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2018.06.07 DC/DC

Noise countermeasures: corner wiring, conducted noise, radiated noise

DC/DC Converter PCB Layout

This is the first of two sessions devoted to an explanation of the relation between board layout and noise.

Measures to Address Noise at Corner Wiring

Wiring patterns will always require corners (bent sections), but EMI can be made worse depending on how the wiring is bent. Persons without experience in PCB layout may be doubtful, but this is the kind of knowhow that comes from PCB layout experience.

The graphic below illustrates better and worse methods of corner wiring. If the wiring is bent at a right angle in a corner, the impedance changes at the corner. This causes disruption of the current waveform, and waveform disruption known as reflection occurs. In wiring that passes high frequencies, such as at switching nodes, reflection can worsen the circuit EMI.

Preferred methods of bending the wiring include using 45° bends and arcs instead of right angles. The larger the radius of curvature of a bend, the smaller is the change in impedance.

Measures to Address Noise Terminal Voltage (conducted emissions)

The noise terminal voltage is noise that is fed back to the input line, and is also called conducted emission noise. The noise bands mainly appear at multiples of the oscillation frequency.

This noise can be suppressed by inserting a ferrite bead or a π filter. Such components to deal with noise must be selected for the band to be suppressed (the noise to be reduced). Hence the noise must first be confirmed, and the frequency determined. The following is an example of measured data for the noise terminal voltage.

Methods to Address Noise Electric Field Intensity (radiated noise)

Another noise type that must be studied is noise electric field intensity (radiated noise). Radiated noise in a DC/DC converter occurs due to the slopes of switching on/off waveforms and ringing; this noise appears in approximately the 100 MHz to 300 MHz range.

Ringing during switching signal rising and falling mainly arises from the wiring inductance between a MOSFET and the input capacitor; the magnitude of the inductance affects the noise.

As explained earlier in the discussion of input capacitor placement, by optimizing the input capacitor placement and wiring, the noise level can be reduced.

Measures that can be taken when radiated noise in a DC/DC converter circuit has exceeded a standard that must be met by the equipment in which the converter will be installed include using a more gradual switching waveform, and adding a snubber circuit.

The following waveform diagram shows an example of measured radiated noise. The results indicate somewhat intense noise in the region below 200 MHz.

Next time, we will explain methods for reducing this radiated noise in somewhat more detail.

Key Points:

・Corner wiring should describe arcs, so that the change in wiring impedance is lessened and noise is not induced.

・Measures to deal with noise terminal voltage (conducted emissions) include beads and π filters selected according to the noise frequencies.

・To cope with noise electric field intensity (radiated noise), the placement of the input capacitor is optimized, and the steepness of the switching waveform is adjusted.

PCB Layout and Selection of Peripheral Parts

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