Selection of input current limiting resistor for switching power supply

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Selection of input current limiting resistor for switching power supply [Copy link]

A switching power supply with an input power of 12W, the input filter capacitor is 47uF/400V. When the capacitor is powered on, it charges and generates a large current.
　　In order to prevent this current impact from interfering with other equipment on the power grid, a "power-on current limiting resistor" is prepared to be connected in series after the fuse and before the rectifier bridge.
　　After the test, I still find it difficult to determine the resistor. I sincerely hope that all seniors and teachers will give me some advice. Thank you!


The analysis is as follows:

1. The necessity of adding an input series resistor
　　If there is no current limiting resistor, AC220V directly charges the input capacitor through the rectifier bridge. If it is powered on at the peak of the AC, and the voltage across the capacitor is 0V, the 310V voltage on the power grid is in a short-circuit state. The short-circuit current depends on the internal resistance Rs of the power grid. With 1 ohm, there will be an instantaneous power-on current of 300A. Although the time is not long, it will shorten the life of the input capacitor and the rectifier bridge, and may also interfere with nearby equipment.
　　Therefore, it is considered necessary to add this series current limiting resistor.


2. Check the relevant reference circuits.
　　The resistors are between 4.7 ohms and 20 ohms. Some use NTC (negative temperature coefficient resistors), some use ordinary resistors, and of course some do not.


3. Considerations for non-power-on conditions.
　　Calculated with 12W input power, the input AC current is 12w/220V=0.055A.
　　In steady state after power-on:
　　　　The voltage drop on the 5 ohm resistor is 0.055·5=0.27V, and the power dissipation is 0.055^2·5=0.015W.
　　　　The voltage drop on the 20 ohm resistor is 0.055·20=1.1V, and the power dissipation is 0.055^2·20=0.06W.
　　So it seems that the series voltage drop and power consumption after power-on are not large, so the selection of the series resistor should focus on the power-on instantaneous working conditions.

　　
4. About the withstand voltage of the series resistor
　　. Check the manual. The working voltage of resistors above 0.5W exceeds 350V
　　. There should be no problem in selecting 1~2W resistors according to this parameter.


5. About the maximum current of the series resistor
　　When the initial voltage of the input capacitor is 0V, the possible maximum voltage added to the current limiting resistor is 220·1.4=310V;
　　　　the maximum current through the 5-ohm resistor is 62A;
　　　　the maximum current through the 20-ohm resistor is 15.5A;
　　the current generates heat. It feels that whether the resistor can pass a large enough current is still related to the pulse power it can withstand.


6. About pulse power
　　I checked a resistor manual (maybe a bit old). For carbon film and metal film resistors, the maximum pulse power does not exceed 500~1000 times the rated power.
　　For a 5-ohm resistor, the rated power should reach 62^2·5/1000=19W
　　For a 20-ohm resistor, the rated power should reach 15.5^2·20/1000=4.8W
　　If calculated by 500 times, it should reach 38W and 10W respectively
　　- this is too big! ! ! Is my manual too old? What is the current indicator of the instantaneous pulse power of the resistor? Where can I find it? Please feel free to give me some advice!


7. About using NTC (negative temperature coefficient resistor)
　　The resistance of NTC resistor decreases as the temperature rises.
　　If it is used as this current limiting resistor, it needs to rely on its own power consumption to heat up. At the moment of power-on, it is a normal temperature resistance, and the entire large current power-on time is not long (20 ohm resistor; charging is estimated by triangle wave, about 2mS). It is estimated that the NTC resistance will not change much in such a short time, and even if it changes, it will be a temperature increase and a decrease in resistance
　　. After power-on, when working normally, even if the power consumption of a 100 ohm resistor is less than 0.3W (12W power input power), such a small power is estimated not to cause the NTC temperature to rise much, and the resistance value will not decrease too much.
　　——I wonder if there is any problem with my consideration?
　　——So what is the use of using NTC resistors?



I did the following experiment:

1. In order not to be affected by other factors, the circuit behind the input filter capacitor is disconnected, and only the fuse, current limiting resistor, rectifier bridge and input filter capacitor are retained.

2. When no current limiting resistor is added, the 0.5A~1A fuses are all blown.

3. When a 1~2W/10~20 ohm current limiting resistor (metal film) is added, the system starts normally, and the 1A fuse passes.
　　But after multiple power-ups, the system is blown.


　　Due to limited space on the board, I don't want to increase the wattage of the resistor, and considering the anti-interference performance, I don't want to reduce the input filter capacitor, so I am in a dilemma.

　　I feel there should be a simple solution, and I hope that teachers and seniors can give me some advice!

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