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, FB, 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, FB = B·A. The magnetic field at a distance, r, encircling a wire is directly proportional to the wire’s electrical current, B = µoI/2pr.
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.(Click here for enlarged image.)
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.
Changing t1 and t2 current-loop areas is the first major source of switching-converter ground noise. Magnetic flux in the Vin-t1-gnd loop grows and collapses on every switch cycle. That changing flux induces voltage everywhere in that loop, including the ideal ground return line. No amount of copper, not even a superconductor, can eliminate this induced voltage. Only a reduction in the changing magnetic flux will help.
Changing magnetic flux has three factors: rate-of-change, magnetic field strength, and loop area. Since the clock frequency and maximum output current may be design requirements, minimizing loop area becomes the best solution.
Inductance is proportional to magnetic flux, so Figure 3 shows an electrical model for Figure 2 where changing current in parasitic inductor Lp1 causes ground noise, while constant current in parasitic Lp2 does not.
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 here for enlarged image.)
Although Figure 3 presents the problem in a familiar way, it makes a poor substitute for the physically enhanced model shown in Figure 2. Figure 3 shows parasitically induced voltage across Lp1 and Lp2; whereas voltage will actually be induced everywhere in a loop enclosing changing magnetic flux. However, this circuit element drawing will still serve the purpose of showing how to reduce induced ground noise.
Figure 4: Careful input-capacitor placement minimizes the changing loop area, and routes the changing current away from ground return to eliminate ground bounce. (Click here for enlarged image.)
As drawn in Figure 3, ground-return current flows and changes in Lp1, and it causes a voltage bounce problem. But a carefully placed input capacitor, as shown in Figure 4, reduces the parasitic magnetic-flux area, and routes changing buck current in a path that does not include ground return.
In this case, current in parasitic inductors Lp1 and Lp2 is constant, so the ground voltage will be stable. Additionally, the reduction in this magnetic flux area proportionally reduces EMI and all other unwanted, induced loop voltages as modeled in Figure 3.
In short, the first important source of switching converter ground noise is a result of changing magnetic flux area. Good printed circuit board (PCB) design uses both trace routing and careful bypass-capacitor placement to minimize changing current-loop areas and changing current in a ground-return path.