An Isolated Feedback Circuit for a Flyback Charging Circuit

Transcription

1 Proceedngs of the 007 WSEAS Int. Conference on Crcuts, Systems, Sgnal and Telecommuncatons, Gold Coast, Australa, January 17-19, An Isolated Feedback Crcut for a Flyback Chargng Crcut LI JIE, HUAG KEWEI, FA IGJU Department of Mechatroncs Bejng Insttute of Technology o.5 Zhongguancun South Street, Hadan Dstrct, Bejng,Chna PEPLE S REPUBLIC F CHIA Abstract: - In a conventonal flyback chargng crcut wth feedback, the nput of feedback crcut s usually sampled from the output voltage across a resstve voltage dvder. Hence the chargng crcut gets unsolated. As solaton problem wll cause safety and stablty problems to the chargng crcut especally when the output voltage gets hgh, a feedback crcut whch samples from the nput sde of the flyback chargng crcut s brought out n ths paper n order to solve the solaton problem. The workng process s analyzed. And the choce of the parameters of that feedback crcut s ntroduced. Smulaton results are presented to show that the crcut s capable of controllng the chargng crcut as well as solvng the solaton problem. Key-Words: - flyback chargng, feedback, solaton, transformer 1 Introducton The flyback chargng crcut s wdely used n many areas, such as swtchng mode power supples, flash lamps and even fuzes. The background of ths paper s Electronc Safety and Armng system(esa) whch s used n fuzes[1]. As t s so wdely used, researches of dfferent aspects have been well done. The workng modes of the crcut whch nclude CCM and DCM have been well known to us[-3]. Dfferent topologes for dfferent control methods have been studed, such as Pulse Tran Control, Adaptve ff-tme Control, Packet Control, PWM Control and so on[4-7]. Control algorthm for optmal chargng s also referred[8]. Auxlary part of the flyback chargng crcut has been developed for some specal purposes such as mnmzaton of power loss as well as mnmzaton of spke voltage on the transformer prmary sde[9]. All of these researches have greatly mproved the performance of flyback chargng crcut. However, as far as our knowledge goes, there are few thess that touch the solaton problem between the nput and output sde of the chargng crcut when a feedback crcut s used[10]. But n ESA, n order to gnte the detonator, the output voltage of the flyback chargng crcut has to get up to 3000 volts. Ths hgh voltage n the output sde may cause some safety and stablty problems when the crcut s not solated between the nput and output sde. In ths paper, n order to solve the solaton problem whle a feedback crcut s used, a feedback crcut whch samples from the transformer prmary sde s brought forward. Problem Formulaton The crcut dagram of a conventonal flyback chargng crcut wth feedback s shown n Fg.1. As t s shown n Fg.1, the exstence of the transformer makes the solaton of the nput sde from the output sde of the chargng crcut possble. But the feedback crcut connects the two sdes by the samplng across a resstve voltage dvder. Fg.1 A fly-back chargng crcut wth feedback As the output of the feedback crcut always have to be sent to the nput sde of the chargng crcut, there are two dfferent ways to solve the solaton problem: 1. Isolatng the nput from output of the feed-back crcut.. Samplng from the nput sde of the chargng crcut..1 Photoelectrc Couplng

2 Proceedngs of the 007 WSEAS Int. Conference on Crcuts, Systems, Sgnal and Telecommuncatons, Gold Coast, Australa, January 17-19, Usually, photoelectrc couplng s used for the solaton purpose. In order to solate the nput from output sde of the feedback crcut, as shown n Fg., a photoelectrc couplng s used. Fg. Feedback crcut usng photoelectrc couplng for solaton purpose bvously, by applyng photoelectrc couplng, the nput and output of the feedback crcut are solated. The problem s that the resstance of the voltage dvder s very hgh n order to reduce ts power loss, hence the current through R 1 s too small to drve the photoelectrc couplng. So ths method s not approprate to solve the solaton problem.. Samplng from the Input Sde Because there are some dffcultes n solvng the solaton problem by applyng photoelectrc couplng, we thus turn to the second method to get the sample from the nput sde of the chargng crcut. In the followng sectons, we begn wth dscussng the workng process of the flyback chargng crcut as well as some waveforms durng ts workng stages...1 Workng Mode of the Flyback Chargng Crcut Frstly, we lst out the followng symbols whch are laterly used : Dran-to-Source voltage of MSFET M U S Dran-to-Source current of MSFET M I S (Transformer prmary-wndng current) Current through dode D I D (Transformer secondly-wndng current) Tme of a workng cycle T S MSFET on tme n each workng cycle t on t off MSFET off tme n each workng cycle Transformer prmary-wndng W 1 Transformer secondly-wndng W Transformer prmary turns 1 Transformer secondly turns Source voltage(nput voltage) U Energy-storage capacty voltage U Durng the work of the fly-back chargng crcut, there are knds of workng modes: Contnuous Conducton Mode (CCM) and Dscontnuous Conducton Mode (DCM)[]. When the flyback chargng crcut works under contnuous conducton mode, there are two stages n each workng cycle. The waveforms are shown n Fg.3. t 0 --t 1 (energy-storng stage):after the MSFET s turned on, U S changes to a value near zero. I S ncreases over tme and I D keeps zero. The startng value of I S s not zero. Energy stored n the transformer prmary wndng ncreases over tme. t 1 --t (chargng stage):after the MSFET s turned off, the transformer begns to transfer energy to the energy-storng capactor C through dode D. The current I D nduces a reverse voltage n the transformer prmary wndng, hence the Dran-to-Source voltage after the MSFET s turned off s as follow: 1 U S + U ( 1 ) When the MSFET s turned on agan, because the energy stored n the transformer has not been totally transferred to the capactor, I D wll not decrease to zero. Fg.3 Waveforms under CCM Whle the flyback-chargng crcut works under dscontnuous conducton mode, there are three stages n each workng perod. The waveforms durng the chargng process are shown n Fg.4. t 0 --t 1 (energy-storng stage):the process of ths stage s just lke the energy-storng stage under CCM. The dfference s that the startng value of I S s zero.

