What are the tricks behind the mos tube fever?

 

Power supply design, or driving circuit, it is inevitable to use the field effect tube, that is, what people often sayMos. Mos have many types and functions. For power supply or driver use, of course, it is to use its switch function.

 

 

Regardless of the n-type or p-type MOs, the working principle is essentially the same. Mos are the voltage added to the gate at the input end to control the leakage current at the output end. Mos are voltage controlled devices. They are applied to the gate to control voltage devices. They do not have the charge storage effect caused by the base current when a transistor is switched. Therefore, they are used in switching applications, the switch speed of the mos tube should be faster than that of the transistor. Its main schematic 1.

 

 

We often use this function in switching power supplies.MosDrain pole open circuit, 2 drain Pole Original sealing load, called Open Circuit drain pole, open circuit drain pole circuit no matter how high the load voltage, can be connected to and off load current. It is an ideal analog switch device. This is the principle of MOS tubes as switching devices. Of course, there are many circuit forms used for the switch of MOS tubes.

 

 

In terms of switching power supply applications, such applications require MOS tubes to be periodically switched on and off. For example, the basic step-down converter commonly used in DC-DC Power Supply relies on two MOs to perform the switching function, these switches alternate in the inductance to store energy, and then the energy is released to the load. We often choose a frequency of several hundred kHz or more than 1 MHz, because the higher the frequency, the smaller the magnetic element. During normal operation,MosIt is equivalent to only one conductor. Therefore, our circuit or Power Supply Designers are most concerned with the minimum conduction loss of MOS.

 

 

We often look at the PDF parameters of MOS tubes. MOS tube manufacturers use RDS (on) parameters to define the on Impedance. For switching applications, RDS (on) is also the most important device feature. The Data Manual defines that RDS (on) is related to the gate (or drive) voltage vgs and the current flowing through the switch, but for a sufficient gate drive, RDS (on) is a relatively static parameter. It is easy to heat up MOs that have been in the conduction state. In addition, the gradual increase in the settlement temperature will also lead to an increase in RDS (on. The MOs Data Manual specifies the thermal impedance parameters, which are defined as the heat dissipation capability of the Semiconductor Junction encapsulated by the mos tube. The simplest definition of rθ JC is the thermal impedance of the junction to the shell.

 

 

1. The problem of circuit design is that the mos Tube works in a linear working state, rather than in the switching state. This is also one of the reasons for the mos tube fever. If the N-MOS is switched, G-level voltage is a few v higher than the power supply, in order to fully turn on, the P-MOS is the opposite. If the voltage drop is not fully enabled and the voltage drop is too large, the power consumption is caused. The equivalent Dc impedance is large and the pressure drop is increased. Therefore, the u * I also increases, and the loss means that the heat is generated. This is the most taboo mistake in circuit design.

 

 

2. The frequency is too high, mainly because sometimes the volume is excessively pursued, resulting in a higher frequency,MosThe loss is increased, so the heat is increased by 3. There is not enough heat dissipation design, the current is too high, and the nominal current value of the mos tube usually requires good heat dissipation. Therefore, the ID is less than the maximum current, and the fever may be severe. Sufficient Auxiliary Heat Sink is required.

 

 

3. incorrect selection of MOS tubes and incorrect power judgment. The internal resistance of MOS tubes is not fully considered, resulting in an increase in the switching impedance. This is my summary recently when dealing with the heat issue of MOS tubes. In fact, these problems are also commonplace. The knowledge of switching power supplies or MOs should be well-developed. Of course, there are sometimes other factors, mainly for the above reasons.

 

What are the tricks behind the mos tube fever?