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An isolated switching regulator has 680p/250V Y caps from both positive and negative terminals to the earth, for common-mode noise suppression:

schematic

simulate this circuit – Schematic created using CircuitLab

Today I noticed that, regardless of the earth connection of the regulator itself and regardless of the earth connection of the input supply voltage source (can be a bench-type isolated DC power source or a PFC pre-regulator which is effectively a non-isolated boost converter), if I remove the output Y-caps the output voltage drops when loaded (e.g. from 12.05 to 11.85V). If I touch to the earth connection of the power supply (even if it's not connected to the real earth) the output increases to 11.92V.

What could be the possible reason? How can an effective capacitance of 340 pF (when there's no earth connection) affect the output stability?

EDIT: Here's the respective FB network:

schematic

simulate this circuit

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    \$\begingroup\$ Maybe the feedback from the output is getting more noise ripple and causing the regulation mechanism to regulate at a slightly lower value. Can't really say without knowledge of this feedback mechanism. \$\endgroup\$
    – Andy aka
    Commented Dec 20, 2022 at 15:37
  • \$\begingroup\$ @Andyaka added the respective feedback network. Hope it can give some idea. \$\endgroup\$ Commented Dec 20, 2022 at 15:54
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    \$\begingroup\$ Yes I thought it might be using a 431. I think the extra spikes that were formerly suppressed by the 680 pF capacitors may now be causing the 431 to pulse the opto LED a bit more and that will result in a regulation DC error. Just a theory of course; not an answer. Maybe if you could shoehorn a decent 330 pF across V+ to V- it could prove it? Maybe the DC voltage might even rise above 12.05 volts? \$\endgroup\$
    – Andy aka
    Commented Dec 20, 2022 at 15:57

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DC voltage shift with AC conditions is a classic bipolar amplifier rectification effect. TL431 is a bipolar amplifier, and susceptible to this effect, as are many others.

While TL431 is marketed as an "adjustable zener", it's anything but; an accurate description is a self-powered open-collector op-amp with a massive but suspiciously stable 2.50V input offset voltage. This makes it much more useful than as a shunt regulator; as seen here, it serves well as an error amplifier for isolated feedback. And, as a not-zener, we can expect to see very different behavior for frequencies above fT; terminal impedance goes up, and eventually, rectification effects occur. (Typically, from ~10MHz to 100s of MHz would be relevant for this effect. Or beyond; I've seen plenty of audio equipment that detected the distinctive beeping and clicking sounds that accidentally come from nearby cell phones.)

Presumably, removing the capacitors is increasing EMI around the regulator, shifting its offset. RF is rectified to a DC offset, increasing the feedback signal and thus decreasing the output voltage.

Probably. Maybe something like that. Is that the correct rectification direction? I forget if I've had a chance to measure that before, actually. In any case -- it's likely to shift somehow; whether up or down, depends on the device.

A good solution (besides fixing the underlying EMI problem itself..) is a small cap between REF and ANODE (or, functionally speaking: -IN and VEE/+IN). This reduces loop phase margin, so not much can be used; typically 100-470 pF is adequate.

As for how the EMI is getting to the regulator in the first place -- who knows. Perhaps it's proximity, the regulator might be near some metal or other traces that have CM noise between them [the regulator and the conductors]. Perhaps it's poor layout (no ground plane?). It's unlikely to be supply ripple (differential mode), given the comparatively low impedance (large capacitors, extra filtering) there. (By extension, using two 'Y' caps here is probably immaterial; one likely will do, or it doesn't matter which rail the two connect to, Vo- or Vo+.)

Also, just to clarify for readers (you probably know this already): it's not material whether EARTH is actually earthed; RF of course doesn't know or care what's at the end of a dozen meter cable (at least, for certain bands of RF). The 'Y' caps serve to close the loop between input (mains) and output, serving as the shunt capacitors in a common-mode low-pass filter, to absorb noise generated by the supply (particularly the isolation transformer, which spans the common-mode path). Typically chassis connections are provided (if not a whole enclosure or partial frame, then screw holes in the PCB), which may serve as the reference plane ("earth" for RF purposes), and so 'Y' caps are often tied to it.

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    \$\begingroup\$ I think the EMIRR if the 431 is particularly bad due to its asymmetric inputs. Regular opamps probably do better, (didn't check if the 431 has EMIRR specs in the datasheet). But it possibly gets a bit too much hate here; I still love it, as I often need the combo of reference and opamp. \$\endgroup\$
    – tobalt
    Commented Dec 20, 2022 at 18:20
  • \$\begingroup\$ Spot on. In my application, reducing or removing the capacitance there leads to an increase of CE in 14-30 MHz band. As for the layout, this is a high density (high W/in³) converter i.e. we are heavily size-limited here, so we did the best we could. (Of course there are power planes). The problem is, we can't increase Y-caps more due to the tight earth leakage requirements. As for FB network modification, we have some margin (>20dB GM, >90° PM) but my only concern is the crossover frequency as the dynamic resp requirement is tight as well i.e. we don't want the fc to decrease. Anyway. Thanks. \$\endgroup\$ Commented Dec 21, 2022 at 7:01

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