Core Concepts
EMI Filter: Short for Electromagnetic Interference Filter. Its function is to suppress unnecessary electromagnetic noise (EMI) generated by electronic equipment, preventing it from interfering with other devices. At the same time, it can block external electromagnetic noise from entering the equipment, ensuring the equipment operates normally and meets the requirements of Electromagnetic Compatibility (EMC) regulations.
Thick-Film Technology: A manufacturing process for electronic components. Special conductive pastes (such as silver, palladium-silver, gold), resistor pastes, and dielectric pastes are printed onto an insulating substrate (usually a ceramic substrate like aluminum oxide) in a designed pattern using screen printing. Then, high-temperature sintering and curing are performed to form the required components (such as conductors, resistors, capacitors) and interconnection circuits. It is characterized by relatively thick formed film layers (usually greater than 10 microns, much thicker than the sub-micron level of thin-film technology).
Thick-Film EMI Filter: A passive EMI filtering network integrally manufactured on a ceramic substrate using thick-film technology. It integrates the components required for an EMI filter (such as common-mode chokes, differential-mode/common-mode capacitors, resistors) and interconnection circuits into a miniaturized ceramic substrate package through multi-layer thick-film printing and sintering processes.
Structure and Working Principle
1. Substrate
High thermal conductivity and high insulation ceramics (such as Al₂O₃ aluminum oxide) are usually used. It provides mechanical support and electrical insulation.
2. Thick-Film Layers
Conductor Layer: Conductive paste is printed to form inductor coils (a key part of common-mode chokes), electrodes, connection pads, ground layers, and shielding layers.
Dielectric Layer: Dielectric paste is printed to form the insulating dielectric layer of capacitors.
Resistor Layer (Optional): Resistor paste is printed to form damping resistors, which are used to suppress resonance peaks or provide discharge paths.
3. Integrated Components
Inductor/Common-Mode Choke: Realized by printing spiral or multi-layer spiral conductors on the substrate. The core is air or ceramic (no magnetic core, or sometimes printed ferrite paste, though this is rare), and it mainly provides high impedance to common-mode noise.
Capacitor: Formed by printing a dielectric layer between two conductor layers. It constitutes X-capacitors (between lines) and Y-capacitors (between line and ground).
Resistor (Optional): Formed by directly printing resistor paste.
4. Packaging
The entire thick-film structure is usually packaged (such as molded plastic, epoxy potting, or metal casing) to provide mechanical protection, environmental sealing, and enhanced insulation performance. There are pins or pads on the outside for surface mount technology (SMD) or through-hole technology (THT) installation.
Working Principle
It is the same as that of EMI filters composed of traditional discrete components: an inductor's inductive reactance (which hinders current changes, especially effective for common-mode noise) and a capacitor's capacitive reactance (which provides a low-impedance bypass for high-frequency noise to the ground or between lines) form a low-pass filter network. The higher the noise frequency, the greater the attenuation of the filter to it, thereby preventing high-frequency noise from propagating along power lines or signal lines.
Application: Used with Thick-Film Power Modules
From the instruction manual of the JLHFH-461-3A EMI Filter developed by ZITN, we found the following usage instructions:"When the EMI filter is used with a DC/DC thick-film power converter, the input current of the DC/DC converter is the output current of the EMI filter. The input current of the DC/DC converter is maximum when the input voltage range is at the low end. The input current of the DC/DC converter at the low end of the input voltage should be calculated, and the power derating design margin should be appropriately considered to reasonably design the output current of the required EMI filter."
It can be seen that the use of thick-film power supplies with EMI filters is an application choice determined by the physical nature of switching power supplies, the mandatory constraints of EMC regulations, and the requirements for system reliability.