a high- power dc- dc converter for high.pdf

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1 9 9 8 年 12 月北方交通大学

学报 Dec. 1 998

第 22卷第 6期 JOUR NA L OF NOR TH ER N JIAOT ON G U NIVER SIT Y Vol . 2 2 N o. 6

A High- Power DC-DC Converter for High

Frequency High- Input Voltage Using IGBT*

P eng Guan gfeng H ao R on gtai ( Co ll ege of M echanica l and Elect rical Eng ineering, North ern Jia otong U niv ersity, Beijing 100044)

Solar inverters Abstract F or th e h ig h f requen cy and h igh- inp ut- volt ag e ( somet im es reach in g several

kilo- volt s) con vert er, it s sw itch ing devices - IGBT s volt age rating can n ot su it t h is re-

qu irement . In th is paper, a new cascaded - inverter - un it s stru ctu re is proposed, in w hich

th e h igh- input-volt age is div ided int o several part s an d each part corresponds t o one DC-

A C converter. S o th e IGBT s operating volt ages are equal to t he volt ag e of each d ivided

part . In order to in crease t he out put pow er, th e ou tpu t- rect if ier - diodes usu ally are used

in parallel, a method t o b alan ce th e cu rrent by changing t h e lin k in g ty pe betw een t he

tran sformer secon dary win din gs and t he out put- rectifier diod es is recommen ded . T his

met hod fully u tilizes t he t ransf ormer leakage indu ctance to realize th e curren t- sharin g

f un ct ion. In t his paper, o perat ion of a h ig h- in put volt age, h ig h- pow er D C- DC con verter

is described . Bot h ex per iment al an d simulat ion results are show n for a s mal-l f u l-l s cale-

mod el converter.

Key words high- inpu t- voltage h ig h- frequency IGBT DC-DC converter

大功率高压输入的 IGBT 高频直-

直变换器

彭光凤郝荣泰 (北方交通大学机械与电气工程学院, 北京 100044)

摘要在高频高压输入的情况下, 开关器件绝缘门极双极晶体管( IG BT )的耐压不能满足要求.

文中论述了一种逆变单元串联的结构, 它将输入的高电压分成几部分,每一部分有一个 DC-A C逆

变器. 此时, IGBT 的工作电压等于各部分电压. 为了提高输出功率,输出整流二极管通常并联连

接, 变压器的副边漏电感作为二极管的均流电感.并分析了高压输入的大功率 DC- DC 逆变器的工

作情况,同时给出了模拟结果和实验结果.

关键词高压输入高频逆变绝缘门极双极晶体管直- 直变

换器

分类号 T M 46

4

In the elect r omot ive, metros, t rolleys etc. , high- voltage f eed is usually used. Generally, the



* 国家自然科学基金资助项目本文收到日期 1998- 09-13 彭光凤女 1972 年生硕士生 email bf xb@ c enter. njtu. edu. cn



北方交通大学学报7 6

h igh- volt age sh ou ld be changed int o t he suitable low volt age. It is f ound t h at t h e low-f requency ,

high- pow er t ran sformers are bu lk y, it could not meet th e requ irement s of th e assist ant power

source.

In order to solve th is p r o b lem, it needs to use the high- f requ ency con verter, but the high- f re-

quency pow er swit ch in g device voltage rat in g is usually low , could n ot meet t he need of th e h igh-

in put voltage. For pow er levels up t o t ens of k V A , IGBT s are economical. Generally a cascaded

devices st ruct ure can improve t he convert er s o perat in g volt age, but t he d evice n eeds voltage-

shar -

ing circuit w hich influences th e con verter s outp ut volt age waveforms and lim it s it s h ig hest f re-

quency, So t his method does not suit the s ituations above.

F or t he h igh- in put volt age, h igh- f requency , high- pow er DC- D C con verter, a more at t ract ive

app roach is t he cascaded - inverter - un it s structu re, w hich can redu ce sw itching d evices applied

volt age to its volt ag e- rat ing range and yet does not in fluence th e converter s oth er characters at t he

same t ime. In t h is st ru cture each invert er u nit h as an in put stage voltage- sharin g capacit or and an

ou tpu t volt aic- isolat in g tran sformer wh ile all t he transformer s secon dary w ind ing s are con nected

toget her. Wh en t he out put voltage is low and t he ou tp ut power is h igh , such as h und reds of volt

and t ens of k V A, t he h igh-

f requency rect ifier diodes curren t-

sharin g in ductance, curren t-

sharin gof these parallel diodes is solved by u sing t he t ran sformers leak age indu ctances.

