Detailed explanation of the working mechanism and application fields of NMOS transistors

MOS transistors can be divided into PMOS transistors and NMOS transistors, with NMOS also known as N-type metal oxide semiconductor. NMOS transistors have the advantages of high input resistance, low noise, low power consumption, large dynamic range, easy integration, no secondary breakdown phenomenon, and wide safe working area, and are widely used in large-scale and ultra large scale integrated circuits. This issue mainly introduces several classic NMOS transistor products, which are widely used in motor control, power supply, inverter, and load switch products. Specific applications include computers, mobile communications, automotive electronics, the Internet of Things, artificial intelligence, power supplies, instrumentation, and energy storage.

Typically, an N-channel MOS transistor is a three terminal device that includes an N-channel region located between the source and drain terminals, with G (gate), D (drain), and S (source) terminals. The symbol of N-channel MOS transistor is shown as N in the following figure. The direction of the arrow symbol is inward, and the arrow symbol specifies the type of channel, represented as P channel or N channel.

The working principle of N-channel MOS transistor is as follows: it consists of a P-type substrate and two high concentration N-diffusion regions. When the gate source voltage is greater than the threshold voltage, an N-type conductive channel is formed in the two high concentration N-diffusion regions, allowing current to flow. It has the characteristics of low on resistance, fast switching response speed, high load current, and high thermal conductivity, which make N-channel MOS transistors perform well in various application scenarios.

It is precisely because N-channel MOS transistors have the above characteristics that their application fields are very broad.

1. Power management

N-channel MOS transistors can be used for voltage stabilization and current control, as well as power switching, to achieve efficient energy management of power circuits through their on and off states.

2. Motor driven

In the motor drive circuit, the low on resistance and fast switching characteristics of N-channel MOS transistors enable precise control of the motor. For example, in DC motor drivers, N-channel MOS transistors are used as switching elements to control the speed and direction of the motor through PWM (Pulse Width Modulation) signals.

3. Signal amplification

N-channel MOS transistors play an important role in applications such as RF amplification, audio amplification, and power amplification. Its high input impedance and low output impedance can achieve precise amplification and driving of signals.

4. Switch circuit

N-channel MOS transistors can be used as switching elements in various circuits, such as VBUS switching circuits. VBUS is an important detection signal that plays a crucial role in the OTG controller, USB device recognition, HOST, and device mode switching process. The N-channel MOS transistor controls the on and off of electronic switching devices through a control circuit, thereby achieving pulse modulation and voltage conversion of input voltage.

5. Photoelectric coupling

In optocouplers, N-channel MOS transistors can be used for output stage driving and signal isolation. For example, at the output of a optocoupler, an N-channel MOS transistor can convert optical signals into electrical signals and achieve signal isolation and amplification.

6. Battery Management System (BMS)

In BMS products, N-channel MOS transistors can serve as protective circuits and other functions. In order to achieve efficient charge and discharge control, the selected MOS transistor should have low on resistance and fast switching response speed, thereby reducing power consumption and improving system efficiency.

In battery management system (BMS) products, the application of N-channel MOS transistors is mainly reflected in battery charge and discharge management, battery protection, and temperature control. One core function of BMS is to control the charging and discharging process of the battery to ensure that it operates in a safe and efficient state. N-channel MOS transistors are widely used in the charging and discharging paths of battery packs. Through precise gate control, N-channel MOSFETs can quickly switch and precisely regulate the flow of current.

During the charging process, N-channel MOS transistors can effectively control the magnitude of the charging current and prevent overcharging. During the discharge process, N-channel MOS transistors can prevent the battery from overdischarging and protect it from damage.

The battery pack may be damaged in abnormal states such as overcurrent, overvoltage, and overheating, and the fast response capability of N-channel MOS transistors enables real-time detection and disconnection of battery connections. By cooperating with sensors and protection circuits, N-channel MOSFETs can quickly respond to danger signals in the battery, protecting the battery from damage. When the battery temperature is too high, the N-channel MOS transistor can cut off the charging and discharging circuit of the battery, preventing further temperature rise and thermal runaway. When the battery experiences overcurrent or overvoltage, the N-channel MOS transistor can also quickly cut off the circuit, protecting the battery pack from damage.

N-channel MOS transistors can also be used in conjunction with temperature sensors to play an important role in thermal management of battery packs. Batteries generate heat during the charging and discharging process, especially in high-power applications. Excessive temperature not only affects battery performance but may also cause safety issues.

N-channel MOS transistors can quickly cut off the current flow between the battery and the load when the temperature exceeds the preset range, thereby helping to control the temperature of the battery and prevent damage caused by overheating.

In BMS with multiple battery cells, N-channel MOS transistors can also be used for voltage balancing between battery cells. Due to the differences in capacity and internal resistance of different batteries, the voltage of each battery cell in the battery pack may become unbalanced after long-term use, affecting the overall performance of the battery pack. The cooperation between N-channel MOS transistors and battery balancing circuits can achieve voltage balancing between battery cells, ensuring the overall efficiency and service life of the battery pack.

Due to the different voltage and current characteristics of different N-channel MOS transistors, when selecting and designing N-channel MOS transistors, it is necessary to consider the operating voltage and current range of the battery pack, as well as parameters such as the switching speed, on resistance, and thermal performance of the MOS transistor. A MOS transistor with appropriate rated voltage and current is a prerequisite for ensuring system reliability.

In high-power applications, the power consumption and heat generation of MOS transistors may affect their performance and lifespan, so it is necessary to design a reasonable heat dissipation mechanism to ensure that MOS transistors operate within a safe temperature range. As a key component in electronic circuits, N-channel MOS transistors play an important role in charge and discharge management, battery protection, switch circuits, motor control, temperature control, and voltage balancing, ensuring the safe, reliable, and efficient operation of circuits.