Technical Information Site of Power Supply Design

2018.05.24 Switching Noise - EMC

Noise Occurring in Switching Power Supplies

Basics of EMC

This time, we will study the noise that occurs in actual switching power supplies.

Noise Occurring in Switching Power Supplies

To begin with, we use the equivalent circuit for a synchronous step-down DC/DC converter to confirm switching current paths.

We label the high-side switch SW1, and the low-side switch SW2. When SW1 is ON (and SW2 is OFF), the current path runs from the input capacitor to SW1, and passes through the inductor L before reaching the output capacitor. When SW2 is ON (and SW1 is OFF), the current path runs from SW2 through L to the output capacitor. In the following diagram, each time the switches are turned ON/OFF, the current in the path indicated by the red line that is the difference between the above two paths changes abruptly. Because the change in current in this loop is sharp, high-frequency ringing occurs in the loop due to the inductance of the PCB wiring.

Below we indicate the relationship between ringing and the parasitic components of the external components and the mounting board that constitute a power supply circuit.

Parasitic components in the loop in which the current changes suddenly are shown in red in the above diagram. In any wiring there is a wiring inductance, and in general the inductance is about 1 nH per mm of wiring. Moreover, an equivalent series inductance ELS is present in a capacitor, and a parasitic capacitance exists between the terminals of a MOSFET. Due to these components, ringing at frequencies between 100 MHz and 300 MHz occurs at a switching node, as indicated in the red box. The current and voltage that occur can be calculated using two equations.

This ringing exerts various effects as high-frequency switching noise. Of course measures to address it can be taken, but parasitic components on the mounting board cannot all be eliminated through power supply ICs, and so innovations are necessary in the PCB wiring layout and through decoupling capacitors. Where the PCB layout is concerned, there is a detailed discussion in the section on “DC/DC Converter PCB Layout” that should be consulted.

Differential mode noise and common mode noise are explained here, and crosstalk is discussed in detail here. Common mode filters will be explained in a future section.

Key Points:

・In a loop in which currents are suddenly turned on and off during switching, high-frequency ringing (switching noise) occurs due to parasitic components.

・This switching noise can be reduced through optimization of the PCB wiring and other measures, but the noise that remains is conducted to an input power supply as common mode noise, and so measures to prevent leakage are necessary.

The EMC Basics

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