T he sn ubber circuit of IGBT , the h ig h f requency converters w it h IGBT , desig n on output

transformer and th e g at e- drive requirement s of IGBT are in troduced respectively in th ese ref er -

ences as[ 1, 2]

.

T his aim of th is paper is t o design an o pt im um con verter s st ruct ure wit h high- out put power

u nder h ig h- in put voltage. First a brief analysis of th e convert er s t opology is present ed, t hen t he

o perat ing process of t he w hole syst em , t he calculat ion f or mu las of t he voltage- sharin g cap acitor s

paramet er, th e s imu lation an d ex p erient ial results to verify th e converter s design is p resen ted

too.

1 The Conver ter 's To pology

T he t opology of t he h igh f requ ency and high inpu t volt age DC-DC converter in w h ich t he K

hy b rid inverters are series con nected w hile K - high f requ ency A C linking b y isolat ing t ransf ormers

is s how n in Fig. 1. T h e IGBT modules are employed to const ruct th e converter s sw it ches. Be-

cause of th e high sw it ch in g sp eed an d low con duction voltage drop of t he IGBT modules, its con-

vers ion loss is low and th e system ef ficiency is h ig h. It is k now n t hat if a volt age- sh aring capacitor

is in p arallel w ith an in verter un it t hen th e volt age of th e capacitor is t h e o perat in g voltage of t he

in verter u nit. H ere t he invert er unit is composed of t wo IGBT s and t wo f ree w heel d iodes; here

th e ad vant age of th is s tructure is t h at th ere is no D C st age short. H owever th e d uty rat io must beless t h an 50% , so t he tran sformers ut ilizat ion factor is low er. It is f oun d t hat if th e DC short

pr otection is ef fective, the bridge converter can be used [ 2]

, so that the system eff iciency w ould be

im p roved. A ll th e IGBT s in Fig. 1 sh ou ld be of fered t he gate- d riving pulse simu ltaneously. It is

found that the max imum applied forw ard volt age of t he IG BTs and the maximum applied reverse



第 6 期彭光凤等:大功率高压输入的 IGBT 高频直- 直变换77

volt age of the free w heel diod es are equal to that of the voltage sharing capacitor. Below w e are to

describe the operation of the converter in detail.

Fig. 1 Circuit schemati c of high input voltage DC- DC converter with

ser ies- connected i nverter units on the input high voltage s id e

In t he ideal cases, t he s harin g- capacitors, IGBT s, th e f ree- wh eel d iodes, th e out put t ran s-

f ormers, and t he rectifier diod es in Fig. 1 should b e t he same. S o t he volt ages of th e sh aring-

ca- p acitors are equal, and th e current flow ing t hrough each IG BT w ou ld be equ al wh en th e IGBT s

are sw it ched on. O n t he ot her h and, w hen t he IGBTs t urn off , t he current s flowing th r oug h each

f ree- w heel d iode w ould be equ al. No matt er every sh aring- cap acitor charge or dis charge sim ulta-

n eously, t he voltages of t he capacitor w ould be alw ays equ al. T he applied voltag es of th e IGBT s

w ould be equal to 1/ K DC bus voltage.

T o analyze the entire o perat ion of F ig . 1, some real cond it ions should be considered as be-

low:

If the sh aring- capacitors w ere not the same, the volt ages of the capacitors w ou ld not be e-

qu al, t h at is t o say: t he smaller - value capacitor correspon ds t o t he large voltage, t he lar ger - value

cap acitor corresponds to th e s maller voltage. W hen t he IG BTs tu rn on , t he out put power of t he

higher - applied - volt age inverter un it is larger than that of the low er applied voltage inverter u nit ,

s in ce t he curren t flow ing t h r oug h t he IGBT of th e former u nit w ould be larger t han t hat of t he

latt er unit . F rom an alysis above, we can see t hat t he convert er mak es t he higher volt age reduce

and t he lower voltag e rise, event ually t he capacitor s volt age and t he out put pow er of th e each u-

n it turn to be equ ivalent .

W h ile the sw it ch ing characteristics of the IGBT s are not the same, even t houg h the IGBTs driving pulses are iden tical, the IGBT s sw itching t im e inst ants are not iden t ical. How ever, when

each IGBT is t urn ing off it w ou ld be applied to the volt age of its resp ect ive sharing capacitor. So,

th e IGBT tu rning of f more qu ick ly can not be applied to the DC bus volt age, on ly its sharing capacitor charges up more quickly than the others mak ing its voltage rise. T he value of the ris ing

volt age depends on the value of the capacitor ch ar ge- t ime and the operating current of the IGBT s .

