Application of MOSFET-Three Key Points of Misunderstanding Power Loss

Application of MOSFET-Three Key Points of Misunderstanding Power Loss

In the data sheet of MOSFET, the total power loss Ptot in the limit table is a very interesting parameter. It is interesting because sometimes the value of Ptot looks powerful, but it is not the decisive parameter to determine whether the MOSFET design is feasible. In this paper, we will talk about the knowledge points about the total power loss of MOSFET, which are easily misunderstood or ignored in the use process.

Myth 1: The power of the Mosfet in the circuit is less than the Ptot in the limit table, and the MOSFET is safe.

The above understanding is wrong. The definition of total power loss Ptot is the power used when the device reaches the maximum junction temperature when the temperature of the solder substrate is maintained at 25℃. It can be expressed by the formula Tj=Tmb+Rth_j-mb*Ptot, and the node temperature is the final parameter to judge whether the MOSFET is overheated or not.

In practical application, it is difficult to keep the temperature of the solder substrate Tmb at 25℃ all the time, so Ptot should be a parameter to characterize the thermal conductivity Rth_j-mb or the maximum junction temperature.

Myth 2: For a MOSFET element, the value of power loss Ptot is fixed.

In fact, from the formula of junction temperature and power loss, we can see that thermal resistance Rth and junction temperature Tj are inherent properties of components, which are unchanged. With the increase of Tmb temperature, the value of power loss Ptot will decrease.

To sum up, it is wrong to understand that the value of power loss Ptot is fixed!

Myth 3: The power of 3：MOSFET = the current flowing through the drain and source * the conduction impedance between the drain and source.

MOSFET can be used in two situations: normally-on state and switch state. The power loss Ptot in the normally-on state should not only consider the power generated by the current flowing through the drain and source, but also consider the power generated by the current flowing in for gate driving.

The power in the switching state only needs to add the power loss in the on and off state. Therefore, the above viewpoint is too one-sided and not rigorous enough.