Summary of Mosfet transistor driver circuit
Time:2023-04-22
Views:1076
When designing switching power supplies or motor drive circuits using MOS transistors, most people consider the conduction 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 as a formal product design, it is also not allowed.
Below is my summary of the basics of MOSFETs and MOSFET driver circuits, which refers to some materials, not all of which are original. Including the introduction, characteristics, driver, and application circuits of MOS transistors.
1. MOS transistor types and structures
MOSFET transistor is a type of FET (another type is JFET) that can be manufactured into either enhanced or depleted types. There are four types of P-channel or N-channel, but the actual applications are only enhanced N-channel MOS transistors and enhanced P-channel MOS transistors. Therefore, NMOS or PMOS are commonly referred to as these two types.
As for why not use depleted MOS transistors, it is not recommended to delve into the root cause.
For these two types of enhanced MOS transistors, the most commonly used one is NMOS. The reason is that the conduction resistance is small and easy to manufacture. So in the application of switching power supply and motor drive, NMOS is generally used. In the following introduction, NMOS is also the main method.
There is parasitic capacitance between the three pins of the MOS transistor, which is not what we need, but is caused by manufacturing process limitations. The existence of parasitic capacitance makes it more troublesome to design or select driving circuits, but it cannot be avoided. We will explain it in detail later.
On the MOS transistor schematic, it can be seen that there is a parasitic diode between the drain and source. This is called a body diode, which is important in driving inductive loads (such as motors). By the way, bulk diodes only exist in a single MOS transistor and are typically not present within integrated circuit chips.
2. MOS transistor conduction characteristics
Conduction means to act as a switch, equivalent to the switch being closed.
The characteristic of NMOS is that Vgs will conduct when it exceeds a certain value, making it suitable for use in the case of source grounding (low end drive), as long as the gate voltage reaches 4V or 10V.
The characteristic of PMOS is that Vgs will conduct when it is less than a certain value, making it suitable for use in the case of source connected VCC (high-end drive). However, although PMOS can be easily used as a high-end driver, due to high conduction resistance, high price, and limited replacement options, NMOS is usually still used in high-end drivers
3. MOS switch tube loss
Whether it is NMOS or PMOS, there is a conduction resistance after conduction, so the current will consume energy on this resistance, which is called conduction loss. Choosing MOS transistors with low conduction resistance will reduce conduction loss. Nowadays, the conduction resistance of low-power MOSFETs is generally around tens of milliohms, and there are also a few milliohms.
MOS must not be completed in an instant during conduction and cutoff. The voltage at both ends of the MOS undergoes a decreasing process, while the current flowing through it undergoes an increasing process. During this period, the loss of the MOS transistor is the product of voltage and current, known as switch loss. Usually, the switching loss is much greater than the conduction loss, and the faster the switching frequency, the greater the loss.
The product of voltage and current at the moment of conduction is large, resulting in significant losses. Shortening the switching time can reduce the loss during each conduction; Reducing the switching frequency can reduce the number of switching times per unit time. Both methods can reduce switch losses.
4. MOS transistor drive
Compared to bipolar transistors, it is generally believed that no current is required to make the MOS transistor conduct, as long as the GS voltage is above a certain value. This is easy to do, but we still need speed.
In the structure of MOS tubes, it can be seen that there are parasitic capacitors between GS and GD, and the driving of MOS tubes is actually the charging and discharging of the capacitors. Charging a capacitor requires a current, as the capacitor can be considered as a short circuit at the moment of charging, so the instantaneous current will be relatively high. The first thing to pay attention to when selecting/designing MOS transistor drivers is the magnitude of the instantaneous short-circuit current that can be provided.
The second note is that NMOS, commonly used for high-end drives, needs to have a gate voltage greater than the source voltage during conduction. When the high-end drive MOS transistor is turned on, the source voltage and drain voltage (VCC) are the same, so the gate voltage is 4V or 10V higher than VCC. If you want to obtain a voltage higher than VCC in the same system, you need a dedicated boost circuit. Many motor drivers integrate charge pumps, and it is important to choose appropriate external capacitors to obtain sufficient short-circuit current to drive MOS transistors.
The above mentioned 4V or 10V is the commonly used conduction voltage of MOS transistors, and of course, a certain margin is required during design. And the higher the voltage, the faster the conduction speed and the smaller the conduction resistance. Nowadays, MOS transistors with lower conduction voltage are also used in different fields, but in 12V automotive electronic systems, 4V conduction is generally sufficient.
The driving circuit and its losses of MOSFETs can be described in detail in Microchip‘s AN799 Matching MOSFET Drivers to MOSFETs. Therefore, I do not intend to write more.
5. MOS transistor application circuit
The most significant characteristic of MOS transistors is their good switching characteristics, which is why they are widely used in circuits that require electronic switches, such as switching power supplies and motor drives, as well as lighting dimming.
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