AN5369

AN5369 DYNEX | Alldatasheet

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www.dynexsemi.com AN5369 Selection Of SCRs For Parallel Operation Application Note Replaces September 2000 version AN5369-2.0 AN5369-2.1 July 2002 Instantaneous forward current - (A) Instantaneous forward volt drop - (V) SCR 1 (V0, rT1) SCR 2 (V0, rT2) V0 V1 V2 Fig. 1 Total current is divided between the two SCRs Selection of Thyristors (and Diodes) for Parallel Operation with non-reactor/resistor sharing. When thyristors are intended to be used in parallel without the use of reactors or resistors to force current sharing then the thyristors must be chosen in such a way that they will share the current in relation to their forward voltage drop at the operating current. This Application Note gives a simple way of doing this from the data presented in the device data sheet, based on a number of simplifying assumptions. The problems of differing turn-on performance, finger voltage etc. have not been considered here but are addressed in Application Note AN4999 “Turn-on Performance of Thyristors in Parallel”. The treatise below is presented in terms of the fully turned on phase of a thyristor conduction cycle. The methodology can equally be applied to rectifier diodes in parallel. Dynex Semiconductor will append a 4 digit selection number to the device type to specify your particular sharing criteria. Consider the case of two thyristors in parallel with voltage V across them. In this situation the total current is divided between the two SCRs such that SCR1 is carrying I2 and SCR2 is carrying I1. At the current I1 SCR 1 has a forward voltage drop of V1. It is assumed here that all the variation in Forward Voltage Drop between the thyristors is due to variation in the slope resistance and that the knee voltage V 0 is the same for both thyristors. SCR 2 is assumed to be the data book limit case device and V0, rt2 are the figures printed in the data book. Now V 2 = V0 + I1.rt2 for SCR2 and V2 = V0 + I2.rt1 for SCR1 i.e. I1.rt2 = I2.rt1 At the current I1 we have: V1 = V0 + I1.rt1 V2 = V0 + I1.rt2 ∴ V2 -V1 = I1.(rt2-rt1) Substituting for rt1 from (1) we have V2 -V1 = I1.(rt2 - I1.rt2/I2)

www.dynexsemi.com Suppose that there is a situation whereby N SCRs are connected in parallel and are required to conduct a current Itot amps with a given cooling system. /circle6In the worst case N-1 SCRs will conduct current I1 and the remaining SCR will conduct /circle6Define a mis-sharing factor (x) such that I Note that different people define mis-sharing in different ways. Some people would say that a 20% mis-sharing results in the current in two devices dividing 80/120. In the above definition this would be a mis-sharing factor of 0.5 because 120 = (1 + 0.5)*80 ! /circle6Itot = I1.(N-1) + (1+x).I1 /circle6The maximum current I2 is Itot - (N-1).I1 /circle6This will be carried by the SCR with slightly higher losses because the voltage across all devices is the same, so the device with the higher current has the higher losses. If the conduction losses of the data book device calculated at I2 are within those that the cooling system can disperse then the design is feasible. /circle6Substituting for I1/I2 from (4) into (2) and for I1 from (5) we get Note that the value of ∆V can be scaled by the factor Itest/I1 to the value of ∆V at any test current point on the linear part of the forward volt drop curve. If ∆V is too tight compared to the production spread indicated in the data sheet then the mis-sharing factor will have to be increased and hence the number of devices in parallel will have to increase. Example 1 : Consider 2 DCR1673SZ28 thyristors in parallel which are required to conduct a total of 40kA peak and share current to within ± 20% of the average i.e. one thyristor can take 24kA while the other takes 16kA. For the DCR1673 rt is 0.093mW To find the mis-sharing factor x : 24kA=(1+x)*16kA i.e. x = 0.5 From equation (6) ∆V = 40x10 The production test point for the DCR1673 is 3kA So ∆V =496 * 3/16 = 93mV @ 3kA Example 2 : Consider running a number of DCR803SG12 thyristors in parallel to conduct a total of 1200A average current half wave with a heat sink that gives a thermal resistance, junction to ambient, of 0.155˚CW per device. The ambient temperature is 40˚C and the desired junction temperature is 100˚C . Data sheet gives V0= 0.85V rt = 0.38mohms The maximum power that may be dissipated is (100-40)/0.155 = 387 Watts. From the dissipation curves in the data sheet 387W is equivalent to 320A average 1/2 wave. This is the maximum current. Assuming a mis-sharing factor of 0.2, the current in the other semiconductors is given by equation 4, namely, 320 = (1+0.2) * I 1 i.e. I1 = 320/1.2 or 267A Number of devices required in parallel for 1200A is given by equation 3 i.e. 320 = 1200 – (N–1) * 267 or N = 880/267 or 4.29 Therefore 5 devices are needed in parallel for a mis-sharing factor of 0.2. Because we have to use 5 thyristors instead of 4.29, the minimum current, given by equation equation 3, will therefore actually be: I1 = (1200-320)/4 = 220A i.e. a mis-sharing factor (x) of (1-220/320) = 0.3125 not 0.2 as originally. Therefore equation (6) gives Volts or 29mV @ I 1= 267A average or 377A peak. The datasheet VF curve shows that the total spread of VF at 377A for the DCR803 is ~50mV so that selecting devices to a 29mV band is reasonable and the use of two bands would utilise 100% of the production spread. If we reduce the number of devices in parallel from 5 to 4,the lower current level would be (1200-320)/3 = 293.33 A The mis-sharing factor (x) is therefore 1–(293.33/320) = 0.0833 and therefore ∆V = 12mV i.e. to use the whole distribution, 4 bands would be required. Using multiple Vf bands can cause problems with the supply of replacement devices and should be recognised before opting for the solution with the least devices in parallel.

