Power supply circuit, how to control the size of the output current

Just used a mos tube.

is far less likely to have used a transistor.

There are a lot of people.

The MOS tube is a voltage control device, that is, the voltage control G pin is required to control the current of the tube.

Generally, the most common on the market is the enhanced N communication MOS tube, you can use a voltage to control the voltage of G, the on-voltage of the MOS tube is generally 2-4V, but to fully control, this value should rise to about 10V. A method is recommended to you.

The power supply of the comparator is connected to 12V and ground, such as LM358 as a comparator), the output of the comparator is pulled up by the resistor and then connected to the G pin, if the control voltage is higher than the reference voltage, the output current of the control MOS tube is turned on.

The reference voltage can come from the sampling resistor, that is, the S pole of NMOS is connected to a high-power and small-scale resistor and then grounded, this resistor is used for current sampling, and when the current flows through the resistor, it will form a voltage, and it will be amplified for reference.

At the beginning, the current is very small, so the control voltage is much higher than the reference voltage, at this time, the G pin is basically added 12V, which can make the tube quickly turn on, after a short time, when the current increases and gradually reaches a certain value, the reference voltage rises rapidly, and when the control voltage is close to and exceeds, the comparator will output a low level (close to 0V) to cut off the tube and the current will decrease. Then when the current decreases, the reference voltage goes down again, the tube turns on again, and the current increases again. And then the cycle repeats.

The current can also be fed back to the primary and controlled by PWM.

Of course, the current limiting circuit is directly used to control the current, so the cost is relatively high, and it is generally a last resort.

Let's briefly talk about the principle of constant current, use a resistor with high power and low resistance value to take a voltage sample in front of the negative output, and then compare it with a reference voltage, and the output of the comparison is connected to the optocoupler (note that it should be isolated with a diode). In this way, the current loop and the voltage loop can be fed back to the primary at the same time for adjustment. The voltage loop works when the current does not reach the current limit point, and the current loop works when the current exceeds the current limit point.

In fact, as long as your load is not a sensitive component, you can connect it directly. As long as the current output of your regulator block is enough for the load, you can do it. Of course, if the load is sensitive to the element, then a current-limiting power resistor needs to be added.

Method of adjusting the output current: In a closed circuit, there are two factors that affect the magnitude of the current: the magnitude of the voltage in the circuit and the magnitude of the resistance.

Therefore, in order to control the output current, we should start from these two points. Among them, in practical circuits, the general resistance (inductive capacitive reactance) is a load electrical appliance, which is not very active. In most cases, the voltage (frequency) of the circuit is changed by thyristor circuits, etc., to control the current magnitude.

Of course, this is not absolute, and there are also resistance increases and decreases in the motor rotor circuit to control the current size and speed.

The switching power supply only has a specified rated current. The specific current value is output by the size of the load.

The easiest way is to string a suitable potentiometer at the output.