3 Proceedngs of the 007 WSEAS Int. Conference on Crcuts, Systems, Sgnal and Telecommuncatons, Gold Coast, Australa, January 17-19, t 1 --t (chargng stage): The process of ths stage s just lke the chargng stage under CCM. The dfference s that the endng value of I D s zero. Durng ths chargng stage, the Dran-to-Source voltage s: 1 U S ( ) t --t 3 (dle stage):input and output current of the transformer take zero. And the Dran-to-Source voltage keeps at the value of U. Fg.5 Smulaton waveforms of U and U S The U S has the connecton wth U only at the chargng stage, whle durng other tme perods of each workng cycle, the connecton dose not exst any more. So we can not get the useful part of U S drectly from the chargng crcut. How the U S should be sampled? It s the frst queston. Fg.4 Waveforms under DCM.. The Use of U S n Isolated Feedback Durng the prevous analyss of the waveforms under both CCM and DCM, much attenton are pad to the change of U S, hence some useful nformaton are detected. The equatons ( 1 ) and ( ) are the same. And accordng to the equatons, under both CCM and DCM, there are some connectons between the Dran-to-Source voltage U S and the energy-storng capactor voltage U durng the chargng stage. That means f we get the value of U S durng the chargng stage, by solvng the equaton ( 1 ), the value of U s determned unquely. Fg.5 shows the smulaton waveforms of U and U S. The upper part of Fg.5 s the waveform of U, whle the lower part shows the waveform of U S. By comparng these two waveforms, we can see clearly that the outlne of the waveform of U S shares the same shape wth the waveform of U durng the chargng process. The connecton between U S and U enables the solated feedback whch samples from the nput sde of the chargng crcut. But there are some problems wth the use of the voltage U S. Fg.6 Detaled smulaton waveforms of U S Fg.6 shows a detaled smulaton waveform of U S under dscontnuous conducton mode. The waveform confrms that the connecton between U S and U descrbed by equaton ( 1 ) only exsts durng the chargng stage of each workng cycle. In addton, after the chargng process stops, the value of U S wll not have any connecton wth U but keeps at the value of U. Ths can be seen clearly from the smulaton waveform of U S whch s shown n Fg.5. So, durng the dschargng process, we can not get the value of U by measurng U S. In what way can the crcut be under control durng the dschargng process? Ths s another queston. 3 Problem Soluton At the end of.., two questons of how to use U S are rased. A smple crcut topology s put forward n ths secton to solve those problems. It s shown n Fg.7.