www.dynexsemi.com POWER ASSEMBLY CAPABILITY The Power Assembly group was set up to provide a support service for those customers requiring more than the basic semiconductor, and has developed a flexible range of heatsink and clamping systems in line with advances in device voltages and current capability 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. The Assembly group offers high quality engineering support dedicated to designing new units to satisfy the growing needs of our customers. Using the latest CAD methods our team of design and applications engineers aim to provide the Power Assembly Complete Solution (PACs). HEATSINKS The Power Assembly group has its own proprietary range of extruded aluminium heatsinks which have been designed to optimise the performance of Dynex semiconductors. Data with respect to air natural, forced air and liquid cooling (with flow rates) is available on request. For further information on device clamps, heatsinks and assemblies, please contact your nearest sales representative or Customer Services. CUSTOMER SERVICE Tel: +44 (0)1522 502753 / 502901. Fax: +44 (0)1522 500020 SALES OFFICES Benelux, Italy & Switzerland: Tel: +33 (0)1 64 66 42 17. Fax: +33 (0)1 64 66 42 19. France: Tel: +33 (0)2 47 55 75 52. Fax: +33 (0)2 47 55 75 59. Germany, Northern Europe, Spain & Rest Of World: Tel: +44 (0)1522 502753 / 502901. Fax: +44 (0)1522 500020 Tel: (949) 733-3005. Fax: (949) 733-2986. These offices are supported by Representatives and Distributors in many countries world-wide. © Dynex Semiconductor 2002 TECHNICAL DOCUMENTATION – NOT FOR RESALE. PRODUCED IN UNITED KINGDOM HEADQUARTERS OPERATIONS DYNEX SEMICONDUCTOR LTD Doddington Road, Lincoln. Lincolnshire. LN6 3LF. United Kingdom. This publication is issued to provide information only which (unless agreed by the Company in writing) may not be used, applied or reproduced for any purpose nor form part of any order or contract nor to be regarded as a representation relating to the products or services concerned. No warranty or guarantee express or implied is made regarding the capability, performance or suitability of any product or service. The Company reserves the right to alter without prior notice the specification, design or price of any product or service. Information concerning possible methods of use is provided as a guide only and does not constitute any guarantee that such methods of use will be satisfactory in a specific piece of equipment. It is the user's responsibility to fully determine the performance and suitability of any equipment using such information and to ensure that any publication or data used is up to date and has not been superseded. These products are not suitable for use in any medical products whose failure to perform may result in significant injury or death to the user. All products and materials are sold and services provided subject to the Company's conditions of sale, which are available on request. All brand names and product names used in this publication are trademarks, registered trademarks or trade names of their respective owners. http://www.dynexsemi.com e-mail: power_solutions@dynexsemi.com Datasheet Annotations: Dynex Semiconductor annotate datasheets in the top right hard corner of the front page, to indicate product status. The annotations are as follows:- Target Information: This is the most tentative form of information and represents a very preliminary specification. No actual design work on the product has been started. Preliminary Information: The product is in design and development. The datasheet represents the product as it is understood but details may change. Advance Information: The product design is complete and final characterisation for volume production is well in hand. No Annotation: The product parameters are fixed and the product is available to datasheet specification.