How to design the EMC of the auxiliary power supply for HMI (Human Machine Delivery)?

The EMC design of HMI auxiliary power supplies needs to consider their low power and multiple output characteristics, with a focus on isolation, filtering, and layout. Auxiliary power supplies typically power peripheral circuits such as relays, indicator lights, and sensors. First, an isolated flyback or reverse flyback topology is recommended, with a shielding layer between the primary and secondary sides of the transformer to suppress common-mode noise coupling. Common-mode filtering circuits must be installed at the AC input or DC high-voltage input, for example, using small-sized CMZ1608 series common-mode inductors and Y capacitors. Each low-voltage DC output should be filtered independently using LC or RC circuits, for example, a PBZ2012E102Z0T power bead connected in series with an electrolytic capacitor in parallel for the 24V relay output.

For 5V or 3.3V outputs powering analog sensors, filtering requirements are higher; a π-type filter can be used, and an LDO stage can be added for voltage regulation and noise suppression. In terms of PCB layout, the auxiliary power supply module should be centrally located and isolated from the main controller and high-speed signal areas using ground lines. Sensitive signal lines should be avoided below the transformer and power switching transistors. The auxiliary power supply's ground wire should be connected to the digital main ground at a single point via a ferrite bead or a 0Ω resistor to prevent noise crosstalk. By using CMZ and PBZ series filter devices from Etymotic suitable for low-power scenarios, and by carefully designing the circuit and layout of the auxiliary power supply, it can be ensured that it meets functional requirements without becoming a source of EMI noise or a weak point in the system's immunity.