AN444 APPLICATION NOTE TRANSISTOR PROTECTION BY TRANSIL: DISSIPATION POWER AND SURGE CURRENT DURATION INTRODUCTION In a great number of applications, we find the diagram in Figure 1 where a Transil is used to protect a switch which controls an inductive load. The switch can be a bipolar or a MOS transistor. The purpose of this paper is to calculate the dissipated power in the Transil and the pulse current duration. Figure 1. Basic Diagram VCC L, r OR CIRCUIT MODELISATION When the switch turns off we use the equivalent circuit represented in Figure 2. The worst case is to consider VCL = VBR min. This hypothesis will be used in all formulas. Figure 2. Equivalent Circuit L r VCC rd VCL VBR VCL = VBRmin Note: VCL: clamping voltage; VBR: breakdown voltage; rd: apparent resistance. April 2004 Rev. D2A - 3582 1/5 AN444 APPLICATION NOTE CURRENT IN THE TRANSIL We can express the current i through the Transil by the following formula: V BR min – V CC V BR min – V CC t i = I p + -------------------------------------- exp – r --- + ------------------------------------- L r r Ip is the current through the coil when the transistor switches off. Figure 3 shows the current variation versus time. Figure 3. Current Waveform Ip t1 t VCC-VBR r t1 can be calculated by: V BR min – V CC t1 = – L --- ln ---------------------------------------------------- V BR min – V CC – rI p r TRANSIL POWER DISSIPATION We can consider two cases, single pulse operation and repetitive pulses operation. Single pulse operation In this case, in order to define a Transil we need peak power Pp and the pulse current standard duration tp. Pp is given by Pp = VBR min x Ip. If we assimilate the pulse current with a triangle the standard exponential pulse duration tp is calculated by the formula: V BR min – V CC 1.4L tp = – ------------ ln ---------------------------------------------------- 2r V BR min – V CC + rI p The energy in the Transil can be expressed by: V BR min – V CC V BR min – V CC V BR min ⋅ L - ln ---------------------------------------------------- [ I p + -------------------------------------W = --------------------------- V BR min – V CC + rI p r r When r tends to zero we find: V BR min 1 2 W = --- LI p --------------------------------------- V BR min – V CC 2 2/5 AN444 APPLICATION NOTE Repetitive pulses operation In repetitive pulse operation the power dissipation can be calculated by the following formula: V BR min ⋅ L V BR min – V CC V BR min – V CC P = F × ----------------------------- [ I p + --------------------------------------- ln ---------------------------------------------------- r r V BR min – V CC + rI p When r tends to zero we find: V BR min 2 1 P = --- LFI p --------------------------------------- V BR min – V CC 2 Where F is the commutation frequency. EXAMPLE OF APPLICATION Commutation of a coil supplied by a battery. The different parameters of the application are: VCC = 14V; L = 10mH; r = 3Ω; Ip = 4A. Transil: 1.5KE36P VBRmin = 34.2V (cf data sheet). Single pulse We find: Pp = 34.2 x 4 = 136.8W –3 – 1.4 ⋅ 10 ⋅ 10 34.2 – 14 tp = – --------------------------------------- ln ------------------------------------------- 34.2 – 14 + 3 × 4 2×3 tp = 1.08ms The data sheet gives Pp 1500W for tp = 1.08ms then this 1.5KE36P can be used in this application. Repetitive pulse operation The commutation frequency is equal to 10Hz so: –3 34.2 ⋅ 10 ⋅ 10 34.2 – 14 34.2 – 14 P = 10 × --------------------------------------- [ 4 + --------------------- ln ------------------------------------------- 34.2 – 14 + 3 × 4 3 3 P = 980mW. Rth = 75°C/W and Tj max = 175°C. So Tj = P x Rth + Tambmax. With Tambmax = 50°C we find: Tj = 0.98 x 75 + 50 = 123.5°C < Tj max So we can also use this Transil in repetitive pulse operation. 3/5 AN444 APPLICATION NOTE REVISION HISTORY Table 1. Revision History 4/5 Date Revision Description of Changes May-1992 1 First Issue 16-Apr-2004 2 Stylesheet update. No content change. AN444 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 5/5