Understand and reduce DC/DC switching-converter ground noise

by Jeff Barrow, Senior Director of Analog IC Design, Integrated Device Technology, Inc. , TechOnline India - January 30, 2012

As the complexity of system and schematic designs increase and applications become more densely populated, the physical-circuit implementation plays a critical role in the electrical integrity of the system. This article illustrates two major sources of ground noise and offers suggestions on how to reduce both.

DC/DC switching-power converters are notorious for physically disrupting an otherwise carefully designed system and circuit schematic designs. These power converters drive unwanted charge onto electrical ground, causing false digital signals, flip-flop double clocking, EMI, analog-voltage errors, and damaging high voltages. 

As the complexity of these designs increase and applications become more densely populated, the physical-circuit implementation plays a critical role in the electrical integrity of the system. This article illustrates two major sources of ground noise and offers suggestions on how to reduce both.
 
Ground noise: Problem #1

Figure 1 shows an ideal buck converter with a constant load current. Switches t1 and t2 toggle back and forth, chopping Vin across Lbuck and Cbuck. Neither inductor current nor capacitor voltage can change instantaneously, and the load current is constant. Hopefully, all switching voltages and currents successfully span Lbuck or pass through Cbuck respectively, since an ideal buck converter produces no ground noise. 

But experienced designers know that a buck converter is a notorious noise source. This fact means that Figure 1 is missing important physical elements.

Figure 1:  Buck converter circuit—inductor current cannot change instantaneously, so identifying a source of ground bounce in an ideal buck converter is not easy.

 
Whenever charge moves, a magnetic field develops. Current in a wire, resistor, transistor, superconductor, and even a capacitor’s plate-to-plate displacement current creates a magnetic field. Magnetic flux, ΦB, is magnetic field, B, passing through a current loop area, A, and equals the product of the field cutting the loop surface at a right angle, ΦB = B·A. The magnetic field at a distance, r, encircling a wire is directly proportional to the wire’s electrical current, B = μoI/2πr.

 

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