1 Introduction

Modern science has led to a new lifestyle with embedded microelectronic devices such as optocouplers almost everywhere in daily life (Sun et al. 2011) Optocouplers are characterized by the following features; noise isolation, small size, rugged construction, and high voltage, so they are used in many applications (Yao et al. 2007; Abd El-Basit et al. 2017). It is a device that can transfer an electrical signal across two galvanically isolated circuits by way of a short optical transmission coupling (Chen et al. 2014; Yashaswini and Rashmi 2022). From the data sheet, the basic design of an optocoupler type 4N25 (Fig. 1), consists of two parts; a gallium arsenide infrared embedded light emitting diode (LED), that produces infrared light which is optically coupled to an embedded monolithic silicon phototransistor detector, which is used to detect the emitted infrared beam (van Drieënhuizen and Wolffenbuttel 1992; Konczakowska et al. 2007; Liu et al. 2014; Lakkireddy and Mathe 2022), as shown in Fig. 1a. Both LED and a monolithic silicon phototransistor detector (Motorola, Inc. 1995) are enclosed in a light-tight body or package with metal legs for electrical connections (Sachin et al. 2019; Katke and Rangdal 2015) as shown in Fig. 1b. The main function of a light emitting diode (transmission medium) is to convert electrical signals into light signals, which are detected by the phototransistor (optical receiver), and then converted back into electrical signals. The output current of the electrical signals is a function of the falling light signal intensity (Reed et al. 2004; Wey et al. 2013; Liu et al. 2014).

Fig. 1
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Optocoupler device a electrical symbol of 4N25 and b it’s typical IC pinout

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Unlike transformers or capacitors, which can only transfer AC signals across the isolation barrier, optocouplers can transfer both DC and AC signals. This makes them very popular in many applications including, isolated power supplies or isolated communication interfaces, space, nuclear facilities, medical and commercial radiologic systems. The main application of optocouplers is used in switching action and data communications (El‑Hageen 2019; Eleazar 2023).

1.1 Isolated electronic switch

In some applications such as motor control, relays, and communication interfaces, the optocoupler is used as an isolated electronic switch, where the phototransistor is driven into conduction and blocking states based on the embedded LED control signal. However, the embedded phototransistor cannot follow the changes in LED current instantaneously because there is a time delay until its collector-emitter voltage starts to change, and it does so at a specific slew-rate as its parasitic capacitance charges or discharges. The most important technical performance parameters of the optocoupler device are the transmission-time characteristics, which includes both turn-on (ton) and turn-off (toff) times as shown in Fig. 2.

Fig. 2
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Optocoupler transmission-time characteristic

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The embedded phototransistor is turned ON by receiving photons from the embedded LED. Phototransistors normally have a large junction to obtain a large photosensitive area, and this brings an associated large internal base–collector capacitance (CCB) as shown in Fig. 2, from the data sheet, the typical value of CCB for an optocoupler type 4N25 is 19 pF.

Once the transistor has been turned ON and then turned OFF, this requires the discharging of CCB, the only discharge path is through the phototransistor base, leading the phototransistor to be turned ON. The large base–collector capacitance is magnified by the transistor gain (hFE). The net effect is that there is usually a fast response of turn ON for LED and phototransistor, but a slow turn OFF while the phototransistor collector–base capacitance is discharged by taking a long time to remove the minority carriers by the recombination process (James 2013; Ahn et al. 2022). The relationship between toff and collector–base capacitance (CCB) is expressed (Renesas Electronics Corporation 2023) by Eq. (1):

$$ t_{off} \propto \,C_{CB} \, \times h_{FE} \, \times R_{L} $$
(1)

where:

toff: turn-off time.

CCB: collector-base capacitance.

hFE: dc-current amplification factor.

RL: load resistance.

The time response of an optocoupler device depends mainly on the embedded output phototransistor. It also depends on the input forward LED drive current and load resistance. Because the load resistance has a greater influence on the time response, careful setting is required when selecting the circuit elements (Godse and Bakshi 2008).

It is well known that, each electronic device generates a certain time delay between its input and output signals, as well as a certain amount of amplitude distortion. Therefore, the present study sheds further light on the factors affecting the switching transient times of a 4N25 optocoupler, which is a popular type, whenever operated under operating conditions such as; the load resistance (RL), forward LED drive current (IF), bias voltage (VCC), base-emitter resistance (RBE) and ambient temperature.

