ST AN1768 handbook
Contents
1. STA AN1768 APPLICATION NOTE ADMISSIBLE AVALANCHE POWER OF SCHOTTKY DIODES INTRODUCTION D JOUVE 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 1 DESIGN RULES The first step for the designer is to estimate in the worst case conditions the following parameters Operating junction temperature Tj Pulse duration of the avalanche current tp a Avalanche energy by pulse generated by the converter in the Schottky diode Eap STMicroelectronics guarantees for each Schottky diode a reference avalanche power given at tp 1us and Tj 25 C Parm 1us 25 C corre sponding to a rectangular current pulse Table 1 gives Parm ius 25 C for some part numbers Table 1 Parm ius 25 C values for some ST Schottky diodes Part number k a rea STPS1545D 2x7 5A 2 7 kW STPS2045CT 2x10A 4 kW STPS3045CT 2x15A 6 kW STPS20H100CT 2x10A 10 8 kW Derating curves figure2. a typical worst case situation the application conditions are Operating junction temperature of the Schottky diode Tj 100 C a Pulse duration of the avalanche current tp 10ns Avalanche energy by pulse through the two diodes connected in parallel Vp 130V lar 1 5A tp 10ns gt Eap 1 95uJ e Table 1 gives Parm 1 us 25 C stps20H100cT 10 8 kW per diode e Figure 2 gives Pray to 100 C _ Pury tp 25 C Parm 1us 100 C Parm 1us 25 C x 0 45 gt Parm 1ys 100 C 4 86 kW 0 45 Parw 10nS T _ Pipu 1S 7 Parm 10ns 100 C Parm 1us 100 C gt Parm 10ns 100 C 4 86 kW e Fig 3 gives Finally Earm 10ns 100 C Parm 10ns 100 C x10ns The maximum admissible avalanche energy of the STPS20H100CT at 10ns and 100 C is Earm 10ns 100 C 48 6uJ per diode Consequently as the guaranteed value Earm 10ns 100 C per diode is higher than EaP measured through the two diodes connected in parallel 48 6uJ gt 1 95uJ the STPS20H100CT will withstand the avalanche energy generated by the converter 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
3. 2 and figure 3 give the ad missible avalanche power versus tp and Tj Parm 1us 25 C 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 Earm tp Tj is greater than the avalanche energy in the application Eap October 2003 Ed 1 Fig 2 Avalanche power derating over tempera ture range Parm tp Tj Panm tp 25 C versus Tj Tj C 25 50 75 100 125 150 175 Fig 3 Avalanche power derating over pulse dura tion range Parm tp Tj PARM 1 S Tj versus tp 10 tp us 0 001 1 2 AN1768 APPLICATION NOTE 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 Fig 4 Topology of a flyback converter Vdiode Idiode FA je Figure 5 shows the corresponding current and voltage waveforms through the two diodes Fig 5 Current and voltage waveforms through the two diodes lar repetitive avalanche current lan corresponding energy in the avalanche area KC ew In
4. 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 2 2
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ST AN1768 handbook