PS2625 datasheet(16/38 Pages) NEC

16 / 38 page

ICBL

ICBL-I1

RBE

VBE

( 1 -

) ................ (5)

As hFE' is equal to hFE if IF = approx. 5 mA, IC = approx. 15 mA, and RBC > 100 k

Ω, expression (5) is simplified as follows:

= 1 -

................ (6)

IC' = hFE' (ICBL-I1) = hFE'

( ICBL-

)

IC' = hFE' • ICBL

( 1 -

) ................ (4)

Note IC'

: Output current of an optocoupler having RBE

hFE'

: 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:

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:

VBE

RBE

ICBL • RBE

VBE

hFE

ICBL • RBE

IC'

hFE'

VBE

ICBL • RBE

IC'

VBE

Figure 3-2.

•

• •

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