Four basic types of feedback circuits for single-chip switching power supplies
(1) Basic feedback circuit;
(2) Improved basic feedback circuit;
(3) Optocoupler feedback circuit with voltage regulator;
(4) Equipped with TL431 optocoupler feedback circuit.
Their simplified circuit is shown in the figure.
Figure (a) shows the basic feedback circuit, which has the advantages of simple circuit, low cost, and is suitable for making miniaturized and economical switching power supplies; Its disadvantage is poor voltage regulation performance, with a voltage regulation rate of SV=± 1.5% to ± 2.5% and a load regulation rate of SI ≈ ± 5%.
Four basic types of graph feedback circuits
a) Basic feedback circuit (b) Improved basic feedback circuit (c) Optocoupler feedback circuit with voltage regulator (d) Optocoupler feedback circuit with TL431
To improve the basic feedback circuit, simply add a voltage regulator VDZ and resistor R1 to achieve a load adjustment rate of ± 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 and meet the needs of the circuit.
It is an optocoupler feedback circuit equipped with a voltage regulator. The reference voltage UZ is provided by VDZ, and 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 the TopSwitch, which can be used in conjunction with an internal error amplifier to adjust the UO. This feedback circuit can achieve a voltage adjustment rate of less than ± 1%.
It is an optocoupler feedback circuit equipped with TL431, which has a more complex circuit but the best voltage stabilizing performance. Here, the TL431 type adjustable precision parallel voltage regulator is used instead of the ordinary voltage regulator to form an external error amplifier, and then fine adjust the UO, which can achieve a voltage adjustment rate and load adjustment rate of ± 0.2%, comparable to linear voltage regulators. This feedback circuit is suitable for constructing precision switching power supplies.
