Uc3842 is a widely used power control chip. Many newcomers and masters are favored by this chip. Although uc3842 looks simple in structure, it also has some complexity. Friends who first started to contact the circuit should not ignore the principle of this chip because of its simplicity.
This article will explain and analyze the voltage feedback circuit in uc3842 in more detail. I hope that designers can fully understand the knowledge points.
The output voltage is directly divided into the input of the error amplifier, as shown in Figure 1.
After the output voltage vo is divided by the resistors r2 and r4, a voltage sampling signal of the output voltage is obtained, and is input to the voltage feedback input terminal 2 of the pwm controller (the reverse input terminal of the error amplifier). The positive input of the error amplifier is connected to the 2.5V reference voltage inside the uc3842. When the sampling voltage is less than 2.5v, the voltage difference between the forward and reverse output terminals of the error amplifier is amplified by the amplifier, and the output voltage is adjusted, so that the duty ratio of the output signal of the uc3842 becomes larger, the output voltage rises, and finally the output is made. The voltage is stable at the set voltage value. The resistor r3 and the capacitor c1 are connected in parallel to form a current type feedback.
As mentioned at the beginning of the article, the biggest advantage of UC3842 is the simple circuit structure. The voltage feedback circuit also has this advantage, but its shortcomings are also very obvious. The input and output voltages cannot be isolated. . The switching power supply works in a high frequency state, so if this design is adopted, it will inevitably cause a certain degree of difficulty to the overall wiring, causing a large area of ​​electromagnetic interference. So designers need to make choices as needed.
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