AN444 APPLICATION NOTE TRANSISTOR PROTECTION BY TRANSIL: DISSIPATION POWER AND SURGE CURRENT DURATION

Transcription

1 AN444 APPLICATION NOTE TRANSISTOR PROTECTION BY TRANSIL: DISSIPATION POWER AND SURGE CURRENT DURATION INTRODUCTION In a geat numbe of alications, we find the diagam in Figue 1 whee a Tansil is used to otect a switch which contols an inductive load. The switch can be a biola o a MOS tansisto. The uose of this ae is to calculate the dissiated owe in the Tansil and the ulse cuent duation. Figue 1. Basic Diagam L, OR CIRCUIT MODELISATION When the switch tuns off we use the equivalent cicuit eesented in Figue 2. The wost case is to conside V CL V BR min. This hyothesis will be used in all fomulas. Figue 2. Equivalent Cicuit L d V BR V CL V CL V BR min Note: V CL : claming voltage; V BR : beakdown voltage; d: aaent esistance. Ail 2004 Rev. D2A /5

2 CURRENT IN THE TRANSIL We can exess the cuent i though the Tansil by the following fomula: I is the cuent though the coil when the tansisto switches off. Figue 3 shows the cuent vaiation vesus time. Figue 3. Cuent Wavefom i V I BR min t ex L -- min V CC I t1 t -V BR t1 can be calculated by: t1 L V BR min -- min I TRANSIL POWER DISSIPATION We can conside two cases, single ulse oeation and eetitive ulses oeation. Single ulse oeation In this case, in ode to define a Tansil we need eak owe P and the ulse cuent standad duation t. P is given by P V BR min x I. If we assimilate the ulse cuent with a tiangle the standad exonential ulse duation t is calculated by the fomula: The enegy in the Tansil can be exessed by: W When tends to zeo we find: t 1.4L min min + I V BR min L [ I + V BR min min min + I W LI 2 V BR min V BR min 2/5

3 Reetitive ulses oeation In eetitive ulse oeation the owe dissiation can be calculated by the following fomula: P F V BR min L [ I + V BR min min min + I When tends to zeo we find: Whee F is the commutation fequency. EXAMPLE OF APPLICATION Commutation of a coil sulied by a battey. The diffeent aametes of the alication ae: 14V; L 10mH; 3Ω; I 4A. Tansil: 1.5KE36P V BR min 34.2V (cf data sheet). Single ulse We find: P 34.2 x W t ln t 1.08ms The data sheet gives P 1500W fo t 1.08ms then this 1.5KE36P can be used in this alication. Reetitive ulse oeation The commutation fequency is equal to 10Hz so: P [ ln P 980mW. Rth 75 C/W and Tj max 175 C. So Tj P x Rth + T amb max. With T amb max 50 C we find: Tj 0.98 x C < Tj max So we can also use this Tansil in eetitive ulse oeation. P 1 2 V BR min --LFI V BR min 3/5

4 REVISION HISTORY Table 1. Revision Histoy Date Revision Descition of Changes May Fist Issue 16-A Stylesheet udate. No content change. 4/5

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