NTC thermistor and inrush current.Stage lens price

　　1. Why does the NTC of the low-power power supply not need to add a relay, and the high-power power supply needs to add a relay? Is it just because the power consumption improves efficiency?

　　2. Does the low-power NTC not need to consider hot start?

　　Let me tell you how I found the answer.

　　In order to take care of the students who are not the power supply, I will briefly introduce what this NTC is and what it is used for, so that everyone can see it.

　　NTC is a negative temperature coefficient thermistor, that is, the higher the temperature, the lower the resistance.

　　The NTC is placed in the above circuit, and its function is to limit the inrush current when it is turned on.

　　Before starting up, the filter capacitor has no electricity, and the voltage across the capacitor is 0V. At the moment of power-on, the voltage at the 2 end of the capacitor cannot change suddenly, it is still 0V, which is equivalent to a short circuit, and the voltage drop of the diode is also very small, so the voltage drop mainly falls on the NTC.

　　It is conceivable that if there is no NTC in the loop, then the current is very large, or if the resistance of the NTC is small, the current will be very large.

　　An NTC thermistor is connected in series on the line. Before the startup, the temperature of the thermistor is relatively low, so the resistance is relatively large, which can well limit the inrush current during startup.

　　After power on, the temperature of the thermistor rises, the resistance value is relatively low, and it will not cause excessive loss.

　　So why suppress inrush current?

　　Because this large current will flow through the diode when it is turned on, if the current is too large, the diode may be damaged. After all, rectifier diodes all have a parameter called IFSM, that is, the maximum inrush current allowed to pass is limited.

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