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.

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

				Absolute Maximum Ratings			Electric Characteristics
		Internal			(TA = 25° C)		(TA = 25° C)
	Product name		Features
		connection						tr (		tr (
				BV	IF (mA)	IC(mA)	CTR (%)		s)		s)
				(Vr.m.s.)				(TYP)		(TYP)
*	PS2601		High isolation
	PS2601L		voltage
			High VCEO	5 k	80	50	80 to 600		3	5
	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		± 80
			A.C. input	5 k		50	80 to 600		3	5
			High VCEO
	PS2606		(80 V MIN.)
	PS2606L		Single transistor
*	PS2607		High isolation
	PS2607L		voltage		± 80
			A.C. input	5 k		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		± 150
			A.C. input	5 k		50	20 to 50		3	5
			Large input
	PS2626		current
	PS2626L		Single transistor
*	PS2633		High isolation
	PS2633L		voltage
			High VCEO				1000 to
			(300 V MIN.)	5 k	80	150	15000	100		100
			High CTR
	PS2634		Darlington-
	PS2634L		transitor
*	PS2651		High isolation
	PS2651L2		voltage
			High VCEO	5 k	80	50	50 to 400		3	5
			(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

* (with a base pin)

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

2

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-B

RL

Figure 2-1. Collector-Base Capacitance

CC-B of Phototransistor

3

		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 µ s	)
( Duty = 1/10
		VCC = 5 V
	PS2601
IF = 5 mA		Input
		monitor
Input	Vo
monitor
51 Ω		Ω
	RL = 100

Vo

Test Circuit 1

Graph 1

Up : Input 0.2 V/DIV

DOWN : Output 0.5 V/DIV

(50 µ s/DIV)

4

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 ).

VCC = 5 V

PS2601

IF = 5 mA

Input

Input monitor

monitor

Vo

51 Ω

RL = 5 Ω

Test Circuit 2

Vo

Graph 2
Up	: Input	0.2	V/DIV
DOWN : Output		2 V/DIV
		(50	µ s/DIV)

As shown in Graph 2, the turn-off time for load resistance of 5 kΩ is about 100 µ s. 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 (ton, toff) and load resistance (RL) in a typical circuit (Test circuit 3) having collector load resistance (5 kΩ ) with the emitter grounded.

	VCC = 5 V
	RL = 5 Ω
PS2601	Vo
IF = 5 mA			Input
			monitor
Input
monitor			Vo
51 Ω
Test Circuit 3
	Graph 3
	Up	: Input	0.2 V/DIV
	DOWN : Output		2 V/DIV
			(50 µ s/DIV)

5

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.

VCC = 5 V

PS2601

IF = 5 mA

Vo

Input monitor

51 Ω

RBE

RL = 5 Ω

Insert resistor of 200 kΩ here.

Test Circuit 4	Graph 4
(Emitter Follower)	Up	: Input	0.2	V/DIV
	DOWN : Output		2 V/DIV
			(50	s/DIV)

		VCC = 5 V
		RL = 5 Ω
	PS2601	Vo
IF = 5 mA
Input
monitor	RBE
51 Ω
Insert resistor
of 200 kΩ here.
Test Circuit 5		Graph 5
(Emitter Grounded)		Up	: Input	0.2 V/DIV
		DOWN : Output		2 V/DIV

(50 s/DIV)

Input monitor

Vo

Input monitor

Vo

6

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

1000

IF =	VCC = 5V

	500	5 mA I x	Vo
		51Ω	RL		tf
	200	IF = 5 mA VCC = 5 V				ts
s)		Sample Solid line:
	100	Current transfer ratio of 166%
		Dotted line:
(
		Current transfer ratio of 274%
Time
	50	at Ir = 5 mA
Switching
	20
	10					tr
	5
	2					td
	1
		100	500	1 k	5 k 10 k	50 k

Load Resistance RL (Ω )

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

160

Vcc = 5 V, IF = 5mA R1 = 5Ω

140Solid line: Emitter follower Dotted line: Emitter grounded

(s)	120
	100
Time				toff
Switching	80
	60
	40
			ton
	20
	0
	100	200	500	1000	8

1000			Vcc=5V
		IF=
	500	10mA I x	Vo			tf
		51Ω	RL
	200	IF =10mAVcc=5V					ts
		Sample Solid line:
s)		Current transfer ratio of 166%
	100	Dotted line:
		Current transfer ratio of 274%
(		at Ir = 5mA
Time
	50
Switching	20
	10
	5						tr
	2							td
	1
100 k		100	500	1 k	5 k		10 k	50 k	100 k
			Load Resistance RL (Ω				)
	160
		VCC = 5 V, IF = 10mA
	140	RL = 5Ω
		Solid line:		Emitter follower
		Dotted line: Emitter grounded
(s)	120
	100
Time							toff
Switching	80
	60
	40
	20						toff
	0
		100	200		500		1000		8

Base-Emitter Resistance RBE (kΩ )

Base-Emitter Resistance RBE (kΩ )

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

7

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 (ICEO) through the baseemitter resistor (RBE).

Fig 2-5 shows the ICEO vs. TA characteristics of a PS2601 optocoupler.

		PS2601 ICEO-TA Characteristics
	10000	IF = 0
(nA)		VCE = 80V (40V for the
		PS2603) 2601
		Solid line: PS2601
	1000	Dotted line: PS2603
ICEO
				RBE =8
Current	100
Dark	10
				RBE =1MΩ		RBE = 1MΩ
Collector
	1				RBE =100 MΩ
	0.1	- 20	0	20	40	60	80	100
			Ambient Temperature TA (° C)

Figure 2-5. ICEO 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/ s 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.

Vo

CBE

RL

Figure 2-6.

Figure 2-7.

8

6a) CBE = No capacitance

Vin

Vo

6d) CBE = 1000 pF

Vin

Vo

Graph 6
Up	: Input Surge Voltage (Vin :1000 V/DIV)
DOWN : PS2601 output		(VO: 1 V/DIV)

Vin

6b) CBE = 10 pF

Vin

Vo

6c) CBE = 100 pF

Vin

Vo

C1-2

5 V

Vo

CBE

470 Ω

Test Circuit

9

Vin (dV/dt = 10	V/ s, 2 V/DIV)
	CCB
	Vin
	Vo
Vo (0.1 V/DIV)	5 kΩ
(500 ns/DIV)

Graph 7-(a)

Input Voltage Fluctuation and Output

Vin (dV/dt = 10	V/ s, 2 V/DIV)	CCB
		1000 pF
Vo (0.1 V/DIV)
(500 ns/DIV)

Graph 7-(b)

Effect of Collector-Base Capacitance on

Voltage Fluctuation

Vin

Vo

5 kΩ

10

Output Voltage, Vo (V)

PS2601

RL = 5 kΩ

1

0.1

0.01

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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