Industry news

Adapter switching power supply

2021-06-25

Adapter switching power supply

    Adapter power supply for laptop HA65NS2-00, input voltage range: 100~240VAC; output voltage is 19.5VDC, maximum working current is 3.34A; rated output power is 65W.

We can decompose the circuit diagram into several parts:

Electromagnetic interference suppression circuit and input rectifier filter circuit: The electromagnetic interference suppression circuit is composed of L1, R1A, R1B, CX1, L2 and a differential mode and common mode low-pass filter to suppress the electromagnetic interference generated by the switching power supply; BD1 and C1 form a bridge The rectifier filter circuit provides smooth DC power for the power conversion circuit.

   Power startup and conversion circuit: the mains power is added to the 8th pin (HV) of the chip after the high voltage tube VD2 half-wave rectifier switch controller and R1 current limit; it provides the starting voltage for the chip, after the chip is started, the 5th pin (DRV) The output PWM controls the on and off of the switch tube VT1. At this time, the high-voltage current source inside the chip is turned off, and the 18V auxiliary power supply is used to continuously power the chip. The auxiliary power supply is composed of R7, VD3, R8, C3 and C10; R4 in the figure is C3 and The discharge resistance of C10.

  Voltage stabilization control circuit: The output voltage of the secondary winding is rectified and filtered by the common cathode double Schottky diodes VD31A, C21A, C21B, C21C to produce a smooth 19.5V voltage. The resistor R21 and the capacitor C21 form an RC absorption network to suppress VD31A The cut-off spikes protect the rectifier devices. In the figure, the linear optocoupler PH1, LM431 and its peripheral components, the internal error amplifier of the chip, and the pulse width control circuit together constitute the output voltage regulator control circuit. According to the working principle of 431 characteristics, we can calculate the output voltage as: Uout=Uref*{1+[R24/(R25//R26)]} =2.5*{1+[69.8/(10.2//909)]} =19.8V.

   We can adjust the output voltage by fixing the resistance of R24 and R25, and by adjusting R26, the output voltage can be reduced by increasing the resistance of R26, and on the contrary, reducing the resistance of R26 can increase the output voltage.

Output protection circuit: I checked this adapter on the Internet and found that this adapter has a complete output protection function: VT3, NTC resistance, and R29 form an output rectifier overheat detection circuit; IC2A, internal 2.5V reference voltage and its peripheral components Output over-voltage detection circuit; IC2B and its peripheral components and current sampling resistor R40 form an output over-current detection circuit.

   The outputs of IC2A and IC2B are gathered together to form a "wire OR" circuit through the common anode double switch diode VD42 and VT3 output. When a certain way of output becomes low, through the feedback of optocoupler PH2, the voltage of IC1 pin 1 is pulled up, and the controller output is turned off. The FA5528 chip protection circuit has a locking function. When the fault is removed, it cannot automatically resume work. Only after the input mains is cut off and the power is turned on can the adapter work quickly.

Output rectifier overheating protection function: When the output rectifier device is overheated, the resistance of the negative temperature coefficient resistor NTC decreases, VT3 is saturated and turned on, and the optocoupler PH2 controls IC1 to turn off the controller output.

   Output overvoltage protection: When the output voltage rises and the midpoint voltage of the voltage divider composed of R41A and R41B is greater than 2.5V, the output of the comparator changes from high level to low level. We can calculate the overvoltage protection voltage according to the formula The value is: Vout*R41B/(R41A+R41B)>2.5V; get Vout>21.25V; that is to say, when the output voltage is greater than 21.25V, the power supply will activate the voltage protection function to prevent the voltage from further increasing.