Application of PTC and NTC in Switching Power Supply Protection Design.magnifier reader price

　　Ordinary switching power supplies use NTC and relays in parallel. NTC is essentially a negative temperature coefficient thermistor. The higher the temperature, the lower the resistance value. The function used in the power supply is to suppress the inrush current during startup. The temperature of the NTC is low at the moment of startup, and the resistance value is large, which suppresses the inrush current. , and then the NTC temperature rises, the resistance value drops, and it has been reduced to a very low level without power consumption. However, if the machine is repeatedly switched on and off in a short time, and the NTC has no time to cool down, the resistance value will always be very low, and the current cannot be suppressed, so it will not play a protective role, so a relay needs to be connected in parallel. It cools down and the next boot is ready to go right away, so it's the best of both worlds.

　　However, in some switching power supplies, such as inverter air conditioners, PTC and relays are used in parallel. PTC is also a thermistor with a positive temperature coefficient, that is, the higher the temperature, the greater the resistance. what about this?

　　Because the air conditioner, especially the inverter air conditioner, needs to charge the large capacitor when it is turned on. In addition, the starting current of the compressor is also large, which has a great impact on the power supply and is easy to burn out the circuit. Therefore, PTC is used. After the PTC temperature rises, the resistance increases. Large, the control current should not rise too fast, let the outdoor unit circuit "slowly" wake up, after normal operation, the relay will pull in, short-circuit the PTC, and prevent a high voltage drop across the PTC, because at this time the PTC resistance high value. Of course, no power is consumed after being short-circuited. If there is an abnormality during the boot process, the main relay does not pull in, and the PTC resistance becomes large as the temperature rises, which plays the role of blocking the current, similar to the fuse blown (of course not completely blown).

　　Therefore, the reason why the air conditioner uses PTC instead of NTC is mainly because the air conditioner has a larger inrush current and a longer time when it is turned on. Therefore, the control requirements for the inrush current at startup are higher than that of ordinary switching power supplies, and PTC can be used to "continuously" control the current. The increase of , gives the back-end main control circuit a "slow" start-up time, and at the same time plays a protective role in the event of an abnormal start-up.

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