A nalys is on the IGBT s operation w hen turning on is similar. T h is ch aracterist ic shows that the

pr o posed st ructure is bett er t han that where the pow er d evices are in series connected.




If th e con duction volt age- drops of t he IGBT s in F ig . 1 are not t he same and t he initial volt-

age of t he sh aring- capacit ors are equ al, th e ou tpu t powers of every invierter un it would not be e-

qu al w hen th e IGBT s are sw it ched on . So t he sh aring cap acitors can n ot char g e or discharge e-

qu ally , t he h igh er voltage corresponds t o t he high er con duct ion voltage- drop. Even tu ally, each

in verter u nit gives t he equal pow er and th e current f low in g t hrough each transformer is t he s ame,

th e differences equal to these of the IG BTs con duct ion voltage-drops.

It is wort h to note that, the operating processes are s imilar w hile the t rans formers, the in it ial

volt ages of the sharing capacitors and the output rectifier d iodes are not the same, and here we do

not analyze them one by one. It is f ound that the considered structure can suit the situations of the

high input st ag e voltage and low power sw itching devices voltage rat ing, the converter can ba-l

ance the condition curren ts and reduce the applied volt ages of the power devices.

Since t he p roposed system considers a high- f requ ency and h ig h- pow er con vert er, th e out put

rect if ier d iodes should b e h ig h-s peed w it h t he large curren t rat ing . Bu t t he high- speed d iodes cu r -

rent rat in g could not suit t he sit uat ion s of th e large ou tpu t current usually. In order to increase t he

ou tpu t power, t he d iodes should be in p arallel, bu t th e assist ant curren t- sharin g inductances can

bring about some disad vant ages such as limit ing the max imum operating frequency, increasing thesw itching sp ik e voltage

[ 3]. Fig. 1 has show n that one transformer has several secondary w ind -

ing s, an d each of t hem is connect ed in series w ith a rectifier diode. Here t he current-sh aring p r o b-

lem is solved b y using t he transformers leakage in duct ances. T he proposed structu re f u nct ions a-

like t hat of cascading an in duct an ce, wit h its resp ect ive diodes in p arallel. Bu t it can overcome t he

disad vant ages men t ioned above.

2 Parameter Com putation

It is t he key issue how to select t he sharin g capacit or, because th e syst em s regulat ing t ime is

mainly in fluenced b y t he value of th e capacit ors . W hen th e pow er device s sw it ch in g speeds are

not ident ical, th e voltage of its respect ive cap acit or s value must b e not too sm all. How ever t he

system is adaptive, it should reach to the balance st atus as quick ly as possib le when some sit ua-

t ion s ment ion ed above occu r, such as d if ference w ith in th e sh aring capacitors, t ran sformers, and

th e init ial volt ages of t he cap acitors etc. it is conclu ded th at t he cap acit or sh ould be as small as

possible.

In add it ion , t he st o ppag e st ates should be considered. For example, w hen on e invert er un it

does n ot o perate and t he ot hers o perate regu larly, th e sh aring capacitor correspond ing to th e b reak

u nit charges up quick ly . In order t o avoid pun ch in g of t he pow er devices, t he sharin g capacitor

must be larger than the m in imum value.

T he value of th e sh aring capacit or is dependent on th e factors men tioned above. N ow weconsider the most d isad vantageous cond it ions.

In case only one pow er d evice is tu rning on and th e ot hers are t urning of f, t he leak age induc-

t ance of th e ou tpu t transformer wou ld be larger. T hen t he value of t he sharin g capacitor sees t he

equation ( 1)




C1 =1 2 ( K - 1)

I 2 t ( 1) U pk K

w h ere K is th e n umber of t he invert er u nits, I 2 th e o perat in g cu rrent of t he pow er device, U pk

th e allow ed rip ple volt age, t t he swit ching- off time g ap b et ween t he f astes t device and t he s low-

est device.

In case only one in verter unit is off, w hile the others are being shorted, the value of the

sh aring capacitor is com puted by the follow ing equat ion ( 2) ,

C 2 =1 K - 1

I s t s ( 2)

Us K

w here K is the number of the inverter un its, I s the short ing current , t s the allowed short ing time,

U s the allow ed ripple voltage. Detailed inf erence of the tw o above equations sees the ref er -

ence[ 4 ]

.

T he maximum of the values of C 1 and C 2 is chosen as the value of the sharing capacitor C d .

