Parallel Operation of Dynex IGBT Modules Application Note Replaces October 2001, version AN AN July 2002

Transcription

1 AN5505 Parallel Oeration of Dynex GB odules Alication Note Relaces October 2001, version AN AN July 2002 NRODUCON GB modules can be connected in arallel to create a switch with a higher current rating. However, successful aralleling of GB modules requires some care. Deending on the alication the system designer has to consider a number of issues to ensure that the system is reliable. hese issues include the module characteristics, gate drive circuitry and circuit layout. he first ste is to detere the otimum number of modules to be connected in arallel to obtain the required current rating for the system. Any solution must ensure that the safe oerating area of the individual modules is not exceeded. Due to variations in module characteristics and circuit layout it cannot be assumed that the arallel connection of N modules each with a current rating of R ams will have a combined current rating of NR ams. For this reason it is necessary to derate the modules. his alication note exlains the theory behind derating with some examles and charts. he derating rocess can be alied to the entire range of Dynex GB modules. his note covers the static (conduction) and dynamic (switching) behaviour of arallel GB modules taking into consideration device characteristics only. SAC SHARNG OF GB ODULES CONNECED N PARALLEL When two or more GB modules are connected in arallel a current imbalance occurs due to the difference in the dynamic and static characteristics of the individual GB modules. n a steady state condition it is rincially the difference in outut characteristics which causes the current to divide unequally between the modules. his is illustrated in figure 1, which shows that the device having the lower V CE(sat) (Q1) carries the largest ortion of the total current. Note that in the static situation the total circuit inductance including the load also influences the dc current sharing but the following analysis assumes that differences in each arm are negligible. C j = 25 C Q1 Q2 WHA S HE SAC (DC CURREN) DERANG FACOR FOR PARALLEL CONNECED GB ODULES? Q1 Q2 f two modules of the same tye but with different V CE(sat) characteristics are connected in arallel the combined current rating is not twice the noal current rating of the module (this assumes the current in neither module exceeds the noal current rating). he reduction in effective current caability is known as the current derating factor. his is defined as: δ = 1 n x [1] Where, δ = the derating factor, V1 V2 V CE(Sat) = total current sustainable by the arallel combination of modules, V 01 V 02 = maximum allowable current for a single module oerating alone (dc current rating), Fig. 1 GB module outut characteristics n = number of modules in arallel. 1/5

2 Examle 1: Suose we have two GB modules with a dc current rating of 800A, and that when oerated in arallel one of the devices conducts 800A and the other 640A. We can calculate the derating factor from [1] thus: = = 1440A = 800A n = 2 Hence, δ = / (2 x 800) = (1 0.9) x 100% = 10% Rearranging equation [1] we obtain: = ( 1 δ )n Knowing the derating factor for a number of modules in arallel, one can detere the total current that can be sustained by the arallel system. For a system of n arallel connected modules where none of them is to exceed the rated current, the worst case situation is when one of the devices is conducting the maximum rated current and the remaining (n - 1) devices are each conducting some imum current. n this case the total current is given by: [2] =imum current for a single module oerating in a arallel connection. We can exress [4] as follows: From [1], [3] and [5] we have δ = 1 Examle 2: What is the derating factor for 4 devices in arallel with a missharing factor of 20%? Using equation [6], we have [5] [6] δ % = 1 ((4 1)(1 0.2) +1)/4 = 0.15 x 100% = 15%. he mis-sharing factor m is an imortant device characteristic for arallel connected GBs and it is related to the sread in GB outut characteristics for a given device tye. t is a function of = (1 m ) ( n 1)(1 m ) + 1 n j = 25 C = + ( n 1) [3] 25 WHA S HE S-SHARNG FACOR? Following on from the derating factor, it is ossible to define in a similar way the amount of mis-sharing between devices when connected in arallel. n this analysis the mis-sharing factor is a measure of the maximum current comared to the imum current: m = [4] Derating factor (%) j = 125 C Where m = mis-sharing factor = maximum allowable current for a single module oerating alone Number of modules in arallel Fig. 2 Static derating factor vs number of D800DD17-A000 GB modules in arallel 2/5

3 saturation voltage V CE(sat), junction temerature, device design and technology. Figure 2 (which is a lot of equation [6]) shows the derating factors for the Dynex D800DD17 GB module for u to 5 modules in arallel. Note that de-rating is more severe at lower temeratures. For a worst case design it is advisable to use derating factors alicable at 25 C junction temerature. he total system current can be estimated for u to 5 modules in arallel by reading off a derating factor from figure 2 and using equation [2]. Examle 3: Estimate total current for a system of four D800DD17 connected in arallel. From figure 2 the derating factor at 25 C for 4 devices in arallel is 30%. From equation [2], = (1 0.3) x 4 x 800 = 2240A DYNAC BEHAVOUR he static rating a system of arallel connected GB modules assumes that none of the devices in arallel combination carries current more than its rated value. A similar aroach can be alied to arallel connected-gb modules in dynamic conditions. he main reason for current imbalance during switching (turn-on and turn-off) assug ideal gate drive conditions and circuit layout is the difference in the transfer characteristics (collector current vs. gate-emitter voltage (V GE )) of the individual modules. Referring to figure 3, if the V GE alied to each of the arallel modules is identical during the switching transistions, the current divides dynamically according to the transfer characteristics. he GB module with the largest value of transconductance (i.e. the steeer transfer characteristic) carries the greater ortion of the current and incurs the highest switching losses. he dynamic current rating ( C(PK) ) is related to the GB rated junction temerature and hence to the total device losses. hus the dynamic current rating deends on the secific alication conditions. f we define C(PK)(max) as the maximum allowable eak current for a single module oerating alone in a secific aication and C(PK)() as the imum eak current for a single module oerating in a arallel connection, then we can define the artial current C(PK) as C(PK)(max) C(PK)() and the dynamic current missharing factor by ϕ = C(PK) / C(PK)(max) [7] hus we can obtain the dynamic derating factor: δ (dynamic) = 1 [(n 1)(1 ϕ) + 1]/n [8] C Static C(max) g (max) 25 Dynamic C() C(dynamic) g () Derating factor - (%) V GE V H() V GE() V H(max) V GE(max) Fig. 3 GB module transfer characteristics V GE 5 0 Oerating conditions: C(PK) = 1300A, f sw = 4kHz j = 25 C Number of modules in arallel Fig. 4 Comarison of static and dynamic derating factors for D800DD17-A000 3/5

