Switch Mode Power Supply Circuit Design Tutorial

Lanka 2020-08-05


Many engineers who have not used the switching power supply may have some worry about it, such as the PCB layout, the parameter and type selection of components and so on. In fact, as long as you understand the basic principle, the use of switching power supply design is very convenient.

A switch mode power supply usually consists of a controller and an output part. Some controllers integrate MOSFET into the chip, which makes it easier to use and simplify the PCB design, but the flexibility of components is weakened.

Switching controller is actually a closed-loop feedback control system, so there is a sampling circuit of output-voltage feedback and a feedback-loop control circuit. Therefore, this part of the design is to ensure accurate sampling circuit, and to control the feedback depth, because if the feedback loop response is too slow, it will have a great impact on the transient response ability.

The output parts include output capacitance, output inductor, MOSFET and so on. The selection of these devices is basically to balance the performance need and cost. For example, high switching frequency can use small inductance (which means small package and low cost), but high switching frequency will increase interference and the switching loss of MOSFET, result in reducing efficiency and increasing cost. Lower switching frequency has the opposite effect.

The selection of Rds_on parameters of MOSFET and the ESR for output capacitance are also very important. ESR is small can reduce output ripple, but the cost of capacitor will increase. And It is important to note that switching power controllers can not be well driven with too much MOSFET.

In general, suppliers of switching power supply controllers will provide specific formulas and usage options for engineers.

1. How to Debug the Switching Power Supply Circuit?

Experiences Sharing 

(1) The output of the power supply circuit is installed to the board through the low resistance and high power resistor, so that the power circuit can be debugged first before welding resistance, avoiding the influence of the latter circuit. 

(2) The switching controller is a closed-loop system. If the output deterioration beyond the range that the closed-loop can control, the switching power supply will work improperly. This situation requires careful examination of feedback and sampling circuits. Especially, if the output capacitance with a large ESR, lots of ripple of power supply will be produced, which will also affect the operation of switching power supply.

2. What Needs to Be Grounded?

At the very start, the introduction of grounding technology is a protective measure to prevent lightning strike on electric power or electronic equipments. The purpose is to introduce lightning current through lightning rod to the earth to protect buildings. And meanwhile, grounding is also an effective way to protect personal safety. When the phase line touches the shell of the equipment causing by some reason (such as poor insulation of wire, line aging, etc.), there will be a dangerous voltage in the shell of the equipment. Having grounding, the resulting fault current will flow to the earth, thus it plays a protective role. For example, in communication systems, the interconnection of signals between a large number of devices requires each device to have a point as reference, and with the complication of electronic equipment, the signal frequency is becoming higher and higher, therefore, grounding design as a special attention paid to the electromagnetic compatibility problems such as mutual interference between signals. In addition, improper grounding will seriously affect the reliability and stability of system operation. Recently, the concept of "grounding" has also been introduced into high-speed signal backflow technology.

3. Definition of Grounding

In the modern concept of grounding, for line engineers, the term usually means "reference point for line voltage"; for system designers, it is often a cabinet or frame; for electrical engineers, it is a green and safe ground line or a wire connected to the earth. A more general definition is that "grounding is the low impedance channel which the current returns its source." Noting that the points are "low impedance" and "channel".

Grounding Mode

There are many ways of grounding: single-point grounding, multi-point grounding and mixed type of grounding. Single point grounding is divided into series single point grounding and parallel single point grounding. In general, single point grounding is used in simple circuits, and low frequency (f10MHz) circuits use multipoint grounding or multilayer (complete a ground plane layer).

4. Introduction of Signal Backflow and Transpartition

For an electronic signal, it needs to find a way with the lowest impedance to return current to the ground, so how to deal with the signal backflow becomes very important.

First, according to the formula, we can know that the radiation intensity is proportional to the area of the loop. Specifically, the longer the path the return is, the bigger the ring is formed, and the greater the external radiation interference is, thus  the power-circuit flowback and signal loop area should as small as possible when design PCB.

Second, for a high-speed signal, providing a good signal backflow can guarantee its signal quality. Because the characteristic impedance of the transmission line on the PCB is generally calculated by reference to the ground (or power layer), if there is a continuous ground plane near the high speed line, the impedance of this line can be kept continuous, and if there is no ground reference near the section line, the impedance will change and the signal will be affected as well. Therefore, the high speed lines should be distributed to the layer near the ground plane, or it should be walked in parallel next to the each other, to shield interference and provide backflow nearly. 

Third, do not divide wires when having power supply in wiring way, this is because the signal backflow path across different power layers will be longer, and be vulnerable to interfere. For low-speed signals, it is not strictly required that, because the resulting interference signal can not be concerned about. But for high-speed signals should be checked carefully, do not cross as far as possible, you can adjust the power part of the wire. (this is for multiple power supplies on multilayer boards).

5. Should Analog Separate from the Digital , and How?

Whether analog signal or digital signal should return to the ground. Because the digital signal changes quickly and the noise caused by the digital signal will be very large, if analog and digital mixing, noise will affect the analog signal. 

In general, the grounding of analog and digital processing must be separated, then connected by a thin line, or a single point. The general idea is to try to block the noise from the digital ground to the analog ground. But this is not a very strict requirement that analog and digital ground must be separated, if the analog section near the digital ground is still very clean, they can be combined.

6. How is the Signal of the Single Board Grounded?

For the general device, the near ground is the best. After adopting the multilayer design with complete ground plane, the grounding of the general signal is very easy. The basic principle is to ensure the continuity of the line, reduce the number of holes, approach the ground plane or the power plane, etc.

7. How Do the Single Board Interface Devices Grounding?

Some veneers will have external input-output interfaces, such as serial port connectors, RJ45 connectors, etc. If their grounding is not well designed, it will also affect normal operation, such as error codes, packet loss, etc. And it will become an external source of electromagnetic interference sending the noise out. In general, a single interface grounding will be made, and the signal is connected by a thin wire connection, string 0 ohms or small resistance. Thin lines can be used to block signal ground noise. At the same time, the interface and the interface power filter should also be considered seriously. 

8. How to Grounding the Shield Layer?

The shielding layer of cables is connected to the interface grounding instead of the signal grounding, because there are various noises on the signal grounding. If the shield layer is connected to the signal ground, the noise voltage will drive the common-mode current to interfere outward along the shield layer. Therefore, the poorly designed cable is generally the maximum noise output source of electromagnetic interference. Of course, the interface ground should keep clean.