LLC holds next output character analysises and the solution that hold output voltage to be able to accuse gently
[introduction] LLC develop attacks to apply extensively in all sorts of power changeover equipment, however LLC develop attacks in carry gently reach carry a circumstance to fall for nothing, although working frequency range is very wide, still often appear the phenomenon that outputs voltage to exceed norms to ask.
LLC develop attacks to apply extensively in all sorts of power changeover equipment, however LLC develop attacks in carry gently reach carry a circumstance to fall for nothing, although working frequency range is very wide, still often appear the phenomenon that outputs voltage to exceed norms to ask. The article from theoretic thorough to causing the reason of this problem to undertake analysis, prove equivalent of transformer former edge is shunt-wound the prime cause that this problem creates when the output electric capacity of capacitance and former edge MOSFET, reduce equivalent electric capacity because of putting forward in the light of the root, shunt-wound capacitance of former edge MOSFET, capacitance of syntonic inductance paralell connection, increase transformer former deputy edge is v/LIT all over the ground than, transformer job answers this question at the measure such as belch mode.
Foreword
In recent years LLC develop attacks to apply extensively at illume drive, TV power source, industrial power source, server / PC power source, the consumption such as communication power source reachs the DC-DC level in industrial domain, because its have the 0 voltage that like completely laden limits former edge switch is in charge of to debut,this is (ZVS) , 0 electric current involve deputy edge diode or synchronous commutate switch (ZCS) , EMI character is good (weight of high frequency noise is less) , circuit structure is simple, cost is lower and exceedingly good characteristic. Develop of typical LLC of commutate of half bridge complete wave attacks as follows plan institute is shown.
Graph 1: Develop of half bridge LLC attacks
Attack to LLC develop, the foundation is current most commonly used base wave approximation (First Harmonic Approximation, FHA, fundamental Element Simplification, the dc gain trace that FES)[1] computation gets, although load is very light laden even for 0, should work only frequency is enough tall so output voltage voltage can accuse certainly, can be in order to stabilize inside norms requirement limits. However, can discover in introducing the product that this develop attacks in great quantities: In carry next output voltage gently to cannot be stabilized inside norms requirement limits namely often the requirement of prep above norms, although LLC has worked,Yu Fei often is not constant tall frequency. This and current theoretical analysis are not of conform to. Because this is necessary to be in,carry gently reach carry a condition to fall to have new analysis and consideration to dc gain trace for nothing, find the factor that affects dc gain from which, find the program that solves a problem thereby.
Shunt-wound parasitism capacitance is opposite equivalent of transformer former edge to spread the effect of gain trace continuously
When the dc gain trace that uses LLC of FHA/FES method computation, the leakage feeling in the model of LLC transformer has been gone in by the consideration actually: It is the application of independent inductance to syntonic inductance, because leak,feeling and syntonic inductance concern for series connection (convert of equivalent of feeling of deputy edge leakage arrives former edge) , because this is syntonic,the quantity of syntonic inductance feeling of antrum is value of overlay of the syntonic inductance of the design and leakage feeling. And compositive to syntonic inductance and transformer plan, transformer leakage feels namely syntonic inductance. But between former edge winding, the output electric capacity of the distributinging capacitance between deputy edge winding and diode of deputy edge commutate or synchronous commutate MOSFET did not consider computation is medium. After considering these parasitism capacitance, the model of transformer and LLC equivalent circuit are like a graph respectively 2 show with 3 [3] .
Graph the transformer model after capacitance of parasitism of 2 considerations deputy edge
Graph the model of LLC equivalent circuit after capacitance of parasitism of 3 considerations deputy edge
According to the graph 2 show a structure, graph Cp of shunt-wound parasitism capacitance is equivalent of 3 medium transformer former edge:
Cpw, csw, csoss circles constituent cloth capacitance for transformer former edge respectively, distributinging capacitance of winding of transformer deputy edge and the equivalent that output rectification diode or synchronous commutate canal output capacitance, nps is transformer of former deputy edge v/LIT all over the ground than,
According to the graph 3 show equivalent circuit, the LLC dc gain that computation gets is:
(2)Among them:
With resonance of a LLC transformer design parameter is exemple: Lp=1400uH; Lr=165uH; Cr=68nF, nps=16.7, to the graph 4 shown LLC transformer, use impedance analyzer is in board check capacitance of equivalent of transformer former edge (the blue dot that shows in the graph is carried for the test) , according to checking gotten impedance curve computation gets capacitance Cp is former edge equivalent:
Graph capacitance of paralell connection of former edge equivalent tests the transformer of 4 switch power source that use LLC develop to attack upright
The basis is formulary (2) can calculate get quality factor Q is worth differring (corresponding 100% laden ~1% are laden) the gain trace below a group of things with common features, if the graph is shown 5 times;
Graph the dc gain trace after capacitance of parasitism of 5 considerations transformer
And the gain trace that parasitism capacitance takes no account of below same condition a group of things with common features if pursue,show 6 times.
