The function and physics behind the buck converter.
PCB design considerations for a buck converter.
How to avoid potential routing issues.
The buck converter is widely adopted for its energy-efficient design
A buck converter is a DC-to-DC converter circuit that reduces DC voltage, which is why it is also commonly referred to as a step-down converter. Among available power options, buck converter PCBs provide a highly efficient circuit, with efficiency ratings typically among the 95th percentile. They are widely used in low power applications on circuit boards where there is a need to drop source voltage to common electronics voltages such as 5V or 3.3V.
Although the circuitry is fairly simple, a circuit board can have performance problems if the buck converter is not placed and routed correctly. A poor layout can result in excessive noise from the circuit and substandard voltage regulation at the output. In extreme cases, a general lack of stability or even device failure could result from poor implementation. In this article, we will discuss the theory behind buck converters as well as some layout tips that can help create a better DC-DC buck converter PCB design.
Buck Converters in PCBs
There are a multitude of power design options for PCBs that depend on factors such as source current and voltage, thermal concerns, and noise susceptibility. Buck converters are classic designs that quickly endear themselves for their utility and efficiency. To understand the wide adoption of buck converters for PCBs, let’s start by looking at how they operate.
How Do Buck Converters Function?
For any time-variant circuitry (typically those containing switches), the change in current and voltage must be examined in the cases when the switch has been closed for long periods and then opened (or vice versa). Traditionally, a flyback diode in parallel with the load is responsible for closing the circuit once the switch opens (and once again, vice versa), but this may be replaced in modern designs with a second transistor for synchronous rectification.
To begin, imagine the situation with a closed switch (the on-state) and what that means for the current and voltage of the components. After a long period in the on-state, the inductor has fully accepted its initial reluctance to the current flowing through it and resembles a short in operation. The capacitor, which initially served as a short circuit, now is functionally an open. Current flows from a source through the inductor and back to load. Switching to the off-state, both the inductor and capacitor wish to maintain their current favored parameter: current for the inductor and voltage for the capacitor. The inductor collapses its magnetic field and stored magnetic energy to essentially exchange voltage for current, as its preference is to see a steady-state current. To recap, at steady on-state conditions, the voltage at the load will be equivalent to the source, and the transient circuit state will have the inductor maintaining the current flow through the load.
Continuous and Discontinuous Mode
There was something of a Chekov’s Gun in the last paragraph–the capacitor’s function in the transient circuit has not been discussed. As it turns out, the buck converter operation outlined above is only one method of operation: continuous mode. In it, the assumption is that the on-state is restored at some point before the current through the inductor reaches zero. When the inductor’s current is allowed to reach zero, the second backup power kicks in, with the capacitor collapsing its electrical field to generate a current to maintain its voltage. In this discontinuous operation, the capacitor restores the current through the inductor, which can then continue to pass current through to the load until the energy stored in the capacitor is exhausted.
Switches and Diodes
As briefly mentioned above, a step-down converter can feature either a switch and a diode or a switch between two switches. The diode, due to its orientation and reverse bias, has a low voltage drop during the on-state and a low current draw during the off-state due to its high resistance. It performs its role well, but there are greater efficiency gains to be had with a second switch. Because the diode cannot be toggled on and off, there is a constant power loss associated with it. For low duty cycle power circuitry, this is less of an issue; a more consistent off-state means the power drop becomes negligible. However, in systems where the duty cycles are appreciable, significant power savings can be realized by replacing the diode with a second switch.
The trade-off is that more control must be enacted to have the second switch dynamically respond to the needs of the circuit. In particular, the double on-state of the two switches can lead to shoot-through, a potentially catastrophic failure event where the source is shorted. In effect, the two-switch design has greater responsiveness and efficiency but needs additional control implementation to function. Luckily, many transistor ICs include additional timing functionality and other fail-safes to prevent this circuit state.
Component Placement in DC-DC Buck Converter PCB Design
A successful step-down converter integration into a layout starts with component placement. Routing should flow naturally from placement, with short, direct traces on critical nets and cascading bypass capacitors outwards by increasing capacitance. Best practices should always follow the layout instructions of the manufacturer’s datasheet, if available. Ultimately, the key is not only proper component placement relative to the associated circuitry, but also to nearby components that may be susceptible to the rapid switching of the transistors. 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 poorly optimized buck converter PCB 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.
Circuit simulation of a buck converter. Note the moderate duty cycle - this might be a good candidate for simultaneous rectification if efficiency is a concern
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 are 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 they can provide an immediate return path to reduce noise sensitivity. Also, beware of having sensitive components or critical signals that do not belong to the step-down converter too close to the circuitry. This will help prevent inductive coupling events that could cause unexpected runtime errors.
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. For thermal and electrical performance, the thermal pad should use the maximum toleranced dimensioning to support greater conductivity. With a greater area, there is more room for vias to connect to the nearest reference plane in multilayer designs. A common misconception is a need to isolate power grounds from the rest of the circuit–this is likely unnecessary and could result in EMI or signal integrity issues. Ground is ground, and efforts to dissuade noise are best handled using more direct solutions available to the layout designer, such as noise filters or best placement practices.
Precision Layout: The Buck Stops With You
How well the power works on a completed printed circuit board will depend in part on the placement, routing, and plane design of buck converter circuitry. Keep parts close together to minimize connections and monitor how you partition the different grounds of the circuit. Create the ground routing of a buck converter PCB 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 network as well as set up spacing rules, components, and different net and component classes. 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 buck converter designs.
Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. If you’re looking to learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.