impedance energy storing devices. In short, this question can be classified as nonsense.


Vive la resistance. But don't overdo it.

[tggzzz]

Group responsibility and democratic decisions are not relevant in such cases.
Understanding and personal responsibility are what matters.

Well, but how about "Some anonymous guy on the internet told me"? Does that count? 

Yes, it does count.

Not much for most of us, but apparently it satisfies treez/faringdon.

[floobydust]

The mind will believe whatever it wants, and the Internet always has enough noise to find supporting baloney.
He's bungling science and engineering. Paschen's law gives the voltage values but one skilled in the art would know the safety standard's generous values for creepage and clearance are based on _____ . 

[tggzzz]

... but one skilled in the art ...

The OP has, apparently, convinced some people that he is skilled in the art.

[Faringdon]

Placing a 4n7 - 70% in front of the switcher you mentioned isn't doing anything except creating high Q circuits which resonate with every bit of stray inductance around.

Thanks, thats an intereting statement.
So placing some small ceramic cap "in tight" to the power switching loop. so as to reduce the area of the power switching current loop...is a bad thing?
Its very common to do it...often you see it in the form of an SMD ceramic in parallel to the leads of a radial electrolytic cap......again, its there to reduce the current loop area.
But  yes , i agree, there is going to be resonance with the stray L.

So , this would raise interesting questions...
1....in 4n7 is too low a value, what value would be OK?
2....Is the concept of putting a small capacitor "In tight" to the power switching current loop of an SMPS, always a lame duck?.....obviously, being a small package, means it can advantageously be squeezed in nice and tight to the power switching current loop....thereby reducing the loop area.

[tggzzz]

Placing a 4n7 - 70% in front of the switcher you mentioned isn't doing anything except creating high Q circuits which resonate with every bit of stray inductance around.

Thanks, thats an intereting statement.
So placing some small ceramic cap "in tight" to the power switching loop. so as to reduce the area of the power switching current loop...is a bad thing?
Its very common to do it...often you see it in the form of an SMD ceramic in parallel to the leads of a radial electrolytic cap......again, its there to reduce the current loop area.
But  yes , i agree, there is going to be resonance with the stray L.

So , this would raise interesting questions...
1....in 4n7 is too low a value, what value would be OK?
2....Is the concept of putting a small capacitor "In tight" to the power switching current loop of an SMPS, always a lame duck?.....obviously, being a small package, means it can advantageously be squeezed in nice and tight to the power switching current loop....thereby reducing the loop area.

You can choose the capacitor based on what you want the resonant frequency to be, as per the standard equation.

And as per the standard equation you can vary the amount of resonance by changing the resistance.

Where did you say you got your degree?

[Faringdon]

Thanks, as you know, RC circuits just upstream of power switch loops are not common.
Neither is changing "inner loop" capacitor values common when trying to pass radiated EMC.

Is there a suggestion that removing a small ceramic from just next to the power switch loop, would help pass radiated EMC?
The loop in which this LC ringing would take place is likely to include the bus electrolytics and their ESR is likely to give the   necessary damping to the ringing.
This is worth another new post, a lot of people will be interested in this.

[tggzzz]

Thanks, as you know, RC circuits just upstream of power switch loops are not common.
Neither is changing "inner loop" capacitor values common when trying to pass radiated EMC.

Is there a suggestion that removing a small ceramic from just next to the power switch loop, would help pass radiated EMC?
The loop in which this LC ringing would take place is likely to include the bus electrolytics and their ESR is likely to give the   necessary damping to the ringing.
This is worth another new post, a lot of people will be interested in this.

"... is likely to ..." is amateurish drivel.

Look at the datasheet to find the ESR.
Measure the ESR capacitance (including as a function of voltage and frequency).
Then calculate using the bog-standard equations that I came across when I was in school (i.e. pre-university).
Then simulate.
Then measure the system to see how it compares with your understanding.
Finally refine your understanding.

That's how engineering is done. Anything less is cargo-cult random meanderings.

You do have a degree in a relevant subject, don't you?

[Faringdon]

Thanks, though ayk,  in this case, it is believed, that no calculation is necessary...a quick  inspection of the situation tells us that no radiated EMC scan, or conducted EMC scan, for an SMPS, could be improved, by removing a low value ceramic capacitor that is placed up tight to the power switching loop.