Constant voltage and constant current control circuit diagram composed of current amplifier

The picture shows a constant voltage and constant current control circuit composed of an LM358 amplifier and a precision voltage regulator TL431.

The induced voltage of the transformer winding N2 is rectified by VD2, and the π filter circuit composed of C2, L1, and C3 obtains the DC output voltage on C3. The purpose of setting the N1 winding is so that IC1 can work normally when the output is short-circuited to ensure the safety of the circuit.

The working principle of the constant voltage circuit: U2, ICIB, R6, R7, VD4, R10, and U1 form a voltage control loop. U2 (TL431) is a precision voltage regulator. The cathode K and the control electrode R are directly short-circuited to form a precise 2.5V reference voltage. R4 is the current limiting resistor of U2. The 2.5V reference voltage is sent to the ICIB inverting input terminal (pin 6) through resistor R5; while the non-inverting input terminal (pin 5) is set by the voltage dividing ratio of R6 and R7. If the output voltage rises, the UR7 voltage also rises. This voltage is compared with the 2.5V reference voltage at the inverting terminal. Pin 7 outputs an error signal, which is then converted into a current signal through VD4 and RIO, flows into the LED in the optocoupler, and is controlled by feedback The network controls the primary side PWM output duty cycle so that the output voltage works in a constant voltage state.

The working principle of the constant current circuit: U2, IC1A, R1, R2, VD3, R10, and U1 form a current control loop. R1 is the output current sampling resistor, and the output current generates the voltage of R1/IOUT on R1.

drop. This voltage is directly sent to the non-inverting input terminal (pin 3) of ICA, while the 2.5V reference voltage is a voltage divider circuit composed of R2 and R3, and then the divided voltage is sent to the inverting input terminal (pin 2), and the output voltage

The voltage drop flowing on R1 is compared with the 2.5V reference voltage divided voltage. Pin 1 outputs an error signal, which is then converted into a current signal through VD3 and RIO, changing the current in the optocoupler LED, and then controlling the primary side through the feedback control network. The PWM output duty cycle makes the output characteristics exhibit constant current characteristics. R8, C4, R9, and C5 are the phase compensation components of IC1A and ICIB respectively.

Using a constant voltage and constant current control circuit composed of amplifiers, high constant voltage and constant current accuracy can be achieved. Since the circuit in the figure adopts the form of an amplifier, the resistance value of R1 can be selected as mΩ level, which has basically no impact on the conversion efficiency of the circuit.

The figure shows a constant voltage and constant current control circuit composed of an LM358 amplifier and a precision voltage regulator TL341.

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