How do MCUs communicate with different level signals

Figure 6

    2. Field effect transistor is a voltage controlled current device controlled by VGS for ID, while ordinary crystal transistor is a current controlled current device controlled by IB for IC. The amplification factor of MOS pipeline is (transconductance gm) how many amperes can the drain current change when the gate voltage changes by one volt. The crystal transistor has a current amplification factor (beta) β) How much change in collector current can occur when the base current changes by one milliampere.

    3. The gate and other electrodes of a field-effect transistor are insulated and do not generate current; During the operation of the transistor, the base current IB determines the collector current IC. Therefore, the input resistance of field-effect transistors is much higher than that of transistors.

    4. Only a majority of charge carriers participate in conduction in field-effect transistors; Transistors have two types of carriers, the majority carrier and the minority carrier, involved in conductivity. Due to the significant influence of temperature, radiation, and other factors on the concentration of minority carriers, field-effect transistors have better temperature stability than transistors.

    5. When the source and drain of a field-effect transistor are not connected to the substrate, they can be used interchangeably with little change in characteristics. However, when the collector and emitter of a transistor are used interchangeably, their characteristics differ greatly, and the value of b will decrease significantly.

    6. The noise coefficient of field-effect transistors is very small, so field-effect transistors should be selected in the input stage of low noise amplification circuits and circuits that require high signal-to-noise ratio.

    7. Both field-effect transistors and ordinary crystal transistors can form various amplification and switching circuits. However, field-effect transistors have simple manufacturing processes and excellent characteristics that ordinary crystal transistors cannot match. They are gradually replacing ordinary crystal transistors in various circuits and applications. Currently, field-effect transistors are widely used in large-scale and ultra large scale integrated circuits.

    8. High input impedance and low driving power: Due to the silicon dioxide (SiO2) insulation layer between the gate sources, the DC resistance between the gate sources is basically the SiO2 insulation resistance, usually around 100M Ω. The AC input impedance is basically the capacitive impedance of the input capacitor.

    Due to the high input impedance, there is no voltage drop on the excitation signal, and it can be driven with voltage, resulting in extremely low driving power (high sensitivity). A typical crystal transistor must have a base voltage Vb to generate a base current Ib in order to drive the generation of collector current. The driving of a crystal transistor requires power (Vb x Ib).

    9. Fast switching speed: The switching speed of MOSFETs is closely related to the capacitive characteristics of the input. Due to the presence of capacitive characteristics of the input, the switching speed slows down. However, when used as a switch, it can reduce the internal resistance of the driving circuit and accelerate the switching speed (the input is driven by a "charging circuit" described later, which accelerates the time for capacitive charging and discharging).

    MOSFETs rely solely on multi carrier conduction and do not have minority carrier storage effects, resulting in a very rapid turnoff process. The switching time is between 10-100ns, and the operating frequency can reach over 100kHz. Ordinary crystal transistors, due to the storage effect of minority carriers, always have hysteresis in the switch, The impact on the improvement of switching speed (currently, switching power supplies using MOSFETs can easily achieve operating frequencies of 100K/S to 150K/S, which is unimaginable for ordinary high-power crystal transistors).

    10. No secondary breakdown: Due to the phenomenon that ordinary power crystal transistors have a positive temperature current characteristic when the temperature rises, the collector current will rise. The increase in collector current will lead to further temperature rise, further temperature rise, and further lead to the vicious cycle of collector current rise.

    The voltage resistance (VCEO) of the crystal transistor gradually decreases as the temperature of the transistor increases, resulting in a continuous increase in transistor temperature and a decrease in voltage resistance, ultimately leading to the breakdown of the crystal transistor. This is a destructive thermoelectric breakdown phenomenon that accounts for 95% of the damage rate of the TV switching power supply tube and row output tube, also known as secondary breakdown phenomenon.

    MOSFETs have the opposite temperature current characteristics to ordinary crystal transistors, that is, when the tube temperature (or ambient temperature) rises, the channel current IDS actually decreases. For example; A MOSFET switching transistor with IDS=10A, when the VGS control voltage remains constant, IDS=3A at a temperature of 250C. When the chip temperature rises to 1000C, IDS decreases to 2A. This negative temperature current characteristic, which causes a decrease in channel current IDS due to temperature rise, prevents a vicious cycle and thermal breakdown.

    That is to say, there is no secondary breakdown phenomenon in MOSFETs. It can be seen that using MOSFETs as switching tubes significantly reduces the damage rate of the switching tubes. In the past two years, after replacing ordinary crystal transistors with MOSFETs in TV switching power supplies, the damage rate of the switching tubes has been greatly reduced, which is also an excellent proof.

    11. After conducting, the conduction characteristics of MOS transistors exhibit pure resistance: when a regular crystal transistor is saturated and conducting, it is almost straight through, with an extremely low voltage drop called saturation voltage drop. Since there is a voltage drop, it is; A regular crystal transistor is equivalent to a resistor with extremely low resistance after saturation conduction, but this equivalent resistor is a nonlinear resistor (the voltage on the resistor and the current flowing through it cannot comply with Ohm‘s law). As a switching transistor, MOSFETs also have a resistor with extremely low resistance after saturation conduction.

    But this resistor is equivalent to a linear resistor, and its resistance value, voltage drop at both ends, and current flowing through it comply with Ohm‘s law. When the current is large, the voltage drop is large, and when the current is small, the voltage drop is small. After conduction, since it is equivalent to a linear element, the linear element can be applied in parallel. When these two resistors are connected in parallel, there is an automatic current balance effect. So when the power of a MOSFET is insufficient, multiple MOSFETs can be applied in parallel without the need for additional balancing measures (nonlinear devices cannot be directly applied in parallel).