MC34063 USB charging circuit diagram

1. Production process

According to the circuit schematic, design a single-sided PCB as shown below.
Before soldering the circuit board, first prepare all the components in Appendix 1. Use a multimeter to test all of them to ensure that there are no problems with the components.

Next, you can start soldering. Follow the principle of soldering from low to high and from small to large. As shown in the figure below, first solder the smaller components such as resistors and ceramic capacitors to the circuit board. Then solder the electrolytic capacitors, transformers, and USB ports in sequence.

For safety reasons, use a 1/2W 1Ω resistor as a fuse (F1).
The following points should be noted during soldering:
(1) The light-emitting diode LED1 is the power indicator light. It needs to be aligned with the round hole on the shell. The pin of LED1 can be left longer, otherwise it cannot be seen on the surface.
(2) Do not mix up the diodes D1, D5, D6, and D7. IN4007 is a low-frequency diode, FR107 is a high-frequency high-voltage diode, IN5819 is a low-voltage high-frequency diode, etc. They cannot be replaced.
(3) Do not install the transistors Q1 and Q2 incorrectly, let alone reversely.
(4) After installation and soldering, carefully check each part to see if there is any cold soldering or other problems.

2. Debugging Methods

After carefully checking that the circuit board is correctly welded, it is time to enter the debugging stage. Pay special attention to safety here, because the circuit is directly powered by 220V, and there is a 22C)V high voltage on the entire circuit board. Once the human body touches the circuit board, an electric shock will occur. So be extra careful in the next steps.

For the convenience of debugging, first solder a 220V plug wire on the PCB. At the same time, for safety reasons, please connect a 10W bulb in series on the plug wire to prevent short circuit or wrong connection, as shown in the figure below.

If the welding is correct, you can use a multimeter to measure that there is a +5V DC voltage output between the 1st and 4th pins of the USB port, and the power indicator LED1 lights up normally. After everything is normal, remove the plug wire, replace it with two wires, and connect the circuit board to the pins on the shell, as shown in the figure below.

Finally, insert the circuit board into the other half of the shell, expose the indicator light outside the round hole, and align the USB port with the square hole reserved in the shell. After completion, you can use it normally to charge MP3 and MP4. The final effect is shown in the figure below. See the component list in the table below.

3. Other solutions
In addition to the solutions described above, you can also refer to the following figure to make a switch-mode USB charger using the DC-DC converter MC34063A as the core. The internal structure of MC34063A can be seen in the right figure. Its internal transistor Q1 plays the role of an analog switch. The internal oscillator sets the switching cycle. At the same time, the Ipeak sensing end limits the current flowing through the inductor L1 by controlling the conduction time of the transistor Q1. The 125V reference voltage and the comparator form a circuit to monitor the output voltage and provide feedback to the chip.
In the figure below. When the power switch S1 is closed, MC34063A obtains the working power supply, the diode D1 can prevent the power supply polarity from being reversed during welding and burning the device, and the transient voltage interference suppressor D2 can suppress the high-frequency pulse noise introduced from the power supply. Capacitors C1 and C2 play a further filtering role.

Three 1Ω resistors R1, R2, and R3 in parallel can help monitor the current flowing through the inductor L1. When the current reaches 1A, the level of pin 7 of MC34063A is 300mV lower than that of pin 6, so that the internal transistor of MC34063A is cut off, and the energy stored in the inductor L1 enters the capacitor C4 through the Schottky diode D3.
Pin 5 of MC34063A stabilizes the output voltage through a voltage divider. The voltage regulator diode D4 and the 10Ω resistor R7 play the role of overvoltage protection when the load suddenly decreases. Under normal circumstances, when the output voltage is 5V, the voltage regulator diode D4 will not conduct. However, when the potentiometer R5 is adjusted to make the divided voltage 1.25V, the output voltage is 5V. If it exceeds 5.1V, the voltage regulator diode D4 will continue to conduct, so it is necessary to be cautious when adjusting R5 to ensure that the output voltage does not exceed 5.1V.

Inductor L1 is made by winding 75 turns of φ0.5mm enameled wire on a round iron core. Since the circuit is relatively simple, you can refer to the device layout in the figure above to design a single-sided PCB. After the PCB is made. Solder the components according to the principle of small to large, and the final product is shown in the figure below. After
the production is completed, enter the debugging stage. First, turn the potentiometer R5 counterclockwise to the end, use the voltage range of the multimeter to measure the voltage of the circuit test point TP, then power the circuit, close the power switch S1, and adjust the potentiometer R5 so that the multimeter reading is 5V±20mV. After debugging, you can install the circuit board shown in the figure below into a plastic box, so that the indicator light D5 is exposed.

4. How to use the USB charger

The finished USB charger is shown in the figure below. It is very simple to use. Just plug the USB charger into a 220VAC socket. If the indicator light on the charger is on, it means it is working properly. Then connect the original USB cable of the MP3 or MP4 to the USB port below the charger and the charging port on the device. You will see a message on the MP3 or MP4 that the battery is being charged. When the MP3 or MP4
prompts that the battery is fully charged, unplug the charger from the socket and disconnect the USB connection between the device and the charger.

5. How does a USB charger work?

It is very easy to use this homemade USB charger to charge MP3 or MP4. So what is the secret in the USB charger? In fact, the circuit of the USB charger is very simple (you can guess it from the cost of less than 10 yuan!), as shown in the figure below is the circuit diagram of the USB charger.
After the 220V AC is rectified and filtered by diode D1 and capacitor C1, there will be a DC voltage of about 300V at C1. After the resistor R2, the base of the transistor Q1 is provided with working current. After Q1 works, its collector generates current in the primary coil 3~4 of the transformer T1, so that the secondary coil 5~6 of T1 generates voltage due to induction, and the other secondary coil 1~2 of T1 also generates induced voltage. These two secondary coils are two independent coils with the same number of turns. The output of coils 1~2 is rectified and filtered by diode D7 and capacitor C5, and finally charged to the load through the USB socket and JP2.
Coils 5-6 of T1, capacitor C3, and resistor R4 also form a positive feedback circuit of transistor Q1, which makes Q1 oscillate at high frequency and continuously supply power to coils 3-4. When the load impedance changes and the output voltage increases, coils 5-6 and voltage-stabilizing diode ZD1 sample and compare to turn on transistor Q2, and the base current of Q1 decreases, and the collector current also decreases, so that the circuit load capacity decreases and the output voltage is reduced, thus playing a role in voltage stabilization.
In addition, capacitor C4, resistor R5, and diode D5 form a breakdown protection circuit to prevent the induced voltage of coils 3-4 from breaking down Q1.