Analysis Of PCB Layout Design Techniques For Non-isolated Switching Power Supplies

- Dec 29, 2019-

A good layout design can optimize efficiency, reduce thermal stress, and minimize noise and effects between traces and components. All this comes from the designer's understanding of the current conduction path and signal flow in the power supply.

When a prototype power board is powered up for the first time, the best case is that it not only works, but also quietly and with low heat. However, this situation is rare.

A common problem with switching power supplies is unstable switching waveforms. Sometimes, the waveform jitter is in the sound band, and the magnetic components generate audio noise. If the problem lies in the layout of the printed circuit board, it can be difficult to find the cause. Therefore, the correct PCB layout in the initial stage of switching power supply design is very critical.

Power supply designers need a good understanding of the technical details and the functional requirements of the final product. Therefore, from the beginning of a circuit board design project, power supply designers should work closely with PCB layout designers on critical power supply layouts.

A good layout design can optimize power supply efficiency and mitigate thermal stress; more importantly, it minimizes noise and the interaction between traces and components. To achieve these goals, the designer must understand the current conduction path and signal flow inside the switching power supply. To achieve the correct layout design of a non-isolated switching power supply, it is important to keep these design elements in mind.

Layout planning

For an embedded dc / dc power supply on a large circuit board, to obtain the best voltage regulation, load transient response, and system efficiency, the power output must be close to the load device to minimize the interconnection impedance and conduction on the PCB traces. Pressure drop. Ensure good airflow and limit thermal stress; if forced air cooling is available, keep the power supply close to the fan.

In addition, large passive components (such as inductors and electrolytic capacitors) must not block airflow through low surface-mount semiconductor components, such as power MOSFETs or PWM controllers. To prevent switching noise from interfering with the analog signals in the system, it is necessary to avoid placing sensitive signal lines under the power supply as much as possible; otherwise, an internal ground layer needs to be placed between the power supply layer and the small signal layer for shielding.

The key is to plan the location of the power supply and the need for circuit board space in the early design and planning stages of the system. Sometimes designers ignore this advice and focus on the more important or exciting circuits on large system boards. Power management is viewed as an afterthought. Placing the power supply on the extra space on the circuit board is a disadvantage to efficient and reliable power supply design.

For multilayer boards, a good method is to place a DC ground or DC input / output voltage layer between the high-current power element layer and the sensitive small-signal trace layer. The ground plane or DC voltage plane provides an AC ground that shields small signal traces from high-noise power traces and power components.

As a general rule, neither the ground plane nor the DC voltage plane of a multilayer PCB should be separated. If this separation is unavoidable, the number and length of the traces on these layers must be minimized, and the routing of the traces must be kept in the same direction as the large current to minimize the impact.

Figures 1a and 1c are the bad layer structures of the six-layer and four-layer switching power supply PCBs, respectively. These structures sandwich the small signal layer between the high current power layer and the ground layer, thus increasing the capacitive noise coupling between the high current / voltage power layer and the analog small signal layer.