AN587 Datasheet, PDF (1/3 Pages) STMicroelectronics – TRANSISTOR PROTECTION BY TRANSIL

AN587
®
APPLICATION NOTE
TRANSISTOR PROTECTION BY TRANSIL™
B.RIVET
INTRODUCTION
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.
Fig. 1: Basic diagram
+Vcc
L, r
or
V_CL
Clamping voltage
V_BR
Breakdown voltage
2. CURRENT IN THE TRANSIL
We can express the current i through the
TRANSIL by the following formula:
( ) ⁱ
=


I
P
+
V_BR
min
r
− V_cc


exp
−r ×
t
L
+


V
cc
− V_BR
r
min


I_p is the current through the coil when the
transistor switches off. Fig. 3 shows the current
variation versus time.
1. CIRCUIT MODELLING
When the switch turns off we use the equivalent
circuit represented in Fig. 2.
The worst case is to consider V_CL =V_{BR min}.
This hypothesis will be used in all formulae.
Fig. 3: Current Waveform
i
IP
Fig. 2: Equivalent Circuit
L
r
t
VCC - VBR
t1
r
Vcc
VBR
VCL = VBRmin
t₁ can be calculated by:
t₁
=
−^L
r
×
In


V_{BR min} − V_cc


^V_{BR min} ^{− V}_cc ^{+ rI} _p 
August 2001
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