answer:
This is a circuit diagram of a laptop power adapter, and here we analyze how this circuit diagram converts the mains into a notebook-powered DC.
We start from the input end, the letter L represents the live wire ,the letter N represents the neutral wire (neutral wire),First through the common mode mutual inductance coil L3, L4 and capacitor C1 and resistance R1, R2 composed of electromagnetic interference suppression circuit, the role of this part is to filter out the common mode signal in the mains, as well as high-frequency signals, to prevent these signals from interfering with the subsequent circuit, play a role in protecting the circuit. It can be seen that the resistance of R1 and R2 is 2.2MΩ is very large, the role of these two resistors is to ensure that after the power is cut off, capacitor C1 can release the stored electrical energy through R1 and R2, without these two resistors, the energy in the capacitor may be released through the back circuit, destroy the back circuit, or generate an electric spark at the plug, which needs to be avoided.
Then the later D1 is called a bridge rectifier circuit, through these four diodes can be rectified ac signal into DC signal, but the DC signal at this time is not stable, the size of the voltage will fluctuate with time.
Later, we get to the core part of the whole circuit, which uses a flyback structure. First, let's introduce the basic structure of the flyback circuit
There is a switch in the flyback circuit, when the switch is turned on, the circuit is in the left state, at this time the power supply is charged to the mutual inductance coil, and the electrical energy stored in the capacitor at this time is supplied to the load; when the switch is disconnected, the mutual inductance coil discharges to the circuit on the right, and the capacitor is charged at this time. Therefore, we can achieve the purpose of regulating the voltage on the load by constantly turning on and off the switch, and the output voltage is related to the on time and the off time. Back in our circuit, extract the core devices in the circuit, you can see the structure of the flyback circuit
In order to stabilize the voltage value of the output, a negative feedback structure is introduced, and the U3A and U3B in the circuit are a set of light-emitting diodes and phototransistors, which play the role of optocoupler isolation, that is, the voltage signal can be fed back from the output to the input, but the mutual crosstalk of the voltage and current at the output and input is ensured. The switch used here can adjust the switching on time and off time through changes in the voltage signal feedback from the output, thereby ensuring that the output voltage can be stabilized at a certain value (the output voltage is stabilized at 19V).
Resources:
65W LAPTOP POWER ADAPTER CIRCUIT DIAGRAM
Diode bridge
Flyback converter
by Paarthurnax
Q.E.D.