2 Experimental setup and methodology

The experimental setup for a typical optoelectronic switching test circuit in a common-emitter configuration is shown in Fig. 4a. For its operation, an input square wave with an amplitude of 0.5 Vp-p and frequency 1 kHz, IF = 10 mA (read from the ampermeter), is applied to the embedded LED in the optocoupler using a function generator type PM5705. A constant bias VCC = 5 Volt is supplied using a regulated DC power supply, Model type PW36-1, manufactured by Kenwood, at a load RL = 5 kΩ. Use a Tektronix Digitizing Oscilloscope, Model TDS420A to simultaneously capture the waveform at the phototransistor collector and compare it to the input square wave from the generator. Record the transient times of the optoelectronic switch (delay (td)-, rise (tr)-, in addition to storage (ts)- and fall (tf)-times) of the output voltage waveform (Vout), where delay- and rise-times corresponding to the time it takes for the output voltage waveform to rise from 10 to 90% of its final value, and conversely, to storage- and fall- times from 90 to 10% of Vout for the fall time. The total on and off times (ton and toff) correspond to the time elapsed from the moment the LED current starts to change until Vout reaches 90% (turn-on) or 10% (turn-off) of its final value (Fig. 3), respectively. The transient times of the optoelectronic switch were investigated under the influence of different operating circuit condition variations (RL, IF, and VCC).

Fig. 3
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Phototransistor internal collector–base capacitance (CCB)

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In addition, the transient times of the optoelectronic switch were studied by inserting a base-emitter resistor (RBE) into the embedded phototransistor (Fig. 4b). In this concern, all the previously mentioned transient times are plotted as functions of RBE.

Fig. 4
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Principle of photoelectric switch a without- and b with-emitter–base resistor

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The work is extended to study the optoelectronic switching transient times under the influence of different temperature levels, within the range of − 175 °C up to 100 °C, while maintaining the RBE at 200 kΩ. The influence of low temperature levels (− 175 °C up to room temperature) was investigated using a liquid nitrogen cooling system (El-Ghanam and Basit 2011). On the other hand, for the effect of high temperature (up to 100 °C), an electric oven is used (Abd El-Basit et al. 2017). An accurate digital thermometer with a thermocouple fixed in a liquid nitrogen cooling system and an electric oven was used for temperature monitoring. For both the low-and high-temperature experiments, to ensure temperature stability, the samples were kept at each temperature level for more than 20 min before measurements were taken at every test temperature.

3 Results and discussions

In this regard, the following factors that affect the switching transient times of the proposed optoelectronic switch were tested: load resistance, forward LED drive current, bias collector voltage, emitter–base resistance, and temperature.

3.1 Load resistance

A snapshot of the input/output voltage waveforms, is plotted (Fig. 5) at different RL values ranging from 100 Ω up to 120 kΩ, while keeping IF = 10 mA and VCC = 5.0 V. From this figure, the switching transient times; turn-on (ton) including, delay (td)-, rise (tr) times and turn-off (toff) including, storage (ts)- and fall (tf)-times, are calculated and presented in Fig. 6 and Table 1. Based on Fig. 6a, it is clearly shown that both the storage and fall times are linearly increasing functions with RL rather than the delay—and rise—times. Concerning turn-on and turn-off times (Fig. 6b), it is obvious that ton time is almost constant with increasing RL. On the other hand, toff is linearly increasing function with increasing RL, which may be attributed to its dependency on the following; CCB, hFE, and RL (Eq. 1). In the designed circuit, CCB and hFE are fixed; accordingly, toff is significantly affected by RL variations.

Fig. 5
figure 5

Influence of RL variations on the Snapshot of the optocoupler switch voltage wave-forms operated at frequency = 1 kHz, VCC = 5 Volt and IF = 10 mA

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Fig. 6
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Dependence of switching transient times on the load resistance, plotted at IF = 10 mA and VCC = 5 Volt a transient switching times and b turn-on/off times

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Table 1 Dependence of transient switching times and turn-on/off times on load resistance values
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Therefore, for designing high-speed optocoupler switching, RL must be chosen to be as small as possible within the allowable rating range (Renesas Electronics Corporation 2023). However, as the load resistance is minimized, a large collector current is recorded. This, in turn, requires a large base current, and thus quickly discharges the base capacitance. However, this requires a commensurately large opto-LED current (James 2013).

3.2 Forward LED drive current

Figure 7 shows the dependence of switching transient times on forward LED drive current (IF) variations from 1.6 mA to 14.4 mA. It is clearly shown that (Fig. 7a), td slightly decreased from 0.52 µs (IF = 1.6 mA) down to 0.496 µs (IF = 14.4 mA), while, tr is decreased exponentially from 3.48 μs down to 0.464 μs (IF = 14.4 mA). On the other hand, both ts and tf increased linearly from 20 μs and 44 μs (IF = 1.6 mA) up to 25 μs and 57 μs (IF = 5 mA), respectively. For higher IF, it is noticed that both ts and tf are almost constant.

Changing the forward LED drive current affects the turn-on time (ton) and turn-off time (toff), as shown in Fig. 7b. Increasing IF increases its luminous efficiency, leading to a slowing down of the cutoff of the embedded phototransistor, accelerating its turn-on i.e., an enlargement of its toff of about 28% and a decrease of ton of about 76% (Zhang et al. 2017). Finally, a high-speed optocoupler switch could be easily achieved by increasing IF leading to a reduction in its turn-on delay time (Venkateswaran et al. 2009).

