Power supply design experience summary

The rapid development of science and technology requires engineers to have higher and higher abilities. For an electronic engineer, the design of the power supply part is the core of the work. To this end, I would like to discuss some questions with you through several questions in this article. Our own experience in power supply design allows us to gain depth and progress in power supply design.

Q1: How to evaluate the power requirements of a system

Answer: For an actual electronic system, its power requirements must be carefully analyzed. Not only care about the input voltage, output voltage and current, but also carefully consider the total power consumption, the efficiency of the power supply implementation, the transient response capability of the power supply part to load changes, the tolerance range of key components to power supply fluctuations and the corresponding allowable Power ripple, heat dissipation issues, etc. Power consumption and efficiency are closely related. The higher the efficiency, the less the total power consumption will be when the load power consumption is the same, which is very beneficial to the power budget of the entire system. Compared with LDO and switching power supply, the efficiency of switching power supply is higher. Some. At the same time, evaluating efficiency not only looks at the efficiency of the power circuit at full load, but also pays attention to the efficiency level at light load.

As for the load transient response capability, there are strict requirements for some high-performance CPU applications, because when the CPU suddenly starts to run heavy tasks, the required starting current is very large. If the power circuit response speed is not enough, causing a sudden The voltage drops too much or too low, causing CPU operation errors.

Generally speaking, the actual value of the required power supply is mostly +-5% of the nominal value, so the allowable power supply ripple can be calculated based on this, of course, a margin must be reserved.

The heat dissipation issue is more important for those high-current power supplies and LDOs, and the suitability can also be evaluated through calculation.

Q2: How to choose a suitable power supply to implement the circuit

Answer: Based on the specific technical indicators obtained by analyzing the system requirements, you can choose the appropriate power supply to implement the circuit. Generally, for the weak current part, it includes LDO (linear power converter), switching power supply capacitor buck converter and switching power supply inductor capacitor converter. In comparison, LDO design is the easiest to implement and has small output ripple. However, the disadvantages are that the efficiency may not be high, the heat generation is large, and the current that can be provided is not large compared with switching power supplies, etc. The switching power supply circuit design is flexible and efficient, but the ripple is large, the implementation is complex, and the debugging is cumbersome, etc.

Q3: How to choose appropriate components and parameters for switching power supply circuit

Answer: Many engineers who have never used switching power supply design will have a certain fear of it, such as worrying about the interference problem of switching power supply, PCB layout problem, parameter and type selection of components, etc. In fact, as long as you understand it, it is very convenient to use a switching power supply design.

A switching power supply generally consists of two parts: a switching power supply controller and an output. Some controllers integrate MOSFETs into the chip, which makes it easier to use and simplifies PCB design, but the design flexibility is reduced.

The switch controller is basically a closed-loop feedback control system, so it generally has a sampling circuit for feedback output voltage and a feedback loop control circuit. Therefore, this part of the design is to ensure an accurate sampling circuit and to control the feedback depth, because if the feedback loop response is too slow, it will have a lot of impact on the transient response capability.

The output part design includes output capacitors, output inductors, MOSFETs, etc. These choices are basically to meet a balance between performance and cost. For example, a high switching frequency can use a small inductor value (meaning a small package and Cheap cost), but high switching frequency will increase interference and switching losses on the MOSFET, thereby reducing efficiency. Using a low switching frequency has the opposite effect.

The selection of the ESR of the output capacitor and the Rds_on parameter of the MOSFET is also very critical. Small ESR can reduce the output ripple, but the cost of the capacitor will increase, and good capacitors will be expensive. We should also pay attention to the driving capability of the switching power supply controller. Too many MOSFETs cannot be driven well.

Generally speaking, the supplier of switching power supply controller will provide specific calculation formulas and usage plans for engineers to learn from.

Q4: How to debug the switching power supply circuit

Answer: I have some experiences that I can share with you.

1: The output of the power circuit is connected to the board through a low-resistance high-power resistor, so that the power circuit can be debugged first without soldering the resistor to avoid the influence of the subsequent circuits.

2: Generally speaking, the switching controller is a closed-loop system. If the output deterioration exceeds the controllable range of the closed-loop, the switching power supply will not work properly. Therefore, in this case, the feedback and sampling circuits need to be carefully checked. Especially if an output capacitor with a large ESR value is used, a lot of power supply ripples will be generated, which will also affect the operation of the switching power supply.

Discussion on grounding technology

Q1: Why is grounding necessary?

Answer: The introduction of grounding technology was originally a protective measure to prevent electric or electronic equipment from being struck by lightning. The purpose is to introduce the lightning current generated by lightning to the earth through lightning rods, thereby protecting buildings. At the same time, grounding is also an effective means to protect personal safety. When the phase line caused by some reason (such as poor wire insulation, aging lines, etc.) comes into contact with the equipment shell, dangerous voltage will be generated in the equipment shell. The generated fault current will flow through the PE line to the earth, thus playing a protective role. With the development of electronic communications and other digital fields, only considering lightning protection and safety in the grounding system is far from meeting the requirements. For example, in communication systems, the interconnection of signals between a large number of devices requires each device to have a base 'ground' as the reference ground for the signals. Moreover, as electronic equipment becomes more complex, signal frequencies are getting higher and higher. Therefore, in grounding design, special attention must be paid to electromagnetic compatibility issues such as mutual interference between signals. Otherwise, improper grounding will seriously affect the reliability of system operation. sex and stability. Recently, the concept of "ground" has also been introduced in signal reflow technology for high-speed signals.

