High-frequency switching power supply layout principle - Database & Sql Blog Articles

Proper layout plays a crucial role in the design of high-frequency switching power supplies. A well-thought-out layout can significantly reduce many common issues associated with these power supplies. These issues often become more apparent at high current levels and are especially noticeable when there is a large difference between the input and output voltages. Common problems include reduced regulation capability under heavy load, increased noise on the output signal, waveform distortion, and potential instability. By following some basic layout principles, these challenges can be effectively minimized.

Inductor

Switching power supplies typically use low-EMI inductors with ferrite-closed cores, such as circular or E-shaped cores. Open-core inductors can also be used if they have lower EMI characteristics and are positioned away from low-voltage traces and components. When using open cores, it’s beneficial to orient them so that their poles are perpendicular to the PCB surface. Stick-type cores are often employed to suppress unwanted noise and improve overall performance.

Feedback

To maintain stability and reduce noise interference, the feedback loop should be kept as far away as possible from the inductor and other noise sources. The feedback trace should be as straight and thick as possible. Although there may be trade-offs in routing, keeping the feedback path isolated from the inductor is more critical for minimizing EMI. Ideally, the feedback line should be placed on the opposite side of the inductor, separated by a ground plane for better isolation.

Filter Capacitor

When using a small ceramic input filter capacitor, it should be placed as close as possible to the VIN pin of the IC. This helps minimize line inductance and ensures a cleaner voltage supply for the internal circuitry. Some designs may require a feedforward capacitor connected from the output to the feedback pin for improved stability. In such cases, the capacitor should also be placed near the IC. Using surface-mount capacitors further reduces lead length, which minimizes the chances of noise coupling into sensitive components through parasitic antennas.

Compensation Components

If external compensation components are needed for stability, they should also be placed as close as possible to the IC. Surface-mount technology is recommended here, similar to the approach used with filter capacitors. Additionally, these components should not be too close to the inductor to avoid interference and noise coupling.

Traces and Ground Plane

All high-current traces should be as short, straight, and wide as possible. On standard PCBs, a minimum trace width of 15 mils (0.381 mm) is generally recommended. The inductor, output capacitor, and output diode should be grouped closely together to reduce EMI generated by high-current switching. This arrangement also lowers lead inductance and resistance, reducing noise spikes, ringing, and resistive losses—factors that contribute to voltage errors. The IC's ground, input capacitors, output capacitors, and output diodes (if present) should all connect directly to a ground plane. It is best to have a ground plane on both sides of the PCB to reduce ground loop errors and absorb EMI generated by the inductor, thereby lowering overall noise. For multi-layer boards, the ground plane can separate the power plane (where power traces and components are located) from the signal plane (where feedback and compensation components reside), improving system performance. Vias should be used to connect traces between layers, and for high-current applications, it’s good practice to add a via every 200mA to ensure reliable current transfer.

Current Loop Orientation

Arrange components so that the primary current loops rotate in the same direction. Depending on the regulator's operation mode, there are two main states: when the switch is on and when it is off. During each state, a current loop is formed by the active power device. By aligning the conduction direction of the current loops in both states, you prevent magnetic field reversal between the two loops, which reduces EMI emissions and improves overall efficiency.

Heat Dissipation

When using surface-mount power ICs or external power switches, the PCB itself can serve as a heat sink. Utilizing the copper area on the PCB helps dissipate heat from the components. Refer to the specific device's datasheet for guidance on how to optimize thermal performance using the PCB. This method can often eliminate the need for an additional heat sink, saving space and cost in the final design.