Harry Truman quoted the phrase “the buck stops here” so much that it became very popular. He further immortalized it by having it engraved in a sign that sat on his desk in the oval office. With apologies to the former president, however, we are going to look instead at how the buck “starts” here. In this case the buck that we are talking about is the DC to DC buck converter design that is often used on printed circuit board designs.
A buck converter is a DC to DC converter circuit that steps down DC voltage. It is widely used in low power applications on circuit boards where there is a need to convert voltage from one level to another. Although the circuitry is fairly simple, your circuit board can have performance problems if you don’t lay the buck converter out correctly during PCB design. A bad layout can result in excessive noise from the circuit, poor output voltage regulation, and a general lack of stability or device failure. Here are some layout tips that will help you create a better DC-DC buck converter design.
Component Placement in DC-DC Buck Converter Design
Starting off with a good component placement of your buck converter circuit will set you up for a successful layout. Some would advocate that you need to route as you place, while others place so that the routing will follow naturally. No matter which way you prefer, the key is to get your components in the correct locations. Many times PCB layout designers will place the components for neat and orderly spacing as opposed to the best circuit flow, and that can result in a bad buck converter layout.
Once the converter IC has been placed on the board, place the power components as close as possible to the IC:
Input Capacitor: The first critical part to place is the input capacitor, and it should be placed on the same surface layer as the IC pins that it is connected to. When this part is placed on the opposite board side, voltage noise can be created by the inductance of the via used to connect it to the IC.
Inductor: To reduce radiated EMI, the inductor should also be close to the IC and on the same board surface layer.
Output Capacitor: As the final power component in the buck converter circuitry, the output capacitor should be close to the inductor. This will minimize the routing distance between the components to help ensure good output voltage regulation.
Once the power components are on the board, place the other small-signal components of the buck converter circuit. These will include parts such as soft-start and decoupling capacitors that and not directly related to the power conversion. These parts are sensitive to noise and should be placed as close as possible to the IC so that they can directly route into it to help reduce their noise sensitivity.
A simple DC-DC converter board
Buck Converter Routing
A DC-DC buck converter used to design a switching power supply should have the goal of keeping inductance low for the critical paths of its routing. This is best done by reducing the length of the paths rather than their widths. With all power components on the same layer, you shouldn’t need to via any connections through the board which is good. Vias used for the power components can add significant inductance to the trace. The corners of your routing paths or traces should be done at a 45 degree angle or even better, rounded. Corners that are at a right angle can cause current waveform reflections and result in impedance changes.
It is also important how you handle the ground routing of the circuit. You should create a ground for the power components that is separate from the ground to the rest of the other components. The power components will be noisy and you will want to contain them within their own ground. The two ground paths can be joined together at one point, usually to the grounded thermal pad under the IC. The thermal vias in this thermal pad will connect the ground to the rest of the board.
DC to DC converter designs are often used in a variety of different PCB applications
Precision Layout: The Buck Stops with You
How well the power works on the completed printed circuit board will depend in part on how well you laid out the buck converter circuitry. This small portion of the layout often doesn’t get the attention it deserves considering how critical it actually is. Remember to keep the parts close together to minimize their connections, and keep an eye on how you partition the different grounds of the circuit. Create the ground routing of your layout so that the current return paths follow a logical progression and don’t push noisy currents through sensitive circuits.
Here is where your PCB design CAD tools can help. You need to use a layout system that allows you to easily set up design rules for the different types of power and ground circuits that you are going to be working with. This way you can assign different routing parameters to each net as well as set up spacing rules, components, and different net and component classes as well.
One such CAD system is OrCAD PCB Designer from Cadence which has all of the capabilities that we’ve been talking about. With its design rules and constraints, it is the PCB design system that you need for success with your buck converter designs.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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