Users of open frame power supplies are often surprised when they test a unit on the bench and measure output noise higher than the published specification. This prompts a call to Technical Support, who will then advise that the product has to be correctly grounded, or earthed.

TDK-Lambda’s open frame ZPSA60 power supply

Enclosed power supplies, like TDK-Lambda’s HWS series, have a metal chassis, and can be easily tested without any such concerns. Why is this?

If a power supply is a Class I type (needing a ground connection) input and output noise suppression usually involves ceramic capacitors between line/neutral to ground and DC output to ground. A typical connection is shown below.

On the AC input, high frequency noise flows through the low impedance path of the capacitors to the earth ground. This avoids the electrical noise being transmitted back down the AC input and causing problems with other equipment in the form of EMI (Electro-Magnetic Interference). On the DC output, the capacitors have the same function, this time avoiding high frequency noise appearing on the output of the power supply and causing problems with the load. Note in some cases there may be only one output to ground capacitor.

Looking at the (unpopulated) printed circuit board of the ZPSA60 power supply, the red arrow indicates the location of the two input side capacitors, CY2 and CY3. A copper trace connects them to the bottom left mounting hole. The blue arrow indicates the location of the output capacitor CY1, this time connected to the bottom right mounting hole.

(We can ignore the top two mounting holes as they have no connection made to them.)

When the power supply is tested on the bench, there is no earth ground connection to the power supply (just line and neutral and the output). An earth connection can be made to the bottom left mounting hole, there still is no connection between the two holes, effectively disconnecting the output noise filter capacitor(s).

Once the unit is mounted on the metal chassis of the system (with metal standoffs and screws), all the capacitors are in circuit. At this point, the power supply will operate within its specification.

Some open frame power supplies like TDK-Lambda’s ZMS100 series are capable of operating with or without an earth ground connection, and can be used within a plastic enclosure. An internal wire (indicated by the pointer) links connects the two mounting-hole points together to ensure compliance to the output noise specification.