Photocouplers

What is a photocoupler?

A photocoupler is a type of optical junction element that combines a light-emitting element and a light-receiving element in one package. (Figure 1 shows an example of a cross-section of an NEC Electronics photocoupler, and Figure 2 shows a sample internal perspective diagram.) Another type of integrated product is a photo interrupter (which features a light-emitting element and a light-receiving element placed facing each other, so that object detection is performed when an object passes between the two elements and blocks off the light).
Generally, a LED is used as the light-emitting element, and a photo transistor, photo MOSFET, or photo IC is used as the light-receiving element. Depending on the light-receiving element that is used, there are types of a photo transistor output, optical MOSFET output, photo IC output, and so on.
Figure 3 shows several equivalent circuits.

<Features>
A photocoupler has the following features because of exchanging electrical signal into optical signal for transmission.

NEC Electronics manufactures a wide range of photocouplers, including photo transistor outputs, optical MOSFETs, and photo IC outputs.

Figure 1.  Photocoupler Cross-Section Diagram


Figure 2.  Sample Internal Perspective Diagram of Photocoupler


Figure 3.  Photocoupler Types

(2007/10)

There is an absolute maximum rating of the forward current (IF) on the light-emitting side of the PS2501 photocoupler, but is there also rating regarding the voltage supplied between the anode and the cathode?

The voltage supplied between the anode and the cathode of the light-emitting element is limited by the power dissipation on the light-emitting side (forward current × forward voltage).

(2007/10)

What is the CTR (Current Transfer Ratio)?

The CTR indicates the current efficiency between the input and the output in a photocoupler.
This ratio is obtained by dividing the output side current IC by the input side current IF (CTR = IC/IF × 100), and it is expressed in %.
Normally, the CTR (abbreviation of current transfer ratio) is the most important performance measure of general-purpose photocouplers.



The CTR has the following specific characteristics.
For details about these characteristics, refer to the following URL.
http://www.ncsd.necel.com/opto/english/technology/ctr/index.html

(2007/10)

Is the CTR rank of part No. PS2801-4-F3 the same as the PS2801-1?

The CTR rank applies only to the PS2801-1, and it does not apply to the PS2801-4.
Refer to the following comments in the data sheet.

(2007/10)

The PS2802-4-A is used under the operating conditions of 0 to 60°C.
I think that the VF of the primary side diode is influenced by temperature, but which values in the data sheet can be used to find mV/°C ?

It can be found by "forward current vs. forward voltage" in the characteristic curves in the data sheet.

Using these characteristic curves, first determine the forward current value (IF) to be used. Next, the forward voltage difference is obtained from the intersections of that forward current value and the curves depending on temperature, so that the mV/°C value can be obtained by dividing the forward voltage difference by the temperature difference at that time.

The value calculated with the above calculation method should be used only as a reference value.

(2007/10)

The CTR characteristic is not listed among the electrical characteristics of the PS9117A high-speed photocoupler (IC output, ultra high speed of 10 Mbps). Why is that?

The PS9117A has a function for outputting a digital signal (H/L). The relationship between the input and the output with regard to transmission of this signal is shown in the following truth table.



To change the output from "H" to "L" here, it is necessary to flow an input current of coupled/threshold input current (H → L): IFHL (TYP.: 2 mA, MAX.: 5 mA) or higher in terms of electrical characteristics. When this current is supplied, the output changes from "H" to "L".
To change the output from "L" to "H", make the input current equal to or lower than IFHL.



Since signal transfer is done by controlling the value of the input current in this way, the CTR characteristic is not needed as a design parameter and is therefore not listed.

(2007/10)

The life of a photocoupler is expressed by using current transfer rate CTR, but CTR is not defined for some products of the high-speed photocoupler (except the analog output type). In this case, by what parameter of the electrical specifications can the life be estimated?

Even the high-speed photocoupler degrades when power is applied. This degradation can be checked by using the threshold input current (I_FHL) of the electrical specifications (transfer characteristics).
As the photocoupler degrades, its output no longer changes from high to low even when a current higher than I_FHL defined by the specification flows. By this phenomenon, the life of the photocoupler can be judged as having expired.
For details, contact your nearest distributor.

