application note AN575/0393 apparent dynamic resistance calculation of transil a b. rivet 1 - introduction to estimate the clamping voltage v cl and the dissipated power in a transil we need the apparent dynamic resistance of the device, rd. this value depends on : - the thermal impedance and therefore the package - the breakdown voltage v br - the pulse current duration tp (standard exponential pulse). the purpose of this note is to explain the means of calculating rd. 2 - expression of the dynamic resistance rd rd is defined by the formula : rd = (v cl -v br )/i pp , where v cl is the peak voltage at ipp and v br is the breakdown voltage of the transil measured at a low level of current ( 1ma) there are two distinct cases : - tp lower than 1 ms - tp higher than 1 ms a) rd (tp) with tp < 1 ms in the data sheet v cl max is specified at tp = 20 m s and 1 ms. we can thus estimate rd 20 m s and rd 1ms with the following formula : rd 20 m s = v cl max ( 20 m s ) - v br nom i pp ( 20 m s ) ( 1 ) rd 1ms = v cl max ( 1 ms ) - v br nom i pp ( 1 ms ) ( 2 ) for tp between 20 m s and 1ms we can calculate rd (tp) as rd ( tp ) = rd 1 ms - rd 20 m s 980 [ tp - 20 ] + rd 20 m s with rd in ohms and tp in m s. the apparent dynamic resistance decreases when the duration decreases. for tp < 20 m s we can use a constant value equal to rd calculated for 20 m s (relation (1)). this is a pessimistic rule. b) rd(tp) with tp > 1ms sgs-thomson transils are built with one chip for the low voltage parts and with two chips in series for the high voltage ones. the two cases need to be considered separately. b.1. low vo ltage devices (up to 213 v for bzw series and up to 220 v for ke series). using thermal criteria we obtain the typical dynamic resistance rd typ for tp higher than 1ms: rd typ = a t rth [ 1 - exp ( - tp t ) ] b v br 2 nom where: - a t is the temperature coefficient of v br . it can be found in the protection devices databook. - rth, t , b define the transient thermal impedance zth. the curve zth = f(tp) is given in the data sheet. (3) (4) 1/3
b.2. high voltage devices (over 213 v for bzw series and over 220 v for ke series) in this case, the following formula is used : rd typ = a t 2 r th ? 1 - exp ? ? - t p t ? ? ? b v br 2 nom note : to estimate the maximum value of v cl and the peak power in the transil we have to use a coefficient k to take into account the dispersion of the various parameters (rd max = k rd typ ). k = 2 is recommended. 3 - example of application : choice of a transil assume the surge current in the transil is an exponential pulse with i pp = 3a and tp = 30ms. in the application (fig.2) we have to check that v rm > v cc = 30v and v cl max < 55v with a maximum ambient temperature of 50 c. try to use a 1.5ke36 p the data sheet gives : v rm = 30.8v ( >30v ) v br max = 39.6v v br nom = 36v a t = 9.9 x 10 -4 /c the table fig.1 gives : r th = 75c/w t = 150s b = 0.49 with the relation (4) we find : rd typ = 1.5 ohm rd max = 3 ohms v cl max = v br max (1+ a t (t amb max -25)) + rd max i pp = 50.2v v cl max = 50.2v < 55v p p = v cl max x i pp = 148.8w the 1.5ke transil datasheet (curves fig.1 and fig.3) indicates (at 50c for a duration of 30ms) a maximum dissipation of 90% x 200w = 180w so a 1.5ke36p can be used in this application. figure 2 : application diagram + - circuit to be protected vmax = 55v t ipp ipp/2 0 t=30ms p rth, t , b depend on the package. their values, assuming that the device is mounted on a printed circuit board, are grouped together in the following table. (5) a application note 2/3
information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics as sumes no responsability for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may resu lt from its use. no license is granted by impli cation or ot herwise under any pat ent 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 l ife support devices or systems wit hout 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. a application note 3/3
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