EMI Filter in power supply

[Sambhav_1]

Hi,

I am building a power supply of 24V,i saw some schematics where the EMI filters are used, How do we calculates the values were not mentioned. And What are the things needed to be taken care while design EMI filter.

 

[stenzer]

Hi,

that's not an EMI filter. A filter would include at least one length element e.g. an inductor and one shunt element e.g. the shown capacitor.

The shown components are used to protect the circuit against high current and voltage events. F1 seems to be a thermal fuse, which blows if the input current is to high. V1 is a varistor/MOV which protects the circuitry against high voltage peaks by providing a low resistive path.

So for example, if the voltage is too high V1 kicks in and provides a low resitive path, which causes a high current wich might destroy F1, depending on the reaction time of the fuse and duration of the high voltage event.

BR

--- Updated Oct 18, 2020 ---

... C1 represents a classical X-capacitor which has indeed an EMI supression purpos.

 

[Akanimo]

Hi,

The combination of L1 and C1 makes an EMI filter.

See these videos:

Although the video series was for flyback, the input filter design method is applicable to any power supply topology.

And also read Chapter 10: Input Filter Design in the book: Fundamentals of Power Electronics by Robert Erickson for optimization.

 

[FvM]

The schematic looks like being copied from TI evaluation board user guide https://www.ti.com/lit/ug/sluuax9b/sluuax9b.pdf, curiously with the common mode choke inductance value deleted.

L1 and C1 can be considered as EMI filter elements.

Allthough L1 is a common mode choke, it has some differential inductance and is in so far filtering differential mode interferences in combination with C1. The common mode inductance is acting in combination with line common mode impedance.

 

[Sambhav_1]

[/QUOTE]

The schematic looks like being copied from TI evaluation board user guide https://www.ti.com/lit/ug/sluuax9b/sluuax9b.pdf, curiously with the common mode choke inductance value deleted.

L1 and C1 can be considered as EMI filter elements.

Allthough L1 is a common mode choke, it has some differential inductance and is in so far filtering differential mode interferences in combination with C1. The common mode inductance is acting in combination with line common mode impedance.

Yes,the schematic is of a TI Evaluation board. I was facing issue in determining the value of L1 and for that i need to know how the value needs to be calculated.

 

[FvM]

Common mode chokes are usually selected for rated current and have typical inductance values. I don't know what your specific problem in "determining the value of L1" is. Copying values from a known working design, e.g. the said evaluation board that comes with EMI measurements would be a practical approach. To calculate anything on your own, you need to understand the mechanism of EMI propagation first, common and differential mode impedances, EMI standards, artificial networks (LISN) used for measurement etc.

 

[Easy peasy]

the DM inductance of the CM choke ( usually about 5% ) forms a DM filter with the cap on the mains, the CM choke attempts to reduce CM ( common mode ) noise to the mains - it is an EMC filter - but perhaps not a very efficacious one.

 

[Sambhav_1]

the DM inductance of the CM choke ( usually about 5% ) forms a DM filter with the cap on the mains, the CM choke attempts to reduce CM ( common mode ) noise to the mains - it is an EMC filter - but perhaps not a very efficacious one.

What could be the alternative for this EMC filter in this design?
Thanks!