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Undervoltage protection circuit of power adapter

source:Industry News     Popular:adapter     release time:2022-01-27 09:18:00     Article author:sznbone    

  3-12W Medical Switching Power

  1. Input undervoltage protection circuit one:

  A type of power adapter input undervoltage circuit: The protection principle is that when the voltage input is lower than the rated value or lower than the protection voltage value, it will pull down the power supply VCC of the control chip to achieve the purpose of shutting down the output.

  Its realization principle is:

  When the power supply input voltage is higher than the undervoltage protection set point, the voltage at point A is higher than the Vref of U4, U4 is turned on, the voltage at point B is low, 04 is turned on, and the Vcc power supply is normal: when the input voltage is lower than the protection voltage At this time, the voltage at point A is lower than the Vref of U4, U4 is off, the voltage at point B is high, and Q4 is off, resulting in no voltage at Vcc. At this time, Vre is low, and the voltage at point A increases as the input voltage increases. When the Vref of U4 is set, the module becomes normal operation, and R4 can set the hysteresis of the undervoltage protection point.

  Advantages: simple circuit structure, accurate protection points

  Disadvantages: the cost is too high

  Application note: Pay attention to the value of R,, RZ when using it. Sometimes it is necessary to connect two resistors in parallel to get the required protection point. It is also necessary to pay attention to the temperature coefficients of R1 and RZ, otherwise the undervoltage protection points will be quite different at high and low temperatures.

  2, input undervoltage protection circuit two:

  Type two under-voltage protection circuit: the input voltage is higher than the under-voltage value and is normally connected, and the output is automatically closed when the input voltage is lower than the under-voltage value.

  Its realization principle is:

  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. At this time, the protection circuit does not work; when the input voltage is lower than the set undervoltage 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, thus realizing the undervoltage protection function. R21, VT6, and R23 form a hysteresis circuit for undervoltage shutdown and recovery. When the undervoltage is turned off, VT6 is turned on, and R21 and R2 are connected in parallel,

  When recovering, VT6 is cut off

  The hysteresis voltage is (Vin’-Vin)

  Advantages: The circuit form is simple and the cost is low.

  Disadvantages: The voltage regulation value of each voltage regulator tube VD4 has an error, which causes the under-voltage protection value to fluctuate. When mass production, the relevant value needs to be adjusted

  Application note: VD4 should choose a regulator tube with a good temperature coefficient, and components that need to be debugged, such as R2, should consider multiple parallel connections to facilitate debugging.

  For the power adapter output over-voltage protection circuit and under-voltage protection circuit is introduced here, if you have any questions, you can contact us, we will answer in detail for you!


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