NEC PS2601, PS2601L, PS2602L, PS2603L, PS2604 Datasheet

...

California Eastern Laboratories

Optocoupler

Applications

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 microcomputers 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.

PS2600 Series Optocouplers (6-Pin Dual-in-Line Package)

Absolute Maximum Ratings Electric Characteristics

A = 25°C) (TA = 25°C)

Product name Features

PS2601 High isolation

PS2601L voltage

Internal

connection

CEO 5 k 80 50 80 to 600 3 5

High V PS2602 (80 V MIN.) PS2602L Single transistor

PS2603 High isolation

PS2603L voltage

High CTR 5 k 80 200 200 to 2500 100 100 PS2604 Darlington PS2604L transistor

PS2605 High isolation

PS2605L voltage

A.C. input 5 k ±80 50 80 to 600 3 5

CEO

High V PS2606 (80 V MIN.) PS2606L Single transistor

PS2607 High isolation

PS2607L voltage

A.C. input 5 k ±80 200 200 to 3400 100 100

High CTR PS2608 Darlington PS2608L transistor

PS2621 High isolation

PS2621L voltage

Large input 5 k 150 50 20 to 50 3 5 PS2622 current PS2622L Single transistor

PS2625 High isolation

PS2625L voltage

A.C. input 5 k ±150 50 20 to 50 3 5

Large input PS2626 current PS2626L Single transistor

PS2633 High isolation

PS2633L voltage

CEO 1000 to

High V

(300 V MIN.) 5 k 80 150 15000 100 100

High CTR PS2634 Darlington PS2634L transitor

PS2651 High isolation

PS2651L2 voltage

CEO 5 k 80 50 50 to 400 3 5

High V

(80 V MIN.) PS2652 Single transistor PS2652L2

PS2653 High isolation

PS2653L2 voltage

High CTR 5 k 80 200 200 to 3400 100 100

Darlington PS2654 transistors PS2654L2

(Vr.m.s.) (TYP) (TYP)

BV I

F (mA) IC(mA) CTR (%) tr (µs) tr (µs)

* (with a base pin)

Note: A product name followed by letter L indicates a product having leads formed for surface mount.

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 Darlingtontransistor 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 highvoltage 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 (CC-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 toff) is low. The relationship between turn-off time toff and collector-base capacitance CC-B is expressed by:

toff × CC-B x hFE x RL ................(1)

where toff : Turn-off time (See Fig. 2-2.) CC-B : Collector-base capacitance hFE : D.C. current amplification factor RL : Load resistance

Cc-

Figure 2-1. Collector-Base Capacitance CC-B of Phototransistor

Input monitor

50%

Input

ton toff

90% 90%

Output

10% 10%

Figure 2-2. ton/toff Measuring Points

As judged from expression (1), the turn-off time toff is affected by collector-base capacitance CC-B, D.C. current amplification factor hFE, and load resistance LR. In actual circuit design, CC-B and hFE are fixed. Accordingly, the turn-off time is significantly affected by the resistance of load RL.

Graph 1 shows the relationship between response speed (ton,toff) and load resistance (RL) in typical emitter follower (test circuit 1) having a load resistance of 100 Ω.

PW = 100 µs

()

Duty = 1/10

VCC = 5 V

PS2601

IF = 5 mA

Input

monitor

51 Ω

= 100 Ω

Test Circuit 1

Graph 1

Up : Input 0.2 V/DIV DOWN : Output 0.5 V/DIV

(50 µs/DIV)

Graph 2 shows the relationship between response speed (ton, toff) and load resistance (RL) in a typical emitter follower (Test circuit 2) having a greater load resistance (5 kΩ).

= 5 V

PS2601

IF = 5 mA

Input

monitor

51 Ω

= 5 Ω

Input monitor

Test Circuit 2

Graph 2

Up : Input 0.2 V/DIV DOWN : Output 2 V/DIV

(50 µs/DIV)

As shown in Graph 2, the turn-off time for load resistance of 5 kΩ is about 100 µs. Similarly, the turn-off time for load resistance of 100 kΩ is 1 to 2 ms. This is also true when the load resistance is connected to the collector of the phototransistor. Graph 3 shows the relationship between response speed (t

on, toff) and load resistance (RL) in a typical circuit (Test circuit 3)

having collector load resistance (5 kΩ) with the emitter grounded.

= 5 V

= 5 Ω

PS2601

IF = 5 mA

Input

monitor

51 Ω

Input monitor

Test Circuit 3

Graph 3

Up : Input 0.2 V/DIV DOWN : Output 2 V/DIV

(50 µs/DIV)

Input monitor

To reduce the turn-off time toff of a test circuit having a greater resistance, insert a resistor RBE between the emitter and the base of the phototransistor. See Test circuit 4 and Test circuit 5. Graph 4 and 5 show their input and output waveforms.

= 5 V

PS2601

IF = 5 mA

Input

monitor

Insert resistor

of 200 kΩ here.

Test Circuit 4

(Emitter Follower)

51 Ω

PS2601

= 5 Ω

Graph 4

Up : Input 0.2 V/DIV DOWN : Output 2 V/DIV

(50 µs/DIV)

= 5 V

IF = 5 mA

Input

monitor

Insert resistor

of 200 kΩ here.

