I have been facing an issue that has been a mystery to me for quite some time, and I wanted to see if someone here can potentially give me more insights on this so I can understand why it is happening. Please bear with me since electrical engineering is not my specialty and I am doing my best to understand this phenomenon.
The issue I am facing is as follows:
I have two laptop chargers. One aftermarket charger from China and an official one from HP. Whenever I use the aftermarket charger from China, my KZ earphones (which are fairly low impedance) connected to the laptop would make electrical buzzing noises. The more power than the laptop draws, the more buzzing can be heard over the earphones. This issue does not happen with lower quality or higher impedance earphones.
As a reference, this was the sound that was heard over the earphones when the device was in use: https://drive.google.com/open?id=1OXwNHcq476hf_FxVr9n_bPVIcNwdNnzm
The Chinese charger is a Class II power supply which lacks a grounding pin. If I shunt 0V on the DC side to AC ground, I get a reading of 59.8V @ 78uA AC on my multimeter. As soon as the shunt is connected, the noise stops. Likewise, the noise stops if I laid my hand on the laptop external chassis (which I suppose had a similar grounding effect).
Thinking that the problem could be due to some kind of leaking AC current, I repeated the test on my HP power supply which was grounded. However, this time I also have lifted the ground prong using a cheater plug so that the leakage current cannot escape. There was no noise on the earphones line with this setup. That said, I was still able to measure 54.1V @ 54.1 uA AC from the ground pin to ground if a shunt was connected in that manner.
To see if the manufacturer would know something about this, I contacted them about the issue and they suggested that I remove the two Y capacitors on CY1 and CY2, or downsize them from 400V 100PF to 47PF to possibly mitigate the noise issue.
Not specifically knowing how this works, here are my questions:
a) What creates the 60V AC voltage on the isolated DC side? Is this caused by the safety capacitors used in the power supply? Why is it specifically half the line voltage?
b) What noise am I specifically hearing over the earphones? I know that it has something to do with the impressed AC voltage on the DC side since the noise is gone as soon as I shunt the 0V side to AC ground but wasn't sure specifically what I was hearing? I am guessing that the AC current is trying to find its way through the ground and parts of that current are flowing through my low impedance earphones and be interpreted as sound...
c) From what the manufacturer is suggesting, it sounds like that they may be shunting the parasitic capacitance from the transformer to the 0V output side and using it as ground. I am assuming the rationale would be that a smaller capacitor would result in smaller leakage current and hence less noise being picked up by the earphones (and perhaps less EMI compliant). Would that be a good assumption? Why did they need 2 capacitors? Is it due to polarity?
d) Would the placement of the Y capacitors matter in this setup? Would it be better to simply place them between L-N and 0V-N so the noise can be diverted from the connected device (laptop)?
e) Why did the HP charger not create this same noise even when it was ungrounded? Could it be due to the positioning of such capacitors or some kind of circuitry design difference? If so, is there a way I can confirm without cracking the charger open?
e) Without knowing anything about the schematic, would there be a better setup than using Y capacitors in reducing EMI? Would the capacitor values suggested by the Chinese manufacturer be a good value to prevent EMI emissions?
f) If I were to view this under an oscilloscope, which two points should I be using to confirm this issue and visualize my findings?
Thank you for your time!