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Introduction to the design process of isolation switching power supply module

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.04.15
Switching power supplies come in various types of circuits and can be designed to meet different application scenarios based on their unique strengths. Below is a brief explanation of the process of designing an isolation switching power supply module.

First, determine the power:

Choose the corresponding topology structure according to specific requirements, such as selecting a flyback type for isolation switch power supply modules, which can basically meet the requirements.

Select the corresponding PWMIC and MOS for preliminary circuit schematic design:

After we determine to use a flyback topology for design, we need to select the corresponding PWMIC and MOS for preliminary circuit schematic design, which can be split or integrated design. Split type PWMIC and MOS are separate, which has the advantage of freely matching power, but the disadvantage is that the design and debugging cycle will be longer. Integrated PWMIC and MOS are integrated in one package, saving designers a lot of computation and debugging steps, making it suitable for engineers who are just starting or developing quickly.

Create a schematic diagram:

After determining the selected chip, start working on the schematic. It's best to take a look at the corresponding datasheet before designing and confirm the simple parameters. Whether choosing integrated PI, discrete designs such as 384x or OBLD, you need to refer to datasheets. Generally, datasheets are accompanied by simple circuit diagrams, which serve as the basis for our design.

Determine the corresponding parameters:

After completing the schematic, we need to determine the corresponding parameters in order to proceed to the next step of PCB layout. Of course, different companies have different processes, and we need to follow the corresponding procedures to develop a good design habit. This step may involve preliminary evaluation, schematic confirmation, and calculation can be carried out after signing off.

Determine the switching frequency and select the magnetic core to determine the transformer:

The frequency of the chip can be set through an external RC, and the operating frequency is equal to the switching frequency. The function of this peripheral is beneficial for us to better design the switching power supply, and external synchronization function can also be adopted. Generally, the operating frequency of ACDC power modules should not be set to exceed 100kHz. This is mainly because if the frequency of the switching power supply is too high, it is not conducive to system stability and even less conducive to EMC passability. If the frequency is too high, the corresponding di/dtdv/dt will increase. For the selection of magnetic cores, the key is based on the switching frequency and power, and more on empirical selection. Of course, in terms of calculation, you need to obtain more magnetic core parameters, including magnetic materials, Curie temperature, frequency characteristics, and so on.

Design transformer for calculation:

Based on the input/output, switching frequency, and selected magnetic core parameters, as well as the efficiency, maximum duty cycle, and magnetic induction intensity changes, we can proceed to the next step of calculation to obtain power, average and peak voltage current, turns, and inductance.

After confirming the number of turns, the thickness of the enameled wire can be directly determined without the need for complex calculations. The wire diameter, like conventional resistors, has a fixed value. Just remember several commonly used fixed value wire diameters. The next step is to determine the size of the input and output capacitors, and then proceed with layout and layout.

Input/output electrolytic capacitor calculation:

Here, based on the input power and output current calculated in the previous step, the specifications of the electrolytic capacitor are finally determined. The frequency and impedance are selected according to the application environment. The larger the theoretical value of the capacitor Cin, the better it is for the subsequent stage. However, from a cost perspective, there will be no unlimited selection of large capacity. Basically, by this point, the components that need to be determined on the PCB have been completed, that is, the PCB packaging is complete. The next step is to define the device packaging through the schematic diagram.



PCB Layout：

The transformer, schematic diagram, and electrolytic capacitor have been determined above, and the following are all standard components. After generating the network table by sch, defining the board edges in PCBfile and loading the corresponding packaging library, the network table can be directly imported for layout.

The ring area formed between the RCD absorption part of the layout and distribution board and the transformer should be as small as possible, which can reduce the corresponding radiation and conduction. The ground wire should be as short and wide as possible to ensure that the corresponding zero level is conducive to the stability of the reference. In areas with significant changes in di/dtdv/dt, minimize loops and widen wiring to reduce unnecessary inductance characteristics.

Determine some parameters:

After the PCB layout is completed, the same name end of the transformer can be determined, the transformer can be fully defined, and it can be sent out for sampling or self winding.

Debugging process:

After completing the above parts, the design of an isolation switch power supply module is basically complete, and the rest is the process of soldering board debugging. The simple equipment required for debugging includes voltage regulators, oscilloscopes, multimeters, etc., while auxiliary equipment includes power meters, LCR bridges, and electronic loads. After welding the board, perform a static inspection. If there is an LCR bridge, you can first measure the parameters of the same name end and inductance of the transformer before welding. Static inspection mainly focuses on whether there is virtual soldering, tin connection, etc. After static testing, you can use a multimeter to test whether the input and output are in a short circuit state, and finally perform a power on test.
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