Applications of KVL and KCL In PCB Design

I have a bad habit of procrastination. I tend to be picky about what I do and those that are not as exciting, such as answering emails, are delegated to a lengthy to-do list. But procrastination always hit you in the end, and I’ve found that far too often. Unfortunately, even if these tasks are far down the to-do list, eventually you’ll still have to get to them. 

Tasks that you put off often gang up together and overwhelm you at the worst time. It’s so widespread, though, that my friends and I often like to have evenings once or twice a year where we write those emails we’ve owed for years and just haven’t gotten around to doing for each other. 

Unfortunately, there aren’t a lot of friends you can count on in your voltage and current simulations for a circuit board. But you can still find support in the form of laws like the applications of KVL and KCL. 

KVL and KCL: Kirchhoff’s in a Nutshell

In our obsession with the latest ICs and tools, we often neglect the fundamentals of electronics. Kirchhoff Voltage Law (KVL) and Kirchhoff Current Law (KCL) are two of the common laws that form the basis of electronics design. If you’re getting rusty with theories, let’s go down the memory lane.

The Kirchhoff Voltage Law states that the total algebraic sum of voltages across a closed loop is equal to zero. Right, it sounds like common sense when you’re educated in electronics, but it is still baffling when new PCB designers commit the grave mistake of omitting limiting-resistors for LEDs. 

The second law, Kirchhoff Current Law, states that the algebraic sum of current at a junction is equal zero. In other words, the amount of current entering a node must equal the total current exiting it. It’s a simple law that is sometimes overlooked in PCB design with often undesirable repercussions. 

Applications of KVL and KCL in Electronics Design

Both KVL and KCL are prominent in every single trace that you created on a PCB. Sometimes, it’s obvious that one of the rules apply while others don’t have KVL and KCL screaming from the circuit. However, it’s evident that forgetting these rules aren’t the best things to do in PCB design.

As mentioned, KVL applies to simple circuits, such as lighting up an LED. As an LED has a specific junction voltage and the voltage source is often way higher, the difference will have to be dissipated elsewhere in the circuit according to the KVL. If a limiting resistor is omitted, the copper trace takes the brunt of the voltage difference and gets overheated or broken in the process.

But KVL is more than lighting up an LED without burning the copper trace. The whole idea that defines KVL is about the conservation of energy. The energy source (power voltage), is being dissipated in a closed loop. However, the unchanging constant is the current flowing through it. This phenomenon has led to the popular usage of the 4-20 mA current loop signaling in industrial application.

KVL is the reason for a limiting resistor in LED circuitry.

Due to the fact that voltage drops over a lengthy cable, it is not a decent choice of signaling over a distance. Using current as a representation of an analog parameter is the better option, as it will remain constant over the entire length. 

KCL, which is all about having an equal amount of currents entering and exiting a junction, is important in planning the copper-trace width. While you’ll get scot-free by using the same minimal width copper trace for low-current applications, designs involving power MOSFET, LED drivers, and other high power components will require the deliberate calculation of the current entering and existing adjacent junctions.

Best Practices When Dealing With KVL and KCL In PCB Design

You could be working on some futuristic design, but the fact is, KVL and KCL will remain as fundamental laws in PCB design. The question is, how do you go about without violating these two fundamental laws of electronics.

Well, the same way when you ace your tests during university days. You’ll want to whip up a calculator, or a spreadsheet to calculate voltage drops or current that relates to both KVL and KCL. Of course, if you’re using advanced PCB tools, you may find simulator tools that calculate the value of the respective nodes and elements. 

But complying with KVL and KCL isn’t all about calculating values. 4-20mA receivers require converting current signals into voltage signals with a resistor and op-amp before being processed by an analog-to-digital converter (ADC). Analog voltage signals must be kept clear from high-speed signals for fear of noise-coupling.

Do it the hard way or use analysis tools for optimizing for KVL and KCL in PCB design.

 For designs involving high-power components, ensuring the proper trace width is the next process after calculating the current flowing through them. This prevents heat point on the PCB, which may affect the functionality of adjacent components.

You’ll be surprised how using the right PCB design software helps with obeying KVL and KCL in a project. For example, Cadence OrCAD PSpice Simulator helps calculate trace width on PCBs based on current flowing through nets in the schematic. PSpice also identifies EMI critical nets in these instances to ensure design integrity. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.