4 Proceedngs of the 007 WSEAS Int. Conference on Crcuts, Systems, Sgnal and Telecommuncatons, Gold Coast, Australa, January 17-19, The workng process of the feedback crcut s lke that: Durng the chargng stage of each workng cycle, the chargng current n W nduces the voltage U S whch s descrbed by the equaton ( 1 ), hence the dode D allows current to flow through t. The capactor s then charged to the value of U S. At other tmes of each workng cycle, dode D s off, current flows through C, R and R 3. If the chargng process s quck enough and the dschargng process s slow enough, the voltage of C wll keeps the value around U +( 1 / ) U. When the chargng process stops, the voltage of C wll declne slowly. And the speed of the voltage declnng s determned by the value of C, R and R 3. If R Df s the forward resstance of dode D, then the chargng tme constant of the feedback crcut s: τ 1 = R D f C ( 3 ) And the dschargng tme constant s: τ = ( R + R3 ) C ( 4 ) In order to keep the voltage of C around the value of U +( 1 / ) U as closely as possble durng the chargng process, τ 1 should be tny enough and τ should be much larger than τ 1. Addtonally, n order to mnmze the nfluence that the feedback crcut has brought to the chargng crcut, the current whch s requred to charge the capactor C should be lmted. So, the capactance of C should be nether too large nor too small. Fg.7 The feed-back crcut whch gets ts nput from the nput sde of the chargng crcut The waveforms shown n Fg.8 are smulaton waveforms of the crcut shown n Fg.7. The upper part shows the voltage of the energy-storng capactor C 1, whle the lower part shows the voltage of C, whch s used as the samplng voltage of the feedback crcut. As shown n Fg.8, durng the chargng process, the waveforms of the voltage of C and the voltage of the energy-storng capactor are the same n shape. Fg.8 Smulaton waveforms The connecton between ther values s descrbed by the equaton below: 1 U C ( 5 ) where U C represents the voltage of C, U =7V and 1 / =1/10. When the chargng crcut stops chargng the energy-storng capactor C 1, U C decreases, and the connecton between the value of U C and U s not avalable any more. But when the chargng process starts agan, U C rses quckly to the value of U +( 1 / ) U. If t s requred that U (U 1,U ) durng the chargng process, U C should be wthn (U +(1/) U 1, U +(1/) U ), then the threshold of the voltage comparator should be: 1 R3 ( U ) R + R3 So, when U rses to the value of U, the output of the voltage comparator wll change from 1 to 0. Tme delay control method s used when the chargng process s paused. When the CPLD gets the nformaton that the output of voltage comparator changes from 1 to 0, t wll control the chargng crcut to stop chargng. After a perod of tme, the CPLD controls the chargng crcut to restart chargng. When U rses to U agan, the output of voltage comparator wll agan change from 1 to 0, then another cycle begns. The length of the delayng tme should be chosen properly to ensure that U keeps hgher than the value of U 1. Tll now, those two questons have got ther answers.

5 Proceedngs of the 007 WSEAS Int. Conference on Crcuts, Systems, Sgnal and Telecommuncatons, Gold Coast, Australa, January 17-19, Concluson In ths paper, a feedback crcut has been presented and analyzed n order to solve the solaton problem between the nput and output sde of the flyback chargng crcut. Smulaton results show that ths crcut s capable of controllng the chargng crcut f the parameters are chosen properly. By samplng from the nput sde of the chargng crcut, ths feedback crcut makes the nput and output sde of the chargng crcut totally solated. For a flyback chargng crcut n fuze, we have to make the chargng process as fast as possble, to choose a proper control method and to make the crcut stable. So we stll have much work to do. Experments have to be done to confrm the functon of ths feedback crcut. In addton, we have to optmze the parameters of the crcut as well as the control method. [9] kolaos P. Papankolaou, Emmanuel C. Tataks, Actve Voltage Clamp n Flyback Converters peratng n CCM Mode Under Wde Load Varaton, IEEE Transactons on Industral Electroncs, Vol.51, o.3, June.004, pp [10]Dongje Lee, Electronc Safety and Armng system-a research on Dynamc Swtch, Hgh Voltage Converter and Frng Crcut, Engneerng and Physcs Academy of Chna, 1998 References: [1]Mke Sowder, Electronc Safety and Armng Fuzes for Conventonal Bomb Weapons, 48 th Annual DIA Fuze Conference. []Xu Yang, Yunqn Pe, Zhaobn Wang, Swtchng Mode Power Supply Technology, Chna Machne Press, 004. [3]Ybn Lu, Xaodan Zhou, Introducton of Converters, Integrated Crcut Communcaton, Vol.4, o.1, Mar 006, pp [4]Mark Telefus, Anatoly Shteynberg, Pulse Tran Control Technque for Flyback Converter, IEEE Transacton on Power Electroncs, Vol.19, o.3, May 004, pp [5] Yur Panov, Mlan M. Jovanovc, Adaptve ff-tme Control for Varable-Frequency Soft-Swtched Flyback Converter at Lght Loads, IEEE Transacton on Power Electroncs, Vol.17, o.4, July 00, pp [6] Kabol, S.; Sadryeh, S.M.R.; Mohammad, A.; Zolghadr, M.R.; Emad, A.; Applcaton of packet control method for a hgh power hgh voltage flyback converter to reduce nose nfluence, Power Electroncs and Applcatons 005 European Conference on Sept.005, pp.10. [7] Wang, C.-M.; Su, C.-H.; Yang, C.-H.; ZVS-PWM flyback converter wth a smple auxlary crcut, Electrc Power Applcatons IEE proceedngs, Vol.153, o.1, Jan.006, pp [8] Sokal,..; Redl, R.; Control algorthms and crcut desgns for optmally flyback-chargng an energy-storage capactor (e.g. for a flash lamp), Appled Power Electroncs Conference and Exposton, Mar.1990, pp