1/2 AN1768 application note ? admissible avalanche power of schottky diodes october 2003 - ed: 1 d. jouve introduction the design of switch mode power supply (smps) is subjected to ever increasing cost and efficiency constraints. one way to respond to these aggressive specifications is to use components closer to their intrinsic limits. the increasing use of schottky diodes in the avalanche area is a good example of this evolution. to help the designer to optimize the choice of the schottky diode in a rectification application, stmicroelectronics is proposing a simple tool to determine if a given st schottky diode can withstand the avalanche energy fixed by the application conditions. 0 0.2 0.4 0.6 0.8 1 1.2 25 50 75 100 125 150 175 tj (c) p (t , t ) / p (tp, 25c) versus t arm p j arm j fig. 2: avalanche power derating over tempera- ture range. 1. design rules the first step for the designer is to estimate, in the worst-case conditions, the following parameters: n operating junction temperature: tj n pulse duration of the avalanche current: tp n avalanche energy by pulse generated by the converter in the schottky diode: e ap stmicroelectronics guarantees for each schottky diode a reference avalanche power given at tp=1s and tj=25c: p arm (1s,25c) (corre- sponding to a rectangular current pulse ). table 1 gives p arm (1s,25c) for some part numbers. part number p arm (1s; 25c) per diode stps1545d (2x7.5a) 2.7 kw stps2045ct (2x10a) 4 kw stps3045ct (2x15a) 6 kw stps20h100ct (2x10a) 10.8 kw table 1: p arm (1s, 25c) values for some st schottky diodes. derating curves figure 2 and figure 3 give the ad- missible avalanche power versus tp and tj. p arm (1s, 25c) for each part number as well as the derating curves are given in the respective datasheet. the designer must ensure that the guaranteed avalanche energy e arm (tp,tj) is greater than the avalanche energy in the application e ap . p (t , t ) / p (1s, ) versus tp arm p j arm t j tp(s) 10 1 1 0.01 0.1 0.1 0.001 0.01 100 1000 10 fig. 3: avalanche power derating over pulse dura- tion range
AN1768 - application note 2/2 2. design example let us consider the use of a stps20h100ct (two 10a, 100v st schottky diodes in to-220 pack- age) used in a flyback converter ( figure 4 ). in a typical worst-case situation , the application conditions are: n operating junction temperature of the schottky diode: tj = 100c n pulse duration of the avalanche current: tp = 10ns n avalanche energy by pulse through the two diodes connected in parallel: v p = -130v, i ar = -1.5a, tp = 10ns t e ap = 1.95j ? table 1 gives: p arm (1s, 25c) stps20h100ct = 10.8 kw per diode ? figure 2 gives: ptp c ptp c arm arm (, ) (, ) . 100 25 045 = t p arm (1s,100c) = p arm (1s,25c) x 0.45 t p arm (1s,100c) = 4.86 kw ? fig.3 gives: pnst pst arm j arm j (,) (,) 10 1 1 m = t p arm (10ns,100c) = p arm (1s,100c) t p arm (10ns,100c) = 4.86 kw finally, e arm (10ns,100c) = p arm (10ns,100c)x10ns the maximum admissible avalanche energy of the stps20h100ct at 10ns and 100c is: e arm (10ns,100c) = 48.6j per diode consequently, as the guaranteed value e arm (10ns,100c) (per diode) is higher than e ap measured through the two diodes connected in parallel (48.6j > 1.95j), the stps20h100ct will withstand the avalanche energy generated by the converter. vout idiode vdiode vin fig. 4: topology of a flyback converter. i ar v r tp i diode corresponding energy in the avalanche area fig. 5: current and voltage waveforms through the two diodes. (i ar = repetitive avalanche current) figure 5 shows the corresponding current and voltage waveforms through the two diodes. information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility 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 stmicroelectronics. specifications mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics products are not au- thorized for use as critical components in life support devices or systems without express written approval of stmicroelectronics. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners. ? 2003 stmicroelectronics - all rights reserved. stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - italy - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - united states www.st.com
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