With the development of the technological age, modular power supplies have become more and more popular in the market. When using modular power supplies, how should the stability and reliability of the modular power supplies be improved? Here are a few directions to introduce how to select external input and output capacitors to improve the service life of the module power supply and the stability and reliability of the entire power supply system.
In the practical application of module power supply, the selection of external input and output capacitors often starts from several aspects:
The rated voltage of the capacitor.
The capacitance of the capacitor.
The service life of the capacitor.
The following focuses on how to select input and output capacitors for points 1 to 2.
1. Select from the rated voltage of the capacitor
1. In the application of DC-DC power supply, we generally choose 1.3-1.4 times the withstand voltage of the external input capacitor.
For example, if the input voltage is 48V, the filter capacitor withstand voltage value we choose is 63V. The reason is that the DC-DC power supply has a large starting current, which easily leads to excessively high secondary impulse voltage, which damages the device.
2. AC-DC rectifier filter capacitor withstand voltage selection: in the AC-DC rectifier filter circuit, according to the formula 
For example: in the input 90-265Vac rectifier circuit, the voltage range after rectification can be 127-375Vdc through formula calculation. Based on experience, we generally choose capacitors with a withstand voltage of 1.1-1.3 times the theoretically calculated voltage value. Considering the cost factor, some designers will also consider a capacitor with 1.0 times the theoretically calculated voltage value.
2. Select from the capacitance of the input capacitor
In many documents, empirical formulas are often used for the selection of filter capacitor C 
Figure 1 Input and output equivalent circuit
In theory, increasing the capacity of the filter capacitor C in the circuit can indeed make the waveform of the output voltage smoother and less undulating, but at the moment the circuit is turned on, the inrush current factor generated in the circuit cannot be ignored. ; In the maintenance of some filter circuits, the replacement of filter capacitors also has the problem of inrush current. Without replacing other components, simply increasing the capacity of the filter capacitor will greatly shorten the actual service life of the entire circuit, or even burn the entire circuit. . Moreover, simply increasing the capacity of the filter capacitor has a limited effect on improving the output voltage. Do a simple experiment on the current curve when the size of the external input capacitor is started. The following figure shows the relationship between the size of the input external capacitor and the starting current.
From this curve, it can be seen that the larger the external input capacitance, the larger the starting current, and the larger the accompanying impact current in the circuit. The threat to the entire circuit is getting bigger.
The above figure shows the start-up current waveforms with different external capacitor sizes. The external capacitor can reach the lowest value under certain conditions, which greatly improves the input and output environment of the entire circuit. When the external capacitor exceeds this range, the starting current will increase, and the threat to other devices will also increase, leading to varying degrees of device damage.
Three, the choice of output capacitor
In the selection of the output capacitor, the general considerations are the output ripple noise of the power supply and the load capacity (capacitive load) of the power supply. When the cost permits, many people think that the larger the external output filter capacitor is, it can reduce the ripple noise interference and make the system power supply more stable. This is a wrong concept.
The selection of the output filter capacitor not only considers the ripple noise factor, but also considers the startup ability of the module power supply and the ability to withstand the input inrush current. As shown in the figure below: It is a product of the ZHIYUAN module that outputs 25w, and the input current waveform and output ripple noise graphs under different output capacitances.
The input and output waveforms of the external output capacitor 470uf test
Input and output waveforms of external output capacitor 4700uf test
From the data in the figure, it can be seen that the external output capacitor will reduce the ripple noise at a certain size, but the ripple noise will be at a balanced value as the external capacitor continues to increase. Looking back at the input current waveform, when the output capacitance is increased, the input current will be maintained at a high current for a longer time. If the used module is resistant to input impulse current, this may cause the module to be damaged by excessive impulse current for a certain period of time. Therefore, the external output capacitor is not as big as possible, which should be selected according to the module's starting ability and the ability to withstand current impact.
Four, summary
In the process of using the module power supply, reasonable external input and output capacitors are a kind of protection for the module module capacitance. It also greatly improves the service life of the power supply and reduces the existence of the entire working system due to the unstable power input and output. Unnecessary hidden dangers.
Advantages of ZLG Zhiyuan power module: In our Zhiyuan power module product description, we will recommend the most suitable peripheral circuits and the capacitance value of external capacitors to customers in detail, which effectively ensures the best environment for the module in the application. It also guarantees the safety and reliability of customer use. Avoid the loss of the module and the entire system due to the incorrect selection of external capacitors. The following figure shows the recommended diagram of the peripheral circuit of the dual output of the Zhiyuan module and the size of the external capacitor.
In order to further stabilize the input power supply, add a capacitor Cin at the input end. To further reduce the output ripple and noise, add a series capacitor Cout with a small equivalent impedance at the output end, but the capacitance value cannot exceed the maximum capacitance of the product. Load, otherwise it will cause poor startup of the power module. The recommended external capacitor value is shown in Table 1.
Table 1 Recommended external capacitor value
The isolated power module independently developed and produced by ZLG Zhiyuan Electronics has been accumulated in the industry for nearly 20 years. The current products have a wide input voltage range, isolate 1000VDC, 1500VDC, 3000VDC and 6000VDC and other series, and have various packaging forms and are compatible with international standard SIP , DIP and other packages. At the same time, Zhiyuan Electronics has built a first-class test laboratory in the industry to ensure the performance of power products, equipped with the most advanced and complete test equipment. The full series of isolated DC-DC power supplies have passed complete EMC tests, and the electrostatic immunity is as high as 4KV and surge immunity. The disturbance level is as high as 2KV, which can be applied to most complex and harsh industrial sites, providing users with stable and reliable power isolation solutions.
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