One of the frequency modulated switching regulated power supply circuits

The picture shows a practical frequency modulation switching regulated power supply circuit. Its main technical indicators are as follows: Output capacity: 5V, IOA; Voltage stability <1%; Load stability <1%; Conversion efficiency: 75%; Output ripple: 25mV (effective value); Frequency adjustment range: 35 ~ 300kHz . In the figure, VT5 and VT6 form a pulse oscillator. Since the base drive and timing loop are separated, the two do their own thing without interfering with each other, so they have high response speed and a wide frequency adjustment range. Frequency adjustment is accomplished through a comparison amplifier composed of VTl, VT2, and VT3 that is linked to the output voltage. The output pulse width is determined by the time constants of Rll~R17 and C4~C5, and is fixed at about 0.9μs. The power switch tubes are VMOS tubes VT10 and VT11. In order to overcome the large input capacitance that affects the turn-on and turn-off speed of the switch tube, a compensation network composed of R22 and Cl0 is added to the primary of the isolation transformer T2, so that when the circuit is turned on, the primary voltage of T2 is about 18V. When the tube is turned off, the amplitude has dropped to about 11V. When it is turned on next time, the amplitude has risen again. The two transistors VTl2, VT13 or VTl4, VTl5 on the secondary side of T2 provide a discharge circuit for the gate input capacitance, thereby greatly shortening the turn-off delay and drop time to less than 60ns. When the output voltage decreases for some reason, the divided voltage of the R1, R2, and R3 sampling circuits also decreases, and the base potential of VTl decreases. This voltage is compared and amplified with the reference voltage provided by resistor R6 and Zener diode VD1, causing the collector voltage of VT2 to decrease and the collector voltage of VT3 to increase. This voltage is added to the bases of oscillators VT5 and VT6, which deepens the degree of saturation conduction of these two tubes, thereby speeding up the charge and discharge speed of R15, C7, VT5 and R13, C6, and VT6 branches, and improving the oscillation The oscillation frequency of the device also increases accordingly the turn-on and cut-off rates of the switching power tubes VTl0 and VTll, causing the output voltage to rise to achieve voltage stabilization. On the contrary, when the output voltage rises, the opposite adjustment is performed according to the above process to keep the output voltage stable.