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APPLICATION NOTE® TRANSISTOR PROTECTION BY TRANSIL™ August 2001 B.RIVET In a large number of applications, we find the circuit in Fig. 1 where a TRANSIL™ is used to protect a switch that 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. INTRODUCTION L, r +Vcc or Fig. 1:Basic diagram When the switch turns off we use the equivalent circuit represented in Fig. 2. The worst case is to consider VCL =VBR min. This hypothesis will be used in all formulae. 1. CIRCUIT MODELLING Vcc L r V= VCL BRmin VBR Fig. 2:Equivalent Circuit VCL Clamping voltage VBR Breakdown voltage We can express the current i through the TRANSIL by the following formula: ( )iI VV r r t LP BR cc=+ − −×min exp + − VV r cc BR min Ip is the current through the coil when the transistor switches off. Fig. 3 shows the current variation versus time. 2. CURRENT IN THE TRANSIL i IP r V- VCC BR t1 t Fig. 3:Current Waveform t1 can be calculated by: t L r In VV VV r I BR cc BR cc p 1 =− × − min min
In repetitive pulse operation the power dissipation can be calculated by the following formula: PF VL r BR=× × ×min I VV r In VV VV r I P BR cc BR cc BR cc P + − − min min min When r tends to zero we find: PF L I V VV P BR BR cc 1 2 min min Where F is the switching frequency. 3.2. Repetitive pulse operation We can consider two cases, single pulse operation and repetitive pulse operation. 3. TRANSIL POWER DISSIPATION In this case, in order to define a TRANSIL we need the peak power PP and the pulse current standard duration tP. Pp is given by:PV IPB R P=× min If we approximate the pulse current with a triangle, the standard exponential equivalent pulse duration tp is calculated by the formula: t L r In VV VV r I P BR cc BR cc P =− × − 14 . min min The energy in the TRANSIL can be expressed by: W VL r BR= × ×min I VV r In VV VV r I P BR cc BR cc BR cc P + − − min min min When r tends to zero we find: WL I V VV P BR BR cc 1 2 min min 3.1. Single pulse operation A typical application would be a switched coil supplied by a battery. The different parameters of the application are: V CC = 14V, L = 10mH, r = 3, IP =4 A TRANSIL: SM6HT36A ,VBR min = 34.2V 4. EXAMPLE OF APPLICATION We find PWP =× =32 4 4 136 8.. tI nP = −× × −+ × −14 10 10 34 2 14 34 2 14 3 4 3.. tm sP = 108. The datasheet gives PP ≈ 450W for tP = 2ms, so the SM6HT36A can be used in this application. 4.1. Single pulse The switching frequency is equal to 10Hz so P =× ×× × 10 3 4 21 01 0 4 34 2 14 34 2 14 34 2 14 3 4 + − − −+ × In Pm W= 980 We have: Tj= P.Rth(j-a)+T amb max With Tamb max =5 0°C, Rth(j-a)= 75°C/W (with copper surface under each lead equal to 2.5cm 2) we find: Tj= 0.98 x 75 + 50 = 123.5°C Tj max beeing equal to 175°C we can also use this TRANSIL in repetitive pulse operation. 4.2. Repetitive pulse operation
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