NewsCompany newsIndustry News

2019

02/25

Why is good PCB layout very important?

Engineering courses generally do not teach how to achieve good 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, ignoring the importance of board layout.


Understanding the rationale behind this board layout guidelines and adhering to it will minimize any PCB related issues with switch mode power supplies.

Switch mode power supplies are used to convert one voltage to another. This type of power supply is typically very efficient, so it replaces linear regulators in many applications.

MetInfo enterprise content manager system | MetInfo CMS

Switching frequency and switching


Switch mode power supplies operate at a certain switching frequency. The switching frequency can be either fixed (eg in PWM type control) or can be varied according to certain factors (eg in PFM or hysteretic control).


In either case, the switching mode power supply works in that it has a certain turn-on time Ton and a certain turn-off time Toff. A typical switching cycle of 50% duty cycle. This means that there is a certain current in the converter during 50% of the full period T; in the other 50%, there is a different current in the converter.


When we consider system noise, the actual switching frequency (in other words, the period length T) is not very important. If it is within the system's sensitive signal frequency range, the switching frequency or its harmonics may affect the system. But in general, the switching frequency is not the biggest factor affecting the system.


In switch mode power supplies, what really matters is the speed of the switching. We can see an enlarged view of the switch transition on the time scale. On a time scale with a period T of 2us, for a 500kHz PWM switching frequency, the transition looks like a vertical line. But after zooming in, we can see that switching conversion usually takes 30 to 90 ns.

 

Why is good PCB layout very important?


Each 2.5cm PCB trace has a trace inductance of approximately 20nH. The exact value of the inductance depends on the thickness, width and geometry of the trace, but empirically it is practical to take 20nH/2.5cm.


Assuming a buck regulator provides 5A of output current, we will see that the current is switched from 0A to 5A. When the switching current is large and the switching time is short, we can use the following formula to calculate the tiny trace. How much voltage offset is generated by the inductor:


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


It can be seen that a trace inductance of only 2.5 cm can generate a considerable voltage offset. This offset often even causes the switch mode power supply to fail completely. Placing the input capacitor a few centimeters from the input of the switching regulator usually results in the switching power supply not working. On a circuit board with improper layout, if the switching power supply still works, it will generate very large electromagnetic interference (EMI).


In the above formula, the only parameter we can change is the trace inductance. We can keep the traces as short as possible, thus reducing the trace inductance. Thicker copper wires also help to reduce inductance. Since the power required by the load is fixed, we cannot change the current parameters. For the conversion time, we can change, but generally do not want to change.


Slowing down the conversion time can reduce the resulting voltage offset, which reduces EMI, but the switching losses increase, and we will have to operate at a lower switching frequency and with expensive and bulky power supplies.

Find the AC current trace


In PCB layout of switch mode power supplies, the most important criterion is to keep the AC traces as short as possible in some way.


If you can follow this guideline carefully, good board layout can be said to have been successful 80%. In order to find these exchanges from "full current" to "no current" in a short time (conversion time) Trace the line and we draw the schematic three times.


It is a simple step-down switching mode power supply. In the schematic at the top, we use a dashed line to plot the flow of current during the turn-on time. In the middle schematic, we use a dashed line to plot the flow of current during the off time. The schematic at the bottom is especially noteworthy. Here, we draw all the traces of the current from the on time to the off time.


In this way, we can easily find AC current traces for any switch mode power supply topology.


When evaluating existing board layouts, a good idea is to print them on paper, put a transparent plastic plate, and then draw the current in the on and off times with different colored pens. Flow direction and corresponding AC wiring.


Although we tend to think that we can accomplish this relatively simple work in our minds, we often make small mistakes in the thinking process. Therefore, it is strongly recommended to draw traces on paper.


Achieve good PCB layout


The AC trace of the buck regulator. It must be noted that some ground traces are also AC traces and 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 via inductance is also quite high. There are very few exceptions to this rule.


If the AC path does not use vias, it will actually result in a larger trace inductance than the via itself, so vias are recommended. Multiple vias are better in parallel than just using a single via.

If you can follow this guideline carefully, good board layout can be said to have been successful 80%. In order to find these exchanges from "full current" to "no current" in a short time (conversion time) Trace the line and we draw the schematic three times.


It is a simple step-down switching mode power supply. In the schematic at the top, we use a dashed line to plot the flow of current during the turn-on time. In the middle schematic, we use a dashed line to plot the flow of current during the off time. The schematic at the bottom is especially noteworthy. Here, we draw all the traces of the current from the on time to the off time.


In this way, we can easily find AC current traces for any switch mode power supply topology.


When evaluating existing board layouts, a good idea is to print them on paper, put a transparent plastic plate, and then draw the current in the on and off times with different colored pens. Flow direction and corresponding AC wiring.


Although we tend to think that we can accomplish this relatively simple work in our minds, we often make small mistakes in the thinking process. Therefore, it is strongly recommended to draw traces on paper.

Achieve good PCB layout


The AC trace of the buck regulator. It must be noted that some ground traces are also AC traces and 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 via inductance is also quite high. There are very few exceptions to this rule.


If the AC path does not use vias, it will actually result in a larger trace inductance than the via itself, so vias are recommended. Multiple vias are better in parallel than just using a single via.


Special considerations for inductors


In terms of EMI, we must also consider the inductance. The actual device is not as symmetrical as many people think. The inductor has a core around which the core surrounds the wire. The winding always has a start end and an end end.


The start is connected to the inner winding of the inductor, and the end is taken out from the outer winding of the inductor. The beginning of the winding is usually marked with a dot on the device. It is important to connect the start end to a noisy switch node and the end to a quiet voltage. For a buck regulator, the quiet voltage is the output voltage.


Thus, the fixed voltage on the outer winding can electrically shield the voltage of the AC switch node on the inner winding, so that the EMI of the power supply will be lower.


By the way, so is the so-called shielded inductor. The outside of the shielded inductor with a certain magnetic permeability does use a shielding material that will tighten most of the magnetic lines of force on the package side. However, this material only suppresses the magnetic field and does not suppress the electric field.


The AC voltage on the outer winding is primarily a problem caused by electrical or capacitive coupling, and the shielding material of the shielded inductor does not inhibit such coupling. Therefore, the shielded inductor should also be placed on the board to connect the high noise switch node to the winding start to minimize EMI.


Switch mode power good board layout and wiring basis


Engineering courses generally do not teach how to achieve good 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, ignoring the importance of board layout.


Most of the problems caused by improper layout of the board can be attributed to the uncontrolled AC current trace being as short and compact as possible. Understanding the rationale behind this board layout guidelines and adhering to it will minimize any PCB related issues with switch mode power supplies.