What to pay attention to when using MOS transistors

Date:2024-11-16 Categories:Product knowledge Hits:195 From:Guangdong Youfeng Microelectronics Co., Ltd


In actual projects, we mostly use enhanced models.

MOSFETs are divided into two types: N-channel and P-channel. We commonly use NMOS because of its low on resistance and ease of manufacturing. 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 for driving inductive loads such as motors. By the way, body diodes only exist in individual MOS transistors and are usually not present inside integrated circuit chips.

1. Conductivity characteristic

The characteristic of NMOS is that Vgs will conduct when it is greater than a certain value, which is suitable for use in situations where the source is grounded (low-end driving), 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 situations where the source is connected to VCC (high-end driving). However, although PMOS can be conveniently used as a high-end driver, due to its high on resistance, high price, and limited replacement options, NMOS is still commonly used in high-end drivers.

2. Loss of MOS switching transistor

Whether it is NMOS or PMOS, there is a conduction resistance present after conduction, and the current will consume energy on this resistance, which is called conduction loss. Choosing MOS transistors with low on resistance will reduce conduction losses. The current on resistance of low-power MOS transistors is generally around tens of milliohms, and there are also several milliohms. MOS does not complete its conduction and cutoff in an instant. The voltage across the MOS has a decreasing process, and the current flowing through it has an increasing process. During this period, the loss of the MOS transistor is the product of the voltage and current, called the switching loss. Usually, the switching loss is much greater than the conduction loss, and the higher the switching frequency, the greater the loss. The product of voltage and current at the moment of conduction is very large, resulting in significant losses. Shortening the switching time can reduce the loss during each conduction; Reducing the switching frequency can decrease the number of switches per unit time. Both methods can reduce switch losses.

3. MOS transistor driver

Compared with bipolar transistors, it is generally believed that conducting MOS transistors does not require current, as long as the GS voltage is higher than a certain value. This is easy to achieve, but we still need speed. In the structure of MOS transistors, it can be seen that there is parasitic capacitance between GS and GD, and the driving of MOS transistors is actually the charging and discharging of the capacitance. Charging a capacitor requires a current, as the capacitor can be viewed as a short circuit at the moment of charging, resulting in a relatively large instantaneous current. The first thing to pay attention to when selecting/designing MOS transistor drivers is the size of the instantaneous short-circuit current that can be provided.

The second point to note is that NMOS commonly used for high-end driving requires a gate voltage greater than the source voltage when conducting. When the high-end driving MOS transistor is turned on, the source voltage is the same as the drain voltage (VCC), so the gate voltage is 4V or 10V higher than VCC at this time. If you want to obtain a voltage greater 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.

 


Previous: Classification, Structure, and Principle of MOSFET

Next: Stable voltage Vz

QQChat
ChatWechat
ConsultTelephone
+86-0769-82730331