Engineering courses generally don't teach how to achieve good PCB layout and routing. High-frequency RF courses may study the importance of trace impedance, but engineers who need to build their own system power supplies typically don't consider them as high-frequency systems and thus overlook the importance of PCB layout and routing.
Understanding the rationale behind the board layout and routing guidelines described in this article and strictly adhering to them will minimize any PCB-related issues with switch-mode power supplies.
Switch-mode power supplies are used to convert one voltage to another. These power supplies are typically very efficient, and therefore, in many applications, they replace linear regulators.
Switching frequency and switching conversion
Switching-mode power supplies operate at a fixed switching frequency. The switching frequency can be fixed (e.g., in PWM control) or it can vary depending on certain factors (e.g., in PFM or hysteresis control).
Regardless of the specific circumstances, the operating principle of a switch-mode power supply lies in its defined on-time (Ton) and off-time (Toff ), which corresponds to a typical 50% duty cycle. This means that during 50% of the complete cycle T, the converter carries a specific current; during the other 50% of the cycle, the converter carries a different current.
When considering system noise, the actual switching frequency (in other words, the period length T) is not very important. While the switching frequency or its harmonics may affect the system if it falls within the system's sensitive signal frequency range, it is generally not the most significant factor influencing system noise.
In switch-mode power supplies, what truly matters is the speed of the switching transition. We can see a magnified diagram of the switching transition on a time scale. On a time scale with a period T of 2µs, for a 500kHz PWM switching frequency, the transition appears as a vertical line. However, upon magnification, we can see that the switching transition typically takes 30 to 90ns.
Why is good PCB layout and routing so important?
Each 2.5cm PCB trace has approximately 20nH of trace inductance. The exact inductance value depends on the trace thickness, width, and geometry, but based on experience, 20nH/ 2.5cm is generally a practical approach.
Assuming a buck regulator provides 5A of output current, we will see the current switch from 0A to 5A . When the switching current is large and the switching time is short, we can use the following formula to calculate how much voltage shift a small trace inductance will cause:
Assuming a trace length of 2.5cm (20nH), an output current of 5A (the 5A switching current in a buck regulator), and a MOSFET power switch switching time of 30ns, then the voltage offset will be 3.33V .
This demonstrates that even a mere 2.5cm of trace inductance can generate a significant voltage shift. This shift can often lead to the complete failure of a switch-mode power supply. Placing the input capacitor a few centimeters away from the regulator input pin will typically cause the switching power supply to malfunction. On a poorly laid-out circuit board, if a switching power supply still operates, it will generate very high levels of electromagnetic interference (EMI).
In the formula above, the only parameter we can change is the trace inductance. We can make the trace as short as possible, thus reducing the trace inductance. Thicker copper wire also helps reduce inductance. Since the power required by the load is fixed, we cannot change the current parameter. We can change the transition time, but generally we don't want to.
Slowing down the switching time can reduce the resulting voltage offset, thereby reducing EMI, but it will increase switching losses, and we will have to operate at a lower switching frequency and use expensive and bulky power supply devices.
Find the AC current trace
In the PCB layout and routing of switch-mode power supplies, the most important principle is to make the AC traces as short as possible in some way.
If this principle is followed carefully, a good circuit board layout and routing can be said to be 80% successful . In order to find these AC traces that change the current from "full current" to "no current" in a very short time (transition time), we drew the schematic three times.
It is a simple buck switch-mode power supply. In the top schematic, we used dashed lines to depict the current flow during the on-time. In the middle schematic, we used dashed lines to depict the current flow during the off-time. The bottom schematic is particularly noteworthy. Here, we have drawn all the traces for the current to change from on-time to off-time.
Using this method, we can easily find the AC current traces for any switch-mode power supply topology.
When evaluating existing circuit board layouts, a good approach is to print them out on paper, place a transparent plastic sheet over them, and then use different colored pens to draw the current flow during the on and off times, as well as the corresponding AC traces.
While we tend to think that this relatively simple task can be done in our minds, we often make small mistakes in the thinking process, so it is strongly recommended to draw the lines on paper.
Achieving good PCB layout and routing
AC routing for buck regulators. It's important to note that some grounding traces are also AC traces and should be kept as short as possible. Furthermore, it's recommended to avoid using any vias for these AC current paths, as vias have relatively high inductance. There are very few exceptions to this rule.
If not using vias in the AC path would result in a larger trace inductance than the via itself, then using vias is recommended. Multiple vias in parallel are better than using only a single via.
Special considerations for inductors
In terms of EMI, we must also consider inductance. Actual devices are not as symmetrical as many people believe. An inductor has a magnetic core around which wires are wound. The winding always has a start and an end.
The start terminal is connected to the inner winding of the inductor, and the end terminal is connected to the outer winding. The start terminal of the winding is usually marked with a dot on the device. It is important to connect the start terminal to a high-noise switching node and the end terminal to a quiet voltage. For buck regulators, the quiet voltage is the output voltage.
In this way, the fixed voltage on the outer winding can electrically shield the AC switching node voltage on the inner winding, thereby reducing the EMI of the power supply.
Incidentally, the same applies to so-called shielded inductors. Shielded inductors with a certain permeability do indeed use some kind of shielding material on the outside, which tightens most of the magnetic field lines on the package side. However, this material can only suppress magnetic fields, not electric fields.
The AC voltage on the outer winding is primarily a problem caused by electrical or capacitive coupling, which the shielding material of the shielded inductor does not suppress. Therefore, the shielded inductor should also be placed on the circuit board to connect the high-noise switching nodes to the winding start, thereby minimizing EMI.
A good foundation for switch-mode power supply circuit board layout and routing
Engineering courses generally don't teach how to achieve good PCB layout and routing. High-frequency RF courses may study the importance of trace impedance, but engineers who need to build their own system power supplies typically don't consider them as high-frequency systems and thus overlook the importance of PCB layout and routing.
Most problems caused by improper PCB layout and routing can be attributed to the failure to control AC current traces to be as short and compact as possible. Understanding the rationale behind the PCB layout and routing guidelines described in this article and strictly adhering to them will minimize any PCB-related problems with switch-mode power supplies.
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