Graph the 6 dc gain trace that take no account of transformer parasitism capacitance
From the graph 5 with the graph the contrast of 6 can get, as a result of the existence of capacitance of parasitism of equivalent of winding of transformer former edge, gain trace is in high frequency appear paralell connection of another inductance capacitance is bit more syntonic, bring about gain trace to be in carry gently the circumstance falls to lift along with frequency and gain becomes tall, and load is lighter, this phenomenon is more apparent. This will be brought about carry voltage of the output below the circumstance gently to cannot be stabilized. Identical load (it is with 10% load exemple) different capacitance of former edge parasitism (500pF~50pF) if the gain trace below pursues show 7 times. From inside the graph knowable, parasitism electric capacity is larger, syntonic point is lower, bigger to the influence of LLC gain trace, be less than 50pF circumstance to fall in parasitism capacitance only, its are negligible to the influence of gain trace not plan.
Graph the dc gain trace below capacitance of 7 different parasitism
As working frequency lift further, more parasitism capacitance in transformer and parasitism inductance produce an effect to the working mode of LLC, make LLC develop attacks turn into develop of much component resonance attacks, gain trace will appear many resonance are nodded, what LLC character will change is more complex.
Equivalent of former edge MOSFET outputs the capacitance influence to dc gain trace
Attack to LLC develop, former edge MOSFET is undertaking switch switch (namely canal of a switch closes, canal of another switch after dead band time is enlightened) in the process, inductance meets excitation produce resonance with the output electric capacity of former edge MOSFET, this resonance energy delivers the part deputy side, make in carry for nothing reach carry voltage of the output below the circumstance gently to lift. Document [the labor in 7] LLC develop attacks the output electric capacity of former edge MOSFET is right the influence of dc gain trace, if the influence of electric capacity of different MOSFET output to dc gain pursues 8 show [7] :
Graph 8 different electric capacity of output of former edge MOSFET is right the influence that carries dc gain trace gently
When the output electric capacity of MOSFET is lesser, carry next dc gain trace gently to appear on become warped phenomenon, make output voltage cannot maintain inside norms requirement limits.
Maintain the measure that outputs voltage stability
According to the article the analysis of the 2nd part, the existence of capacitance of because transformer is equivalent former edge and electric capacity of output of former edge MOSFET are lesser, the gain trace of LLC is in high frequency paragraph of phenomenon that appears to rise along with working frequency, bring about carry voltage of the output below the circumstance gently to cannot maintain inside norms limits. Most switch power source cannot accept this. Next the part will introduce a few measure to solve this problem:
4.1 reduce electric capacity of transformer equivalent paralell connection
This is the directest the plan that solves a problem, also be the plan that implements the hardest however however. Because anyhow the parasitism capacitance of transformer is of existence, the step that can take accordingly is to reduce this electric capacity as far as possible, document [3] gave out the circles law ” for “ detached type transformer that its weigh is circled make a proposal, its parasitism capacitance has a tradition only and circle a method very one of. Document [4] put forward ” of “ progressive type to circle formulate a method, parasitism electric capacity is very small. But often these circling make means can be brought circle the complexity that make to rise, make thereby the price of transformer rises.
4.2LLC works at belch mode
In carry develop of the LLC below the circumstance gently to attack enter belch (Burst) mode is more a kind of adoption control is politic, this strategy can maintain output voltage to be inside norms limits on one hand, reduced on the other hand carry the power input below gently, rose to carry the efficiency below gently. Graph the key below the 9 belch mode that are a model is undee [5] . However belch mode can bring output voltage ripple to greaten, this is in a few application, for example server power source, PC power source cannot be accept.
Graph the LLC key below 9 belch mode is undee sketch map
Capacitance of paralell connection of 4.3 syntonic inductance
In the application that is independent inductance to syntonic inductance, document [the program that 6] offerred a kind of many syntonic LLC, go up in syntonic inductance namely shunt-wound a capacitance, if the graph is shown 10 times, generate a new LLC resonance to nod F02 thereby, and
Graph develop of 10 much more syntonic LLC attacks
Original and syntonic frequency also has change a bit, its value is:
New gain trace shows 11 times like the graph:
Graph gain trace of dc of transformer of 11 much more syntonic LLC
Be in as a result of gain trace FW=f02It is when 0, it is OK that because this is theoretic,develop of this much more syntonic LLC attacks to output voltage below any loads low to 0. The need in the design chooses appropriate C_p, ensure the top job frequency of LLC does not exceed F02.
