Six Kinds of Universal MOS Tube Circuit Analysis

PublishTime： 2024-02-12 Article Source：HI-SEMICON News

For MOS tube, many people are impressed by the small signal analysis model learned in the textbook on mode electricity, as well as various kinds of amplifying circuits. In practical projects, MOS transistors are often used in full-bridge drives, full-bridge inverters, Buck/Boost circuits, and other circuits.

In addition to the above applications, MOS transistors also have the following six functions: anti-backconnection circuit, power switching circuit, timing control circuit, logic level conversion circuit, charging state monitoring circuit, and phase-locked circuit.

As long as we understand that MOS meets the MOS transistor circuit, we will never be stumped by circuit analysis. For the MOS tube circuit function, Xiao Sheng has some tips to share with you, such as an anti-reverse connection circuit, logic level conversion circuit, and phase-locked circuit, you can use it easily!

1. Why not use diodes for anti-back connection circuits

Fig.1

The simple anti-backconnection circuit can directly connect the diodes in series. When the power is connected, the diodes are cut off to prevent the circuit from forming and protect the electrical devices. However, the conduction voltage drop of the diode cannot be completely ignored. When the load current is large, such as 1A, the 700 milliwatt dissipation power may cause serious heat of the diode. However, MOS transistors are characterized by small conduction voltage drops and low conduction resistance. At present, many MOS transistors have achieved a millivolt level of conduction voltage drop, such as 5 millivolt. The voltage drop generated by the current passing through 1A is only 5 millivolts, with very small dissipation and voltage loss.

2. MOS tube selection in the level conversion circuit

Fig.2

For different circuits, a sufficient margin should be left when choosing MOS transistors, especially some limit parameters given in the data manual, such as V_DSS (leakage source voltage), V_GSS (gate source voltage), and Vth (opening voltage) of NMOS transistors.

If MOS transistors are used to form anti-backconnection circuits or power switching circuits, generally the voltage will be relatively high, such as 12V or 24V. When choosing MOS transistors, it is necessary to pay attention to whether the three parameter allowances mentioned above are sufficient. If the voltage is exceeded, the MOS tube will be broken down. In addition, the opening voltage of an MOS tube with high voltage will not be too low. When IO is used to control the power sequence, the gate source voltage may be insufficient and the MOS tube cannot be switched on or off.

Similarly, when used as a level conversion device, if the selected MOS tube has too high an opening voltage, it will also make the MOS tube unable to work normally and realize the circuit's due functions.

In the use of MOS tube do level conversion, or logic devices, also notice MOS tube junction capacitance C_GD, C_GS, C_DSand distributed parameter C_ISS, C_RSS, and C_OSS, also should choose when communication speed junction capacitance smaller MOS tube, to avoid caused excessive junction capacitance IO capacitive load is too large, makes the rise or fall velocity have an obvious change, produce logic errors.

3. The realization technique of MOS tube phase-locked circuit

Fig.3

In the actual test of the PLL circuit, it is found that the frequency performance of THE MOS PLL circuit is not consistent due to the existence of the junction capacitance. For different frequency requirements, it is necessary to select the appropriate resistance size according to the actual situation to ensure the best performance.

However, the pins of STM32 and some models of FPGA can be set to the open-leak output mode. According to this characteristic, a resistor can be directly used in series to form a phase-locked circuit, and the phase of the reference signal can be controlled directly on the chip through a program.

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