The heat dissipation and impedance of MOS are critical design elements

　　In electronic products, semiconductor devices are common components. The use of semiconductor devices will generate heat, engineers need to solve the problem of power consumption and heat dissipation. Then we need to pay attention to something called thermal resistance. When checking the specifications of semiconductors, there are almost all parameters related to thermal resistance. The three parameters Rja, Rjc, and Rjb are often seen. Many people are not clear about these three parameters, and they are not used in practical applications. Know which parameter to use to calculate.

　　First, let's first understand a few basic concepts.

　　Ta (Temperature Ambient) ambient temperature

　　Tc (Temperature Case) case temperature

　　Tj (Temperature Junction) junction temperature

　　Ta is the ambient temperature of the switch tube, and the general specification is 25°C.

　　Tc is the temperature of the package surface of the semiconductor device. If the MOSFET is metal encapsulated, this TC generally refers to the temperature on the metal sheet on the side close to the heat sink.

　　Tj is the temperature of the internal wafer of the semiconductor.

　　The concepts of Rja, Rjc, and Rjb in the specification.

　　Rja is the total thermal resistance of the semiconductor wafer to the environment.

　　Rjc is the thermal resistance from the semiconductor wafer to the case.

　　Rjb is the total thermal resistance from the semiconductor wafer to the PCB.

　　After knowing the concepts of Rja, Rjc, and Rjb, you will know how to apply them in actual applications. Heat is conducted from the inside of the wafer to the outside. The temperature of the wafer is the highest, so power semiconductor devices often have to calculate the maximum temperature, because the wafer temperature cannot exceed the maximum junction temperature. Generally, the junction temperature of semiconductors is 150℃ will be given in the specification. Then different packages of semiconductors are calculated differently, such as To-220 packaged MOS tube, if the heat sink is not used, the wafer will dissipate heat to the surrounding air through the shell, so when calculating the temperature, use Rja, the calculation formula Is Ta=Tj-P×Rja,

　　If the total loss of the MOS tube is known, then the temperature of the wafer can be calculated. For example, Rja is 62°C/W. If the loss is 1.2W and the ambient temperature is 65°C, then the junction temperature of the MOSFET can be calculated as much as possible. Up, Tj=Ta+P×Rja=65℃+1.2×62℃/W=139.4℃

　　Conversely, the junction temperature cannot exceed 150°C. If the maximum ambient temperature is 65°C, without a heat sink, Rja=62°C/W

　　The maximum loss on the MOS tube is P≤(Tj-Ta)/Rja=1.37W, and the maximum loss cannot exceed 1.37W by calculation.

　　If mos is the case with a radiator, then there are two specific scenarios:

　　1. The radiator is very large and the contact is good, so the contact thermal resistance is very small and can be ignored. According to Tj=Tc+P×Rjc, then TC at this time is the temperature of the surface of the semiconductor shell. Rjc=0.6℃/W, if the test case temperature is 25℃, then the maximum power P=(Tj-Ta)/Rjc= (150-25)/0.6=208W, the maximum power in the specification is calculated like this from.

　　2. The radiator is limited and has thermal resistance. In this case, Tj=Ta+P×(Rjc+Rcs+Rsa). Here Rcs refers to the thermal resistance of the MOS tube in contact with the radiator, and Rsa is the thermal resistance of the radiator to the ambient temperature. In actual use, the heat sink is often limited, and there are often insulating sheets that increase the thermal resistance. Therefore, the maximum temperature is required to be between 120°C and 130°C. If the surface temperature is high, the junction temperature inside will exceed 150°C.

　　MOS is a very important component of PCM and BMS, the real operator of the actual protection of lithium batteries, although the current lithium iron phosphate batteries are inherently very safe, and can be used in electric vehicles, large energy storage, UPS, etc. However, after all, lithium batteries are dangerous goods, so we must be very familiar with the temperature parameters of MOS, only in this way can we design a reasonable circuit board.