4 Where δ = dynamic derating factor n = number of modules in arallel ϕ = is-sharing factor he dynamic mis-sharing factor ϕ is related to the transfer characteristics (V H and g fe ) of the GB. Figure 4 comares static and dynamic derating for D800DD17 GB modules at 25 C junction temerature under given oerating conditions (in this case a PW motor drive inverter). Note that the static derating factor is greater than the dynamic derating factor. his is true in general and so for most alications users need only consider the static derating factor when deterg the number of modules required in arallel. EXERNAL NFLUENCES ON SHARNG his note considers only the effects of device characteristics on static and dynamic sharing. However, it should be noted that dynamic sharing is more sensitive to external circuit factors (esecially the stray inductance in the gate-emitter circuit loo) than the GB module dynamic characteristics (transconductance). Figure 5 illustrates how unequal stray inductances in the gate-emitter loos roduce dynamic imbalance due to non equal gate emitter voltages (i.e. V V ) during GE1 GE2 switching and thus an unbalanced current in dynamic switching. he use of searate gate resistors R g1 and R g2 (figure 6) hels to restore the dynamic balance. SUARY When a number of GB modules are connected in arallel, the total current caability must be derated due to mismatching of device characteristics and non-symmetrical external circuit layout. he derating factors for static and dynamic oeration are resented for arallel combinations of u to five modules. Using the derating charts a system designer can estimate the total sustainable current for a system of arallel-connected GB modules. Fig. 5 Static and dynamic collector current imbalance for two D800DD17-A000 modules in arallel Gate Drive R g1 R g2 V GE1 1 L 1 V GE2 2 L 2 Fig. 6 Use of R g1 and R g2 to restore dynamic balance 4/5

5 POWER ASSEBLY CAPABLY he Power Assembly grou was set u to rovide a suort service for those customers requiring more than the basic semiconductor, and has develoed a flexible range of heatsink and clag systems in line with advances in device voltages and current caability of our semiconductors. We offer an extensive range of air and liquid cooled assemblies covering the full range of circuit designs in general use today. he Assembly grou offers high quality engineering suort dedicated to designing new units to satisfy the growing needs of our customers. Using the latest CAD methods our team of design and alications engineers aim to rovide the Power Assembly Comlete Solution (PACs). HEASNKS he Power Assembly grou has its own rorietary range of extruded aluium heatsinks which have been designed to otimise the erformance of Dynex semiconductors. Data with resect to air natural, forced air and liquid cooling (with flow rates) is available on request. For further information on device clams, heatsinks and assemblies, lease contact your nearest sales reresentative or Customer Services. htt:// ower_solutions@dynexsemi.com HEADQUARERS OPERAONS DYNEX SECONDUCOR LD Doddington Road, Lincoln. Lincolnshire. LN6 3LF. United Kingdom. el: +44-(0) Fax: +44-(0) CUSOER SERVCE el: +44 (0) / Fax: +44 (0) SALES OFFCES Benelux, taly & Switzerland: el: +33 (0) Fax: +33 (0) France: el: +33 (0) Fax: +33 (0) Germany, Northern Euroe, Sain & Rest Of World: el: +44 (0) / Fax: +44 (0) North America: el: (613) Fax: (613) oll Free: DYNEX (39639) / el: (949) Fax: (949) hese offices are suorted by Reresentatives and Distributors in many countries world-wide. Dynex Semiconductor 2002 ECHNCAL DOCUENAON NO FOR RESALE. PRODUCED N UNED KNGDO Datasheet Annotations: Dynex Semiconductor annotate datasheets in the to right hard corner of the front age, to indicate roduct status. he annotations are as follows:- arget nformation: his is the most tentative form of information and reresents a very reliary secification. No actual design work on the roduct has been started. Preliary nformation: he roduct is in design and develoment. he datasheet reresents the roduct as it is understood but details may change. Advance nformation: he roduct design is comlete and final characterisation for volume roduction is well in hand. No Annotation: he roduct arameters are fixed and the roduct is available to datasheet secification. his ublication is issued to rovide information only which (unless agreed by the Comany in writing) may not be used, alied or reroduced for any urose nor form art of any order or contract nor to be regarded as a reresentation relating to the roducts or services concerned. No warranty or guarantee exress or imlied is made regarding the caability, erformance or suitability of any roduct or service. he Comany reserves the right to alter without rior notice the secification, design or rice of any roduct or service. nformation concerning ossible methods of use is rovided as a guide only and does not constitute any guarantee that such methods of use will be satisfactory in a secific iece of equiment. t is the user's resonsibility to fully detere the erformance and suitability of any equiment using such information and to ensure that any ublication or data used is u to date and has not been suerseded. hese roducts are not suitable for use in any medical roducts whose failure to erform may result in significant injury or death to the user. All roducts and materials are sold and services rovided subject to the Comany's conditions of sale, which are available on request. All brand names and roduct names used in this ublication are trademarks, registered trademarks or trade names of their resective owners.