Power supply circuit diagram

The following is my summary of the basis of the mos and MOS driving circuit. I have referred to some materials, not all original. This includes the introduction, features, drivers, and application circuits of MOS tubes.

When using MOS to design a switching power supply or motor drive circuit, most people will consider the mos on-resistance, maximum voltage, maximum current, and so on. Many people only consider these factors. Such a circuit may work, but it is not excellent. It is not allowed as a formal product design.
 
1. Type and structure of MOS
One type of FET (the other is JFET), which can be manufactured as an enhanced or depletion type. There are four types of p-channel or N-channel, however, in practice, only enhanced NMOS and enhanced p-mos are used. Therefore, NMOS or PMOS are usually used. The right figure shows the symbols of the two MOS tubes.
 
We do not recommend that you use the depletion MOS.
For the two Enhanced MOs, NMOS are commonly used. The reason is that the conduction resistance is small and easy to manufacture. Therefore, NMOS are generally used for switching power supplies and motor drives. In the following introduction, NMOS is also used.
A parasitic diode exists between the drain pole and the source pole. This is called a body diode, which is very important in driving inductive loads (such as motors. By the way, the diode only exists in a single MOS tube and is usually unavailable in an integrated circuit chip. It is the structure diagram of the mos tube, which is usually shown in the right diagram in the schematic diagram. (Diode used for gate protection is sometimes not painted)
 
Parasitic capacitors exist between the three pins of the mos tube, as shown in the right figure. This is not what we need, but because of manufacturing process limitations. The existence of Parasitic Capacitance makes it difficult to design or select the driving circuit, but there is no way to avoid it. We will introduce it in detail when designing the driving circuit of the mos tube.
 
2. On-going characteristics of MOS Tubes
As a switch, the switch is closed.
For NMOS, if the value of vgs is greater than a certain value, it will be turned on. It is suitable for the case when the source pole is grounded (low-end drive), as long as the gate voltage reaches 4 V or 10 v.
When the PMOS feature is smaller than a certain value, the vgs will be turned on. When the source pole is connected to VCC (high-end driver ). However, although PMOS can be easily used as a high-end driver, NMOS is usually used in high-end drivers due to the large conduction resistance, expensive price, and few replacement types.
The figure on the right shows the relationship between the vgs voltage of rensa 2sk3418 and the VDS voltage. It can be seen that when the small current, the vgs reaches 4 V, and the voltage drop between DS is very small, it can be considered as conduction.
 
3. MOS switch losses
Both NMOS and PMOS have an on-resistance after being turned on. Therefore, when the current flows between ds, the two ends also have a voltage (as shown in the 2sk3418 characteristic diagram ), in this way, the current will consume energy on this resistor. the energy consumed is called the conduction loss. Selecting a MOS tube with a small conduction resistance reduces the conduction loss. At present, the on-resistance of low-power MOS tubes is usually around dozens of S, and several s are also available.
Mos must not be completed in an instant at the end of the tunnel and tunnel. The voltage at both ends of MOS has a descent process, and the flowing current has a rising process. During this period, the loss of the mos tube is the product of the voltage and the current, which is called the switch loss. Generally, the switch loss is much greater than the conduction loss, and the faster the switch frequency, the larger the loss.
It is the waveform when the mos tube is turned on. It can be seen that the product of the transient voltage and current is very large, resulting in a great loss. Reducing the switch time can reduce the loss during each turn-on. Reducing the switch frequency can reduce the number of switches per unit time. Both methods can reduce the switch loss.
 
4. MOS driver
Compared with bipolar transistor, it is generally considered that the mos do not require current for conduction, as long as the GS voltage is higher than a certain value. This is easy to do, but we still need speed.
In the structure of the mos tube, it can be seen that there is a parasitic capacitor between GS and Gd, while the driving of the mos tube is actually charging and discharging the capacitor. Charging a capacitor requires a current, because the capacitor can be regarded as a short circuit in an instant, so the instantaneous current will be relatively large. When selecting/designing the mos drive, the first thing to note is the size of transient short-circuit current.
Second, it is generally used for NMOS of high-end drivers. The gate voltage must be greater than the source pole voltage during conduction. The source voltage is the same as the drain voltage (VCC) when the high-end drive MOS tube is turned on, so the gate voltage is larger than VCC by 4 V or 10 v. If you want to obtain a voltage greater than that of VCC in the same system, you need a dedicated Boost Circuit. Many motor drivers are integrated with a charge pump. It should be noted that the appropriate external capacitor should be selected to get enough short-circuit current to drive the mos tube.
The 4 V or 10 V above is the on-voltage of the commonly used MOS tube. Of course, there must be a certain margin during the design. The higher the voltage, the faster the conduction speed, and the smaller the conduction resistance. At present, MOS with lower conduction voltage are also used in different fields, but in 12 V automotive electronic systems, it is enough to conduct 4 V.
For more information about the driving circuit and loss of MOS, see the Microchip Corporation's an799 matching MOs drivers to ipvets. The description is very detailed, so I don't plan to write more.
5. MOS Application Circuit
The most notable characteristic of MOS is that they have good switching characteristics, so they are widely used in circuits requiring electronic switches. Common ones include switching power supplies and motor drives, and lighting dimming. These three applications are described in detail in various fields. I will not write much here. I will make a summary later.