I. Main Functions of EMI/EMC Devices
The main functions of EMI/EMC devices can be broadly categorized into the following four points:
1. Reducing radiated and conducted noise at the power input or output;
2. Mitigating the surge effects of voltage transients on the power input or output;
3. Weakening inrush current generated when voltage is applied to the power input;
4. Protecting the input power supply and downstream circuitry in case of power supply failure.

II. Protection Devices
1. Fuses
Fuses are devices used for current protection and are connected in series with the power supply input. To ensure effective protection, the fuse and the upstream power supply should ideally be directly connected and connected in series with the ungrounded input terminal to ensure that no current flows through the power supply when the fuse blows.

Currently, in addition to traditional fuses, there are self-resetting PPTC fuses. Compared to fuses, PPTC fuses can return to their pre-operation state after the fault is cleared, reducing after-sales and replacement costs. In practical applications, attention should be paid to voltage, current, response time, and operating temperature to select the appropriate fuse type. Choosing the right package can further optimize the design and reduce costs.

2. Metal Oxide (MOV) Resistors
Metal oxide resistors (MOVs), also called varistors, are primarily placed at the input terminal to absorb voltage transients that may occur from the input voltage source. When using AC-DC power supplies, the input voltage source is the AC power line, so the MOV needs to absorb the transient energy generated by transient current surges or other damage.

MOVs have high impedance during normal operation, but become low impedance when the voltage exceeds the rated value (this occurs when input transient voltages occur). A fuse with appropriate input parameters is placed between the MOV and the input power supply. When the MOV is subjected to excessive power and is at risk of damage, the fuse triggers the protection circuit to prevent overcurrent burnout.

The most commonly used type of resistor at the power supply terminal is the TVS (Transient Voltage Suppressor), which is mainly placed at the power supply terminal to shunt external surge voltage transients on the power supply terminals, thereby protecting downstream circuitry. Although MOVs often have similar uses at the power supply terminal, for applications requiring fast response and lower voltage clamping, TVS diodes are a more suitable solution.

Three EMI Filter Components
1. Differential Mode Inductor
The inductor is placed in series with the input power supply path. This component, together with the input capacitor, forms a low-pass LC filter to reduce annoying conducted noise voltage on the input conductors and prevent it from being transmitted to the external power supply. This requires the inductor to have a sufficiently large saturation current rating to withstand the maximum input current during normal operation. A differential mode inductor with a sufficiently low DCR (parasitic DC resistance) should be selected to ensure adequate power dissipation. A flat-wire integrated T-CORE inductor is a suitable choice for this requirement.

2. Common Mode Inductor
A common mode inductor is used to generate high impedance to reduce common mode noise flowing along the input conductor. The principle is that when a differential mode signal passes through the common mode inductor, the magnetic flux generated in the core cancels out because the magnetic fluxes of the two windings are in opposite directions, and the differential mode signal is hardly impeded. When common mode noise passes through the common mode inductor, the magnetic flux generated in the core is superimposed because the currents of the two windings are in the same direction, resulting in a larger magnetic flux. This larger magnetic flux generates higher impedance in the core, thus suppressing common-mode noise.

Choosing a common-mode inductor during normal power supply operation allows for handling maximum current and achieving acceptable power dissipation. Unlike differential-mode chokes, very little current flows in common-mode chokes, so saturation current ratings are generally not a concern.

IV. Summary
Selecting appropriate components to prevent EMI and EMC issues in a circuit is not difficult, but many details need to be considered. The final chosen devices and parameters are usually trade-offs between performance, cost, size, and reliability.