# USB multi-channel output power supply --- 3 circuit diagram and other device selection calculation

2023-10-29Archive

5V --> 3.3V

Typically, microcontroller is powered by 3.3V, and accuracy requirements are relatively high. This uses an AMS1117 three-pin voltage regulator IC, as shown in figure below:

The input and output are connected to a 10uF+104 capacitor, 10uF is used for energy storage and 104 is used for filtering.

5V_USB --> -12V

Use SY8120 to create a negative voltage circuit, calculate resistors R6 and R9 according to IC manual, let Vfb = 0.6V, create a negative -12V power supply, working principle was mentioned earlier. The difference here is that output of chip is connected to GND, and "GND" of the chip is a negative voltage.

-12V --> -5V

Use 79L05 three-terminal voltage regulator IC to create a step-up circuit to convert -12V to -5V. Resistor R10 should divide some of power drawn into U3.

Finally, a resettable PTC fuse has been added to USB input to protect computer's USB interface. When current in loop exceeds a certain threshold, transistor Q1 turns on and LED D3 lights up. At moment, scheme is almost ready. The selection of key components has been calculated. If there are other calculations that are unclear, please discuss with us. The following figure shows layout of final version:

Calculating selection of other key circuit components:

When load changes, C14 will charge and discharge. The change in potential of pin 6 is actually result of integrating current C14, which plays role of an integral adjustment. For small load changes, DC level adjustment at this time needs some damping delay effect, and adjustment occurs after a few cycles. The delay time of this adjustment depends on RC circuit, so resistance R17 should not be too large, otherwise integration period will be very long. After all, it still needs to be tuned with a test signal when the actual circuit is debugged.

When 12V power supply is just installed, a loop is formed through Q2, R15 and D5, but working current of zener diode is 1-2mA, so when debugging actual circuit, it is necessary to actually match resistance R15 and voltage regulator tube voltage regulation threshold. Although transistor Q2 is on, blue circuit on left is also on, and CE of transistor Q6 short circuits voltage regulator D5, so that a new circuit is formed through Q2, R15 and CE Q6, then current in control circuit should not be too large, so resistor R15 should not be too small, but moderate. Then, to select resistor R19, it is also necessary to pay attention to fact that power consumption in circuit is not too large. The current on positive circuit of Boai is 1-2mA. Resistor R21 in above figure should match corresponding resistance value during actual debugging, but usually it is from tens of R to 100 R and should not be too large, otherwise PWM on and off waveform will be very slow.