PCB Tech - PCB layout of switching regulator design

Why is good PCB layout important?

Every 2.5cm PCB trace has a trace inductance of about 20nH. The exact inductance value depends on the thickness, width and geometry of the trace, but based on experience, it is generally feasible to take 20nH/2.5cm. Assuming that a buck regulator provides an output current of 5A, you will see the current switch from 0A to 5A. When the switching current is large and the switching transition time is short, the following formula can be used to calculate the tiny trace inductance How much voltage offset is generated:

Assuming the wire length is 2.5cm (20nH), the output current is 5A (5A switching current in the buck regulator), and the switching time of the MOSFET power switch is 30ns, then the voltage offset will be 3.33V.

It can be seen that a trace inductance of only 2.5cm can produce a considerable voltage offset. This shift even often leads to complete failure of the switch-mode power supply. Placing the input capacitor a few centimeters away from the input pin of the switching regulator will usually cause the switching power supply to fail to work. On a circuit board with improper layout, if the switching power supply can still work, it will produce very large electromagnetic interference (EMI).

In the above formula, the only parameter that can be changed is the trace inductance. The traces can be made as short as possible to reduce trace inductance. Thicker copper wires also help reduce inductance. Since the power required by the load is fixed, the current parameters cannot be changed. For the conversion time, it can be changed, but generally do not want to change. Slowing down the switching time can reduce the voltage offset that is generated, thereby reducing EMI, but the switching loss will increase, and you will have to work at a lower switching frequency and use expensive and bulky power devices.

find the AC current trace

In the PCB layout of the switch mode power supply, the most important criterion is to make the AC traces as short as possible in some way. If you can carefully follow this rule, good circuit board layout can be said to have succeeded 80%. In order to find these ACs that change the current from "full current" to "no current" in a short time (conversion time) Route, draw the schematic diagram three times. It is a simple step-down switch mode power supply. In the schematic diagram at the top, the current flow during the on-time is drawn with a dashed line. In the middle schematic diagram, the current flow during the off time is drawn with a dashed line. The schematic at the bottom is particularly noteworthy. Here, all traces where the current changes from on time to off time are drawn.

Through this method, you can easily find the AC current traces of any switch-mode power supply topology.

When evaluating the existing circuit board layout and routing, a good way is to print it on paper and place a transparent plastic board, and then use different colored pens to draw the current during the opening time and closing time. Flow direction and corresponding AC wiring. Although we tend to think that this relatively simple task can be done in our minds, we often make some small mistakes in the thinking process. Therefore, it is strongly recommended to draw lines on paper.

Realize good PCB layout and routing

The AC wiring of the buck regulator. It must be noted that some ground traces are also AC traces, and also need to be kept as short as possible. In addition, for these AC current paths, it is recommended not to use any vias, because the inductance of the vias is also quite high. There are very few exceptions to this rule. If the AC path does not use vias, it will actually cause a larger trace inductance than the via itself, so it is recommended to use vias. Multiple vias in parallel is better than using only a single via.

The layout example of the circuit board using the ADP2300 step-down regulator. Check whether the AC wiring in the figure is laid out according to the absolute shortest path.

Connection A is laid out in the shortest possible path, because the high-side connection of C2 can be connected to the switching MOSFET (pin 5 of ADP2300, namely the Vin pin) with the shortest trace.

Connection B is the trace between pin 6 (SW pin) and the cathode side of diode D1. We also see that the trace is as short as possible to reduce trace inductance.

Connection C is the trace between the anode of diode D1 and the ground connection of C2. The PCB pads of these two devices are adjacent to each other and have the lowest trace inductance. In addition, it also helps that the AC current does not pass through a quiet ground plane. The ground plane should only be used as a reference voltage, and preferably no current (especially no AC current) flows through the ground plane. The vias next to C2 connect the ground area on the top layer of the PCB to the ground on the bottom layer, but no AC current flows through these vias.

Special considerations for inductance

In terms of EMI, inductance must also be considered. The actual device is not as symmetrical as many people think. The inductor has a magnetic core, and the magnetic core surrounds the wire. The winding always has a start end and an end end. The start end is connected to the inner winding of the inductor, and the end end is connected to the outer winding of the inductor. The beginning of the winding is usually marked with a dot on the device. It is very important to connect the start end to the noisy switching node and the end end to a quiet voltage. For a buck regulator, the quiet voltage is the output voltage. In this way, the fixed voltage on the outer winding can electrically shield the AC switch node voltage on the inner winding, so that the EMI of the power supply will be lower.

Incidentally, the so-called shielded inductor is the same. The outside of the shielded inductor with a certain magnetic permeability does use a certain shielding material, which will tighten most of the magnetic lines of force on the package side. However, this material can only suppress the magnetic field, not the electric field. The AC voltage on the outer winding is mainly a problem caused by electrical or capacitive coupling, and the shielding material of the shielding inductor does not inhibit such coupling. Therefore, the shielding inductor should also be placed on the circuit board to connect the noisy switching node to the start of the winding to minimize EMI.

The basis of a good circuit board layout for a switch mode power supply

Engineering courses generally do not teach how to achieve good circuit board layout. High-frequency RF courses will study the importance of trace impedance, but engineers who need to build their own system power supply usually do not regard the power supply as a high-frequency system, and ignore the importance of circuit board layout and routing. Most of the problems caused by improper circuit board layout and wiring can be attributed to uncontrolled AC current traces as short and compact as possible. Understanding the reasons behind the circuit board layout guidelines described in this article and strictly following them will minimize any PCB design-related issues with switch-mode power supplies.