MOSFET drive circuit design and application

The following is a summary of my understanding of the basics of MOSFET and MOSFET drive circuits, which includes some reference materials, not all original, including the introduction, characteristics, drive and application circuits of MOS tubes.

When using MOS tubes to design switching power supplies or motor drive circuits, most people will consider the on-resistance, maximum voltage, maximum current, etc. of MOS, and many people only consider these factors. Such a circuit may work, but it is not excellent, and it is not allowed as a formal product design.

1. Types and structures of MOS tubes
MOSFET tubes are a type of FET (the other is JFET), which can be made into enhancement type or depletion type, P channel or N channel, a total of 4 types, but in actual application, only enhancement type N channel MOS tubes and enhancement type P channel MOS tubes are used, so NMOS or PMOS usually refers to these two types. The picture on the right is the symbol of these two MOS tubes.

As for why depletion-type MOS tubes are not used, it is not recommended to dig into the details.
Of the two types of enhancement-type MOS tubes, NMOS is more commonly used. The reason is that the on-resistance is small and it is easy to manufacture. Therefore, NMOS is generally used in switching power supplies and motor drive applications. In the following introduction, NMOS is also the main focus.
In the schematic diagram of the MOS tube, you can see that there is a parasitic diode between the drain and the source. This is called a body diode, and this diode is very important when driving inductive loads (such as motors). By the way, the body diode only exists in a single MOS tube and is usually not present inside an integrated circuit chip. The figure below is a structural diagram of a MOS tube, and the schematic diagram is usually drawn as shown in the figure on the right. (The gate protection diode is sometimes not drawn)

There is parasitic capacitance between the three pins of the MOS tube, as shown in the right figure. This is not what we need, but is caused by the limitation of the manufacturing process. The existence of parasitic capacitance makes it more troublesome when designing or selecting the drive circuit, but there is no way to avoid it. It will be introduced in detail when designing the drive circuit of the MOS tube.

2. MOS tube conduction characteristics
Conducting means acting as a switch, which is equivalent to closing the switch.
The characteristics of NMOS are that it will be turned on when Vgs is greater than a certain value, which is suitable for the case when the source is grounded (low-end drive), as long as the gate voltage reaches 4V or 10V. The characteristics of PMOS are that
it will be turned on when Vgs is less than a certain value, and it is used when the source is connected to VCC (high-end drive). However, although PMOS can be used as a high-end drive very conveniently, NMOS is usually used in high-end drives due to its large on-resistance, high price, and few replacement types.
The right figure is a relationship diagram between the Vgs voltage and the Vds voltage of Renesas 2SK3418. It can be seen that when the current is small, Vgs reaches 4V, and the voltage drop between DS is already very small, which can be considered to be turned on.

4. MOS tube drive
Compared with bipolar transistors, it is generally believed that MOS tubes do not require current to turn on, as long as the GS voltage is higher than a certain value. This is easy to do, but we also need speed.
In the structure of MOS tubes, we can see that there is a parasitic capacitor between GS and GD, and the drive of MOS tubes is actually the charging and discharging of capacitors. Charging a capacitor requires a current, because the capacitor can be regarded as a short circuit at the moment of charging, so the instantaneous current will be relatively large. The first thing to pay attention to when selecting/designing MOS tube drivers is the size of the instantaneous short-circuit current that can be provided. The
second thing to note is that NMOS, which is commonly used for high-end drive, needs to have a gate voltage greater than the source voltage when it is turned on. When the MOS tube of high-end drive is turned on, the source voltage is the same as the drain voltage (VCC), so the gate voltage should be 4V or 10V greater than VCC. If you want to get a voltage greater than VCC in the same system, you need a special boost circuit. Many motor drivers have integrated charge pumps. It should be noted that you should choose a suitable external capacitor to get enough short-circuit current to drive the MOS tube.
The 4V or 10V mentioned above is the commonly used MOS tube conduction voltage, and of course a certain margin is required during design. Moreover, the higher the voltage, the faster the conduction speed and the smaller the conduction resistance. Now there are MOS tubes with lower conduction voltage used in different fields, but in 12V automotive electronic systems, 4V conduction is generally sufficient. For the drive
circuit and loss of MOS tubes, you can refer to Microchip's AN799 Matching MOSFET Drivers to MOSFETs. It is described in great detail, so I don't plan to write more.
5. MOS tube application circuit
The most notable feature of MOS tubes is their good switching characteristics, so they are widely used in circuits that require electronic switches, such as switching power supplies and motor drives, as well as lighting dimming. These three applications have been introduced in detail in various fields, so I won't write more here for the time being. I will summarize them later when I have time.