Questions and Answers in the Design of Switching Power Supply
Time:2023-01-17
Views:1406
How to select appropriate components and parameters for switching power supply circuit?
Many engineers who have not used the design of switching power supply will have some fear of it, such as the interference of switching power supply, PCB layout, parameter and type selection of components, etc. In fact, as long as you understand, it is very convenient to use the switch power supply design.
A switching power supply generally includes two parts: switching power supply controller and output. Some controllers will integrate MOSFET 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 generally there will be a feedback output voltage sampling circuit and a feedback loop control circuit. Therefore, the design of this part is to ensure accurate sampling circuit and 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 design of the output part includes output capacitance, output inductor and MOSFET, etc. The selection of these devices is basically to meet the balance between performance and cost. For example, high switching frequency can use small inductance value (meaning small package and cheap cost), but high switching frequency will increase interference and switch loss on MOSFET, thus reducing efficiency. The result of low switching frequency is opposite.
For ESR of output capacitor and Rds of MOSFET_ The on parameter selection is also very important. A small ESR can reduce the output ripple, but the cost of capacitance will increase. A good capacitor will be expensive. The driving ability of switching power supply controller should also be noted that too many MOSFETs cannot be driven well.
Generally speaking, the supplier of switching power supply controller will provide specific calculation formula and use scheme for engineers to use for reference.
How to debug the switch power supply circuit?
Some experiences can be shared with you:
(1) The output of the power circuit is connected to the board through a low resistance value and high power resistance, so that the power circuit can be debugged first without welding the resistance to avoid the influence of the following circuits.
(2) Generally speaking, the switch controller is a closed loop system. If the output deterioration exceeds the range that can be controlled by the closed loop, the switch power supply will not work normally, so it is necessary to carefully check the feedback and sampling circuits in this case. Especially 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 switching power supply.
Why grounding?
The introduction of grounding technology was originally a protective measure taken to prevent electric or electronic equipment from being struck by lightning. The purpose was to introduce the lightning current generated by lightning to the ground through the lightning rod, thus playing the role of building protection. At the same time, grounding is also an effective means to protect personal safety. When the phase line (such as poor wire insulation, line aging, etc.) and the equipment shell touch for some reason, the equipment shell will have a dangerous voltage, 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 that each device should have a reference ‘ground‘ as the signal reference ground. Moreover, with the complexity of electronic equipment, the signal frequency is getting higher and higher. Therefore, in the grounding design, electromagnetic compatibility issues such as mutual interference between signals must be given special attention, 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.
Definition of grounding
In the modern grounding concept, for line engineers, the term usually means "reference point of line voltage"; For system designers, it is often a cabinet or rack; For electrical engineers, it means green safety ground wire or connecting to the ground. A more general definition is "grounding is a low impedance channel for current to return to its source". Note that the requirements are "low impedance" and "path".
Common grounding symbols
PE, PGND, FG - protective ground or enclosure; BGND or DC-RETURN-DC-48V (+24V) power supply (battery) return; GND-workplace; DGND - digital ground; AGND - analog ground; LGND - lightning protection ground.
Proper grounding method
There are many ways of grounding, including single-point grounding, multi-point grounding and mixed type grounding. The single-point grounding can be divided into series single-point grounding and parallel single-point grounding. Generally speaking, single-point grounding is used for simple circuits, grounding differentiation between different functional modules, and low-frequency (f10MHz) circuits, which require multi-point grounding or multi-layer board (complete ground plane layer).
Introduction to signal backflow and cross-division
For an electronic signal, it needs to find a way to return the current with the lowest impedance to the ground, so how to deal with this signal return becomes very critical.
First, according to the formula, the radiation intensity is proportional to the circuit area. That is to say, the longer the path that the return flow needs to take, the larger the loop formed, and the greater the interference of the external radiation. Therefore, when PCB layout, the area product of the power circuit and the signal circuit should be reduced as much as possible.
Second, for a high-speed signal, providing a good signal return can ensure its signal quality. This is because the characteristic impedance of the PCB transmission line is generally calculated by 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 section line, the impedance will change, The discontinuous impedance will affect the integrity of the signal. Therefore, when wiring, the high-speed line should be distributed to the layer close to the ground plane, or the high-speed line should be next to the ground and walk one or two ground wires to play the function of shielding and providing return current nearby.
Second, for a high-speed signal, providing a good signal return can ensure its signal quality. This is because the characteristic impedance of the PCB transmission line is generally calculated by 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 section line, the impedance will change, The discontinuous impedance will affect the integrity of the signal. Therefore, when wiring, the high-speed line should be distributed to the layer close to the ground plane, or the high-speed line should be next to the ground and walk one or two ground wires to play the function of shielding and providing return current nearby.
Thirdly, 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 that the power supply division cannot be crossed. It is acceptable for low-speed signals, because the generated interference can be ignored compared with the signal. Check the high-speed signal carefully and try not to cross it. You can adjust the wiring of the power supply. (This is for multiple power supplies of multilayer boards.)
Why and how should we separate analog and digital?
Both analog signal and digital signal should return to the ground, because the digital signal changes rapidly, so the noise generated 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 shall be processed separately, and then connected together through thin wiring or single point connection. The general idea is to try to block the noise on the digital ground from channeling to the analog ground. Of course, this is not a very strict requirement that 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.
How is the signal on the board grounded?
For general devices, the nearest grounding is the best. After the multilayer board design with complete ground plane is adopted, the grounding of general signals is very easy. The basic principle is to ensure the continuity of wiring and reduce the number of vias; Close to ground plane or power plane, etc.
How is the interface device of the board grounded?
Some boards will have external I/O interfaces, such as serial port connectors, network port RJ45 connectors, etc. If their grounding is not well designed, it will also affect the normal operation, such as network port interconnection, error code, packet loss, etc., and will become an external source of electromagnetic interference, sending the noise inside the board. Generally speaking, an independent interface ground will be separated separately, and the connection with the signal ground will be connected with thin wiring, which can be connected with a resistance of 0 ohm or small resistance value. The thin wiring can be used to block the noise from the signal ground to the interface ground. Similarly, the filtering of interface ground and interface power supply should also be carefully considered.
For cables with shielding layer, how to ground the shielding layer?
The shielding layer of the shielded cable should be connected to the interface ground of the board rather than 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 to interfere outward along the shielding layer. Therefore, poorly designed cables are generally the largest noise output source of electromagnetic interference. Of course, the premise is that the interface should also be very clean.
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