PS2625 datasheet(25/38 Pages) NEC

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Time (Hr)

Figure 6.

Figure 5.

CTR vs. TA Characteristics (TA: Ambient Temperature)

The CTR-Temperature characteristic is greatly affected by the total characteristics of light-emission efficiency of the LED and hFE

of the phototransistor as the light-emission efficiency has a negative temperature coefficient and hFE has a positive temperature

coefficient. See Figure 7.

Figure 7. CTR vs. TA Characteristics

Current Transfer Ratio (CTR)

The current transfer ratio (CTR) of an optocoupler is the ratio of the value of output current IC to the value of input forward current

IF (IC/IF x 100%). The CTR is a parameter equivalent to the D.C. current amplification factor hFE of a transistor.

The CTR is one of the most significant characteristics of optocouplers, as well as isolation voltage. In circuit designing, CTR must

be considered first of all because the CTR:

varies as time goes by,

is affected by ambient temperature, and

is dependent upon forward current IF flowing through the LED.

Both PS2505 and PS2506 optocouplers (bidirectional input type) have two current transfer ratios (CTRs) because they have two

LEDs in the input. For further information, refer to Applications of Optocouplers for A.C. input.

Change of CTR over time

The current transfer ratio (CTR) of an optocoupler is determined by the light-emission efficiency of the LED (emitting infrared

light), efficiency of light transmission between the LED and the phototransistor, light sensitivity of the phototransistor, and hFE of

the transistor.

The change of a CTR over time is mainly caused by reduction of the light-emission efficiency of the LED. Generally, the CTR is

reduced to a greater extent as the forward current IF increases or as the operating temperature increases. Figure 5 and 6

respectively show estimated changes of CTRs of PS2501 and PS2505 optocouplers over time.

Estimated change of CTRs with time lapse (Standard values)

CHARACTERISTICS OF PS2501 AND PS2505 OPTOCOUPLERS

Standard value

Continuous supply of 20 mA (IF)

TA = 60˚C TA = 25˚C

1.2

1.0

0.8

0.6

0.4

0.2

1.2

1.0

0.8

0.6

0.4

0.2

010

Standard characteristics

IF = 5 mA

TA = 60˚C

IF = 20 mA

TA = 25˚C

IF = 5 mA

TA = 25˚C

PS2625 Datasheet(PDF) 25 Page - NEC

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