Differential-Mode Conducted EMI Filterįigure 3 shows a typical differential-mode conducted EMI noise filter on the input side of a switch-mode supply. Typical EMI plot of a switch-mode buck supply without an input EMI filter. To meet the standard, an EMI filter is required to attenuate the differential-mode EMI. Figure 2 shows an EMI plot where the peak values of these spikes exceed the CISPR 22 EMI limits. The most significant EMI spike occurs at the switching frequency of the supply, followed by additional spikes at its harmonic frequencies.

Conceptual overview of LISN-based measurement of differential-mode and common-mode conducted EMI of a switch-mode supply.įigure 2 shows a typical EMI noise plot of a switch-mode, step-down buck supply without an input EMI filter. Fortunately, unlike the other EMI types, the pulsating input current and the resulting relatively low frequency EMI generated at the input capacitors and LISN circuit can be predicted by software, such as LTpowerCAD, with acceptable accuracy. DM conducted noise arises from the high di/dt, pulsating input current of the switch-mode supply. Like radiated EMI, the high frequency switching node ringing and parasitic capacitance cannot be easily and accurately modeled in a paper design.ĭM noise is measured differentially between two input lines. CM noise is generated at high dv/dt switching nodes, couples through the device’s parasitic PCB capacitance, C P, to earth ground, then travels to the supply input LISN. CM conducted noise is measured between each input line and earth ground. To quantify conducted input EMI, a line impedance stabilization network (LISN) is placed at the regulator’s input, providing a standard input source impedance. Figure 1 shows the generic conduction paths of the common-mode and differential-mode noise of a dc-to-dc power supply (the DUT in an EMI lab). Standard industry limits for conducted emissions usually cover a lower frequency range than radiated emissions, namely from 150 kHz to 30 MHz. Other than what is inherently built into good layout practices, it is nearly impossible to precisely predict how much radiated EMI a switch-mode supply will transmit “on paper.” One must simply build the board and measure its EMI in a sufficiently well-designed EMI lab to quantify its radiated noise level.Ĭonducted EMI results from the rapid changes in a switching regulator’s conducted input current, including common-mode (CM) and differential-mode (DM) noise. At these frequencies, radiated EMI from switching regulators is produced mainly by switching voltage ringing and spikes, and can depend heavily on PCB board layout. Industry standards for radiated emissions usually cover the frequency band from 30 MHz to 1 GHz. In a switch-mode supply, radiated EMI is usually generated by high dv/dt noise at the switching nodes. There are two major type of EMI: radiated and conducted.

Different Types of EMI: Radiated and Conducted Noise, Common Mode, and Differential Mode

To speed up the process of EMI filter design to meet EMI specifications, this article shows how conducted EMI noise analysis and filter design can be easily estimated and prebuilt using ADI’s LTpowerCAD ® program. Unfortunately, EMI analysis and filter design can be a difficult task, typically requiring a time-consuming iterative process of design, build, testing, and redesign-that is, assuming one has proper test equipment. To meet these limits for a switch-mode supply, one must first quantify its EMI performance and, if necessary, add proper input EMI filtering to attenuate the EMI. Because of this, many standards have arisen to set acceptable limits on EMI. The problem is that high frequency noise can couple to other devices in the system, degrading the performance of sensitive analog or digital signal circuits. Fast turn-on and turn-off of power transistors create sudden interruptions of current flow, resulting in high frequency voltage ringing and spikes. For example, the input side switch current of a typical buck converter is a pulsating current rich in harmonic content. This is commonly referred to as electromagnetic interference (EMI), EMI noise, or just noise. One side effect of the proliferation of switch-mode supplies is the noise they produce.

Switch-mode power supplies are used throughout modern electronics systems mainly because of their high efficiency power conversion. Speed Up the Design of EMI Filters for Switch-Mode Power Supplies

Designing pi filter