Four basic types of feedback circuits for monolithic switching power supplies
The circuits of a monolithic switching power supply can vary widely, but there are only four basic types of feedback circuits:
(1) Basic feedback circuits;
(2) Improved basic feedback circuits;
(3) Optocoupler feedback circuit with voltage regulator;
(4) with TL431 optocoupler feedback circuit.
Basic feedback circuit, the advantages of the circuit is simple, low cost, suitable for the production of miniaturized, economical switching power supply; its disadvantage is the poor performance of voltage regulation, voltage adjustment rate SV = ± 1.5% to ± 2.5%, load adjustment rate SI ≈ ± 5%.
(a) basic feedback circuit (b) improved basic feedback circuit (c) with the voltage regulator optocoupler feedback circuit (d) with TL431 optocoupler feedback circuit
Improved basic feedback circuit, only need to add a voltage regulator VDZ and resistor R1, you can make the load adjustment rate of ± 2%. VDZ stabilization voltage is generally 22V, you must correspondingly increase the number of turns of the feedback winding, in order to obtain a higher feedback voltage UFB, to meet the needs of the circuit.
is an optocoupler feedback circuit 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 then be used in conjunction with the internal error amplifier to adjust UO. This feedback circuit enables a voltage adjustment rate of ±1% or less.
Is equipped with TL431 optocoupler feedback circuit, its circuit is more complex, but the best voltage regulation performance. Here with TL431-type adjustable precision shunt regulator to replace the ordinary voltage regulator, constituting an external error amplifier, and then the UO for fine tuning, the voltage adjustment rate and load adjustment rate can reach ± 0.2%, comparable to the linear regulated power supply. This feedback circuit is suitable for the composition of precision switching power supply.
