Case 2 - Detailed description of protection circuit for switching power supply

Input under voltage protection circuit

1. Overview (description of circuit type and realization of main function):
Input under voltage protection circuit. When the input voltage is lower than the preset under voltage value, cut off the output. When the input voltage rises to the preset recovery value, the output automatically returns to normal.
2. Circuit diagram:



3. Analysis of working principle (main function, performance index and realization principle):
When the input voltage is within the normal operating range, Va is greater than the regulated value of VD4, VT4 is turned on, Vb is at 0 potential, and VT5 is turned off, the protection circuit does not work at this time. When the input voltage is lower than the preset under voltage value, Va less than the voltage value of VD4, VT4 is off, Vb is high, VT5 is on, COMP (pin 1 of the chip) is pulled to 0 potential, and the chip turns off the output to realize the under voltage protection function. R21, VT6, and R23 form a hysteresis circuit for under voltage shutdown and recovery.

4. The advantages and disadvantages of the circuit:
Advantages: simple circuit form and low cost.
Disadvantages: Due to the difference in the voltage regulation value of the voltage regulator tube VD4 between batches, to result in the under voltage protection point fluctuates up and down, and the relevant parameters need to be adjusted frequently during mass production.