PS2601 Datasheet, PDF (16/38 Pages) Clare, Inc. – AC Power Switch

REDUCTION OF CURRENT TRANSFER RATIO (CTR) BY INSERTION OF BASE-
EMITTER RESISTOR
A resistor (RBE) connected to the base and emitter pins of an optocoupler causes the reduction of the output current (reduction of
current transfer ratio). This is because a part (I1) of the base current flows through the base-emitter resistor and causes a voltage
equivalent to the emitter-base voltage (VBE). The base current is reduced by this current component (I1) and, as the result, the
current transfer ratio (CTR) goes down. The output current IC' is expressed as follows:
ICBL
RBE
ICBL-I1
VBE
I1
Figure 3-2.
IC' = hFE' (ICBL-I1) = hFE' ( ICBL- ^VBE )
RBE
( ••• IC' = hFE' • ICBL 1 - ^VBE
) ................ (4)
ICBL • RBE
Note IC'
hFE'
: Output current of an optocoupler having RBE
: Amplification factor of an optocoupler having RBE
Accordingly, the ratio of output current IC' (of the optocoupler having RBE) to output current IC (of the optocoupler with the base
open) is expressed by:
IC' hFE'
VBE
=(1-
) ................ (5)
IC hFE
ICBL • RBE
As hFE' is equal to hFE if IF = approx. 5 mA, IC = approx. 15 mA, and RBC > 100 kΩ, expression (5) is simplified as follows:
IC'
VBE
=1-
................ (6)
IC
ICBL • RBE
15