application note AN586/0393 use of transils as rectifier diodes ? transil p6ke 1n5635a to 1n5665a 1.5ke bzw50 case cb-417 do 13 cb-429 ag rating of equivalent diode (a) 1 1.2 3 6 transils are diodes especially designed to dissipate high peak power in avalanche operation. in direct conduction they have the properties of very good conventional diodes (with the possibility of handling very high surge currents) and perform as well as rectifiers. in general, there could be a voltage surge due either to the power supply or to the load. power supply overvoltages can be clamped by means of a transil on the ac side : see fig.1. wherever possible, it is more attractive to use transils directly as rectifiers instead of protecting separate rectifiers with transils since the number of components is reduced : see fig.2. the characteristics given in the data-sheets (i fsm , forward voltage drop characteristic) enable calculations to be made in this mode of operation. the following table gives the equivalence between transils and common rectifiers. neither of these two methods protects against overvoltages originating from the load. fortunately, a study of bridge rectifier operation shows that it is sufficient to limit the voltage on + - figure 1 : the diodes are not protected from overvoltages due to the load + - figure 2 : mixed solution which reduces the component count. the 2 transils work as rectifiers the dc side to protect the bridge whatever the origin of the overvoltage : see fig.3. in all cases, a single transil is sufficient to protect a single phase or a 3-phase bridge rectifier. 1/2
figure 3 : a single transil protects the rectifying components whatever the origin (power supply or load) of the overvoltages + - + - information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics assumes no responsability for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specifications mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. sgs-thomson microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of sgs-thomson microelectronics. ? 1995 sgs-thomson microelectronics - printed in italy - all rights reserved. sgs-thomson microelectronics group of companies australia - brazil - france - germany - hong kong - italy - japan - korea - malaysia - malta - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. ? application note 2/2
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