Introduction to the four basic types of feedback circuits for monolithic 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.
As a basic feedback circuit, its advantages are simple circuit, low cost, and suitable for making miniaturized and economical switching power supplies; Its disadvantage is poor voltage regulation performance, with a voltage regulation rate SV of ± 1.5% to ± 2.5% and a load regulation rate SI of ≈ ± 5%.
(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, only one voltage regulator VDZ and resistor R1 need to be added 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.
Optocoupler feedback circuit with 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 TOpSwitch, which can be used in conjunction with an internal error amplifier to adjust UO. This feedback circuit can achieve a voltage regulation rate of less than ± 1%.
The optocoupler feedback circuit with TL431 is relatively complex, but it has the best voltage regulation performance. Here, TL431 adjustable precision parallel regulator is used instead of ordinary regulator to form an external error amplifier, and then UO is finely adjusted, which can achieve a voltage adjustment rate and load adjustment rate of ± 0.2%, comparable to linear regulator power supply. This feedback circuit is suitable for constructing precision switching power supplies.
