System power supply design problems
Time:2022-08-10
Views:1883
For an electronic system, the design of power supply is becoming more and more important. I would like to discuss with you some of my own experiences on power supply design, so that we can have in-depth and progress in power supply design.
Q1: how to evaluate the power demand of a system
Answer: for an actual electronic system, we should carefully analyze its power requirements. Not only the input voltage, output voltage and current, but also the total power consumption, the efficiency of the power supply, the transient response ability of the power supply to the load change, the tolerance range of the key devices to the power supply fluctuation and the corresponding allowable power ripple, as well as the heat dissipation. Power consumption and efficiency are closely related. If the efficiency is high, the total power consumption will be less when the load power consumption is the same, which is very beneficial to the power budget of the whole system. Compared with LDO and switching power supply, the efficiency of switching power supply is higher. At the same time, the efficiency evaluation is not only based on the efficiency of the power supply circuit at full load, but also on 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 response speed of the power supply circuit is not enough, the transient voltage will drop too much and too low, resulting in the CPU running error.
Generally speaking, the actual value of the required power supply is mostly + - 5% of the nominal value, so the allowable power ripple can be calculated based on this, and of course, a margin should be reserved.
Heat dissipation is very important for high current power supply and LDO, and it can also be evaluated through calculation.
Q2: how to select a suitable power supply circuit
Answer: according to the specific technical indicators obtained by analyzing the system requirements, you can choose the appropriate power supply implementation circuit. Generally, the weak current part includes LDO (linear power converter), switching power capacitor buck converter and switching power inductor capacitor converter. In contrast, the LDO design is the easiest to realize, with small output ripple, but the disadvantages are that the efficiency may not be high, the heat generation is large, and the current available is not large compared with the switching power supply. The switching power supply circuit design is flexible and efficient, but the ripple is large, the implementation is complex, and the debugging is cumbersome.
Q3: how to select appropriate components and parameters for switching power supply circuit
Answer: many engineers who have not used the design of switching power supply will have a certain fear of it, such as worry about the interference of switching power supply, PCB layout, parameters and type selection of components. In fact, as long as you understand, it is very convenient to use a switching power supply design.
A switching power supply generally includes a switching power supply controller and an output. Some controllers integrate MOSFETs into the chip, which makes the use easier and simplifies the PCB design, but reduces the flexibility of the design.
The switching controller is basically a closed-loop feedback control system, so there is usually a sampling circuit for feedback output voltage and a control circuit for feedback loop. Therefore, the design of this part is to ensure the accurate sampling circuit and 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 design of the output part includes the output capacitance, output inductance and MOSFET, etc. these choices are basically to meet a balance between performance and cost. For example, a high switching frequency can use a small inductance value (meaning a small package and cheap cost), but a high switching frequency will increase interference and switching loss to MOSFET, thus reducing efficiency. The result of using low switching frequency is opposite.
ESR for output capacitance and RDS for MOSFET_ On parameter selection is also very important. A small ESR can reduce the output ripple, but the capacitor cost will increase. A good capacitor will be expensive. The driving ability of the switching power supply controller should also be noted that too many MOSFETs cannot be well driven.
Generally speaking, the supplier of switching power supply controller will provide specific calculation formula and use scheme for reference by the engineer.
Q4: how to debug the switching power supply circuit
Answer: I have some experience to share with you
1: The output of the power supply circuit is connected to the board through a low resistance high-power resistor, so that the power supply circuit can be debugged first and avoid the influence of the later circuit without welding resistance.
2: Generally speaking, the switching controller is a closed-loop system. If the output deterioration exceeds the range that the closed-loop can control, the switching power supply will not work normally. Therefore, in this case, the feedback and sampling circuits need to be carefully checked. In particular, if the output capacitor with large ESR value is used, a lot of power ripple will be generated, which will also affect the operation of the switching power supply.
Discussion on Grounding Technology
Q1: why grounding?