Q2: Definition of grounding

Answer: In the modern concept of grounding, to a line engineer, the term usually means the 'reference point of the line voltage'; to a system designer, it is often a cabinet or rack; to an electrical engineer, it is It means green safety ground wire or connected to the earth. A more general definition is "Ground is a low-impedance path for electrical current to return to its source." Note that the requirements are "low impedance" and "passage".

Q3: Common grounding symbols

Answer: PE, PGND, FG-protective ground or chassis; BGND or DC-RETURN-DC-48V (24V) power supply (battery) return flow; GND-working ground; DGND-digital ground; AGND-analog ground; LGND-protection Thunder protection ground.

Q4: Suitable grounding method

Answer: There are many ways of grounding, including single-point grounding, multi-point grounding and mixed types of grounding. Single-point grounding is divided into series single-point grounding and parallel single-point grounding. Generally speaking, single-point grounding is used for simple circuits, grounding areas are distinguished between different functional modules, and for low-frequency (f10MHz) circuits, multi-point grounding or multi-layer boards (complete ground plane layers) are used.

Q5: Introduction to signal reflow and cross-segmentation

Answer: For an electronic signal, it needs to find a path with the lowest impedance for the current to return to the ground, so how to handle this signal return becomes very critical.

First, according to the formula, we can know that the radiation intensity is proportional to the loop area. That is to say, the longer the path that the reflow needs to take, the larger the ring formed, and the greater its interference with external radiation. Therefore, when laying out the PCB Minimize the area of ​​power loop and signal loop as much as possible.

Second, for a high-speed signal, providing good signal return can ensure its signal quality. This is because the characteristic impedance of the transmission line on the PCB is generally calculated with the ground layer (or power layer) as a reference. If the high-speed line There is a continuous ground plane nearby, so that the impedance of this line can remain continuous. If there is no ground reference near the segment line, the impedance will change, and the discontinuous impedance will affect the integrity of the signal. Therefore, when wiring, the high-speed lines should be allocated to a layer close to the ground plane, or one or two ground wires should be run alongside the high-speed lines to provide shielding and provide reflow nearby.

Third, why it is said that when wiring, try not to separate it across power supplies. This is also because after the signal crosses different power layers, its return path will be very long and it will be susceptible to interference. Of course, it is not strictly required to not cross the power supply division. It is possible for low-speed signals because the interference generated can be ignored compared to the signal. For high-speed signals, check carefully and try not to cross them. You can adjust the wiring of the power supply part. (This is for the situation of multiple power supplies for multi-layer boards)

Answer: For general devices, it is best to ground them nearby. After adopting a multi-layer board design with a complete ground plane, it is very easy to ground general signals. The basic principle is to ensure the continuity of wiring and reduce unnecessary Number of holes; proximity to ground plane or power plane, etc.

Q6: Why should analog ground and digital ground be separated, and how to separate them?

Answer: Both analog and digital signals must flow back to the ground. Because digital signals change quickly, the noise caused on the digital ground will be very large, and analog signals require a clean ground reference to work. If analog and digital grounds are mixed together, noise will affect the analog signal.

Generally speaking, analog ground and digital ground should be processed separately, and then connected together through thin traces, or connected together at a single point. The general idea is to try to block noise on the digital ground from spreading to the analog ground. Of course, this is not a very strict requirement that the analog and digital grounds must be separated. If the digital grounds near the analog part are still very clean, they can be combined.

Q7: How are the signals on the board grounded?

Answer: For general devices, it is best to ground them nearby. After adopting a multi-layer board design with a complete ground plane, it is very easy to ground general signals. The basic principle is to ensure the continuity of wiring and reduce unnecessary Number of holes; proximity to ground plane or power plane, etc.

Q8: How are the interface devices of the single board grounded?

Answer: Some boards have external input and output interfaces, such as serial port connectors, network port RJ45 connectors, etc. If their grounding is not designed well, it will affect normal operation. For example, there are errors in the network port interconnection. , packet loss, etc., and will become a source of external electromagnetic interference, sending the noise in the board outward. Generally speaking, an independent interface ground will be separated separately, and the connection with the signal ground will be connected by thin wiring, and a 0 ohm or small value resistor can be connected in series. Thin traces can be used to block signal ground noise from passing to the interface ground. Similarly, the filtering of the interface ground and interface power supply must also be carefully considered.

Q9: How to ground the shielding layer of cables with shielding layer?

Answer: The shielding layer of the shielded cable must be connected to the interface ground of the single board instead of the signal ground. This is because there are various noises on the signal ground. If the shielding layer is connected to the signal ground, the noise voltage will drive the common mode current along the shielding layer. External interference, so poorly designed cables are generally the largest noise output source of electromagnetic interference. Of course, the premise is that the interface ground must also be very clean. The above is a collection of new power supply designs from some engineers, which will be helpful to beginners.