UC3842 is a new control device developed by Unitrode company, and it is a current-controlled pulse width modulator widely used in China. The so-called current-mode pulse width modulator adjusts the pulse width according to the feedback current. At the input end of the pulse width comparator, the current signal flowing through the output inductor coil is directly compared with the output signal of the error amplifier, so as to adjust the duty ratio and make the peak current of the output inductor change with the error voltage. Because of the double-loop system of voltage loop and current loop, the voltage regulation rate, load regulation rate and transient response characteristics of switching power supply are improved, and it is an ideal new controller. Circuit design and principle 1. 1 UC3842 working principle uc3842 Chinese data download uc3842 is an integrated chip with single power supply, forward current compensation and single channel modulation output. Its internal composition block diagram is shown in Figure L. 1 pin is connected with a resistance-capacitance element to compensate the frequency characteristics of the error amplifier. Pin 2 is the feedback voltage input terminal, and the sampling voltage is applied to the inverting input terminal of the error amplifier, and then compared with the reference voltage at the noninverting input terminal to generate an error voltage. Pin 3 is the current detection input, which cooperates with the resistor to form an overcurrent protection circuit. The external timing resistor with 4 pins and the timing capacitor of sawtooth oscillator determine the oscillation frequency, the reference voltage VREF is 0.5V, and the output voltage will determine the transformation ratio of the transformer. As can be seen from figure 1, it mainly includes functional circuits such as high-frequency oscillation, error comparison, under-voltage locking, current sampling comparison, pulse width modulation latching, etc. UC3842 is mainly used for high frequency and small capacity switching power supply. When driving a single-ended switch with isolated output, the traditional off-line flyback converter circuit usually compares the signal obtained by the reverse input of the error comparator through the resistance voltage division of the feedback winding with the internal 2.5V reference. The output of the error comparator is connected with the reverse input to form a PI compensation network, and the output of the error comparator is compared with the current sampling voltage, so as to control the duty cycle of the PWM sequence and achieve the purpose of circuit stability. 1.2 system principle in this paper, a single-ended flyback switching power supply with 220V AC input and 24v DC output is designed with UC3842 as the core control element. The control circuit of switching power supply is a voltage and current double closed-loop control system. The amplitude-frequency characteristic of the converter is changed from two poles to one pole, which improves the gain-bandwidth product, has great stability and has good frequency response characteristics. The main functional modules include: starting circuit, over-current, over-voltage and under-voltage protection circuit, feedback circuit and rectifier circuit. The principle and function of each module are analyzed below. The schematic diagram of the circuit is shown in Figure 2. The starting circuit of 1.2. 1 is shown in Figure 2. The AC power supply is low-pass filtered by C 16, L 1, C 15, C 14 and C 13. C 14, C 13, L 1 constitute an anti-* mode interference circuit, which is used to suppress the * * state noise interference. Their combined application has a strong attenuation bypass effect on electromagnetic interference. The filtered AC voltage is filtered by D 1 ~ D4 bridge rectifier and electrolytic capacitor C 1 and C2, and becomes a pulsating d C voltage of 3lOV. This voltage is reduced by R 1 and then charged to C8. When the voltage of C8 reaches the threshold of starting voltage of UC3842, UC3842 starts to work, and provides driving pulse, which is output by 6 pins to push the switch tube to work. With the start-up of UC3842, the work of R 1 basically ends, and the remaining tasks are handed over to the feedback winding, which generates voltage to supply power to UC3842. Because the input voltage exceeds the working voltage of UC3842, in order to avoid accidents, D 10 regulator is used to limit the input voltage of UC3842, otherwise it will be damaged. 1.2.2 short-circuit, over-current, over-voltage and under-voltage protection circuit Due to the instability of input voltage or other external factors, short-circuit, over-voltage and under-voltage sometimes occur, which is not conducive to the circuit work. Therefore, the circuit must have a certain protection function. As shown in Figure 2, if the output terminal is short-circuited and overcurrent occurs for some reason, the leakage current of the switch tube will increase greatly, the voltage at both ends of R9 will increase, and the voltage at the three pins of UC3842 will also increase. When the voltage at this pin exceeds the normal value by 0.3V and reaches 1V (that is, the current exceeds 1.5A), the PWM comparator of UC3842 outputs a high level, thus resetting the PWM latch and closing the output. At this time, pin 6 of UC3842 has no output, and MOS tube S 1 is turned off to protect the circuit. If the power supply voltage is over-voltage (above 265V), UC3842 can't adjust the duty cycle, and the primary winding voltage of transformer rises sharply, so does the 7-pin power supply voltage of UC3842 and the 2-pin voltage, thus turning off the output. If the grid voltage is lower than 85V, the voltage at the 1 pin of UC3842 will also drop. When the voltage drops below lV (the normal value is 3.4V), the PWM comparator outputs a high level, thus resetting the PWM latch and closing the output. If the output terminal is accidentally short-circuited, the output current will increase exponentially, so that the internal heat of the automatic recovery switch will surge, and it will immediately disconnect the circuit and play an overvoltage protection role. Once the fault is eliminated, the automatic recovery switch RF quickly recovers the impedance within 5 seconds. Therefore, this circuit has triple protection of short circuit, overcurrent, overvoltage and undervoltage. 