Overheat protection circuit of power adapter

The temperature resistance and fire resistance of the power adapter are not only directly related to the reliability and service life of the power adapter, but also directly related to the risk of fire and the safety of the user's life and property.

The heat sources of the power adapter are mainly Schottky rectifier diodes, high frequency switching transformers, power MOS tubes, and filter electrolytic capacitors. Among them, the temperature rise of power MOS tubes, high frequency transformers and rectified Schottky diodes is more prominent. In order to prevent the power adapter from being damaged by overheating, the design of the switching power adapter not only requires the use of electronic components with good high-temperature characteristics, but also requires reasonable design of printed PCB boards, heat sinks, high-frequency transformers, etc., advanced production processes, and overheating Protective measures, these are all necessary conditions to ensure safety.

In order to suppress the temperature rise of the power adapter, in addition to selecting the power MOS tube with small leakage current and short storage time, it is also necessary to install a heat sink for the power MOS tube. Facts have proved that after the MOS tube is equipped with a heat sink, the stability of the power adapter will be greatly improved, and the loss rate will be significantly reduced. The function of electronic switch overheating protection measures is to cut off the AC input line of the power adapter, or forcibly turn off the modulated pulse output, and stop high-frequency oscillation before the temperature of the components or the power shell that is easy to heat in the power adapter exceeds the specified limit value.

The overheat protection type of the power adapter can be divided into the following categories: automatic reset type, manual reset type, non-renewable, non-reset type, and various other types that can provide equivalent overheat protection.

The overheat protector and the Switching Power Supply form a whole, and you need to pay attention to the placement position, the distance to the heat source, and the ease of replacement. The most basic placement requirement is that it cannot be mechanically impacted and is easy to disassemble; when the function of the protector is related to polarity, use a flexible cord to connect, and the plug without polarity should have overheat protection on both leads When the circuit of the protector is disconnected, it will not affect the normal operation of the power adapter, let alone cause fire or damage to electrical equipment. Usually the circuit board area of the power adapter and the space in the plastic shell are relatively small, and it is difficult to use an overheat protector. If the overheat protector is difficult to fit inside, a temperature fuse or thermistor can be used as the overheat protector. Stick it on the surface of the heat sink of a high-frequency transformer or power MOS tube. When the temperature rises to a certain value, the overheat protection switch can automatically cut off the power, or force the modulation pulse output to shut down to stop high-frequency oscillation.

If the power adapter adopts a control driver IC with overheating protection function, then there is no need to add any peripheral components or only a very small number of peripheral components need to be added, which can play the role of overheating protection. Because the control drive integrated circuit adopted by the power adapter has a built-in PN junction temperature sensor, it contains a thermal shutdown circuit. The driver IC without a built-in PN junction temperature sensor needs to connect a temperature sensing element outside it. The use of a driver integrated circuit with an overheating shutdown circuit makes the overheating protection very simple, and the price of the integrated circuit itself is also very low, and its cost-performance ratio is very high.

Best regards,

Ada Zheng