【正文】 r Inductor Design and Transformer Design, Part 2. Powerconversion International. January/[17]Ayyanar Mohan，and Jian Threephase Power Factor Circuit Using Three Isolated SinglePhase SEPIC Conerters Operating in CCM. Proceeding of 附錄A外文文獻(xiàn)與譯文 英文原文Based on the switching power supply protection circuit UC3842 ImprovementAbstract: The paper discusses the current control chip UC3842 PWM switching power supply consisting of the principle of the defects of its protection circuit and an improved method. Key words: UC3842。 switching power supply。 protection circuit UC3842 Introduction Unltmde produced in the United States an excellent current control PWM chip, which has a small number of pins, and simple external circuit, which has been widely used . But with the UC3842 switching frequency increases, it posed by the protection circuit switching power supply there have been many problems. This paper analyzes the protection circuit UC3842 defects and improved methods. 1 UC3842 typical application of the typical application circuit mainly by the bridge rectifier circuit, high frequency transformer, MOS power transistor and currenttype PWM chip UC3842 position. Its working principle is: 220V AC through the bridge rectifier filter circuit, got about +300 V DC high voltage, the DC voltage is M0S power tube and through the highfrequency transformer stepdown chopper into a frequency of tens of kHz rectangular wave voltage, and then through the output rectifier filter to obtain a stable DC output voltage. The high frequency transformer feeder loop from the voltage induced in N2, obtained by the D2 rectified DC voltage is fed back to the UC3842 error amplifier and internal reference voltage and pared by the error voltage Vr, while the establishment of the sampling resistor R11 DC voltage is also fed back to the UC3842 current measurement parator input , the detection voltage and error voltage Vt pared to generate the drive signal pulse width adjustable to control the switching power transistor turnon and turnoff time to decisionoff state of highfrequency transformer, so as to achieve output voltage regulation purposes. In Figure l, R5 charge to limit the peak current generated by C8. Vref Vi and taking into account the noise on the output voltage will also affect the pulse width, therefore, UC3842 feet 8 feet 7 and then there is noise cancellation, respectively, capacitor C4 and the C2. R7 is a MOS power transistor gate limit resistor. In addition, the UC3842 between the input and ground, as well as 34V of the regulator, once a highpressure input, the regulator was reverse breakdown, the Vi clamp at 34V, protection of the chip will not be damaged. 2UC3842 overload protection circuit defects in the defect overload when the power supply or output short circuit, UC3842 protection circuit operation, the output pulse duty cycle decreases, the output voltage drops, UC3842 lower supply voltage also followed, When the low to UC3842 not work, the entire circuit is off, then stick in the beginning of the next R6 startup process. This protection is known as hiccup type (hiccup) protection. In this protection, the supply of work only a few switching cycles, and then enter a long time (a few hundred ms to several s) of the boot process, so it39。s average power is low. However, there is the transformer leakage inductance and other reasons, some switching power supply during each switching cycle, the switch has a very high voltage spikes, even in the case of very small duty cycle, the auxiliary power supply voltage can not fall low enough it can not achieve the desired protection 3 improved protection circuit for the above ，the circuit has the following features. 1) by sampling the voltage at the UC3842 access to an emitter follower, resulting in the addition of a control voltage pulse width modulation clock synchronization with artificial slopes, it can followup period will be reduced to zero perturbation ?I . Therefore, even if the system works in more than 50% duty cycle or continuous inductor current conditions, the system will not instability. However, the slope of the slope of the pensation must be equal to or slightly greater than m2 / 2, the system can have real stability. 2) no sense of resistance sampling switch resistance. No sense resistor is a twolane and around the winding resistance, and its high accuracy and easy power. With no sense of resistance, its resistance will increase with increasing frequency. Thus, even under highfrequency sampling resistor in the power consumption of no more than its nominal power and, therefore, will not blow up phenomenon. 3) Add optocoupler feedback circuit to control the use TL43l. We all know that when the amplifiers are required for signal transmission time, both input and output is not established. If the feedback signal received UC3842 voltage feedback end, the feedback signal must pass through two highgain error amplifier, the transmission time of growth. TL431 itself as a highgain error amplifier, therefore, used in Figure 3, pin 1 as feedback directly from the UC3842 feet 8 (the reference voltage pin), a resistor to pull the foot l, 2 foot ground through R18. The benefits of doing so, skip the UC384239。s internal amplifier, the feedback signal to the transmission time in half, so that the dynamic response of power faster. In addition, the direct control of the feet l UC3842 can simplify the system and the output power of the small frequency pensation and other issues. 4 Experimental results Figure 4 shows the sense resistor voltage UC3842 waveform and the sampled signal waveform. It can be seen from Figure 4, an improved circuit, the sampling waveform closely following the sense resistor voltage waveform, does not appear very large voltage spikes. Therefore, the circuit can effectively avoid the transformer leakage inductance and abnormal interference problems caused by misuse of power, but also can avoid the duty cycle of ex