All Silicon Marx-bank opology for high-volage, high-frequency recangular pulses L.M. Redondo Cenro de Física da Universidade de Lisboa Insiuo Superior de Engenharia de Lisboa Rua Conselheiro Emídio Navarro 1 195-62 Lisboa, Porugal Email: lmredondo@deea.isel.ipl.p J. Fernando Silva Cenro de Auomáica da UTL Insiuo Superior Técnico Av. Rovisco Pais 1 149-1 Lisboa Email: fernandos@alfa.is.ul.p P. Tavares Indúsrias Lever Poruguesa S.A. R. Cidade de Goa, 22-24 2689-52 Sacavém, Porugal Email: Pedro.Tavares@unilever.com Elmano Margao Cenro de Auomáica da UTL Insiuo Superior de Engenharia de Lisboa Rua Conselheiro Emídio Navarro 1 195-62 Lisboa, Porugal Email: emargao@deea.isel.ipl.p Absrac - This paper discusses he operaion of a fully inegraed solid-sae Marx generaor circui, which has been developed for high-frequency (khz), high-volage (kv) applicaions needing recangular pulses. The convenional Marx generaor, used for high-volage pulsed applicaions, uses inducors, or resisors, o supply he charging capaciors volage, which has he disadvanages of size, power loss and frequency limiaion. The proposed circui akes advanage of he inensive use of power semiconducor swiches, replacing he passive elemens in he convenional circui, o increase he performance, srongly reducing losses and increasing he pulse repeiion frequency. Also, he proposed opology enables he use of ypical half-bridge semiconducor srucures, while ensuring ha he maximum volage blocked by he semiconducors is he volage of each capacior (i.e. he power supply volage), even wih mismaches in he synchronized swiching, and wih faul condiions. A laboraory prooype wih five sages, 5 kw peak power, of his all silicon Marx generaor circui, was consruced using 12 V IGBTs and diodes, operaing wih 1 V d-c inpu volage and 1 khz frequency, giving 5 kv pulses, wih 1 µs widh and 5 ns rise ime. I. INTRODUCTION high-volage pulses are applied o he sample, resuling in he acceleraion of he ions ino he surface of he sample and furhemplanaion of he maerial [1]. This and oher applicaions (food reamen, wase serilisaion, ) increase he need of efficien and suiable pulsed power supplies, based on power semiconducor swiches and on new opologies brough from power elecronics [2]. High volage pulses can be generaed using several echniques. The mos widely used one, combines a high volage power supply wih semiconducor swiches, eihen series or resonan circui associaions o overcome he high volage limiaions of semiconducor devices. Sep-up ransformers can be applied o furhencrease he oupu volage pulses. However, he ransformer non-ideal behaviour worsens he pulse shape [3]. The Marx generaor concep [4], as shown in Fig. 1, charging capaciors (C i ) in parallel and discharging hem in series ino he load (hrough a number of swiches, S i ), where he subscrip i {1, 2,, n-1, n}, provides anoher widely used mehod for generaing high-volage pulses, because i requires only a relaively low-volage power supply,, for charging and does no require pulse ransformers o achieve he desired high-volage. Nowadays, high volage pulsed power supplies have a broad range of applicaions. One aracive applicaion in surface reamen echniques, paricularly, plasma immersion ion implanaion (PIII), is a versaile new mehod for implaning ions, which can be used o modify he surface properies of maerials inended o form new compounds and o devise new semiconducors. Wih his echnique, he sample is immersed in a discharge chamber (where plasma is generaed) and shor, almos recangular, negaive Z 1 Z 2 Z (n-1) Z n S 1 S 2 S (n-1) S n C (n-1) Z 1 Z 2 Z (n-1) Z n v i This work is suppored by FCT POSI/ ESE/38963/21 Fig. 1. Basic opology of he EMG circui, wih n sages, for negaive oupu pulses o he load. -783-933-4/5/$2. 25 IEEE. 117
