AN869 APPLICATION NOTE Tj MAX LIMIT OF SCHOTTKY DIODES INTRODUCTION This application note is about the limit of Tj max given in the datasheet of SCHOTTKY rectifiers. It explains the real meaning of this parameter and why in some applications, the component can operate with a junction temperature higher than Tj max. Table 1 shows the Tj max specified in the datasheet for the different families of SCHOTTKY diodes. Table 1. Tj max versus SCHOTTKY family VRRM (V) Tj Max (°C) STPSxxL10 10 100 STPSxxL25 25 125 STPSxx45 45 150 STPSxx100 100 125 SCHOTTKY FAMILY For a classical ultra fast rectifier, this limit is equal to 150°C. For a SCHOTTKY diode this limit is lower (for instance 125°C for a STPSxxL25). This lower limit of Tj max is only due to thermal runaway phenomenon linked to the leakage current. This phenomenon is explained in the application note: "THERMAL RUNAWAY IN RECTIFIER". This application note describes the rules for the calculation of the limit before thermal instability is reached. This limit depends on the characteristics of the diodes (leakage current: IR, junction to case thermal resistance Rth(j-c) ...) and application parameters (reapplied voltage across the diodes: VR, duty cycle of the blocking of the diode: (1 - δ), thermal resistance of the heatsink: Rth(c-a) ...). The following two examples show that for the same diode (STPS10L25D) different conditions lead to different limits for Tj max. First example: Application parameters: (1-δ) = 0.5 VR = 15V Rth(c-a) = 8.5°C/W Diode parameters: (STPS10L25D): IR max (15V, 125°C) = 210mA Rth(j-c)=1.6°C/W REV. 2 May 2004 1/4 AN869 APPLICATION NOTE The limit of the reverse current at VR and Tj max before reaching thermal runaway is given by: 1 I R ( 15V ,Tjmax ) = --------------------------------------------------V R ( 1 – δ )cRth ( j – a ) Where c is a thermal coefficient c ≈ 0.055 Rth(j-a) = Rth(j-c) + Rth(c-a) We have: IR (15V, Tj max) = 242mA Tj max is given by: I R ( 15V, Tjmax ) 1 Tjmax = 125 + ---- In -------------------------------------------------C I R max ( 15V, 125°C ) Tj max = 127°C Second example: Application parameters: (1-δ) = 0.5 VR = 5V Rth(c-a) = 18.5°C/W Diode parameters (STPS10L25D): IR max (5V, 125°C) = 125mA Rth(c-a=) = 1.6°C/W We have IR (5V, Tj max) = 363mA and Tj max = 144°C CONCLUSIONS This application note shows that the maximum limit of Tj of the SCHOTTKY diodes given in the datasheet is mainly due to the thermal runaway phenomenon. This limit doesn’t only depend on diode parameters but also application parameters. In the first example corresponding to a typical application we will find Tj max given in the datasheet. The second example shows that in some applications SCHOTTKY diodes can be used with a junction temperature higher than the Tj max given in the datasheet. 2/4 AN869 APPLICATION NOTE REVISION HISTORY Table 2. Revision History Date Revision Description of Changes December-1996 1 First Issue 10-May-2004 2 Stylesheet update. No content change. 3/4 AN869 APPLICATION NOTE 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 authorized 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 © 2004 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 4/4