The traditional switching power supply uses passive power factor correction technology, and the power factor is only about 0.75. There is a large phase difference between the input voltage and the input current. The input current is pulse-shaped, non-sinusoidal, and the harmonic component is high, which not only causes serious pollution to the power system, but more importantly, causes waste of the grid energy. In the context of the current vigorous promotion of green power, increasing the power factor of switching power supplies has become a top priority for power supply manufacturers. The active power factor correction circuit with power factor control chip as the core has the advantages of small size, light weight, wide input voltage adaptation range and high efficiency (power factor close to 1). It is gradually replacing passive power compensation technology and obtained in various fields. widely used. First, the working principle of power factor compensation The power factor compensation circuit shown in the following figure is mainly composed of a power factor control chip MC33262, a power transistor Q1, a boost inductor L3, a boost diode D1, an output filter capacitor C2, and a feedback loop. Because of the physical mapping, it is inevitable that there are errors and omissions. The two inputs of the multiplier in the chip MC33262 and related components constitute an internal and external feedback control circuit. The inner loop implements input current shaping to make it a standard sine wave in phase with the voltage. The full-wave rectified output DC ripple voltage is sampled by a resistor divider consisting of R58 and R17 and sent to pin 3 of the MC33262, which is applied to an input of the multiplier. Therefore, the current reference output by the multiplier is a double half-wave sinusoidal voltage as the current reference value of the zero current comparator. Compare the waveform of the real reaction alternating current (AC) input voltage. When the AC input voltage changes from zero to sinusoidal to the peak value, the output of the multiplier is controlled to control the threshold of the current sensing comparator, forcing the current through L3 to track the input voltage as a sinusoidal trajectory change, passing the triangular high frequency current The peak envelope is proportional to the input AC voltage, and its average current is sinusoidal, which means that the power input current is also sinusoidal, thus achieving power factor compensation. The current flowing through the power transistor Q1 is converted into a voltage signal on the resistor R4, and is input to the fourth pin of the chip through the R9 and R12 resistors, and is input to the MC33262 chip current detecting comparator through the chip built-in RC low-pass filter. At the forward input end, the waveform of the inductor L3 current is a high-frequency sawtooth triangle wave. In the process of increasing the current value from zero to the peak value, Q1 is turned on, as long as the sensing voltage on R10 exceeds the threshold voltage of the current detecting comparator. The on-chip logic circuit operates, and the output power transistor Q1 is turned off; the secondary winding Ns of the boost inductor L3 is used as a high-sensitivity Current Sensor of L3, and the current flowing through L3 is detected, and then input to the chip through the current limiting resistor R11. The internal zero current detector, as soon as the inductor current drops to the "zero" level set by the chip, the zero current detector will be turned on by setting the gate lock drive 01. During Q1 conduction, the boost diode D1 is turned off, and the filter capacitor C1 is discharged through the load. When 01 is turned from turn-on to turn-off, the abrupt potential generated by L3 causes D1 to be forward-biased and turned on, and energy storage in L3. After Dl release, charge C2, because Ql and Dl alternately conduct, the rectifier bridge output current flows continuously through L3, which means that the rectifier diode tends to 1800 in the half cycle of the AC power supply, the power factor Close to l. The induced voltage on the secondary winding Ns of the boosting inductor L3 is rectified by D6 and C5 filtered, and serves as the auxiliary operating power source after the MC33262 chip is started and the starting voltage of the UC8342 chip. The outer loop is used as a feedback regulator control for the output DC voltage of the active power factor correction technology converter. The DC voltage across the filter capacitor C2 is sampled by a resistor divider consisting of R18 and R7, R15, and sent to the inverting terminal of the chip's internal error amplifier, and compared with the error amplifier's non-inverting terminal precision reference voltage Vref to generate a direct current (DC). The error voltage, as another input to the multiplier, controls the MC33262 to output a PWM drive signal to adjust the duty cycle of the power transistor Q1 to stabilize the output voltage. When the output voltage rises, the output voltage of the error amplifier drops, the base current value of the multiplier output decreases, the on-time of the switch tube is shortened, and the current flowing through the inductor is decreased, thereby causing the output voltage to drop, and vice versa, causing the output voltage to rise. In order to achieve the purpose of stabilizing the output voltage. The MC33262 has an undervoltage and overvoltage lockout protection circuit. When the input voltage drops to make the 8th pin voltage lower than the built-in 8V reference voltage, the PWM pulse output is blocked. When the output voltage rises, the 1st pin voltage exceeds 1.08V. When the reference voltage is applied, the chip 7 pin driving pulse output is prohibited to ensure the safety of the power factor compensation circuit and the main power circuit. Second, the main power supply works 2. The overcurrent protection circuit is composed of resistors R26, R40, R41 and related transistors such as transistors Q4 and Q6. When the load current rises or shorts for some reason, the current of the switching transistor Q7 rises, the voltage on the current sampling resistor R41 rises, and the voltage sent to the 3rd pin of the UC3842 protection terminal also rises. When the voltage of the 3 pin rises to 1V, The internal protection circuit operates, causing the duty ratio of the 6-pin output drive pulse to decrease, and the output voltage to decrease. The supply voltage Vaux of the UC3842 is also reduced. When the UC3842 is low, the entire circuit is turned off. At the same time, the overcurrent self-locking protection transistor Q4 and Q6 are turned on, and the 5V reference voltage outputted by the 8th pin of the UC3842 is applied to the 3-pin protection terminal through Q4 to realize overcurrent self-locking protection. 3. The 5V, 12V overvoltage self-locking protection circuit is composed of D11, D16, z5, 26 and optocoupler components. If the 12V output voltage is overvoltage for some reason, the Zener diode 26 breaks down, the diode in the photocoupler P2 (4N35) turns on and emits light, and the triode is lighted and turned on, causing the diode D11 to also conduct, UC3842 The voltage of pin 1 is clamped below 1V, and the duty cycle of the 6-pin output pulse is 0, and the main power supply stops working. At the same time, the transistor Q5 is turned on, thereby achieving overvoltage protection self-locking. 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MC33262 power factor compensation control chip current control mode is the peak current control mode in CCM.
The internal control circuit includes a self-starting timer, a zero current detector, a current detection comparator, an error amplifier, a multiplier, a drive output, and undervoltage and overvoltage. The specific internal structure is shown in the right figure. 
1. The main power circuit is a typical single-switched switching circuit. The core of the whole circuit is the pulse width modulation (PWM) control chip UC3842. As can be seen in the figure on the right, the feedback input of the UC3842 is grounded. Therefore, the input error voltage of the error amplifier is always constant. The output of the amplifier is changed by adjusting the gain of the error amplifier, thereby changing the duty of the switching signal. Compared with the output voltage, when the output voltage changes greatly, the output voltage is basically unchanged, which has a good voltage regulation effect. The feedback regulator circuit is composed of C11, U4, optocoupler Pl and other related components. When the output voltage rises for some reason, the output voltage is sampled by a resistor divider composed of R50, R51, and R55. The voltage input to the U4 control terminal also rises, and the current flowing through the photocoupler rises. UC3842 is the first. When the voltage of the foot drops, the gain of the UC3842 decreases, the pulse duty ratio of the output of the seventh pin decreases, and the on-time of the power transistor Q7 is shortened, so that the output voltage is lowered, and vice versa, the output voltage is raised to achieve the purpose of stabilizing the output voltage. The transformer Tl has three outputs, which are rectified and filtered to obtain 12V and 24V working voltages. The voltage of the N4 winding is 16V voltage obtained by D9 rectification and Cl0 filtering, which is used as the working voltage after the UC3842 is started.