PS2625 datasheet(14/38 Pages) NEC

14 / 38 page

ELIMINATION OF INPUT SURGES

Unwanted external noise and output leakage currents (e.g., collector current IC) of a preceding transistor may cause the light-

emitting diode (LED) of an optocoupler to light involuntarily. Usually, a circuit (connecting a resistor in parallel to the LED) is

provided to absorb such input surges. To prevent malfunction of an optocoupler, it is also effective to insert a resistor (RBE) that

increases the input threshold current (by the use of the input-output characteristics) between the base and the emitter of the

phototransistor. In this case, the current transfer ratio (CTR) must be low. (See 3.3 for Reduction of CTR.)

RBE =

200 k

Ω

100 k

Ω

50 k

Ω

30 k

Ω

20 k

Ω

10 k

Ω

5 k

Ω

VCE = 5 V

(PS2601)

4 5

30 40 50

Figure 2-9. IC vs. IF Characteristics (Example)

Forward Current IF (mA)

APPLICATION TO HIGH POTENTIAL CIRCUIT

The withstanding voltage between the collector and the emitter of the PS2601 optocoupler is 80 V (MAX). To make the

optocoupler available to higher withstanding voltages, use the collector-base junction photodiode as a light-sensitive element and

connect a high-voltage circuit to the output of the optocoupler. In this case, the output of the photodiode must be amplified

because it is smaller than the usual output.

Fig. 2-10 shows an example of an optocoupler applied to a high-voltage circuit. In this sample circuit, the photocurrent (ICBL) of

the optocoupler is fed to the base of the high-voltage transistor and a current (IF) passes forward through the light-emitting diode

(LED). Fig. 2-11 shows the ICBL vs. IF characteristics. Before working on applications outside the rated values of the

optocouplers, evaluate the practical circuits fully by contacting CEL.

Figure 2-10. Application to a

High Voltage Circuit

Figure 2-11. ICBL vs. IF Characteristic

PS2601

ICBL

200

100

4 5

30 40 50

VCB = 100V

(PS2601)

ICBL

CTR = 274%

CTR = 166%

100V

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