Answer: the introduction of grounding technology was originally a protective measure taken to prevent power or electronic equipment from being struck by lightning. The purpose is to introduce the lightning current generated by lightning to the ground through the lightning rod, thus playing the role of protecting buildings. At the same time, grounding is also an effective means to protect personal safety. When the phase line (such as poor insulation of the wire, aging of the line, etc.) caused by some reason touches the equipment shell, dangerous voltage will be generated in the equipment shell, and the resulting fault current will flow through the PE line to the ground, thus playing a protective role. With the development of electronic communication and other digital fields, only considering lightning protection and safety in the grounding system is far from meeting the requirements. For example, in the communication system, the interconnection of signals between a large number of devices requires each device to have a reference ‘ground‘ as the reference ground of the signal. Moreover, with the complexity of electronic equipment, the signal frequency is getting higher and higher. Therefore, in the grounding design, special attention must be paid to electromagnetic compatibility problems such as mutual interference between signals. Otherwise, improper grounding will seriously affect the reliability and stability of system operation. Recently, the concept of "ground" has also been introduced into the signal return technology of high-speed signals.
Q2: definition of grounding
Answer: in the modern grounding concept, for the line engineer, the term usually means‘ reference point of line voltage ‘; For the system designer, it is often a cabinet or rack; For electrical engineers, it means green safety ground or connected to the ground. A more general definition is "ground is the low impedance channel through which current returns to its source". Note that the requirements are "low impedance" and "path".
Q3: common grounding symbols
Answer: PE, PGND, FG - protective ground or enclosure; Bgnd or dc-return - dc-48v (+ 24V) power supply (battery) return; GND - place of work; DGND digital ground; Agnd analog ground; LGNd lightning protection ground
Q4: proper grounding mode
Answer: there are many ways of grounding, including single point grounding, multi-point grounding and mixed type 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 distinction between different functional modules, and low-frequency (f < 1MHz) electronic circuits. When designing high-frequency (F > 10MHz) circuits, it is necessary to use multi-point grounding or multilayer board (complete ground layer).
Q5: introduction of signal backflow and cross division
Answer: for an electronic signal, it needs to find a path for the lowest impedance current to return to the ground, so how to deal with the signal return becomes very critical.
First, according to the formula, the radiation intensity is directly proportional to the loop area, that is, the longer the return path, the larger the loop formed, and the greater the interference to external radiation. Therefore, the area between the power circuit and the signal circuit should be reduced as much as possible during PCB layout.
Second, for a high-speed signal, providing a 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 reference to the stratum (or power supply layer). If there is a continuous ground plane near the high-speed line, the impedance of the line can remain continuous. If there is no ground reference near the segment line, the impedance will change, The discontinuous impedance thus affects the integrity of the signal. Therefore, when wiring, the high-speed line shall be distributed to the layer near the ground plane, or one or two ground wires shall be laid beside the high-speed line to play the role of shielding and providing return flow nearby.
Third, why should we try not to divide the wiring across the power supply? This is also because after the signal crosses different power supply layers, its return path will be very long and easy to be disturbed. Of course, it is not strictly required not to cross the power supply division. It is possible for low-speed signals, because the generated interference can be ignored compared with the signal. For high-speed signals, it is necessary to carefully check them and try not to cross them. You can adjust the wiring of the power supply. (this is for multi-layer board power supply)
Q6: why and how to separate analog ground from digital ground?
Answer: both the analog signal and the digital signal should be returned to the ground. Because the digital signal changes rapidly, the noise caused on the digital ground will be very large, and the analog signal needs a clean reference work. If analog and digital 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 wiring or single point. The general idea is to try to block the noise from digital ground to analog ground. Of course, this is not very strict. The analog ground and the digital ground must be separated. If the digital ground near the analog part is still clean, it can be combined.
Q7: how is the signal on the single board grounded?
Answer: for general devices, the nearest grounding is the best. After adopting the multi-layer board design with complete ground plane, the grounding of general signals is very easy. The basic principle is to ensure the continuity of wiring and reduce the number of vias; Near the ground plane or the power plane, etc.
Q8: how is the interface device of the single board grounded?
Answer: some boards have external I / O interfaces, such as serial port connectors and network port RJ45 connectors. If their grounding is not well designed, their normal operation will also be affected. For example, there are code errors and packet losses in the network port interconnection, and they will become external electromagnetic interference sources, sending the noise in the board to the outside. Generally speaking, an independent interface ground will be separated separately, and the connection with the signal ground is connected by a thin wiring, and a resistance of 0 ohm or small resistance can be connected in series. The thin wiring can be used to block the signal ground noise from passing to the interface ground. Similarly, the filtering of interface ground and interface power supply shall also be carefully considered.
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