1.2.3 feedback circuit The feedback circuit adopts precision regulated power supply TL43 1 and linear optocoupler PC8 17. The error voltage amplifier is composed of TL43l adjustable precision regulator, and the output is precisely adjusted by linear optocoupler. As shown in Figure 2, R4 and R5 are external control resistors of the precision regulator, which determine the output voltage, and together with TL43 1 form an external error amplifier. When the output voltage rises, the sampling voltage VR7 also rises, and the set voltage is greater than the reference voltage (the reference voltage of TL43 1 is 2.5V), which makes the output voltage of the error amplifier in TL43 1 rise, which leads to the drop of the output voltage of the on-chip driving transistor and the drop of the output voltage Vo, and finally Vo tends to be stable. On the contrary, when the output voltage is lower than the set voltage, the output voltage of the error amplifier decreases and the output voltage of the on-chip driving transistor increases, which finally changes the compensation input current of the 1 pin of UC3842, prompting the on-chip PWM comparator to adjust and change the duty ratio, thus achieving the purpose of voltage stabilization. The resistance values of R7 and R8 are calculated as follows: first fix the resistance value of R7, and then calculate the resistance value of R8, that is, 1.2.4 output rectifier filter circuit directly affects the voltage ripple and the performance of output voltage. The influence of switching power supply output on ripple amplitude mainly includes the following aspects. (1) Input power supply noise refers to the AC component contained in the input power supply. The solution is to add a capacitor C5 at the power input to filter out this noise interference. (2) High frequency signal noise. In switching power supply, DC input is chopped at high frequency and then transmitted through high frequency transformer. In this process, high-frequency noise interference will inevitably be mixed. There is also high frequency noise generated by power tube devices during switching. The solution to this high frequency noise is to use π-type filtering at the output. The filter inductance is 150μH, which can filter out high-frequency noise. (3) Fast recovery diodes D6 and D7 are used for rectification. Based on the characteristics of low voltage, low power consumption and large current, it is beneficial to improve power efficiency, and its reverse recovery time is short, which is beneficial to reduce high-frequency noise.

Shunt rectifier diode reduces peak voltage. In the high-power rectifier circuit, the secondary rectifier bridge circuit has large stray inductance. When the output rectifier commutates, the rectifier will bear a large peak voltage due to parasitic oscillation in the circuit. The existence of peak voltage improves the withstand voltage requirement of rectifier diode, and also brings additional circuit loss. The parasitic oscillation of rectifier bridge is generated between the leakage inductance (or additional resonant inductance) of transformer and the winding capacitance of transformer and the junction capacitance of rectifier tube. When the secondary voltage is zero, all four diodes in the full-bridge rectifier are turned on, and the inductance current of the output filter is in a natural freewheeling state. However, when the secondary voltage becomes high voltage Vin/K(K is the transformation ratio), the two diodes in the rectifier bridge will be turned off and will continue to conduct. At this time, the leakage inductance (or additional resonant inductance) of the transformer begins to resonate with the capacitance of the turned-off rectifier diode. Even if the fast recovery diode is used, the diode will still withstand at least twice the peak voltage. Therefore, an effective buffer circuit must be adopted. Many literatures have studied this, and there are five ways to sum it up: RC buffer circuit, RCD buffer circuit, active clamping buffer circuit, tertiary winding plus diode clamping buffer circuit, primary side plus diode clamping buffer circuit. Here, another effective method to reduce the peak voltage of diodes is proposed: rectifier diodes are connected in parallel, and its specific circuit diagram is shown in Figure 3. This method has been applied in the project of high-power full-bridge phase-shifted DC/DC power converter. The experimental waveform verifies this method, and the experimental result is shown in Figure 4, in which Figure 4(a) is the rectifier bridge voltage waveform. It can be seen that the diode has a high peak voltage due to the high frequency oscillation between the leakage inductance of the transformer, the junction capacitance of the diode and the winding capacitance of the transformer. Fig. 4(b) shows the voltage waveform of the rectifier bridge with parallel rectifier diodes, and the obvious peak voltage is reduced a lot, which verifies the effectiveness of this method. Experimental results and analysis The designed circuit is tested, and the experimental waveform is shown in Figure 5. In fig. 5(a), the upper waveform is the four-pin triangular wave oscillation waveform of UC3842, and the lower waveform is the PWM wave of the six-pin driving switch tube of UC3842; The upper waveform in fig. 5(b) is the DC component of the output voltage at full load, and the lower waveform is the AC ripple Vripp.

UC3842 is a high-performance fixed-frequency current-mode controller with single-ended output, which can directly drive transistors and MOSFET. It has the advantages of few pins, simple peripheral circuit, simple installation and debugging, excellent performance and low price, and has a good application prospect in switching power supply below100W. Details:/Dian gong/article/2009-8-12/11597.