Electronics > Power/Renewable Energy/EV's

Power supply EMC document is erroneous?

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Faringdon:
Hi,
The following article makes statements about EMC.
https://www.ti.com/lit/an/snva489c/snva489c.pdf?ts=1713454091161

On page 2 it says
 QUOTE>>>In a typical IC based non-isolated DC-DC SMPS, only two lines connect to the input port. So any current
 going in through one terminal has to go out through the other. In this configuration, the common mode
 noise VCOMM will always be zero.<<<<UNQUOTE

..Surely it cant be zero?....
Though we see "two lead" offline SMPS's, and they have common mode chokes, and Y caps across the transformer isolation barrier,
So they do have Common mode noise...so surely a non-isolated DCDC also does?, and can radiate to its surroundings...and when it
does so, its "go" and "return" currents will not be equal and so we have common mode noise?

..On page 3 it says the following....

QUOTE>>>Conducted EMI involves the normal operation of DC-DC converters. It does not involve circuit parasitics
 except input or output capacitor ESR and ESL. PCB layout itself is not going to help reduce conducted
 EMI. Further, conducted EMI is only related to the current level, not the voltage level at input or output
 ports. In another words, with the same power level buck converter, lower input voltage means higher input
 current, thus worse input conducted EMI.<<<<UNQUOTE

...surely these 3 sentences are filled with innaccurate statements? For starters, conducted EMI involves more strays than just capacitor related
ones, also  PCB  layout can surely help reduce conducted emissions? High voltage , low current SMPS's can have just as much conducted EMC problem
as Low voltage, high current ones, for example the dv/dt of the switching node can cause severe conducted emissions problems.

AnalogTodd:
Read and pay attention to the article. It is talking in those places about CONDUCTED EMI, not radiated.

--- Quote from: Faringdon on April 19, 2024, 07:34:07 pm ---
..Surely it cant be zero?....
Though we see "two lead" offline SMPS's, and they have common mode chokes, and Y caps across the transformer isolation barrier,
So they do have Common mode noise...so surely a non-isolated DCDC also does?, and can radiate to its surroundings...and when it
does so, its "go" and "return" currents will not be equal and so we have common mode noise?
--- End quote ---
You are ignoring KCL for a non-isolated DC-DC SMPS. It doesn't matter what the load looks like, it doesn't matter what the SMPS looks like, if you have one wire in and one wire out, the currents MUST always be equal. You don't get electrons magically flying off into the ether unless you are actually getting to where you are turning things into a plasma, and even then they must be returning through another path.

--- Quote ---...surely these 3 sentences are filled with innaccurate statements? For starters, conducted EMI involves more strays than just capacitor related
ones, also  PCB  layout can surely help reduce conducted emissions? High voltage , low current SMPS's can have just as much conducted EMC problem
as Low voltage, high current ones, for example the dv/dt of the switching node can cause severe conducted emissions problems.

--- End quote ---
You are mixing up conducted and radiated EMI. Conducted EMI is passed through the circuit in question and is not part of the magnetic fields that create radiated EMI coming from large loops on the board or fields created by switching currents. Remember, at zero current it is impossible to have conducted EMI, there has to be electrons flowing to get conducted EMI.

Please, review your physics.

A simple way to look at this is to think about a linear regulator on a board; conducted EMI is often viewed as the power supply rejection (PSRR) of the part. Conducted EMI should solely be a function of the circuit response and with perfect models will match between simulations and reality. I have personally seen that to hold true in the design of very high PSRR parts. But what was also found to be true is that the PSRR on early boards didn't match what simulations said it should. It was found that having a capacitor right at the input of the part killed the PSRR. This was because at frequency the capacitor was a low impedance and ran significant current. This raised the AC voltage at the input of the part (parasitic R and L of the traces coupled with the cap) and also created a magnetic field that coupled into other sections of the board. We are not talking about a 6dB difference in PSRR, but instead 40dB. It became very important to separate conducted versus radiated EMI and know which was which and how to correct each one.

tggzzz:

--- Quote from: Faringdon on April 19, 2024, 07:34:07 pm ---For starters, conducted EMI involves more strays than just capacitor related
ones, also  PCB  layout can surely help reduce conducted emissions? High voltage , low current SMPS's can have just as much conducted EMC problem
as Low voltage, high current ones, for example the dv/dt of the switching node can cause severe conducted emissions problems.

--- End quote ---

The answer to that will be a vital part of your company's IPR, so get the answer straight for when you sell it "for a very large sum" to the "Chinese consortium":
https://www.eevblog.com/forum/renewable-energy/intelligent-product-of-switch-mode-power-supplies/msg5456486/#msg5456486

Faringdon:

--- Quote ---You are ignoring KCL for a non-isolated DC-DC SMPS. It doesn't matter what the load looks like, it doesn't matter what the SMPS looks like, if you have one wire in and one wire out, the currents MUST always be equal. You don't get electrons magically flying off into the ether unless you are actually getting to where you are turning things into a plasma, and even then they must be returning through another path.
--- End quote ---
Thanks, so why do 2-lead input offline SMPS have common mode chokes and Y caps?


