The role of the thermistor in the switching power supply

When the switching power supply is turned on, 220V AC power, after the fuse and the thermistor, after rectification, the capacitor is charged, and the characteristic of the capacitor is that the instantaneous charging current is large, thus causing impact on the front rectifier diode and the fuse, which is easy to cause Damage, in order to improve the coefficient of the power supply design, often add a resistor after the fuse to limit the current, the larger the resistance, although the current limiting effect is good, but the power consumed by the resistor is also larger, after the switching power supply starts, the current limiting resistor has no The role is to waste electricity.

In order to achieve better current limiting effect and save power, the current switching power supply often uses a negative temperature thermistor for current limiting (absorption of inrush current). The characteristic of the negative temperature thermistor is that the higher the temperature, the smaller the resistance. . At normal temperature, the resistance is generally 8-10 ohms, which is relatively large. When starting up, it has a good current limiting effect. After the power is turned on, the working current passes through the thermistor to make it heat up, and the resistance value of the thermistor drops sharply. 1-2 ohms), so that the thermistor is reduced to low after the power is turned on.

In order to reduce the consumption of this current limiting resistor after the switching power supply is started, in addition to using the negative temperature thermistor, there is also the use of a relay (after the power is turned on, the short circuit of the current limiting resistor is connected), and in the TV set. In particular, a bidirectional silicon circuit is used. After the power is turned on, the operating current drops to a certain value, and the bidirectional silicon is turned on, thereby short-circuiting both ends of the current limiting resistor, so that the current limiting resistor consumption is reduced to zero.