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Minimising DC/DC switching-converter ground noise

Posted: 08 Dec 2011     Print Version  Bookmark and Share

Keywords:buck converter  Magnetic flux 

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, analogue-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.

However, 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.

Electrical components have length and charge must flow from one device to the next in the various wire segments. But moving charge creates a magnetic field, so the schematic in figure 1 can be improved. Figure 2 shows a better model of a simple buck converter.

Figure 2: Magnetic flux = (B-field) × (current loop area). Changing flux induces voltage. As a buck switches, the changing current-loop path causes a changing flux and induces ground bounce.

In figure 2, the wire remains ideal in every way, except current must flow some distance in each segment while traveling from one electrical component to the next. As this charge flows, magnetic field wraps around the energized wires and is magnetic flux passing through the t1 and t2 switch loops.

Figure 3: Parasitic inductance models energy stored in the magnetic field. Changing current in Lp1 induces ground bounce whereas constant current in Lp2 does not. (Click on image to enlarge.)

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