Bi-Directional Discrete Parts in Your PCB Design

The old story is that elephants are afraid of mice. In truth, an elephant isn’t really afraid of anything due to its great size, but that story makes for a great metaphor; that something so small could control something so large. There are a lot of other examples of this though, including hydraulic systems that lift great weights, and small rudders which steer large ships.

Another example is the small discrete parts on a circuit board. They don’t perform all the functions of the large active integrated circuit components on the board, but without them, the circuit board wouldn’t work. A discrete part is an electronic component that has only one circuit element within it, such as resistors, capacitors, inductors, and other passive parts. Let’s take a closer look at these bi-directional discrete parts, and how they are used in PCB layout.

What are Bi-Directional Discrete Parts?

Before integrated circuits (ICs) were introduced, all components such as resistors, capacitors, diodes, and transistors were single element parts, and therefore they were considered discrete parts. With most of these functions now being incorporated into ICs though, there are fewer active discrete parts used on a circuit board. There is still an important need however for passive discretes such as resistors, capacitors, and inductors on most every circuit board.

Passive discrete parts are almost always two-pin components. They do not generate energy, nor do they need any additional input or power to operate as an active device would. A passive component does, however, consume and dissipate the energy that is fed into it. Because of their function, most passive devices are bi-directional, meaning that energy can flow in either direction through them. There are some exceptions, however, such as electrolytic capacitors which do have a specific polarity of their energy flow, but for the most part, passive parts can be connected either way.

Although the resistors, capacitors, and inductors are the three main categories of passive parts, those categories are made up of several smaller groups of components. These include fixed resistors, variable resistors, ceramic capacitors, film capacitors, coils, and chokes. All of these components have different functions and are important in the operation of a circuit board.

Bi-directional discrete resistors and capacitors on a circuit board

How Passive Bi-Directional Discrete Components are Used in PCB Design

Passive components are used for a variety of purposes on a circuit board, from filtering a signal to providing temporary storage for power. Here are some of the more common purposes that you might see in a PCB design:

• Resistors: As the name would suggest, resistors restrict the flow of current in an electrical circuit. This could be to reduce the amount of current, or to control it. In addition to controlling current, resistors can serve specific purposes in a circuit. For instance, to prevent a signal pulse from reflecting back through a transmission line and interfering with the next pulse, a termination resistor will be used. This resistor dampens the signal pulse instead of allowing it to reflect back down the line.

• Capacitors: These components store energy in an electrical field when power is conducted through them, and discharge that energy when the voltage drops. This makes a capacitor an important part of controlling current fluctuations in a circuit in order to help preserve signal integrity. With the use of bypass capacitors on active components that are actively switching signal states, you can prevent ground bounce and other noise-related problems. In a capacitor, energy is calculated in terms of voltage.

• Inductors: To slow down current spikes, an inductor will temporarily store energy in an electromagnetic field before releasing it back into the circuit. This action will help oppose sudden changes in current and filter out unwanted frequencies. In an inductor, energy is calculated in terms of current, and where a capacitor works best at higher frequencies, an inductor is used more for lower frequencies.

That is a brief explanation of how bi-directional discretes function in a design, but how they are used in the physical layout of the board is just as important.

Resistors that are used to terminate a transmission line must be placed either at the driver of the signal or at the end of the trace. Bypass capacitors should be as close as possible to the power pin of the device that they are connected to, and their trace routing should go from the device pin straight to the capacitor, and then to the power plane. For inductors and other passive components used in power supplies, you need to keep their distances short with their routing as much as possible on the same layer as the components.

As with the placement of all PCB components, you need to make sure that you adhere to the recommended spacing guidelines between parts in order for the board to be completely manufacturable. At the same time though, you need to keep some of these parts as close as possible. The best way to accomplish this task is to use PCB design tools that have a full set of design rules and constraints to help you.

OrCAD’s design for assembly constraints gives you control over your placement spacing

Working with Passive Discrete Components in Your PCB Design Tools

To make sure that your PCB layout is going to be manufacturable and work as intended, you need to make sure that your design tools are set up correctly for the job. This starts with the PCB footprints that you will use, which must be correct so that you can place them exactly as you need. Next, you need to fully set up the design rules and constraints for these parts. Advanced PCB design tools will allow you to set up general placement and routing rules, as well as specific rules for nets, parts, and classes of nets and parts. This will allow you to exercise a very precise amount of control over how you place and route these parts.

Fortunately, there are PCB design systems already available to you that can handle this level of precision control over place and route that we’ve been talking about. OrCAD PCB Designer from Cadence has the advanced rules and constraints to give you the level of precision that you need in your layout. In addition, OrCAD has many other tools and features in it to expertly take your design from concept to the final manufacturing files.

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