Shunt-wound capacitance of MOSFET of 4.4 former edges
According to the analysis of the 3rd part, the output electric capacity of former edge MOSFET is larger, gain trace of the dc below same job frequency is lower, output voltage to jump over easy control to be inside norms limits namely. Because this is below the premise of MOSFET of former edge of make choice of, still can increase equivalent to output capacitance through shunt-wound capacitance, control output voltage thereby. This method goes easily simply, but weakness is more apparent also: The accretion that outputs capacitance brings the addition of MOSFET switch loss, reduced thereby change efficiency, be in especially carry gently below, of efficiency reduce more apparent.
4.5 increase transformer former deputy edge is v/LIT all over the ground than
Pursue according to graph 6~ 8, no matter be equivalent of transformer former edge,shunt-wound parasitism capacitance or electric capacity of output of former edge MOSFET are opposite the influence of dc gain trace, it is to happen below the circumstance of frequency of resonance of working frequency prep above. Because this passes the former deputy limit that raises transformer to circle,compare (majority is to carry the v/LIT all over the ground number that increases winding of transformer former edge) , your LLC develop attacks in carry the job below the circumstance gently to be nodded at resonance around, criterion parasitism parameter is negligible to outputting the influence of voltage, it is easier to carry next output voltages gently thereby stability is inside norms limits. Of course this design needs to consider to be fully loaded with the peak value that has reached the dc gain that the circumstance waits for to fall before shedding protection enough tall, assure to the stability of voltage is outputted below these circumstances.
4.6 reduce deputy edge diode / capacitance of parasitism of synchronous commutate canal
According to the analysis of the 2nd part, the equivalent that shunt-wound capacitance has equivalent of transformer former edge to be deputy edge diode or synchronous commutate canal partly outputs capacitance, because this chooses the diode of lesser output electric capacity or MOSFET will conduce to stability output voltage. Document [8] put forward shunt-wound on output diode or synchronous commutate canal a MOSFET and the circuit that diode establishs ties, this circuit feedbacks partial energy former side side, be in thereby carry gently reach carry next stability that maintained output voltage for nothing.
4.7 carry next shutting gently synchronous commutate canal
It is synchronous commutate to deputy edge (MOSFET is in charge of for switch of deputy side side) design, shut in the drive that carries a circumstance gently to issue will synchronous commutate will conduce to the stability that maintains output voltage, after the drive of synchronous commutate is shut, deputy side side carries the body diode follow on current of MOSFET, of body diode pressure fall interpose at 0.7V~1.2V, when outclass synchronism commutate is enlightened pressure fall (V=ID*R(DS(on)) ) , the output voltage of the deputy edge winding that because this is identical,needs below output voltage is taller. The drive that shuts synchronous commutate of course also can have additional problem, in laden and abrupt aggravating need drive of will synchronous commutate is opened when, because afore-mentioned pressing,poor presence can bring about output voltage to had appeared to develop a phenomenon, because need in this design,consider this measure integratedly can carry out a gender.
Summary
The article attacks to LLC develop in carry gently reached the phenomenon that carries voltage of the output below the circumstance to exceed norms to ask for nothing to undertake theoretical analysis, prove equivalent of transformer former edge is shunt-wound the existence generation of capacitance and electric capacity of output of former edge MOSFET gave this issue. Accordingly, the article offerred a variety of feasible solutions, will realize the stability that outputs voltage. The article will develop an engineer to solve LLC develop to attack to carry the problem of output voltage on the high side below gently to provide beneficial reference to power source.
Bibliographical reference:
[1] Bo Yang, fred C. Lee, a.J. Zhang And Guisong Huang, LLC Resonant Converter For Frond End DC/DC Converter, 17th APEC, 2002
[2] Ya, liu, high Efficiency Optimization Of LLC Resonant Converter For Wide Load Range,
[3] Heng Yue, min Chen, research On Effects Of Transformer Parasitic Capacitance On LLC Light Load Stability, journal Of Power Supply, 2013.3
[4] Shan Yu, jiqing Dong, the Analysis Of High-Frequency Transformer Parasitic Capacitance On LLC Non-load Characteristics, magnetic Elements&Power Supply, 2014.03
[5] Datasheet Of IDP2322, infineon Technologies
[6] Dianbo Fu, fred C. Lee, ya Liu And Ming Xu, novel Multi-Element Resonant Converters For Front-end DC/DC Converters, 2008 IEEE Power Electronics Specialists Conference.
[7] Jae-Hyun Kim, chong-Eun Kim, jae-Kuk Kim And Gun-Woo Moon, analysis For LLC Resonant Converter Considering Parasitic Components At Very Light Load Condition, 8th International Conference On Power Electronics-ECCE Asia, 2011
[8] Yiqing Ye, chao Yan, jianhong Zeng And Jianping Yin, a Novel Light Load Solution For LLC Series Resonant Converter, 29th International Telecommunications Energy Conference, 2007