3 Simulation R esults

T he simulation circuit consists of t hree invert er un its, i. e. K = 3 . T he p r o posed convert er s basic operation is simu lated by the PSP ICE, the Circuit S imu lation T oolk it. The models of the

devices have been described in the handbook [ 5 ]

. The leak age and the line indu ct ance are subst it ut-

( a) ( b)

Fig. 2 Simula tion wavefor ms




ed by some lu mped in duct ances. T he s imu lation result s see Fig. 2. F ig. 2 ( a) shows t he applied

volt ages of t he IG BTs, Fig. 2 ( b ) shows t he flow ing cu rrent of th e IGBT s, w hich b elong to t he

different inverter units.

4 Cir cuit Control Str ategies

V arious cont rol schemes shou ld correspond t o various w ays of inp ut ting th e voltage. Wh en

in put t ing a DC voltag e, th e simplest con trol w ay is t o modify th e pu lse-

w idth according to t he

need of t he out put voltage. W hen in put ting an AC volt age, we sh ould rect ify and filter at f irst , b

ut it s is wort h t o n ote t h at th e DC b us also has rippled wh ere t he PWM con trol schem e should be u

sed to mak e th e act ive curren ts of t he IGBT s iden tical during each period. T he w ider pulse cor -

respon ds to t he smaller volt age. A lso t h e f iltering capacit or sh ould be sm aller, t hen t he out pu t DC

volt age can be smoot her [ 3 ]

.

5 A Smal-l scale Pr ototy pe

A smal-l scale converter has been bu ilt w ith the follow ing object specif ications:

Inpu t AC volt ag e: 50 H z, 380 V output DC voltage: 200 V .T ransf ormer turn rat io: n = 1 ( for simplicity) output cu rrent : 15 A ( DC) .

Sw itch ing frequency: fixed 20 kH z.

T he pr otot ype is illus trat ed in Fig. 3, w hich was b uilt w it h w in d - cooled h eat sink s. It con-

sists of tw o invert er un it s, stripline d esigns to min imize circuit indu ctance. In ou r p r ot oty p e, FU-

J I GBT b rid g e legs are used , and FU J I EX B841 driver is applied to realize a sim plif ied driving of

IGBT modules. T h e valu e of s nubb er capacitors are 1. 0 F across each ar m , and t he value of t he

in put f ilt ering cap acit or is 300 F. T he value of sh aring- capacit ors is 40 F, given th at t he value

of t he ou tpu t f ilterin g capacit or is 50 F. T h e t ransf ormer consists of U o- 3 non- crystal core

( mad e in P R Ch in a) . T he ex p erimental results see Fig. 4 . T he d if ference of the t wo output rec-

t ifier diod es is sm all, and un balanceness rate is less t h an 10% . T he major limit at ion for o perat in g

is given by the diode reverse time and the curren t, package inductances and int er -package induc-

t ances.

Fig. 3 Ex per imental Cir cui t

T he simulat ion and ex periment al resu lt s illust rat es t hat t he IGBT s volt ages and currents of

different inverter un it are similar, t he ap plied voltages of th e IGBT s are equal to 1/ K DC- b us

volt age.




( a ) S witch vo lt age and current of Unit 1

( b) Sw itch v o lt age and current of Unit 2 ( c) Se conda ry v olt age and current of the t ransf ormer

Fig. 4 Ex per imental wa vefor ms

7 Conclusion

A h ig h frequency, h ig h- in put volt age and h ig h-ou tpu t power DC-D C con vert er w it h th e con-

n ect ed in series invert er u nits is presented in t h is paper. A conn ect ed in series in vert er un it s struc-

t ure h as been put forward w hich can suit t he sit uations of low devices voltage- rat ing un der t he

high- inpu t volt age. T he proposed circuit also adopts a special st ru ctu re to b alan ce cu rrent s of t he

diodes in parallel by ut ility of t he t rans formers leakage indu ctances. Bot h th e princip les and t he

charact erist ics of th e convert er are d iscussed, also th e resu lt s of s imu lation and ex perimen ts f or

each unit of the pr ototype are given.

R efer ences

1 Zhang Y , et al. Snubber considerat ions f or IGBT applicat ion. IP EMC, 1994. 261~ 269

2 M shan N , et a l. P ower Elect r onic Co nv erters A pplica tion and Design. John W iley & S ons, 1989.

3 D ewan S B, St ra ughen A . P o wer Se miconduct or Circuits. New Y ork: Int ersciences, 1975.

4 Peng G F . Research on Hig h- Frequency Co nve rte r of the Air Co nditioner s Po w er Source on the T ra in. M . Eng. thesis, No rt h-

ern Jia otong U niv ersity, 1996.

5 P SPICE ( V4. 02) H andbook, M icroSim Co. , Sea Publicat ion.

6. www.newsinoenergy.com solar inverters

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