Layout tips for switching power supplies

Layout is very important when designing high frequency switching power supplies. Good layout can solve many problems with this type of power supply. Problems caused by layout usually manifest themselves at high currents and are more obvious when the voltage difference between the input and output voltage is large. Some of the main problems are reduced regulation at high output currents and/or large input/output voltage differences, excess noise on the output and start waveforms, and instability. These problems can be minimized by applying the following simple guidelines.

Inductors

Try to use low EMI (Electro Magnetic Interference) inductors with closed ferrite cores. For example, round or closed E-cores. Open cores can also be used if they have lower EMI characteristics and are far away from low-power wires and components. If using open cores, it is also a good idea to make the poles of the core perpendicular to the PCB board. Stick cores are usually used to eliminate most of the unwanted noise.

feedback

Keep the feedback loop away from the inductor and noise sources. Also keep the feedback line as straight and thick as possible. Sometimes you need to compromise between the two solutions, but keeping the feedback line away from the inductor's EMI and other noise sources is the more critical of the two. Keep the feedback line on the opposite side of the PCB from the inductor and separate it with a ground plane.

Filter capacitors

When using a small ceramic input filter capacitor, it should be placed as close to the IC's VIN pin as possible. This will eliminate as many trace inductance effects as possible, giving the internal IC circuits a cleaner voltage source. Some designs require the use of a feedforward capacitor from the output to the feedback pin, usually for stability reasons. In this case, it should also be placed as close to the IC as possible. Using surface mount capacitors will also reduce lead length, thereby reducing noise coupling into the effective antenna created by through-hole components.

compensate

If external compensation components are required for stability, they should also be placed as close to the IC as possible. Surface mount components are also recommended for the same reasons discussed for filter capacitors. These components should also not be too close to the inductor.

Traces and Ground Planes

Keep all power (high current) traces as short, straight, and thick as possible. On a standard PCB board, it is best to keep the absolute minimum width of the traces to 15mil (0.381mm) per ampere. The inductor, output capacitor, and output diode should be as close together as possible. This can help reduce the EMI caused by the power traces when large switching currents flow through them. This will also reduce lead inductance and resistance, thereby reducing noise spikes, ringing, and resistive losses, which will produce voltage errors. The ground of the IC, input capacitors, output capacitors, and output diodes (if any) should be connected directly together to a ground plane. It is best to have ground planes on both sides of the PCB. This will reduce ground loop errors and absorb more of the EMI generated by the inductor, thereby reducing noise. For multi-layer boards with more than two layers, the power plane (the area where the power traces and components are located) and the signal plane (the area where the feedback and compensation components are located) can be separated by a ground plane to improve performance. On multi-layer boards, through-holes are required to connect traces to different planes. If the trace needs to carry a large current from one side to another, it is a good habit to use one standard via for every 200mA of current.

Arrange the components so that the switch current loops rotate in the same direction. Depending on how the switch regulator operates, there are two power states. One state is when the switch is closed and the other state is when the switch is open. During each state, a current loop will be generated by the power device that is currently turned on. Arrange the power devices so that the conduction direction of the current loop is the same during each state. This prevents the routing between the two half loops from causing magnetic field reversal and can reduce EMI radiation.

Heat dissipation

When using surface mount power ICs or external power switches, the PCB can often be used as a heat sink. This is done by using the copper surface on the PCB to help dissipate heat from the device. Refer to the specific device manual for information on using the PCB to dissipate heat. This often eliminates the need for an external heat sink.