Difference Between AC and DC Current in Circuit Boards
The debate on AC current vs. DC current dates back to the 1880s during the War of the Currents.
Designers need to be aware of the issues that AC or DC currents can cause in complex PCB designs.
There are multiple ways designers can mitigate the variety of issues that come along with using either AC or DC currents.
Thomas Edison and Nikola Tesla argued their conflicting ideas on electric power in the War of the Currents
In 1880, a battle known as the War of the Currents occurred between Thomas Edison and Nikola Tesla. While Edison firmly believed that the future of electric power depended on the use of direct current, Tesla championed the use of alternating current.
Where are the present-day battle lines drawn? Today, large power distribution systems rely on AC voltages that change direction at specific intervals in time. Electric utilities can generate power through different types of facilities and then transmit very high voltages over long distances with minimal energy losses. In addition, increasing or decreasing an AC voltage is much easier than increasing or decreasing a DC voltage. However, DC powered equipment—such as tablets, smartphones, and portable lights—can run more efficiently with DC power that runs in only one direction and does not change.
Problems Caused by AC and DC Currents
DC vs. AC current
Consider high-frequency PCB design and multilayer boards—high-frequency AC signals from oscillators can couple from one circuit to another. Low-frequency DC signals can also couple from one circuit to the next and impact the operation of processors. In addition, leakage resistance might allow alternating currents and direct currents to enter signal circuits. Capacitance might also allow alternating current to seek a home within signal circuits.
The combination of AC and DC components in a complex circuit can lead to electromagnetic interference (EMI). Connecting a DC voltage of any magnitude to a load allows momentary transient, or ripple, currents to flow through traces and wires. These transient currents lead a double life; on one hand, the movement of the current changes it to an AC current, on the other hand, the transient current acts as a propagating electromagnetic wave that generates electromagnetic fields and noise.
In addition to DC transients, another DC-related problem exists: in digital circuits, integrated circuits draw current from the DC rail. Those currents include both a DC component and radio frequency (RF) currents. These RF currents can cause emissions.
Fixing AC and DC Current Problems
Winning the War of the Currents requires designers to take advantage of power and ground planes, use decoupling and coupling capacitors, and bypass the power supply at the supply terminals of an amplifier.
While separating multiple power planes allows better power and thermal management, it also isolates different regulated power supplies from noise. The use of different regulated power supplies prevents transmitter noise from coupling into receiver circuits and reduces opportunities for digital noise entering into analog circuits. Achieving noise control also requires separate reference or ground planes for each regulated voltage.
Separate analog and digital ground planes prevent the injection of digital noise signals into an analog trace. Without separation, fast-rising edges can generate current spikes that flow in the ground plane and harm the performance of analog circuits. Connecting the separate analog and digital grounds at a common ground point mitigates the possibility of circulating ground currents and noise.
Thomas Edison championed the use of direct current.
Decoupling capacitors decouple circuits from a power supply and filter any voltage spikes or AC voltage components from a regulated DC power supply voltage. The capacitors connect in parallel to the signal path and absorb noise that can cause digital logic circuits to have errors. Decoupling capacitors can also protect power supplies from any electric noise generated as logic circuits change states.
Coupling capacitors connect in series with the signal path and filter DC components from analog and digital circuits. In particular, coupling capacitors prevent DC signals from entering transmission lines for communication systems. When used with amplifiers, coupling capacitors prevent any incoming AC signals from changing transistor bias voltages.
Bypassing the rails-to-ground power supply involves connecting multiple parallel capacitors from the power supply of an operational amplifier to ground. The technique shows a low AC impedance to the power supply pins across a range of frequencies. With a low AC impedance path to ground across frequencies, unwanted noise stays away from the amplifier. The capacitors also compensate for any roll-off of the power supply rejection (PSR) for the op-amp. Using multiple capacitors with different values ensures the frequency response necessary for covering a wide range of frequencies. Most configurations place the capacitor with the lowest value and smallest size at the IC power supply pins and then continue with the next-higher value capacitor.
It’s critical that design teams remain up-to-date on problems caused by AC and DC currents. Find the latest research, tips, and materials at the PCB Design and Analysis overview page. See the latest from OrCAD PCB Designer and make sure you are on track and working with the best products for your next project.
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