AN1768 APPLICATION NOTE ® ADMISSIBLE AVALANCHE POWER OF SCHOTTKY DIODES 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. 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: EAP Fig. 2: Avalanche power derating over temperature range. PARM(tp, Tj) / PARM(tp, 25°C) versus Tj 1.2 1 0.8 STMicroelectronics guarantees for each Schottky diode a reference avalanche power given at tp=1µs and Tj=25°C: PARM(1µs,25°C) (corresponding to a rectangular current pulse ). Table 1 gives PARM(1µs,25°C) for some part numbers. Table 1: PARM(1µs, 25°C) values for some ST Schottky diodes. Part number PARM(1µs; 25°C) per diode STPS1545D (2x7.5A) 2.7 kW STPS2045CT (2x10A) 4 kW STPS3045CT (2x15A) 6 kW STPS20H100CT (2x10A) 10.8 kW 0.6 0.4 0.2 0 25 50 75 100 125 150 Tj (°C) 175 Fig. 3: Avalanche power derating over pulse duration range PARM(tp, Tj) / PARM(1µs, Tj) versus tp 10 tp(µs) 1 Derating curves figure 2 and figure 3 give the admissible avalanche power versus tp and Tj. PARM(1µs, 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 0.01 0.1 1 10 100 1000 0.1 0.01 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 package) used in a flyback converter (figure 4). Fig. 4: Topology of a flyback converter. Vdiode Idiode Vout Vin In a typical worst-case situation, the application conditions are: n Operating junction temperature of the Schottky diode: Tj = 100°C n Pulse duration of the avalanche current: tp = 10ns n Avalanche energy by pulse through the two diodes connected in parallel: VP = -130V, IAR = -1.5A, tp = 10ns ⇒EAP = 1.95µJ • Table 1 gives: PARM(1µs, 25°C)STPS20H100CT = 10.8 kW per diode • Figure 2 gives: Figure 5 shows the corresponding current and voltage waveforms through the two diodes. Fig. 5: Current and voltage waveforms through the two diodes. (IAR = repetitive avalanche current) PARM (tp ,100 °C ) = 0.45 PARM (tp ,25 °C ) ⇒PARM(1µs,100°C) = PARM(1µs,25°C) x 0.45 ⇒PARM(1µs,100°C) = 4.86 kW • Fig.3 gives: IDiode PARM (10ns ,Tj ) =1 PARM (1µs ,Tj ) ⇒PARM(10ns,100°C) = PARM(1µs,100°C) ⇒PARM(10ns,100°C) = 4.86 kW tp IAR VR corresponding energy in the avalanche area 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) = 48.6µJ 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 (48.6µJ > 1.95µJ), 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 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. © 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