It is important to know about MOSFET

It is important to know about MOSFET

MOSFETs are commonly known as MOSFETs, which are commonly used as switching devices

We usually choose MOSFET transistors with the following parameters:

1. High enough pressure resistance to avoid DS breakdown.

2. The conduction current is large enough to ensure sufficient power output.

3. Small size with low heat generation to avoid overheating and damage to components.

4. The switching speed should be fast, as it needs to pass through the variable resistance zone, so it is necessary to avoid excessive heat loss as soon as possible.

Introduction to various parameters MOSFET:

1、 Breakdown voltage

The maximum voltage that DS can withstand, and this parameter is not necessarily better. Pipes with high voltage resistance generally have a higher on state resistance!

2、 Conducting current

Id, continuous drain current, is usually closely related to the on state resistance.

3、 Volume and heat generation

Thermal resistance: The blocking effect of heat during the conduction process, denoted by R. Unit ℃/W

We naturally hope that the smaller the thermal resistance, the better, so that the heat will be dissipated and the tube will not overheat and damage the device. Usually, MOS tubes with the same parameters have smaller thermal resistance and smoother heat dissipation with larger volume. Usually, we also provide auxiliary heat dissipation for MOS tubes, such as increasing the heat sink and laying copper on the circuit board area.

4、 Switching speed

If the switch speed is not handled properly, the corresponding heat generation may be greater than that caused by the on state resistance!

The reason for this is still because MOS transistors are not ideal for conduction and cutoff, but require a period of time. For example, during conduction, the voltage gradually decreases and the current gradually increases. During this period, if the voltage and current are not zero, a certain amount of heating power will be generated.

Of course, it doesn't matter if it's a long-term continuous on or off state, but generally MOSFET transistors are in the state of high-speed switching. At this point, the corresponding losses should not be underestimated.

1. Switching loss and conduction loss of MOSFET transistors

The usual solutions include zero voltage switching technology (ZVS) and zero current switching technology (ZCS).

MOS transistor principle:

A MOSFET transistor is a device that controls the gate voltage and drain current.

From an external perspective, the gate can be equivalent to a capacitor, and controlling the MOS transistor switch is the charging and discharging of the capacitor. Once the opening voltage (2-4V) is exceeded and further increases are made, it will enter the variable resistance region, and further increases will enter the constant current region.

The voltage at both ends of the capacitor does not suddenly change, and the rise and fall require a process. The faster the process, the faster the switching speed, and the lower the corresponding switching loss.

2. Grid charging and discharging equivalent circuit

Normally, the larger the current, the larger the corresponding gate capacitance, which means the switching speed is slow. However, the larger the current, the greater the corresponding power. They are interdependent and interrelated, so in parameter selection, balance should be achieved.

The role of gate resistance

To say a thousand words is to increase the switching speed as high as possible, and the presence of gate resistance will inevitably affect the switching speed, that is, the charging and discharging speed of the capacitor. Why should we keep it?

3. Parasitic oscillation of the gate

If there is no gate resistance, parasitic oscillation of the gate will occur due to the influence of various stray parameters. There are even serious instances of overshoot.

Diodes inside MOS tubes

Usually, there is a diode between the DS of the MOS transistor, which is comparable in performance to a fast recovery diode. This is why MOS transistors driving inductive loads do not require additional parallel diodes.

MOSFET transistor driver circuit

The driving circuit requires sufficient charging and discharging current. In high-speed conditions, pull-down resistors cannot be used to provide corresponding high and low levels, otherwise it will seriously reduce the switching speed, increase the time to pass through the variable resistance region, and reduce efficiency. So what should we do?

Slow opening and fast closing driving circuit

4. Simplified slow opening and fast closing circuit

On the premise of avoiding parasitic oscillations (retaining the gate resistance), parallel a diode on the gate resistance.

This can achieve a fast cutoff effect when it is at a low level.

Driving circuit for negative pressure cutoff occasions

Let's first talk about the benefits of negative pressure cutoff

① Ensure fast cut-off. According to the RC charging model, within the reference range of 0-5V, the turning off speed of -5-12 is much greater than that of 5-12.

② Ensure reliable cut-off. Due to the presence of a large capacitance between the GDs of the MOS transistor, high-frequency interference from the load may generate an induced voltage at the gate through the Cgd, which may cause the MOS transistor to mislead, and negative pressure cutoff can to some extent avoid this situation.