In this concern, the following factors which affected the switching transient times of the proposed optoelectronic switch are tested; load resistance (RL), forward LED drive current (IF), as well bias collector voltage (VCC), emitter-base resistance (RBE) and temperature are illustrated through Figs. (5-11).
3.1. Load Resistance
The 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 Volts. 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). Based on Fig. (6a), it is clearly shown that, both the storage– and fall- times are linearly increasing function with RL rather than the delay- and rise –times. Concerning turn-on and turn-off times (Fig. 6b), it is obviously shown that, ton time is almost constant by 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). Where in the designed circuit, CCB and hFE are fixed; accordingly, toff is significantly affected by RL variations.
So, for designing a high-speed optocoupler switching, RL must be chosen to be as small as possible within the allowable rating range [19]. However, as the load resistance is minimized, a large collector current is recorded. This in turn requires a large base current, and so quickly discharges the base capacitance. This however, requires a commensurately large opto-LED current [17].
3.2. Forward LED Drive Current
Figure (7) shows the dependence of the switching transient times on the forward LED drive current (IF) variations from 1.6 mA up to 14.4 mA. It is clearly shown that (Fig. 7a), td is 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 are 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 shown to be almost constant.
Changing the forward LED drive current (IF), will affect the turn-on time (ton) and turn-off time (toff) as shown in Fig. (7b). Increasing IF lets to increase its luminous efficiency, leading to slowing down the cutoff of embedded phototransistor there for accelerating its turn-on i.e. enlargement its toff of about 28% and decrease of ton of about 76% [22]. Finally, high-speed optocoupler switch could be easily achieved by increasing IF leading to reducing its turn-on delay time [23].
3.3. Bias Voltage
Figure (8) shows the dependence of the transient switching times of phototransistor on the bias voltage (VCC), at RL=5 kΩ, and IF=10 mA. Fig. (8a) illustrates bias voltage influence on the phototransistor switching transient times. It is clearly shown that, as VCC increased from 3.0 Volts up to 5.0 Volts, both tr and td is slightly increased from 0.74 μs up to 0.84 μs. On the other hand, both ts and tf are shown to be decreased 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 the ratio of 21.0%, while toff is a decreasing function with ratio of 58.0%, respectively.
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 on its switching times. In this concern, considering Fig. (10a), the values of ts and tf, are shown to be a function of RBE. Where, for 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 are shown to be decreased 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 shown to be almost unaffected by 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 reducing the current transfer ratio [17].
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. In this concern, the dependency of the switching transient times; td, tr, ts, and tf on the ambient temperature ranges from -175 °C up to 100 °C are shown Fig. 11 (a &b), while the switching on/off times dependency are illustrated in Fig. 11(c). Based on Fig. 11(a & b), it is clearly shown that, both the switching transient times td and ts are less dependency on the ambient temperature variations rather than tr and tf times [24].
As a matter of the fact that, 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 the ambient temperature i.e., it has a positive temperature coefficient. It is to be noted that, decreasing hFE is more predominant than increasing LED efficiency [25].