Offline SMPS in earthed metal enclosure but cant use Y caps.

[cupoftea]

What do you do when you have an
Offline SMPS in earthed metal enclosure but cant use Y caps.?
You cant use Y caps because of their AC current , which will trip the RCDs if too many connected Y caps overall.

Is it maybe you just connect the neutral Y cap, (obviously theres no current in that one) and hope that the current through the X2 caps gives it some connection to Line...so you get an overall Y cap effect?

Can you compensate by just using an enormous Common mode choke and hoping the "stray" y caps will act with that to combat the dreaded common mode noise?
We all know the dreaded common mode noise is the cause of all noise_ills in electronics.

 

[cupoftea]

Obviously theres shield windings in the transformer...but you need the room in the bobbin.
Any more "reduce y caps" tips?

 

[cupoftea]

Hi, On a related note.....
The attached shows an electronic switch unit.
It comprises three units..

1..A plastic manual control box
2..An earthed metal enclosed "Mains Switch box"
3..An earthed metal enclosed "Offline 24V PSU"

The "Mains switch box" has no Y capacitors at its mains input, nor any
common mode choke there either.

Please confirm that this is a disaster awaiting to happen? It works in certain installations
but in other more noisy locations, (poor mains wiring and high mains transients), the unit
is likely to go fatally wrong. Would you agree?

The "Mains Switch Box" comprises no capacitive (Y cap) connection of its earthed metal enclosure
to the circuitry inside it. As such, this is a disaster waiting to happen. And you concur?

 

[BradtheRad]

As your recent threads bring up dubious electrical practices in commercial installations, I can't help but connect unseen dots. Someday a mains electrician will come along and agree it's a disaster. Perhaps he'll show up because a disaster (or injury or death) occurred.

The reckless approach by the other consultancy you observed is consistent with the designer later being out-of-reach, having since 'moved on'. The designer might be one man, or a business... unavailable in the future.

However you, if you personally remain involved, may get blamed for something, perhaps all of it.

I wouldn't know how to advise what precautions you might take at this time, legally or professionally or whatever.

 

[mtwieg]

Electrical safety standards generally mandate that capacitors connecting to mains (either line-line or line-PE) have specific ratings (X, Y, etc). But AFAIK no safety standard mandates the use of capacitors on the mains in the first place, so therefore IMO a lack of any filtering caps on mains isn't a safety issue (unless this is some life-sustaining system). It could, however, be an EMC issue, but only proper testing per the standards can say for sure.

 

[cupoftea]

Thanks, it is known that eg SMPS's inside metal enclosures often dont work if they are not y_capacitor_connected to the metal enclosure.
(ie, say their ground to be connected to the enclosure via a y cap).
As is known, the metal enclosure acts like an antenna, and bombards the internal circuitry with noise at close range....but if you y capacitor connect the circuit ground to an enclosure, then the circuit is "bouncing" with the metal enclosure, and so doesnt "see" the "antenna noise emissions".
This is the Microwave Engineering principle which tells us that we absolutely must Y capacitor connect a circuit to its metal enclosure.
Sometimes you can get away with not doing it, but we do this at our peril.

 

[KlausST]

Thanks, it is known that eg SMPS's inside metal enclosures often dont work if they are not y_capacitor_connected to the metal enclosure.

It is known? Please provide a link to this information.
AfaIk: (Y-) capacitors to metal case are not functionally relevant. Their main job is to reduce EMI.

As is known, the metal enclosure acts like an antenna, and bombards the internal circuitry with noise

the enclosure bombards the circuitry with noise?
AfaIk: It´s the other way round. The SMPS generates the noise and the metal enclosure acts like a shield to prevent the noise to go outside.

Klaus

 

[cupoftea]

it is known? Please provide a link to this information.

...it is very very well known...so many times you get an SMPS or even just a PCB, where it doesnt work because it is in a metal enclosure and not y capacitor connected to it. All microwave engineers know this. And i have seen sneaky engineers getting a cct working by soldering the earthed rest rings to the cct ground and not telling anyone thats how they got the PCB working!!!

Its a secret of the trade....so many reading this will know...but will not let on...as they wish to keep the secret...but so many know it now that its not a secret any more.

It´s the other way round. The SMPS generates the noise and the metal enclosure acts like a shield to prevent the noise to go outside.

...thanks but ....any noisy bit of a circuit will couple into the metal enclosure...and that will then re-radiate the noise into all other parts of the cct, because much of the cct is likely to be near to the enclosure. -You must "Y cap to enclosure" to prevent this. Yes, once youve Y cap'd to enclosure, then the metal enclosure does indeed also act as a shield as you kindly say.

As we all know, "Y capping to metal enclosure" is not just about passing emc tests...you have to do it to prevent the cct thats in the enclosure from being made to malfunction by the enclosure.

 

[KlausST]

...it is very very well known...

so where is the link? It should be easy...
I did not find something useful for: "SMPS malfunction metal case"

not telling anyone thats how they got the PCB working!!!

Its a secret of the trade

Sorry, this sounds like a modern myth. (My opinion without prove)
Like: Many others have found a way to use free energy from space. But they hide it as a secret. Some say the governmet has stolen the invention (And now they can´t remember how they did it !!??) ..... The internet is full of such (beep).

All microwave engineers know this.

First: your headline is focussed on "SMPS" now you talk about microwave, which starts at about 300MHz.
On the other hand: How does this work that all MW engineers know this? Does this happen at birth? Or are there documents. .... one could provide a link to?

I know that usually microwave circuits have their metal case "HF wise shorted to GND". But they don´t use "Y capacitors" (your headline).
The "Y" in Y capacitors referes to safety standards for mains voltage. But MW devices only in rare cases involve mains voltage.
And I doubt that Y capacitors are useful for 300+ MHz. A TDK Application Note talks about an effective frequency range up to 8..10MHz.There surely may be other Y capacitors for higher frequency.
(Again: ON MICROWAVE circuits, then HF wise connection to the case may have influence on it´s function. Mainly stability of amplifiers and frequency flatness)

My personal opinion: If your SMPS PCB is that bad that the function get´s confused by a metal case (HF wise connected to GND or not), then you should change your job.
(correctly designed regarding all other design and safety rules)


Klaus

 

[cupoftea]

if your SMPS PCB is that bad that the function get´s confused by a metal case

Thnaks, As you know, FETs often have to be heatsunk to metal case....and this is a very noisy situation that cant be helped (FETs have the switching node with them), because one may not have room for separate heatsinks...so noise pollution into the metal case is often forced upon ones self.

 

[KlausST]

Hi,

I agree.
Thus there are isolated MOSFET packages, there are other methods to electrically isolate the MOSFETs from the heatsinks.

If this is no option then we mainly talk about "Capacitive coupling" from heatsink to case. (less "electro magnetic").
This HF currents need to be "short circuited" with your "Y capacitor". It does only work if you allow AC currents to flow.
It´s the job of the designer to select a capacitor that lets enough current flow to reduce HF noise .. while keeping mains frequency AC currents low.
Since the frequencies are so far apart .. this usually is no problem.

The better solution is to keep the capacitance between heatsink and case low. --> increase distance, decrease area.
Maybe an additional shield between heatsink and case can improve performance.

Klaus