With multi-electric aircraft and all-electric aircraft becoming the development trend of modern aircraft, and due to the rapid development of modern power electronics technology, DC stabilized power supply has been widely used in the aviation field. According to the actual demand of DC power supply for a certain type of aircraft, this paper designs a DC power converter. The converter mainly converts the three-phase alternating current outputted by the aircraft generator into a direct current power source for use by some DC loads on the aircraft. The three-phase AC power generated by the aircraft generator is rectified and filtered, and then input to the DC/DC module. The current-type pulse width modulator UC3842 is used to complete the stable DC output. At the same time, the overcurrent, overvoltage and undercurrent are also designed in combination with the characteristics of UC3842. Pressure protection and other functions.

1, chip introduction

UC3842 is a high performance single-ended output current controlled pulse width modulator manufactured by Unitrode Corporation of the United States. Current mode control compares the output inductor current directly sensed at the input of the pulse width comparator with the error amplifier output signal to control the duty cycle of the output pulse so that the output inductor peak current follows the error voltage. The UC3842 is a high-performance fixed-frequency current-mode controller that uses a single-ended output to directly drive bipolar transistors and MOSFETs. It has a small number of pins, simple peripheral circuitry, easy installation and commissioning, excellent performance, and price. Low cost and other advantages, with input and reference undervoltage lockout, phase-by-phase current limit and other protection characteristics. In the application, the UC3842 is an 8-pin dual in-line plastic package. The internal is mainly composed of a 5.0V reference voltage, an oscillator for accurately controlling the duty cycle adjustment, an attenuator, an overcurrent detection comparator, and a PWM latch. The high-gain E/A error amplifier and the high-current push-pull output circuit for driving the power MOSFET are shown in Figure 1. The internal structure and pin diagram are shown in Figure 1.

Power converter design based on UC3842

Figure 1 UC3842 internal structure and pin diagram

As shown in Figure 1, the 1st end of the chip is the COMP end, which is the output of the chip's internal error amplifier. The external RC network can be used for loop compensation to improve the closed-loop gain and frequency characteristics of the error amplifier. For VFB, it is the voltage feedback pin, which is the inverting input terminal of the error amplifier, which is compared with the internal 2.5V reference voltage as the output of the error amplifier; the 3 pin is CURRENTSENSE, which is the current measuring terminal, used for sampling current. When the voltage on the sampling resistor connected in series is greater than 1V, the UC3842 will stop outputting the trigger pulse and cut off the switch tube, thus protecting the switch tube from damage due to overcurrent, so it can be used to design overcurrent protection; On the RT/CT side, the CT is charged by the 5V reference voltage output from the 8-pin by RT, and then discharged by an internal current to generate oscillation. The oscillator frequency f can be calculated by the following equation: f=1.8/(RT×CT) kHz, Medium: RT is in kΩ, CT is μF; 5 is GROUND, which is the common ground for control circuit and power supply; 6 is OUTPUT, which is the push-pull output, which is used to drive the gate of the switch Control switch The 7-pin is VCC, which provides the power supply for the chip. The chip can work at 8~40V. In addition, the UC3842 also has an input voltage undervoltage lockout threshold of 16V (pass) and 10V (break); 8 is VREF. For the reference output, the reference voltage is 5V to the outside, and the load capacity is 50mA. It supplies the charging current to the timing capacitor through the timing resistor RT.

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