How to find the power switch loop

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How to find the power switch loop [Copy link]

When it comes to designing a successful power supply, power supply layout is one of the most important aspects. However, everyone has their own opinions and reasons on how to do it. The fact is that many different solutions lead to the same result; most power supplies will work if the design is not a real mess. Of course, there are some general rules, such as: Don't run sensitive signals in the middle of fast switching signals. In other words, don't run feedback traces under the switch node. Make sure the power load traces and ground plane size are sufficient to support the current being applied. Try to maintain at least one continuous ground plane. Use enough vias (usually start with 1A per via) to connect the ground planes. In addition to these basic layout rules, I usually start by identifying the switching loops and then determining 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: Buck Power Supply Schematic and Layout

There are two states in a buck power supply (assuming continuous conduction mode): when the control switch (Q1) is on and when the control switch is off. When the control switch is on, current flows from the input to the inductor. When the control switch is off, current continues to flow in the inductor and through the diode (D1). Current continues to flow out. However, there is input pulse current, which is what you need to pay attention to in the layout. In Figure 1, this loop is labeled the "high frequency loop" and is shown in blue. Your first layout goal is to connect Q1, D1, and the input capacitors with the shortest, lowest inductance path possible. The smaller this loop is, the lower the noise generated by the switching. If this is ignored, the power supply will not operate effectively. The procedure for identifying switching loops applies to all power supply topologies. The steps in the procedure are: ●Determine the current path in the on state. ●Determine the current path in the off state. ●Find the location of continuous current. ●Find the location of discontinuous current. ●Minimize discontinuous current loops. This list shows the key loops for a given power stage configuration: ● Buck – Input capacitor loop. ● Boost – Output capacitor loop. ● Inverting Buck-Boost – Input and output capacitor loops. ● Flyback – Input and output capacitor loops. ● Fly-Buck – Input capacitor loop. ● SEPIC – Output capacitor loop. ● Zeta – Input capacitor loop. ● Forward, Half-Bridge, Full-Bridge – Input capacitor loop. Power supply layout is an art form, everyone has their own way, and it works a lot. Just be sure to identify the high-frequency switching loop first when you are determining the locations of the components for the power stage; you will save yourself time and frustration.