Issue link: https://iconnect007.uberflip.com/i/1523825
JULY 2024 I DESIGN007 MAGAZINE 27 It is important to understand that fluctuat- ing DC cannot exist; anything that fluctuates is actually AC. As designers, we oen use the term DC rather too loosely. At the instant of connection of a DC voltage to a load, a tran- sient current flows through the conductors (PCB traces), charging up their distributed capacitances. is transient current is AC and is a propagating EM wave that generates fluc- tuating EM fields (magnetic and electric) as it travels along the transmission line. Aerward, the current is DC and the EM fields become static, not fluctuating. Electric and magnetic fields are present around any electrical circuit, whether it car- ries AC or DC electricity. Since DC is static and AC varies in direction, fields from DC and AC sources have significant differences. Static fields, for example, do not induce cur- rents (crosstalk) in stationary objects, while AC fields do. Static magnetic fields do not vary over time and thus do not have a frequency (0 Hz). erefore, DC signals do not exhibit the skin effect, where current tends to flow only in the surface of a conductor. As the frequency increases, current is delegated to the surface. Charges do not move in a conductor unless there is an electric field to push them along. In a vacuum, electrons are unimpeded and accel- erate across the available space. ey give up their energy on impact at the end of their jour- ney. In a dielectric, the moving charges con- stantly collide with atoms, creating thermal energy (heat). On average, they attain a fixed velocity. is steady flow of charge is called a direct current. A direct current that flows in a conductor flows uniformly through the entire conductor cross-section (not only on the sur- face). In a DC circuit, energy flows due to the inter- play of electric and magnetic fields. e energy transfer occurs through the interplay of the electric field (due to the presence of charged particles) and the magnetic field (due to the motion of charged particles). It is not current that delivers the energy to the load, but rather electrons moving through the copper create both fields, facilitating the energy transfer. Figure 3: Electromagnetic fields of a DC power circuit. (Source: Juliano Mologni, Ansys)