I grew up eating doughnuts and loving the sweet, savory texture of each bite. And when bagels came to town, I had trouble telling them apart. In fact, I thought bagel was just another name for the doughnut until I sunk my teeth into it. It was then that I realized that the hole in the middle is the only thing in common.
It’s alright if you can’t tell doughnuts and bagel apart. I wouldn’t suggest taking any if you’re on a diet. But you’ll have a tough time if you can’t tell a voltage source and a current source apart. Worse, some PCB designers thought that a current source is a myth that only exists in the textbook.
I’ve dealt with both voltage and current sources throughout my PCB design career. Let’s take a look at what each means when it comes to PCB design.
What Is A Voltage Source
A voltage source can be defined as a device that supplies a constant voltage to a load. In theory, a voltage source is ideal where it will maintain the specific voltage regardless of the current drawn. An ideal voltage source has no internal resistance.
In reality, an ideal voltage source does not exist. What you have are practical voltage sources, which do have internal resistance affects the voltage level across the load. A voltage source can produce varying voltage, such as an AC power supply, or it can provide a constant voltage like a battery.
A battery is a typical voltage source
In PCB design, using a voltage source is mandatory, because all ICs require a certain level of voltage to function. Voltage sources like switching regulators are defined by their current capacity, which is the maximum current. the load could handle reliably without significant drop in its output voltage or increase in temperature due to power loss in the internal resistance.
What Is A Current Source
Unlike a voltage source, it is harder to visualize a current source physically. In theory, it is defined as a device that could source a steady amount of current regardless of the voltage required for a load. The theoretically ideal current source can supply infinite voltage.
However, it is impossible for an ideal current source to exist in the physical world. A practical current source has a huge resistance value, but it is nevertheless finite. This means that a practical current source couldn’t sustain the required current if the voltage across the load rises to a certain level.
A current source usually involves transistors.
You don’t have a single component that is specifically a current source. A current source in PCB design is usually derived from transistors.
For instance, the emitter current of a BJT transistor is defined by the current that flows through the base. By providing a positive voltage at the base-emitter junction, a constant current will flow through the emitter which is a direct function of base current.
Designing With Voltage And Current Source In PCB
Now that we’re done with theories let’s get to dealing with voltage and current sources in PCB design. There are a few best practices that you’ll want to keep in mind when designing with both types of components.
The most common voltage sources are voltage regulators and batteries. When selecting a voltage source, you’ll need to ensure that it meets the power requirements of the load. For batteries, the critical parameter that you’ll want to consider is its capacity. As for voltage regulators, the maximum rated current is important to ensure it reliably supply the required voltage.
As mentioned, voltage sources do not have zero resistance. A voltage regulator, which is a voltage source, does have internal resistance. This means that heat will be dissipated from the voltage dropped across the internal resistance. You’ll want to ensure that heat dissipation is well-taken care of when designing with a voltage regulator.
An excellent example of a current source in PCB design is the current loop driver. A current loop driver is used to transmit an analog signal over a long distance. In industrial applications, 4-20 mA drivers are commonly used to transmit operational parameters to a centralized controller.
When designing a current source, such as a 4-20 mA driver, it is important to remember that the driver’s resistance is not infinite. This means that the current splits into two parts, one going through the internal resistance and the other going through the load. When the internal resistance decreases, more current will flows through internally, and lesser to the load.
When designing a current source, it is also equally important to remember that it will only source out current that is limited by the voltage the driver can support. A 4-20mA driver will continuously adjust its current to the level that is specified. However, when the length of the cable wire increases to a certain threshold, the driver will not be able to produce the voltage needed to push the required current through.
Whether you are designing with a voltage or current source, heat dissipation is an important consideration, and you’ll want an advanced PCB design software for that. PSpice SMOKE option will give you a clear perspective of thermal hotspots in the design.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.