This circuit only gives the complete circuits of oscillation, sampling, comparison and adjustment after DC input. As the AC step-down and rectification circuit is relatively simple, please prepare it yourself.
Circuit working principle:
R 1, R2, C 1 and 555 time base circuits form an unstable multivibrator. The oscillation frequency is determined by the values of R 1, R2 and C 1. Select appropriate component values to make the oscillation frequency reach 15KHz. R3 is the load resistance of IC 1, and the output pulse trigger signal waveform of IC 1 is shown in Figure (b).
R4, C2 and IC2 form a monostable trigger, and the pulse trigger signal output by IC 1 directly enters the second pin of the low-level trigger terminal of IC2. When the trigger level is high, the monostable trigger is in the steady state, the output level is low, and the switching regulator V 1 is in the off state. The diode VD 1 in Figure (a) is a level shift tube to ensure that V 1 is reliably turned off when the flip-flop outputs low level. When the low level of the trigger terminal comes, the monostable trigger flips and outputs a high level (transient), and V 1 is saturated, and the power supply charges the capacitor C2 through R4. When the potential on C2 is higher than the potential on the fifth pin of IC2, the monostable trigger returns to the steady state, the output is low, V 1 is turned off, and the capacitor C2 is rapidly discharged through the discharge tube inside IC2. This reciprocating cycle makes V 1 work in the on-off state. In Figure (a), L is the energy storage inductor and VD2 is the freewheeling diode. The relationship between the output waveform of the monostable trigger, the trigger signal at the trigger terminal and the potential Vco2 at the fifth pin of the voltage-controlled terminal IC2 is shown in Figure (b).
Automatic voltage stabilization process: the output voltage Vo is detected by the sampling circuit and compared with the reference voltage, and the detected error signal is amplified by the error amplifier circuit, which automatically adjusts the level of the potential Vco2 at the fifth pin of the voltage control terminal of IC2, so that the pulse width tpo of the trigger output signal changes accordingly. Adjust the conduction time of V 1 tube, so as to achieve the purpose of automatic voltage stabilization. The output voltage is regulated by potentiometer RP.
Component selection and manufacturing:
Energy storage inductor L, which can use high-frequency magnetic ring and cross-talk with 0.4mm enameled wire for 40 turns;
The freewheeling diode VD2 is a Schottky or fast recovery diode, for example, 2CN series.
The key of this circuit is to ensure that the period t of the trigger signal is greater than the pulse width tpo of the monostable trigger transient, otherwise the circuit will not work.
When debugging, first disconnect pin IC25, connect the oscilloscope to the output end of IC2, and adjust the timing element R 1 or R4 to make the transient pulse width tpo of the trigger output waveform on the oscilloscope as close as possible to T, but just ensure T & gtTpo.