Four basic types of switching power supply feedback circuits
The circuit of a monolithic switching power supply can be varied, but its feedback circuit has only four basic types:
(1) Basic feedback circuit;
(2) Improved basic feedback circuit;
(3) Optocoupler feedback circuit with voltage regulator tube;
(4) Optocoupler feedback circuit with TL431.
The basic feedback circuit has the advantages of simple circuit and low cost, and is suitable for making miniaturized and economical switching power supplies; its disadvantage is that the voltage stabilization performance is poor, the voltage regulation rate SV=±1.5%～±2.5%, and the load regulation rate SI ≈±5%.
The improved basic feedback circuit only needs to add a regulator tube VDZ and resistor R1 to make the load adjustment rate reach ±2%. The stable voltage of VDZ is generally 22V, and the number of turns of the feedback winding must be increased accordingly to obtain a higher feedback voltage UFB to meet the needs of the circuit.
Optocoupler feedback circuit with Zener tube. The reference voltage UZ is provided by VDZ. When the output voltage UO fluctuates, an error voltage can be obtained on the LED inside the optocoupler. Therefore, this circuit is equivalent to adding an external error amplifier to TOpSwitch, and then working with the internal error amplifier to adjust UO. This feedback circuit can make the voltage regulation rate reach below ±1%.
The optocoupler feedback circuit with TL431 is more complicated, but the voltage stabilization performance is the best. Here, the TL431 adjustable precision shunt voltage regulator is used to replace the ordinary voltage regulator tube to form an external error amplifier, and then to fine-tune UO, so that both the voltage adjustment rate and the load adjustment rate can reach ±0.2%, which can be compared with the linear regulator. comparable to piezoelectric sources. This feedback circuit is suitable for forming a precision switching power supply.
