In the application of switching power supply, the application of MOS is required to conduct regular and off. For example, the basic buck converter used in DC-DC power supply relies on two MOS tubes to perform the switching functions, which are stored in the inductor to store energy, and then the energy released to the load. We often choose the frequency of hundreds of kHz and even more than 1 MHz, because the higher the frequency, the magnetic components can be smaller and lighter. During normal operation, the MOS tube is only equivalent to a conductor. As a result, we are most concerned about the circuit or power design is the minimum conduction loss of MOS.

We often look at the PDF parameters of the MOS tube, the MOS control maker uses RDS (ON) parameters to define the conduction impedance, the switch application, RDS (ON) is the most important device characteristics. The data sheet is defined by RDS (ON) which is related to the gate (or driving) voltage VGS and the current flowing through the switch, but for the full grid drive, the RDS (ON) is a relatively static parameter. MOS tube has been in a very easy fever. In addition, the increase of the temperature of the junction temperature will lead to the increase of RDS (ON). The thermal impedance parameter is defined as the thermal impedance parameter, which is defined as the thermal resistance of the MOS tube, which is defined as the MOS tube. R 6 JC the most simple definition is to heat impedance tube shell.

1, fever, circuit design problem, is to let the MOS pipe work in a linear working state, rather than in the switch state. This is one of the reasons leading to the heat of the MOS tube. If N-MOS do switch, the G level of the voltage to be higher than the power of a few V, in order to completely lead, P-MOS is the opposite. Not fully opened and the pressure drop across the big success rate of consumption, the equivalent of the DC impedance is relatively large, the pressure drop increases, so U*I also increased, the loss of heat. This is the design circuit of the most taboo error.

2, the frequency is too high, the main is sometimes excessive pursuit of the volume, resulting in increased frequency, MOS tube on the loss increased, so the heat has increased

3, do not do enough heat dissipation design, the current is too high, MOS pipe nominal current value, generally need a good heat dissipation to achieve. So ID is less than the maximum current, it may have a serious fever, the need for adequate auxiliary heat sink.

The selection of 4, the MOS tube is wrong, the power of judgment is wrong, MOS did not fully consider the internal resistance, resulting in increase in impedance switch