When doing power supply design or driving circuits, it is inevitable to use MOS tubes. There are many types of MOS tubes and many functions. For the use of power supply or drive, of course, its switching function is used. Regardless of N-type or P-type MOS tube, its working principle is essentially the same.
We often look at the PDF parameters of MOS tubes. MOS tube manufacturers use the RDS(ON) parameter to define the on-resistance. For switching applications, RDS(ON) is also the most important device characteristic. The data sheet defines RDS(ON) in relation to the gate (or drive) voltage, VGS, and the current flowing through the switch, but for adequate gate drive, RDS(ON) is a relatively static parameter. The MOS tube that is always on is easy to heat up. In addition, slowly increasing junction temperature will also lead to an increase in RDS(ON). The MOS tube data sheet specifies the thermal impedance parameter, which is defined as the semiconductor junction heat dissipation capability of the MOS tube package. The simplest definition of RθJC is the thermal resistance from junction to case.
The reason why the MOS tube heats up with a small current:
1. Circuit design problem
It is to make the MOS tube work in a linear working state, not in a switching state. This is also one of the reasons for the heat of the MOS tube. If N-MOS is used as a switch, the G-level voltage is a few V higher than the power supply to be fully turned on, while P-MOS is the opposite. It is not fully turned on and the voltage drop is too large, resulting in power consumption, the equivalent DC impedance is relatively large, and the voltage drop increases, so U*I also increases, and the loss means heat generation. This is the most taboo mistake in designing circuits.
2. The frequency is too high
The performance is sometimes excessive pursuit of volume, resulting in increased frequency, increased loss on the MOS tube, and increased heat generation.
3. Not doing enough heat dissipation design
If the current is too high, the nominal current value of the MOS tube generally requires good heat dissipation to achieve. Therefore, if the ID is less than the maximum current, it may also generate serious heat, and sufficient auxiliary heat sinks are required.
4. The selection of MOS tube is wrong
The power judgment is wrong, and the internal resistance of the MOS tube is not fully considered, resulting in an increase in the switch impedance.
How to solve the serious heat generation of mos tube small current:
1. Do a good job in the heat dissipation design of the MOS tube, and add enough auxiliary heat sinks.
2, paste heat dissipation glue.
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