Test Circuit 5

(Emitter Grounded)

51 Ω

Graph 5

Up : Input 0.2 V/DIV DOWN : Output 2 V/DIV

(50 µs/DIV)

The turn-off time can be greatly reduced by the base-emitter resistance (RL). In Test circuit 4, the turn-off time of the test circuit having resistance RL is about 1/30 of that of the test circuit without the resistance. This is because the carrier (photocurrent) stored in the collector-base capacitor (CC-B) is quickly released through the base-emitter resistor (RBE). However, note that part of a photocurrent generating on the base of the phototransistor flows through the R

BE resistor and reduces the current transfer

ratio (CTR). Compare the voltage level of the output waveform in Photo 4 with that of the output waveform in Photo 2. The current transfer ratio of the test circuit having a base-emitter resistor of 200 kΩ is half or less of that of the test circuit without the resistance. (See 3.3 for reduction of the current transfer ratio CTR.) For reference, Fig. 2-3 shows the switching-time vs. R

L characteristics and Fig. 2-4 shows the switching-time vs. RBE characteris-

tics.

1000

F =

5 mA

500

51Ω

200

= 5 mA VCC = 5 V Sample Solid line: Current transfer ratio of 166%

100

Dotted line: Current transfer ratio of 274% at Ir = 5 mA

VCC = 5V

20 10

Switching Time (µs)

100 500 1 k 5 k 10 k 50 k 100 k

Load Resistance RL (Ω)

Fig. 2-3 Switching-Time vs. RL Characteristics

1000

F =

10 mA

500

IF = 10 mA Vcc = 5 V

200

Sample Solid line: Current transfer ratio of 166%

100

Current transfer ratio of 274% at Ir = 5mA

51Ω

Dotted line:

Vcc = 5V

t f

t s

20 10

Switching Time (µs)

t r

t d

100 500 1 k 5 k 10 k 50 k 100 k

Load Resistance RL (Ω)

160

Vcc = 5 V, IF = 5mA R

140

1 = 5Ω

Solid line: Emitter follower Dotted line: Emitter grounded

120

100

Switching Time (µs)

ton

100 200 500 1000

Base-Emitter Resistance RBE (kΩ)

Fig. 2-4 Switching-Time vs. RBE Characteristics

160

VCC = 5 V, IF = 10mA R

140

= 5Ω Solid line: Emitter follower Dotted line: Emitter grounded

120

100

toff

off

Switching Time (µs)

off

100 200 500 1000

Base-Emitter Resistance RBE (kΩ)

STABILIZING OUTPUT LEVELS

When an optocoupler is used with the base pin of its phototransistor open, the collector dark current (ICEO) flows as a base current. The current is amplified as a collector current and could make the output level of the phototransistor unstable. To eliminate this unwanted base current and make the output level stable, flow the collector dark current (I emitter resistor (RBE). Fig 2-5 shows the I

CEO vs. TA characteristics of a PS2601 optocoupler .

PS2601 ICEO-TA Characteristics

10000

IF = 0 V

= 80V (40V for the

PS2603) 2601

(nA)

CEO

Solid line: PS2601 Dotted line: PS2603

1000

100

RBE = 1M

=1M

Ω

CEO) through the base-

=100 M

Ω

Collector Dark Current I

0.1

- 20 0 20

40 60 80 100

Ambient T emperature TA (°C)

Figure 2-5. I

CEO vs. TA Characteristics

ELIMINATION OF INDUCED NOISE

Generally, machine-controlling equipment generates induced noise which may cause malfunctions. This unwanted noise in input signals can be isolated by means of optocouplers. However, if the noise is too strong, it may be switched to the output through the input-output capacitance C1-2 of the optocoupler. This unwanted noise in the output can be removed in the following manner. Insert a capacitor (preferably 100 pF) between the base and the emitter of the phototransistor of the optocoupler. This capacitor delays response and suppresses malfunctions.

Graph 6-(a) to 6-(d) show how an external noise (surge voltage of 1000 V/µs at rise time) is eliminated as the capacitance of the base-emitter capacitor. A fluctuation in the collector-emitter voltage caused by the on/off operation of a power switch at the output of the optocoupler causes a base current to flow through the collector-base capacitor (CCB), which causes a malfunction. In Fig. 2-7, for example, an instantaneous base current flows through the collector-base capacitor (CCB) of the optocoupler. The current is multiplied by hFE (as a collector current) and causes an output voltage on both ends of the load resistance. It seems as if an input signal was applied to the optocoupler. Graph 7-(a) shows the waveforms. This unwanted instantaneous induction current can be eliminated by inserting a capacitor CBE between the emitter and the base of the phototransistor. Graph 7-(b) shows the waveforms. Fig. 2-8 shows the output-voltage vs. CBE characteristics.

Figure 2-6.

Figure 2-7.

6a) CBE = No capacitance

Vin

6b) CBE = 10 pF

Vin

6d) CBE = 1000 pF

Vin

6c) C

BE = 100 pF

Graph 6

Up : Input Surge Voltage (Vin :1000 V/DIV) DOWN : PS2601 output (VO: 1 V/DIV)

Vin

1-2

CBE

Test Circuit

5 V

470 Ω

Vin (dV/dt = 10 V/µs, 2 V/DIV)

CCB

Vin

(500 ns/DIV)

Graph 7-(a) Input Voltage Fluctuation and Output

Vo (0.1 V/DIV)

Vin (dV/dt = 10 V/µs, 2 V/DIV)

Vo (0.1 V/DIV)

1000 pF

5 kΩ

Vin

(500 ns/DIV)

Graph 7-(b) Effect of Collector-Base Capacitance on Voltage Fluctuation

0.1

Output Voltage, Vo (V)

0.01

PS2601 R

= 5 kΩ

100 1000

Base-Emitter Capacitance, CBE (pF)

Figure 2-8. Vo vs. CBE Characteristics

As mentioned above, noise induced by the fluctuation of supply voltage can be removed by proper treatment of the base pin. For switching of input free from induced noise at normal switching speed, optocouplers with a base pin such as the PS2602 series are available. If the base pin of an optocoupler is left unused or opened, it typically will pick up external noise. Cutting off the base pin is also effective in order to prevent it from picking up external noise. See Graph 8-(b).

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