The Isolated Power Supply in Your PCB Design

In September of 1991, the world was first introduced to the classic phrase “More Power” on the TV show “Home Improvement.” I remember laughing so much that I could barely breathe during the first episode when “Tim the Tool-Man Taylor” applied more power to his dishwasher. His enhancement promptly blew out the back of the kitchen cabinet, and sent a plate flying across the room. The lesson here; more power is fine, but it must be designed correctly.

When designing a circuit board, the components on the board will require power at specific voltages to function. Usually one or more power supplies are designed onto the board to fill this need. There are different types and configurations of power supplies that can be designed, with one of the simpler versions known as an isolated power supply. Here are some more details on what that is, and how it can be incorporated into a printed circuit board.

Why Use an Isolated Power Supply?

There are two types of power supplies that can be designed into a circuit board; isolated and non-isolated. Although isolation can refer to separating the power supply parts from the rest of the design on the PCB, it usually refers to isolation between the inputs and outputs of the power supply itself. This isolation is typically done with a transformer, which by its design provides a barrier that dangerous voltages can’t cross to invade the rest of the board circuitry A non-isolated power supply on the other hand relies on integrated circuits for separation, which don’t provide the same level of protection.

The separation that an isolated power supply affords in turn provides safety to the user. For devices that are stepping down high voltages, such as medical equipment, you don’t want any chance that a problem could shoot that high voltage straight into the user. On the other hand, the components of an isolated power supply, such as the transformer, take up extra space and add cost. If the device doesn’t have the system or safety standards that require an isolated power supply, they may be better off without it. Non-isolated power supplies are usually much more efficient than isolated supplies, and will even be used downstream of an isolated supply in a device.

Isolated power supplies therefore are going to be used in applications that have to meet safety requirements. Non-isolated supplies will be used on their own only in applications where the entire device is enclosed for user safety, such as wireless smart-home IoT devices.

The use of a transformer in an isolated power supply will protect users from electrical shock

PCB Layout Considerations for an Isolated Power Supply Design

When you are laying out an isolated power supply on your printed circuit board, you are going to have to accept the fact that you are going to be working with some larger components. Depending on the needs of the design, those transformers can get rather large and they could throw your placement strategy out the window. 

You should also plan ahead for the PCB layer stackup that your power supply will need. You will want to make sure that in multi-layer board configurations that you have either a power or a ground layer between the outer layer of the board where the power supply components are, and the inner layers where sensitive routing may be passing through.

Here are some other layout considerations that you should keep in mind with your power supply design:

• Placement: The goal is to keep your power connections as short and direct as possible, so keep the placement as tight as you can. Start with the main components of the power supply first, and then place the remaining parts. You also will want to keep the parts on the same side of the board for direct routing and to eliminate any impedance that would be caused by vias.

• Routing: In conjunction with the placement, your routing should be as short and direct as possible. Make sure that you use trace widths that are wide enough to handle the current load. You should also route corners in your power traces at a 45 degree angle, or round them. Another important point is to avoid routing other signal traces through the power supply area as much as possible.

• Ground planes: Creating a good solid grounding scheme for your power supply is important in controlling noise. To create the most optimum power supply ground, it is best to use a solid plane instead of traces. It is also a good idea to create a ground plane for the power supply that is separate from the common ground plane for the rest of the board. This will isolate the noise of the current return paths from the rest of the design, and prevent regular signal return paths from trying to punch through an area of noisy ground on the common plane. The two ground planes should then connect at a single point to isolate them from each other as much as possible.

• Thermal: A power supply will run hot, and you will want to use thermal vias under hot parts that will run into the ground planes to dissipate the heat. You may also need to design in extra mechanical heat sinks as needed.

There’s a lot to consider when designing a power supply; different component clearances, trace widths, and ground plane connections are just the start. In order to manage all of these requirements, you need a versatile design tool that can be configured for the greatest amount of control.

The right PCB design tools will help to place and work with the components in your design

Best Use of Your PCB Design Tools for Power Supplies

To help you with the design of your isolated power supply, use as much of the design rules and constraints in your CAD system as possible. Create power supply component classes so that you can set separate clearances for placement. In the same way, create as many individual net rules and net class rules to control all of the trace widths that you need. By setting these up before you start, you won't have to worry about fighting the online DRC’s while you are designing your power supply.

Another useful feature is the ability to view and check your power supply layout in 3D. This will tell you right off the bat about any clearance violation that you may have. Since your transformers and other power components are taller and larger than the other parts on your board, you may encounter problems that you normally don’t run across. The last thing that you want to find out during prototyping is that your transformer is busting through the device enclosure because it was in the wrong location.

Fortunately, the PCB design tools that you need for isolated power supply layouts areis already available to you from the Cadence line of high performance EDA tools. OrCAD PCB Designer has the features that we’ve discussed here, including a full range of design rules and constraints as well as interactive 3D viewing and checking.

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