This approach has been used inensively hrough he years, changing only he swich echnology, from spark gaps o vacuum or gas ubes and nowadays o solid-sae semiconducors, and alernaing resisive charging sysems wih inducive ones, Z i. These echnological upgrades increased he life-ime of he circui and permied higher pulse repeiion frequency, meaning an improved performance [5-9]. However, he use of passive elemens (resisors or inducors, Z i ), as shown in Fig. 1, for charging he energy soring capaciors, C i, and o limi he self-discharging of he capaciors, during he series operaion, conribues o he low yield and efficiency of he circui, limiing he pulse frequency, due o he long charging ime consans, and degrading he generaion of almos recangular pulses. Thus, in he circui here proposed, Fig. 2, o increase he performance of he classic Marx-bank generaor opology, Fig. 1, no charging resisors onducors are used. Insead, volage increase is achieved by charging capaciors in parallel, hrough power semiconducor swiches (IGBTs and diodes), and hen discharging hem in series by opening he charging swiches, and closing he discharging ones. The circui opology and operaion mode block any self-discharging capacior pah. Due he power semiconducor opology used, almos recangular high-frequency pulses can be obained. Also, he proposed opology enables he use of ypical half-bridge semiconducor srucures, while ensuring ha he maximum volage blocked by he IGBTs is he volage of each capacior (i.e. he power supply volage), even when he swiching is no well synchronized, and even in faul condiions. A laboraory prooype wih five sages, 5 kw peak, of his all silicon Marx generaor circui, was consruced using 12 V IGBTs and diodes, operaing wih 1 V d-c inpu volage and 1 khz repeiion frequency. Firs experimenal resuls show almos recangular pulses wih 5 kv, near 5ns rise ime and 1 µs widh, giving 1 A ino a resisive load. II. CIRCUIT TOPOLOGY The innovaive concep, in he Marx-bank ype pulse generaor circui presen here, is he use of jus solid-sae swiches o charge and discharge he energy soring capaciors sages. For his reason he circui will be named here as Elecronic Marx Generaor (EMG). The basic opology of he EMG, wih n sages, able o deliver negaive high-volage oupu pulses o a load, is presened in Fig. 2. Each sage of he EMG consiss of a energy soring capacior C i, a diode D ci and wo IGBTs (T ci and T di ), where he subscrip i {1, 2,, n-1, n}. Oupu posiive pulses are simply obained by invering he polariy of all semiconducors as well as changing D ci wih T ci, as shown in Fig. 3. In relaion o Fig. 2, he circui in Fig. 3 needs on diode less, D cn, and can be replaced wih an ani- T c1-1 D cn Fig. 2. Basic opology of he EMG circui, wih n sages, for negaive oupu pulses o he load. T c T c1-1 v i v Fig.3. Basic opology of he EMG circui, wih n sages, for posiive oupu pulses o he load. parallel diode. The inclusion of T c, guaranees ha, during he pulse, he power supply is no in parallel wih. The EMG operaion in Fig. 2 can be undersood, considering only wo differen operaing modes. In he firs one, swiches T ci and T di are, respecively, on and off. During his period, capaciors C i are charged from he dc power supply,, hrough T ci and D ci, as shown in Fig. 4, wih curren limied by he inernal resisance of he elemens, resuling in a small ime consan ha enables khz operaion. The on sae of D ci ensures ha, during his period, he volage, v, applied o he load is approximaely zero, as shown in Fig. 6, for a resisive load. Due o he parallel charging opology of he capaciors during his period, he charge currens are largen he firs sages. During saring on, he volage is slowly increased o limi he charging curren on he semiconducors T ci and D ci. In he second operaing mode, swiches T ci and T di are, respecively, off and on. During his period, capaciors C i are conneced in series and he volage applied o he load is, approximaely, v = n, (1) considering ha all capaciors are charged wih, as shown in Fig. 5. However, his holds: i) on he characerisics of he componens; ii) on he operaing frequency; iii) on he capaciors charge ime, c, being much longer ha discharge ime, d, meaning ha T ci and T di operae, respecively, wih a long (δ c = c /T) and shor (δ d = d /T) swiching duy cycle, as shown in Fig. 6. The off-sae of D ci ensures, during his period, ha capaciors are no shor-circuied by T di swiches. i 1171
