groundgroundingpower supply
Have a nice piece of Motorola obtainium that I want to use to power a Raspberry Pi, and I also happen to have a left over ground wire (the case has a three-lead power connector). The case is 100% plastic.
I assume the metal bit on the top, pointed at by the arrow, is a heat sink, and am wondering if it would be a good idea to connect the ground to this?
Below is a typical schematic of an AC/DC power supply EMI filter.
You can see that the X-capacitors (between line and neutral) plus the leakage inductance of the common-mode inductor give the differential-noise rejection, and the CM choke inductance combined with the Y-capacitors give the common-mode noise rejection.
I would also not be surprised if the output return is directly connected to earth.
Any long cables with RF clocks/data will radiate noise and when connected to AC ground could also radiate via AC cable from path ground loops.
Smart idea is make generous use of common mode chokes for interface cables and DC outputs to raise impedance of the common mode signals from AC ground.
Beware that Hipot testing of AC/DC PSU may cause failure on secondary port when grounded. This is due to dielectric breakdown of Hipot from primary to secondary. Floating secondary is easier to protect if there is capacitive leakage and tester uses fast rising input. In this case modify Hipot tester to avoid discharge surge that would destruct PSU by using a current limiting series resistor.
Leakage can be minimized with prudent gaps, quality caps, and moisture protection on conductors. (moist dust causes leakage)
Best Answer
If users can touch the heatsink, such as at the back of some linear power supplies, it should be grounded and electrically insulated from the semiconductors mounted to it.
A common way to do this is by using aluminum oxide plates, TO-220FP packages (plastic housing) or a sil-pad.
Or semiconductors, such as IGBT modules, that are mounted on a ceramic base.
The power supply you have shown is intended to be mounted inside an enclosure, where users cannot touch it.
Heatsinks inside power supplies are often not insulated, since it add costs and users are unable to touch it anyway.
So no, it would not be a good idea to ground this heatsink, as it will most likely cause a short circuit.