PSU - AC/DC front end noise considerations

Projects / Smart Bench Supply

Over the years I have seen a lot of DIY bench supplying using modified PC PSU or laptop supply as their AC/DC frontend.  Instead of the blanket statement of "SMPS is bad news", I am going to sit down and explain why I won't be doing that and rather be using a big heavy hunk of copper and iron for my front end AC/DC conversion.

This is the block diagram of a common AC/DC converter.  There are two type of high frequency noise: Differential and Common Mode.  The differential noise is filtered with an inductor (or leakage inductance of a common mode choke) and a "X-cap". The common mode noise is filtered with a common mode choke and forces to go to "Earth" ground a pair of "Y-caps".  (see pdf for filtering)  The amount of conductive and radiated noise is governed by regulatory standards such as FCC part 15.

The story doesn't end there as some of the high frequency common mode noise can reach the second side by means of parasitic capacitive coupling.  The problem is when the secondary side is not grounded.

Those old boat anchor 50/60Hz transformers don't have a high enough frequency for the capacitive coupling to work.

The following picture shows such a power supply.  There is a yellow Y-cap sitting between the Primary and Secondary side.  Usually it is right next to an opto-isolator that is part of the feedback loop.  The Y-cap is connected  between the low voltage 0V and the "Earth" ground.  It allows the Common mode noise to flow back to the "Earth" ground.  The AC grounding isn't perfect as the capacitor has a non-zero impedance.  Some of the noise still persists on the secondary side.  It'll try to go to the "Earth" ground through whatever least impedance path e.g.  I/O in your device.

It gets even worse for those laptop supplies without a "Earth" ground connection. Let's say that tingling sensations when you touch the metal part of a laptop are not your spider senses.  It is small enough to not hurt you too much but will cause enough of a noise problem for sensitive analog circuit.

Reference:
Block diagrams are from:

Journal of Engineering and Development, Vol. 16, No.1, March 2012  ISSN 1813- 7822 , "Practical Approach in Designing Conducted EMI Filter to Mitigate Common Mode and Differential Mode Noises in SMPS", By: Nidhal Y. Nasser Assistant Lecturer, Electromechanical Eng. Dept.,University of Technology
(pdf)