Fig. 7
figure 7

Switching transient times dependence on the forward embedded LED drive current, at RL = 5 kΩ, and VCC = 5 Volt a transient switching times and b turn-on/off times

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3.3 Bias voltage

Figure 8 shows the dependence of the transient switching times of the phototransistor on the bias voltage (VCC), at RL = 5 kΩ, and IF = 10 mA. Figure 8a illustrates the influence of the bias voltage on the phototransistor switching transient times. It is clearly shown that, as VCC increased from 3.0 up to 5.0 Volts, both tr and td slightly increased from 0.74 μs up to 0.84 μs. On the other hand, both ts and tf decrease exponential from 31.1 μs and 56.0 μs down to 22.0 μs and 34.0 μs, respectively. For higher VCC values greater than 5.0 Volt, it is clear shown that, td, tr, ts, and tf are almost constant. Accordingly (Fig. 8b); it is clearly noticed that, ton is an increasing function of VCC with a ratio of 21.0%, while toff is a decreasing function with a ratio of 58.0%, respectively.

Fig. 8
figure 8

Phototransistor transient switching times dependence on the bias voltage, at RL = 5 kΩ, and IF = 10 mA a transient switching times and b turn-on/off times

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3.4 Controlling base-emitter resistance

Figure 9 shows the screen shots of the input/output voltage waveforms of a typical switching circuit (Fig. 4) based on the tested optocoupler, when its base pin of its opened i.e. RBE = ∞ (Fig. 9a) and with RBE = 200 kΩ (Fig. 9b). The dependence of the switching times on RBE is illustrated in Fig. 10. It is clearly observed that, insertion of RBE leads to a pronounced improvement in its switching times. In this concern, considering Fig. 10a, the values of ts and tf, are shown to be a function of RBE. For the open base-emitter condition (RBE = ∞), the reported values of ts and tf are 25 µs and 57 µs, respectively. On the other hand, at RBE equals 200 kΩ, their values decrease down to 14.0 µs and 22.3 µs. For lower RBE values, down to 12.5 kΩ, the times are shown to be unaffected. While, it is noticed that both td and tr are almost unaffected by the insertion of RBE values.

The turn-off time can be greatly reduced by the base-emitter resistance (RBE), as shown in Fig. 10b. This is because the carrier (photocurrent) stored in the internal collector–base capacitor (CCB) is quickly released through the base-emitter resistor (RBE). However, part of the generated photocurrent follows through RBE and hence reduces the current transfer ratio (CTR) (James 2013).

Fig. 9
figure 9

Screen shots of the optocoupler switch input/output voltage waveforms dependence on the controlling base-emitter resistance at frequency of 1.0 kHz a RBE = ∞ and b RBE = 200 kΩ

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Fig. 10
figure 10

Dependence of the operating switching times on REB, plotted at RL = 5 kΩ, IF = 10 mA and VCC = 5 Volt a transient switching times and b turn-on/off times

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3.5 Ambient temperature

In the present part of the work, a trial has been carried out to shed further light on the effect of ambient temperature on the switching characteristics of the proposed optocoupler devices. From the data sheet, the operating range of the ambient temperature is -55 °C up to + 100 °C. In this concern, the dependency of the switching transient times; td, tr, ts, and tf, on the ambient temperature ranges studied from -175 °C up to 100 °C are shown Fig. 11a, b, while the switching on/off times dependency is illustrated in Fig. 11c. Based on Fig. 11a, b, it is clearly shown that, both the switching transient times td and ts are less dependent on the ambient temperature variations than the tr and tf times (Ranbir and Baliga 1996).

As the ambient temperature decreases lower than the room temperature, the embedded LED becomes more efficient, thereby providing more photocurrent to force the phototransistor further into saturation. At the same time, the phototransistor gain (hFE) reduces with decreasing ambient temperature i.e., it has a positive temperature coefficient. It should be noted that, decreasing hFE is more predominant than increasing LED efficiency (Vishay Secmiconductors 2012).

Fig. 11
figure 11

Optocoupler switching times dependency on the ambient temperature variations a delay time & rise time, b storage time & fall time and c turn-on/off times

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4 Conclusions

From the experimental studies, the analysis of its results, and discussions, it could be concluded that the switching time speed of an optoelectronic switch based on an optocoupler of type 4N25 is closely affected by the load resistance, forward LED drive current, bias collector voltage, controlling base-emitter resistance, and ambient temperature. The experimental results concluded that, the more sensitive transient time to these factors was the turn-off time than the turn-on time, i.e. the turn-on time appeared much faster than turn-off time. However, obtaining a faster switching speed using a lower load resistance requires a large forward LED current. Increasing IF reduces turn-on time and increasing the turn-off time. In addition, increasing the bias voltage (VCC) up to 5 Volts lead to an increase in the turn-on time and a decrease in the turn-off time. For higher VCC values, turn-on time and turn-off times were almost constant. Additionally, by inserting a resistor between the base and emitter (RBE), an extra terminal is added and the phototransistor base is outside the optocoupler package, which increases the switching speed of the proposed photoelectronic switch.

Finally, lowering the temperature from 100 °C down to -175 °C could be used to improve the switching times of the imbedded phototransistor for application to high-frequency switches. So, optocoupler devices are suitable for space applications.