--- Quote ---You are mixing up conducted and radiated EMI. Conducted EMI is passed through the circuit in question and is not part of the magnetic fields that create radiated EMI coming from large loops on the board or fields created by switching currents.
--- End quote ---
Thanks, i tend to agree, though as is known, a power supply board that is going to fail radiated EMC, is highly likely to have a high level of common mode emissions as detected on the conducted EMC scan.
In fact, a power supply PCB that gives out  radiated EMI, is bound to show common mode conducted emissions.
And as page 17 of the following states, an EMC problem above 1MHz is likely due to common mode conducted emissions...
https://emcfastpass.com/wp-content/uploads/2017/04/an15.pdf




--- Quote ---A simple way to look at this is to think about a linear regulator on a board; conducted EMI is often viewed as the power supply rejection (PSRR) of the part. Conducted EMI should solely be a function of the circuit response and with perfect models will match between simulations and reality. I have personally seen that to hold true in the design of very high PSRR parts. But what was also found to be true is that the PSRR on early boards didn't match what simulations said it should. It was found that having a capacitor right at the input of the part killed the PSRR. This was because at frequency the capacitor was a low impedance and ran significant current. This raised the AC voltage at the input of the part (parasitic R and L of the traces coupled with the cap) and also created a magnetic field that coupled into other sections of the board. We are not talking about a 6dB difference in PSRR, but instead 40dB. It became very important to separate conducted versus radiated EMI and know which was which and how to correct each one.
--- End quote ---
Thanks for this, it sounds interesting, although you seem to be saying that you had a linear regulator and its performance became worse when a higher capacitance  decoupling capacitor was fitted at its input terminals(?)......i must admit, i am used to adding such a capacitor to solve  a problem. I wasnt aware of any need to limit the capacitance at the "input" to a linear regulator. When you look at Middlebrooks theorems, there is nothing in there that says limiting the capacitance at input to a power supply (or linear reg)  solves problems.....more the other way round.
I am guessing that theres more to this than you are going into?...and that perhaps the load of the linear reg was a pulsating load or something(?)


--- Quote ---you don't get electrons magically flying off into the ether
--- End quote ---
Thanks,  yes they dont magically do that i agree....but i am sure you would agree, any DCDC , even two lead input ones, supplied even by a battery,  can cause radiated EMC problems.
A "2 lead input" DCDC which causes radiated EMC problems (eg the authorities detect it and say no to it), is highly likely to have a conducted common mode EMC problem.

As we all know, KCL and all that , is part of the well known "low frequency approximation" that we all use to develop things because its easy and quick to do so....but as you know, KCL is not in fact accurate at all in real terms...it has its "older  cousin" form by way of Maxwell's equations...and these describe and condone why a 2 lead DCDC can cause radiated EMC problems, and can have a common mode conducted EMC problem.


--- Quote ---Remember, at zero current it is impossible to have conducted EMI, there has to be electrons flowing to get conducted EMI.
--- End quote ---
Thanks yes, i would agree, and i am sure you would agree that its di/dt that causes the EMC problem. A totally "flat" DC current would not cause an EMC issue.
The article "Common mode EMI noise suppression for bridgeless PFC converters" on page 292 gives the causation of
common mode noise. (this IEEE article is  now hard to find on www)

QUOTE>>>As we all know, CM noise is due to the voltage pulsating
generated by high-frequency switching. Such high dv/dt generates
CM noise currents which go through the parasitic capacitance
from converter to ground.<<<QUOTE

So yes, its di/dt, but interwoven with the current is the changing voltage aswell

As is known, DCDC's in cars and trains and aeroplanes all can produce radiated emissions....and that any  DCDC power supply PCB that produces radiated emissions, will inevitably present common mode conducted emissions. The article of the top post, says that common mode conducted emissions from a 2_lead_input DCDC would be zero...this is not the case.

As can be seen on page 36 of the following....common mode noise currents can indeed be created in a 2_lead_input DCDC,
This page (36) also shows how they can be caused by the drain of the switching mosfet......
https://www.vicorpower.com/documents/design_guides/DG-DCM-Design-Guide-VICOR.pdf
________________________________________________________ ______________________________________________________________
As is known, with a 2_lead_input DCDC, the module is often housed in a metal enclosure/heatsink.
The metal of the enclosure is often not permitted to be connected to any of the circuit conductors.
In such cases, the metal enclosure is still declared as the "EMI ground" or "chassis ground".
And as such, the circuit power inputs and outputs are often connected to the metal enclosure via Y capacitors.
This is to reduce common mode noise of the 2_lead_input DCDC.

So, the actual chassis GND  , as discussed, is often not directly (DC) connected to the circuit conductors,
but often is made to be a copper plane in a certain layer of the DCDC's PCB. This "chassis plane"
, in part, serves to capacitively couple to the circuit conductors so as to re-route disturbances
that would otherwise  manifest as common mode noise. Ie capacitively re-route noise back to the
circuit so that it doesnt go off as common mode noise.

ejeffrey:

--- Quote from: Faringdon on April 20, 2024, 12:16:26 pm ---As we all know, KCL and all that , is part of the well known "low frequency approximation" that we all use to develop things because its easy and quick to do so....but as you know, KCL is not in fact accurate at all in real terms...it has its "older  cousin" form by way of Maxwell's equations...and these describe and condone why a 2 lead DCDC can cause radiated EMC problems, and can have a common mode conducted EMC problem.

--- End quote ---

The difference between KCL and ampere's law / Maxwell's equations is pretty much the definition of "radiative emissions".  Specifically, KCL + displacement current is amperes law.  If you have only two wires and they have a mismatched current, it must be matched by displacement current. Displacement current can be from capacitive coupling to ground, but if we are testing an isolated device on a non conductive table, capacitance is negligible.  The only other option is radiation.

Two wire power supplies have chokes and filters to avoid turning into radiators and also because they tend to get plugged into devices, not used floating in space.  So to some extent the distinction is a bit artificial indeed. 

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