What is the role of constant current circuit in power supply?

Definition of ideal constant current source

An ideal constant current source is one in which the current does not change with changes in input voltage, is not affected by ambient temperature, and has infinite internal resistance. However, there is still a gap between the actual constant current circuit and the ideal one, so an appropriate constant current source circuit must be selected according to the actual application.

Two transistors of the same model form a constant current source with a relatively constant current based on the relatively stable Vbe voltage of the transistor and the small base current of the transistor relative to the collector current. The current Io=Vbe/R1; this constant current The source does not use special devices. It consists of two transistors and two resistors. It has low cost and the current Io is adjustable. The disadvantage is that the size of Vbe will change with changes in current and temperature. The larger the current, the larger the Vbe, and the lower the temperature, the larger the Vbe. Therefore, Not suitable for use in places with high accuracy requirements.

This constant current circuit mainly uses the relatively stable voltage characteristics of the zener diode and the stability of the transistor Vbe to form a constant current circuit, Io=(Vd-Vbe)/R3; the advantage of this circuit is that it is low cost and the current can be The disadvantage is that the temperature characteristics are poor and the steady flow accuracy is not high, so it is suitable for occasions that do not require high accuracy.

TL431 provides a reference voltage Vref to form a constant current source, and the current Io=Vref/R2.

The three-terminal voltage regulator provides a constant voltage Vout to form a constant current source, Io=Vout/R1.

The above are some common and simple constant current sources, and they have one thing in common: the voltage stabilization accuracy is not high, and the current Io is not large. In addition to the ones listed above, there are other similar constant current sources, but they are all based on a constant voltage source, so I will not list them one by one here.

In the application process, if a high-precision, high-current constant current source is needed, an operational amplifier can be used to form a high-precision, high-current constant current source, as shown in circuit diagram 5

Use a constant current source composed of an op amp, Io=Vref/R1.

Application of constant current source in wide voltage input module

In module power supplies, the short-circuit protection of low-power power supplies generally does not require an external short-circuit protection circuit. This kind of module is characterized by small power, small size, and low cost; it is suitable for the current fiercely competitive market; however, they themselves have a fatal characteristic, short circuit There is a contradiction between the protection function and the starting ability. If the starting ability is strong, the short-circuit protection will become worse; if the short-circuit protection becomes stronger, the starting ability will become weaker. Especially when an ultra-wide voltage range input is required, the startup capability and short-circuit capability are even less compatible.

For example, E4805UHBD-15W, 18~72VDC input, 15W output module power supply, if the RC startup circuit is composed of resistors and capacitors as shown in Figure 6, the current will change with the input voltage. When the low voltage and high voltage are short-circuited, the hiccup cycle will There is a big difference. The short-circuit power will be larger when the high-voltage input is used. After adjusting the low-voltage starting capability and short-circuit protection, the high-voltage short-circuit protection will become worse and the starting ability will be super strong. In turn, the high-voltage starting and short-circuit capabilities will be adjusted and the low-voltage short-circuit protection will be improved. The ability is very good, but the startup ability is very poor, and poor startup will occur.

In order to solve the above contradiction, the starting circuit is replaced by a constant current circuit, as shown in Figure 7. The input current basically does not change with the change of the input voltage. For the two starting circuits, the low voltage provides the same starting current. When the high voltage is short-circuited, , the short-circuit power consumption of the second starting circuit will be much smaller, and the short-circuit periods of low voltage and high voltage will be closer.