[Example in Electrical Specifications of PS9115 Data Sheet]

(2008/02)

What are rise time t_r and fall time t_f?

These parameters indicate the transient change time of the output voltage when a pulsating forward current (I_F) flows into the light-emitting side and the output turns ON or OFF.

(2008/02)

What are propagation delay time (H → L) t_PHL and propagation delay time (L → H) t_PLH?

These refer to the delay time from the change of forward current until the change of output voltage, when a pulsating forward current (I_F) flows into the light-emitting side.
These two times may seem to refer to "rise time t_r" and "fall time t_f", but the definition is different. They actually refer to the time "from the change of input until the change of output."

The transfer delay time and test circuit shown on the data sheet of the PS9115 are reproduced below.

(2008/02)

What is pulse width distortion (|t_PHL - t_PLH |)?

This parameter indicates how much the signal input and propagated from the light-emitting side changes at the light-receiving side.
This change is expressed as | t_PHL - t_PLH| by using the propagation delay times.
It can be said that the lower the value of |t_PHL - t_PLH| is, the less distorted the photocoupler is, i.e., the less its pulse width changes.

[Propagation delay time and test circuit in the PS9115 data sheet]



Related FAQ:What are propagation delay time (H → L) t_PHL and propagation delay time (L → H) t_PLH?

(2008/02)

What is Power Dissipation Derating (ΔP_D/°C or ΔP_C/°C)?

This is a rate of decrease in Power Dissipation (the light-emitting side P_D and the light-receiving side P_C) when the ambient temperature rises 1°C.

In the case of the PS2502, the following data is shown as Power Dissipation Derating (of the light-emitting side) in the maximum absolute ratings in the data sheet.



This power dissipation derating of the light-emitting side (ΔP_D/°C) is shown as a linear inclination of at least 25°C on the data sheet (characteristic curve).



Power Dissipation Derating of the light-receiving side is also shown on the data sheet.

(2008/02)

The data sheet (absolute maximum ratings) of the photocoupler PS2502-4 says that the collector current (Ic) is 160 mA/ch. Is it correct to understand that a collector current of up to 160 mA is allowed to flow into the four channels at the same time?

Yes, exactly. A current of up to 160 mA is allowed to flow into each channel. When the photocoupler is used, it is also necessary to check the power dissipation (Pc) of 160 mW/ch.

(2008/02)

Ordering Information on the Data Sheet of the PS2701A-1 photocoupler shows the following descriptions of safety standards.



Does this table mean that "PS2701A-1-V", "PS2701A-1V-F3", and "PS2701A-1-V-F4" have earned the UL approval?

Yes. Most NEC Electronics' photocoupler products have obtained UL approval. Some of these products in question have also obtained VED approval (when you order them, "-V" must be suffixed to the part number).

For the products (photocouplers) that have obtained approval of standards, go to the following URL.

(2008/04)

Where can the CTR rank of the delivered "PS2801-1" be checked?

The rank marking is not shown on the package. The CTR rank is indicated by the digit following "(Z)S/NO:" on the label of the packing box.
In the following example, the first character, K, means "rank K". The other character and numeral are an NEC Electronics in-house control code.

(2008/04)

What kinds of sets can photocouplers be used in?

A major feature of photocouplers is electrical insulation between the output and the input. As illustrated in Figure 1, the absence of an electrical connection between the input and the output has the advantage of allowing linking between circuits with different GND potentials, and preventing the effect of GND noise (which could occur in the case of a shared design) due to separation of the both GNDs.
Using these merits, photocouplers can be applied to air conditioners and switching power supplies.
For specific application examples, refer to the following URL.
http://www.ncsd.necel.com/opto/english/application.html

Figure 1

(2007/10)

What are the phenomenon and the possible countermeasures for the noise generated in so-called "general-purpose photocouplers" that receive light with a phototransistor?

Compared to general transistors, general-purpose photocouplers that receive light with a phototransistor are operated in so-called "floating" state in which the base is completely free of circuit wires, so that noise may occur in unexpected situations. The following phenomena may occur.
[Phenomenon]

[Countermeasures]

For details, refer to the following URL.
http://www.ncsd.necel.com/opto/english/technology/noise/index.html#CCB

(2007/10)