The DC/DC switching power supply is a power supply form that continuously controls the power switching 

tube at a certain frequency to perform an on-off operation so as to supply power to the inverter or the load 

through an energy storage element such as an inductor and a capacitor. The average output voltage or current 

can be controlled by changing the duty cycle, switching frequency or associated phase. The switching power 

supply's switching frequency ranges from 20kHz to several MHz.

         For applications where the power supply is greater than 90W, the switching power supply usually takes 

a two-stage conversion. That is, a power factor correction (PFC) control converter and a DC/DC converter. 

In particular, it should be mentioned here that it is a power factor correction circuit. It is set to ensure that the 

input voltage and current work in phase. As a result, the power factor is close to 1, and the apparent power is 

all converted to active power, so the system efficiency is improved.If there is no PFC correction circuit, the 

input current will enter the switching power supply in the form of a narrow pulse width and high peak pulse, 

causing severe harmonic interference components. These harmonic components not only do not provide any 

energy to the load, but also cause heat generation in transformers and other equipment. The power 

factorcorrection circuit is divided into active and passive types. The switching power supply of FM broadcast 

transmitter mostly adopts active power factor correction circuit, which is composedof two parts: AC/DC 

converterwith active power factor correction and independent DC/DC converter.

         DC/DC converters mainly include: EMI filter, slow start circuit, bridge rectifier, PFC controller, power drive 

circuit and converter circuit (by power switch MOSFET, energy storage inductor L, fast recovery rectifier diode 

and filter capacitor And so on).It consists of switching transformer, MOSFET power switch, rectifying component, 

sensing circuit (including voltage, current and temperature sampling), auxiliary power supply, UC3843PWM 

controller and related drive circuit. The DC voltage input from the front stage is applied to the drain of the parallel 

power switch MOSFET, and the gate input is provided by the UC3843 control chip set frequency switching signal 

via the drive circuit. After boosting by the switching transformer, the rectification and filtering obtains the required 

DC voltage.
          The UC3843 control chip is a current mode PWM control regulator. It features optimized DC/DC converter, 

low starting current, automatic feedforward compensation, current limit, low voltage blocking, pulse rejection, 

high current drive and switching frequency up to 500kHz.

         From the internal circuit analysis of UC3843, the internal reference signal and the transformer secondary 

rectified and filtered voltage sample value are processed in the error amplifier, and the processed error voltage 

andthe voltage formed by the sense resistor are input to the PWM comparator, and the output and the clock 

signal are The waveform processing is performed in the trigger circuit, and the DC/DC finally outputs a switching 

frequency signal whose frequency is consistent with the clock frequency.

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