Successful power supply design, layout is one of the most important links

Power supply layout is one of the most important aspects of successful power supply design. However, everyone has their own views and reasons on how to do this. The truth is that many different solutions are the same; if the design is not really a mess, most power supplies will work.

Of course, there are some general rules in this, such as:

Do not run sensitive signals in fast switching signals. In other words, do not run feedback tracking under the switch node.

Ensure that the power load tracking and ground plane size is sufficient to support the current.

Try to keep at least one continuous ground plane.

Use enough vias (usually starting with each via 1A) to connect the ground planes.

In addition to these basic layout rules, I usually first identify the switching loops and then determine which loops have high frequency switching currents. Figure 1 shows an example of a simplified power stage for a buck power supply (schematic and layout).

Figure 1: Step-down power supply schematic and layout

There are two states in the buck power supply (assuming continuous conduction mode): when the control switch (Q1) is turned on and when the control switch is turned off. When the control switch is turned on, current flows from the input to the inductor. When the control switch is turned off, current continues to flow through the inductor and through the diode (D1). The current is continuously output.

But there is an input pulse current, which is what you need to pay attention to in the layout. In Figure 1, this loop is labeled "High Frequency Loop" and is shown in blue. The primary goal of your layout is to connect Q1, D1 and the input capacitor to the shortest, lowest inductance loop. The smaller the loop, the lower the noise generated by the switch. If you ignore this, the power supply will not work effectively.

The procedure for identifying the switch loop applies to all power supply topologies. The various steps of the procedure are:

The current path is determined in an on state.

The current path is determined in the off state.

Find the location of the continuous current.

Find the location of the intermittent current.

Minimize intermittent current loops.

The key loops for a given power stage configuration are listed in this list:

Buck - Input capacitor loop.

Boost - the output capacitor loop.

Inverting Buck-Boost - Input and Output Capacitor Loops.

Flyback - input and output capacitor loops.

Fly-BuckTM - Input Capacitor Loop.

SEPIC - Output capacitor loop.

Zeta - input capacitor loop.

Forward, half bridge, full bridge - input capacitor loop.

Power layout is just like an art form. Everyone has their own way, and it often works. One thing to be sure of is that when you determine the position of the power stage, first determine the high frequency switching loop; this saves you time and trouble.