T c1 v gs (T di ) V i -1 v v gs (T ci ) i D cn Fig. 4. Capaciors charging operaion mode of he EMG in Fig. 2. c V i T d v c) -1 v -n i Fig. 5. Pulse operaion mode of he EMG in Fig. 2. I is imporan ha, during he pulse, he volage drop, due o he discharge of he energy soring capaciors, is only a few percen of each capacior volage. To guaranee his, he energy sored in he capaciors, E = n, (2) cap 2.5Civc where v c is he volage in he n capaciors, mus be 1 imes greaer han he energy delivered by each volage pulse, o he load [2], E pulse nvdci =, (3) where d is he on sae period of T di and i is he pulse curren, d i = n Z load, (4) considering a resisive load and all capacior charged wih, as shown in Fig. 6. For he above condiions, he plaeau of he pulse volage decreases exponenially, during he duraion of he pulse, described by v ( / C eq R eq ) = n e, (5) where C eq is he capaciance equivalen o he series of C i, and R eq represens he equivalen series resisance of he circui during his period, which is normally relaively low. The opology of he EMG, in Fig. 2, guaranees ha, if problems wih he swiching synchronizaion occur on i d) n /Z carga Fig. 6. Theoreical wave forms for he operaion of he EMG of Fig. 2, considering a resisive load: Drive signal of semiconducors T di ; Drive signal of semiconducors T ci ; c) load volage, v ; d) load curren, i. fauly condiions, he maximum volage ha each semiconducor holds is (maximum charge volage of capaciors C i ). As an example, if swich swiches o onsae somewha laer han he remaining T di swiches, diode D n says on during his period, mainaining he volage a he erminals of equal o he capacior volage. During his condiion he load volage is, roughly, ( n ) v = 1. (6) In addiion o he above described advanages, he swiching sequence and swich configuraion, seen in Fig. 2, enables he use of ypical half-bridge semiconducor srucures currenly inegraed in modular packages, which is advanageous o buil he circui and o drive he semiconducors. Due o he circui opology, Fig. 2, i is imporan o avoid cross conducion beween T di and T ci swiches. Hence, an auxiliary circui provides a delay ime (i.e. dead ime), beween swiching inpu conrol signals, so ha he urn-on conrol inpu o T di IGBTs is delayed wih respec o he urnoff conrol inpu of T ci IGBTs, and vice-versa. Also, due o he increase number of semiconducor, in he circui of Fig. 2 in comparison o he circui of Fig. 1, he complexiy of he driving circui is enhanced. Firs, here are wo drive signals, v gs(tdi) and v gs(tci), respecively, o T di and T ci, which mus be driven synchronously, Fig. 6. Second, all he swiches are a differen poenials, requiring gae circuis wih galvanic isolaion (opic fibres are used o ransmi he gae signals). 1172
III. EXPERIMENTAL RESULTS A laboraory prooype of he EMG circui of Fig. 2, wih five sages, 4.5 µf capaciors, was buil using 12 V IGBTs and diodes, operaing wih =1 V, 1% duy cycle and 1 khz repeiion rae. Fig. 6 shows he pulse pulse, v, and pulse curren, i, for a resisive load. The volage pulse, in Fig. 7, exhibi an almos recangular shape wih - 5 kv ampliude and 1 µs widh, giving 1 A, ino a resisive load, Fig. 7. The 1 khz pulse frequency is observed in Fig. 8, and he 5 ns pulse rise ime is shown in Fig. 8. IV. CONCLUSION A new all-in-silicon Marx-bank opology for high-volage, high frequency pulse generaor circui for recangular pulsed applicaions has been proposed. The circui uses only power semiconducor swiches o increase he performance of he classical circui, where he inducive, or resisive, charging sysem is replaced by solid-sae swiches, srongly Fig. 8. Experimenal resuls for EMG of Fig. 2, Volage pulse, v, 1 (V/div), horizonal scale: 5 (µs/div), 1 (ns/div) reducing losses and increasing he repeiion frequency. The proposed opology enables, also, he use of ypical half-bridge semiconducor srucures while ensuring ha he maximum volage blocked by he IGBTs is he volage of each capacior (i.e. he power supply volage), even when he swiching is no synchronized, and in faul condiions. A laboraory prooype wih five sages, 5 kw peak power, of his all silicon Marx generaor circui, was consruced using 12 V IGBTs and diodes, operaing wih 1 V d-c inpu volage and 1 khz frequency, giving 5 kv pulses, wih 1 µs widh and 5 ns rise ime. Using sae-of-he-ar kv IGBTs and diodes, high volage pulses reaching dozens of kv can be obained using he EMG concep. ACKNOWLEDGMENT Fig. 7. Experimenal resuls for he EMG of Fig.2, horizonal scale 2 (µs/div): Volage pulse, v, 1 (V/div); Curren, i, 2 (ma/div). The auhors would like o hank Insiuo Superior Técnico, Insiuo Superior de Engenharia de Lisboa and Fundação da Ciência e da Tecnologia for supporing his work. 1173
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