Orient transistor optocoupler introduction- Current transfer ratio (CTR) and response time

Ray Wang [email protected] www.orient-opto.com

1. Current Transfer Ratio (CTR)

The Orient transistor optocoupler (ORPC-817) current transfer ratio (CTR) is a parameter similar to the direct current transfer ratio (hFE) of a transistor and is expressed as a percentage indicating the ratio of the output current (IC) to the input current (IF).

CTR (%) = (IC/IF) x100%

CTR is a very important characteristic parameter of the optocoupler, which has the following characteristics.

* It is varied by the current input of the LED (IF)

* It varies depending on the operating temperature

* It varies with operating time (device life)

Therefore, the design must leave a margin for CTR; If the design does not allow for sufficient margin, the output Ic may be too small and cause failure.

In addition, an optical coupler with AC input capability (such as the ORPC-814) has two leds (light emitting diodes) at the input side, so each led also has a corresponding CTR.

Even IF the input is the same positive or negative value, the value of the output current IC will be different for each polarity of IF.

i.CTR depends on LED input current (IF)

CTR is strongly correlated with LED input current (IF), as shown in Figure 1, when the input current continues to increase, CTR will increase to a peak, and then fall from the peak.

FIG. 1 Change diagram of ORPC-817 optocoupler CTR-IF

It should be noted that the CTR and the input current change slope are not constant. The slope is large in the low current region (IF < 1 mA) and the high current region (IF > 20mA), and the CTR peak gradually reaches when the IF=5mA~20mA.

1. If values should be designed with sufficient margin

b. The output current IC decreases with the decrease of the IF value in the IF < 5mA current region.

c. On the contrary, in the IF=5~20mA region, even IF the IF value increases, the output current IC will not be as large as expected, so the design of the IC value should be smaller than expected.

ii.Effect of temperature on CTR

The luminous efficiency of the LED is negatively correlated with the temperature coefficient, while the hFE of the transistor is positively correlated with the temperature coefficient. Therefore, the effect of temperature on CTR is a combination of these two parameters. As shown in Figure 2, the influence of temperature on CTR is usually reflected by combining the above two temperature coefficients.

FIG. 2 Influence of temperature on CTR

The following figure shows an example of a real product.

Figure 3 Shows the temperature characteristics of ORPC-817 CTR

iii.Change of CTR with running time (lifetime)

The CTR of the optical coupler is mainly affected by the following factors:

* Luminous efficiency (Power) of LED (Light emitting diode)

* Efficiency of optical coupling between LED and optical transistor (e.g. photoconduction path)

* Photoelectric conversion efficiency and DC magnification (hFE) of transistors

Figure 4 Example of ORPC-817 CTR estimation curve with running time (lifetime) (typical value)

Figure 5 shows a graphical representation of the estimated life of the optical coupler at different LED input currents (IF) and ambient temperatures (TA).

Figure 5 Estimated life of ORPC-817 optical coupler based on CTR

2. Response time

The response time of an optocoupler is similar to that of a transistor and is expressed as follows.

tf // RL × hFE × CCB

RL: load resistance, hFE: DC amplification, CCB: capacitance between collector and base,

It can be seen from this formula that tf increases as the load resistance increases, as shown in Figure 6, so for high-speed signal transmission, the load resistance must be designed as small as possible within the allowable rating range.

Figure 6. Response time vs.. RL characteristic

However, when the load resistance is very small, the transistor may not turn on fully, causing the output signal to be unstable.

Unless the input current IF and output current IC have fully taken into account factors such as CTR specification range, temperature characteristics, and lifetime variation over time. Here are some examples of these features.

Figure 7. Response time vs.. TA characteristic

Figure 7 shows an example of how ORPC-817's response time varies with ambient temperature (TA).

Figure 8 shows an example of how the response time of ORPC-817 varies with the input current (IF).

Figure 8. Response time and IF characteristics

Figure 9 shows an example of how the response time of the ORPC-817 varies according to the power supply voltage (V CC).

Figure 9.ORPC-817 response time and VCC characteristics

Ray Wang [email protected] www.orient-opto.com