introduction in spite of the high operating frequency, the conduction losses remain the main cause of the junction's temperature increase in the majority of the applications. therefore, it is important to accurately estimate these losses. the purpose of this note is to give data to calculate the conduction losses in the diodes. the forward characteristic of a diode is shown in fig.1 we can define two areas : 1) the peak current i m is lower than 3 i f(av) : the forward characteristic of a diode may be assimilated to a straight line defined by vto and rd (fig.1). the forward voltage can be expressed by v fm =vto+rdi fm vto and rd are given in the datasheet for each part number. with this model the expression of the conduction losses is : pcond = vto i f(av) +rdi f 2 (rms) i f(av) : average forward current in the diode i f(rms) : rms forward current in the diode fig.2 shows the average and rms values for different current wave forms. application note b. rivet ? AN604/0893 the conduction losses in a power rectifier fig.1 : forward characteristics of a diodes fm i fm v 2i f(av) i f(av) vto rd real characteristic i f (av) : average forward current of the diode fig.2 : average and rms values for different cur- rents wave forms. t t t c i m t t t i m a t t t b i m i f ( av ) = i m d 2 i f 2 ( rms ) = i m 2 d 3 i f ( av ) = 2 i m d p i f 2 ( rms ) = i m 2 d 2 i f ( av ) =d i m i f 2 ( rms ) =d i m 2 1/2
example : with a stta1206d : vto = 1.15v rd = 0.029 ohm and a rectangular current : i m =20a d =0.5 we find : pcond = 17.3w 2) the peak current i m is higher than 3 i f(av) : when the peak current i m is higher than 3 i f(av) , the forward voltage and the conduction losses values calculated with vto and rd becomes very pessimistic (fig.1). a more accurate estimation of the conduction losses can be done with the curve v fm ,i fm given in the datasheet (fig.3). in the case of a rectangular current conduction losses can be expressed by : pcond = v fm (i m )xi f(av) where v f (i m ) is the v fm value when i fm =i m example : with a rectangular current : i m =70a d = 0.1 and a stta806d v fm (70a) = 2.75v (fig.3) pcond = v fm (70a) x d xi m we find : pcond = 19w in these conditions, conduction losses calculated with vto and rd give : pcond = 29w ! conclusion this short note provides the designer with the rules to properly estimate the conduction losses in a power diode. it also highlights the limitation of the traditional forward characteristic model v f = vto + rd i f , gives a value very pessimistic at high level current. fig.3 : forward voltage drop versus forward cur- rent of a stta806d. 0.1 1 10 100 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 maximum values ifm(a) tj=125 c o vfm(v) 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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