How to Accurately Control Circuit Current with MOS Diodes

How to Accurately Control Circuit Current with MOS Diodes

MOS transistors can be used to amplify current, as variable resistors, and as electronic switches, and are now widely used in electronic devices. During use, MOS transistors control the current of the output drain by applying a voltage to the input gate.

MOS transistor is a voltage control device, which requires the use of voltage control G pin to control the current of the transistor. The most common type on the market is an enhanced N-communication MOS transistor, and manufacturers can use a voltage to control the voltage of G.

Basic method: Use a control voltage (same phase input terminal of the comparator) and a reference voltage (opposite phase input terminal of the comparator) to simultaneously enter the voltage comparator. The output of the comparator is pulled up by a resistor and connected to the G pin. If the control voltage is higher than the reference voltage, the MOS transistor is controlled to conduct the output current.

The reference voltage can come from a sampling resistor, which is grounded after connecting a high-power small resistor to the S pole of NMOS. This resistor is used for current sampling. When the current flows through the resistor, it forms a voltage, which is amplified and processed as a reference.

At the beginning, the current is very small, so the control voltage is much higher than the reference voltage, which can quickly conduct the diodes. After a short period of time, when the current gradually increases to a certain value, the reference voltage rises rapidly. When it approaches and exceeds the control voltage, the comparator outputs a low level (close to 0V) to cut off the diodes and reduce the current. After the current decreases, the reference voltage drops again, the diodes conducts again, and the current increases again. And then it goes back and forth.