DESIGNING FOR OPTOCOUPLERS WITH BASE PIN
GENERAL
Optocouplers (optical couplers) are designed to isolate electrical output from input for complete elimination of noise. They have
been used conventionally as substitutes for relays and pulse transformers. Today's current technology in the area of microcom-
puters creates new applications for optocouplers.
This manual describes the characteristics of typical optocouplers. Also included are notes on designing application circuits for
typical optocouplers (with a base pin) for better comprehension. NEC's typical optocouplers with or without base pins are listed
on the following pages.
1
There are two kinds of optocouplers (a light emitting diode (LED) as an input and a phototransistor as an output) according to the
type of output transistor: Single transistor type and Darlington-transistor type.
The single-transistor type optocouplers are used to perform high-speed switching (with high-speed response). The Darlington-
transistor type optocouplers are used to obtain a large output current by utilizing a small input current (independently of switching
speeds).
Designing the circuits properly will improve the PS2601 optocoupler (Single Transistor type) by having a base pin in terms of
switching speed, elimination of noise in input signals, and output leakage current (collector dark current, and application to high-
voltage circuits).
APPLICATIONS OF OPTOCOUPLER BASE PINS
INCREASING SWITCHING SPEED
The switching speed of an optocoupler with a base pin can be increased by inserting a resistor between the base and the emitter
of its phototransistor even when the optocoupler is applied to a large load resistance.
Generally, the phototransistor of an optocoupler such as the PS2601 has a large photo-sensitive area on it. Accordingly, the
junction capacitance (C
C-B
) between the collector and the base of the phototransistor is great (up to 20 pF) and as a result its
response speed (turn-off time t
off
) is low. The relationship between turn-off time t
off
and collector-base capacitance C
C-B
is
expressed by:
t
off
×
C
C-B
x h
FE
x R
L
................(1)
where
t
off
: Turn-off time (See Fig. 2-2.)
C
C-B
: Collector-base capacitance
h
FE
: D.C. current amplification factor
R
L
: Load resistance
Cc-
B
R
L
Figure 2-1. Collector-Base Capacitance
C
C-B
of Phototransistor
3
50%
Input
ton
90%
90%
toff
Output
10%
10%
Figure 2-2. t
on
/t
off
Measuring Points
As judged from expression (1), the turn-off time t
off
is affected by collector-base capacitance C
C-B
, D.C. current amplification
factor h
FE
, and load resistance L
R
. In actual circuit design, C
C-B
and h
FE
are fixed. Accordingly, the turn-off time is significantly
affected by the resistance of load R
L
.
Graph 1 shows the relationship between response speed (t
on
,t
off
) and load resistance (R
L
) in typical emitter follower (test circuit 1)
having a load resistance of 100
Ω.
(
PW ==100
µs
)
Duty 1/10
V
CC
= 5 V
PS2601
I
F
= 5 mA
Input
monitor
Vo
Input
monitor
51
Ω
R
L
= 100
Ω
Vo
Test Circuit 1
Graph 1
Up
: Input 0.2 V/DIV
DOWN : Output 0.5 V/DIV
(50
µs/DIV)
4
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