| As the advantages of small size, high efficiency, low cost, switching power supply is widely used in airborne electronic equipments, while the actual aircraft power supply system has the characteristics of high voltage surge, under voltage surge, instantaneous power interruption, etc., and switching power supply in work producing electromagnetic noise, which will affect the sensitive equipments. So airborne switching power supply must meet the military standards of GJB181 A and GJB151 A.This thesis designs a airborne DC switching power supply with multi outputs, which is small size, wide input voltage, high efficiency, 50 ms uninterrupted power supply, protection functions, low ripple and meet the electromagnetic compatibility(EMC) standard. The main research contents and results are as follow:1. For small size and wide input voltage requirements, comparing different power supply topologies and ultimately determine the flyback convertor to achieve DC-DC. Research focuses on flyback transformer design; circuits of PWM feedback control, current sampling, slope compensation and RCD snubber is also investigated. With the design of 50 ms uninterrupted power supply, the input voltage of DC-DC can widely from 10 V to 50 V.2. Loss compositions and influence factors of switching power supply are analyzed to guide the circuit design and component selection. For high efficiency, synchronous rectification is applied to the flyback converter. And all The PWM controllers of flyback converters are powered by a Buck converter, which is working in a pulse skip mode that can improve light load conversion efficiency. This product with 33 W total output power achieves the efficiency up to 85% in the final test.3. A circuit based on capacitor energy storage is designed to maintain 50 ms uninterrupted power supply and 12 V low voltage starting, which is composed of charging circuit, large capacity tantalum capacitor and discharging circuit. This structure extremely reduces the capacity, volume and cost of capacitor. By means of decreasing the PWM duty cycle of charging circuit, the charging surge current is less than 0.5A and charging time is only 3s.The issue of maintaining voltage oscillation is analyzed, a under voltage protection circuit of ultra-wide hysteresis voltage is presented accordingly.4. For low output ripple requirement, the formation mechanism of ripple is analyzed, phenomenon of beat frequency and multi outputs Crosstalk interference that can increase ripple are also discussed, methods of filtering, grounding and shielding are effective to damping the ripple. EMI filter is the key to realize EMC, focusing on its filtering principle and equivalent circuit, analyzing the different flow path of common mode interference and differential mode interference, designing the parameters of EMI filter by comparative experiments. In the final product test, the ripple is less than 20 mV and all the EMC test is qualified.Reliability design is significant to airborne power supply, protection functions as like output overvoltage lockout, state indicating and output current limit are designed and simulated by software. With the application of derating design, thermal design, structural design and tolerance analysis, a highly reliable airborne switching power supply is achieved. |
