The integrated circuit KA7500B described in the article is a dedicated pulse width modulation switching power supply integrated controller from Samsung. It is fully compatible and interchangeable with TL494. The circuit scheme is shown in the figure below. 
The following describes the working principle of the circuit:
The circuit depicted in the figure is essentially a power supply circuit with constant current / constant voltage output. It has two feedback circuits, which are current feedback and voltage feedback. The positive and negative poles of current feedback correspond to the first and second legs of KA7500B. The output current produces a voltage drop across resistors R12 and R20. The voltage drop is R9. R10 and R14, R15 resistors are fed back. When the voltage of pin 1 of KA7500B is greater than the voltage of pin 2, KA7500B will reduce the output pulse width (pins 8 and 11) to reduce the current, otherwise increase the pulse width and make the output The current is constant at a preset value, and its constant current value conforms to the following formula:
Where R is the resistance of R12 and R20 in parallel, so the theoretical value of constant current is 735mA.
The positive and negative voltages of the voltage feedback in the circuit correspond to the 16th and 15th pins of the KA7500B. After power-on, the 14th pin of the KA7500B outputs a stable 5V voltage, which causes the LED to emit light as a power supply indication, and the 5V voltage is used as a reference. The voltage is supplied to the KA7500B's 15 pin as a voltage reference. After the output voltage is divided by R19, R10, VR1 and R17, it is compared with the voltage reference. When the voltage is too large, the pulse width is reduced. If it is too small, the pulse width is increased. To maintain a constant output voltage value, the output voltage value meets the following formula:
Since the two-way feedback of the KA7500B is controlled after the phase is "AND", when the output voltage is lower than the constant voltage value, the current feedback plays a controlling role. When the output voltage reaches 8V4, the voltage feedback plays a controlling role. Thus, the circuit completes the constant current/constant voltage control function, and its principle is exactly the same as that of the regulated power supply, except that the circuit is a switching power supply control mode, so the efficiency is high and the temperature rise is low.
In the figure, D1 is a diode that prevents polarity inversion, D2 is a freewheeling diode that operates from a switching power supply, and T1 operates in a switching state.
After understanding the working principle, it is much simpler to expand and apply it. It is described as follows:
1. When using normally, adjust the VR1 potentiometer to get the corresponding output voltage.
2, charge different capacity lithium-ion battery: can change the resistance of R12 and R20, the required current value can be set according to the formula described above. If the current is too large, a heat sink should be added to the T1, and the freewheeling diode should be replaced with 1N5822 to withstand more current.
3, charge high voltage lithium battery: just change R10 or R17, the specific required output voltage value can be calculated according to the formula.
4, for high-precision regulated power supply: its output voltage principle is consistent with the above, but need to pay attention to ripple, if possible, increase the inductance of the inductor and filter capacitor in the circuit to reduce ripple.
Dongguan Guancheng Precision Plastic Manufacturing Co., Ltd. , https://www.dpowergo.com