Research On Several Reliability Problems Of DC Bus Power Supply System For LEO Satellite

Low earth orbit(LEO)satellites are widely used in positioning,navigation and communication systems,and are closely related to human life and industrial production.In recent years,although the rapid development of space technology,but the satellite manufacturing and launch costs are still extremely expensive,which determines the reliability of the system is one of the core technology of satellite technology development.As the "Power Source" of the satellite,the power supply system improves the energy for the payload,and the reliability of the power supply directly determines the reliability of the satellite system.Based on the summary of the current researches and technology development,this paper focuses on the improvement of the reliability of the DC bus power system for LEO satellites,and makes an in-depth study around the four parts of the system: shunt unit,lithium battery equalizer,discharger unit and secondary power supply,making a series of theoretical analysis and experiments,and several new ideas and solutions were proposed.Firstly,in the paper,the working principle of S3 R commonly used on satellites is described in detail,the system control model is given,and the loss analysis is made.It is pointed out that the S3 R reliability can be improved in following two ways: first,the bus diode is the biggest factor restricting the system efficiency,and improving efficiency and reducing losses can improve the heat dissipation design and system reliability;second,the double-section phenomenon of the S3 R increases the DC bus ripple,which makes the filter design difficult and generates EMI interference,which affects the stability of secondary power supply and the surrounding system.Then,the paper gives a detailed explanation of the causes of the double-section phenomenon,which is caused by the combination of hysteresis crossover,control loop integration delay and parasitic capacitance of the solar panel,etc.Finally,the paper proposes an improved S3 R regulator: first,by replacing the bus series diodes with synchronous rectifier MOSFETs,the loss can be reduced by 70%;second,the controller adopts fixed-frequency control to specifically compensate the frequency point of the sampled signal and reduce the phase shift,thus reducing the delay;third,a stepped decimator is used instead of stepped hysteresis loop to reduce the loop crossover;the three aspects are combined to achieve the purpose of suppressing the doublesection phenomenon and improving the efficiency of the shunt,thus improving the system reliability.In order to improve the reliability of the Li-ion battery equalizer,firstly,the paper proposes four candidate subsets based on the specific structural differences of the equalization topologies.Secondly,the operating principles and characteristics of the selected topologies within the subset are introduced,and the number of devices,equalization rate,volume and reliability are compared horizontally and vertically among the topologies.Based on this,four topologies are selected and proposed.Based on the analysis and comparison of the four topologies,it is concluded that the proposed active-passive equalization topology based on the switch matrix is most suitable for the application in the LEO satellite DC bus power system.The proposed active-passive topology adopts passive equalization in the light period,which can rapidly and reliably charge the battery;in the ground shadow period,in view of the characteristics of numerous charge-discharge cycle and shallow discharge of lithium batteries in LEO satellites,active equalization is adopted to slowly recharge the battery cell with deeper discharge depth,thus delaying its life decay.In order to improve the reliability of Li-ion battery dischargers,the efficiency of the dischargers can be improved to reduce the heat generation and extend the device life on the one hand,and to reduce the depth of discharge of Li-ion batteries and extend the battery pack life on the other hand.Firstly,the paper points out that the LLC converter can be applied as a Li-ion battery discharger in satellite power supply due to its simple control and high efficiency.Then,an improved LLC topology(LLC-MPR)using magamp as an auxiliary rectifier is proposed for the problem of narrow working range of LLC converter.The proposed LLC-MPR operates in variable frequency mode mode when the input voltage is low,which is the same with the conventional LLC converter.At this time,the magamp does not operate and is equivalent to a section of wire,which has little effect on converter efficiency.At higher input voltages,the LLC-MPR changes to a fixed frequency mode,operating at the resonant frequency point or at a frequency near the right side,and the output voltage is regulated by the magamp.After analysis,the magamp’s loss is very small,approximately 1% of efficiency loss can increase the converter operating range to1.75 times of the original.Moreover,the frequency characteristics of the magamp circuit and the LLC converter are the same,and the two can share a set of feedback circuit,which can achieve low overshoot and fast dynamic switching.In order to improve the reliability of the secondary power supply,first,the paper analyzes that the output retention of the secondary power supply during bus drop requires a large capacity bus capacitor,pointing out that this increases the size and weight,thus sacrificing cost and reliability.Then,taking the forward converter,which is widely used in the secondary power supply of satellite system,as the research object,it is concluded that its duty cycle cannot exceed50% due to the magnetic reset of the transformer,so that the energy stored in the bus capacitor cannot be fully utilized,and further research finds that the magnetic reset current of the forward transformer is very small,and a capacitor with a higher voltage than the bus but a smaller capacity is used to reset the transformer during the bus drop phase,which can shorten the reset time and make the duty cycle of the forward converter break through the traditional 50% limit,thus delivering more energy from the bus capacitor to the secondary and increasing the output holdup time.Based on the above idea,this paper proposes two forward topologies to break through the traditional duty cycle limit,with a small auxiliary circuit,which can increase the output holdup time by about 35% or reduce the bus capacitance by about 25% at the same hold time.The working principle and parameter design principles of the two topologies are given in the paper,and the comparison shows that topology I is more suitable for satellite secondary power supply.