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AC and DC Current Paths
Figure 1. Recommended Layout Starting Points
3 AC and DC Current Paths
Referring to Figure 2a, the bold lines represents the main (power) current flow in the converter during the
time the switch is ON. As the switch turns ON, the edge of the of the current waveform is provided largely
by CBYPASS, the remainder coming mainly from CIN. Some slower current components come from the
input DC power supply (not shown) and also refresh these input caps. Figure 2b represents the situation
when the switch is OFF. We can therefore see that in certain trace sections, current has to start flowing
suddenly during the instant of switch turn-off and in some sections it needs to stop flowing equally
suddenly. Figure 2c represents the ‘difference’, that is, traces shown bold in Figure 2 are those where the
current flow changes suddenly. During the turn-on transition the picture reverses, but the 'difference'
trace sections are the same. Therefore during either switch transition, 'step changes' of current take place
in these difference sections. These traces encounter the harmonic-rich rising or trailing edges of the
current pedestal waveform. The difference traces are considered 'critical' and deserve utmost attention
during PCB layout. It is often stated colloquially, that ‘AC current’ flows in these trace sections, and 'DC
current' in the others. The reason is that the basic switching PWM frequency forms only a fraction of the
total harmonic (Fourier) content of the current waveform in the 'AC' traces. In comparison, where ‘DC
current’ flows, the current does not change in a stepped fashion and so the harmonic content is lower. It is
also no surprise that the DC sections are those in series with the main inductor, because it is known that
inductors have the property of preventing sudden changes in current (this is analogous to a capacitor
which ‘resists’ sudden changes in voltage).
3
SNVA054C–June 2002–Revised April 2013 AN-1229 SIMPLE SWITCHER
®
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