by Christine C. Lee, senior business development engineer, Toshiba America Electronic Components, Inc.
The current development trend in photocouplers is targeting smaller packages, lower cost, higher reliability, higher operating speed, and lower power consumption, which corresponds with the requirements for electronic devices in the future.
Defining the Photocoupler
The primary function of a photocoupler is to provide electrical isolation through an optical source. A photocoupler comprises both a light emitting diode (LED) and a photodetector in one package. When a voltage (current) is applied, the LED converts an electrical signal into an optical one. The photodetector receives the optical signal and converts it back into an electrical signal, forming an isolation interface in the circuit (see Figure 1).
The internal structure of the photocoupler can be generally divided into two categories - reflective and face-to-face. In reflective structures, the LED and photodetector are mounted horizontally on the same surface of the frame. When light is emitted from the LED, it passes through the transparent silicon resin and reflects to the photodetector through a barrier located between the epoxy resin and the external mold. In face-to-face structures, the LED and photodetector are mounted on separate frames facing each other (see Figure 2). During production, the reflective structure has the advantage of a single frame, enabling an increase in the capacity and efficiency when compared to the face-to-face structure.
Figure 1: The Basic Function of Photocouplers
Figure 2: Internal Structure of Photocouplers
Advantages of Photocouplers in System Design
Photocouplers are typically designed to isolate electrical output from the input in order to eliminate the noise, which was formerly a function for relay and pulse transformers. In today's technology environment, widespread usage of the microcomputer has created a number of new applications for photocouplers. The components can solve various system design issues, including the connection of different voltages, elimination of noise, and the protection of the end-user from electrical injuries. Each of these issues will be addressed throughout the article as they relate to the various photocoupler types.
Choosing a Photocoupler
Essentially, photocouplers are categorized by their output function. Photocouplers available to today's design engineers include:
phototransistor output couplers
high-speed photo IC couplers
phototriac-output couplers and photothyristor-output couplers
MOSFET-output couplers (also called photorelays)
There are many applications for photocouplers. For example, a cell phone battery charger is one of the key applications within the household equipment category, primarily because the charger functions as a switching power supply device and requires a photocoupler to stabilize the output voltage. In addition, inverters inside air-conditioners and other white goods applications utilize photocouplers because there is usually a microprocessor controlling the alternating current (a.c.) loads (e.g., inverter motor, fan motor, heater, and electromagnetic valve). If the microprocessor is directly connected to the a.c. load electrically, it may be influenced by the noise from the a.c. load, and thereby result in a malfunction. The photocoupler is able to protect the microprocessor from the noise by isolating it from a.c. loads electrically.
In computer and office automation equipment, a high-quality power supply unit is provided to supply the stabilized direct current ( d.c. ) power. The switching power supply is used in the power unit to guarantee the stabilization in general, and a photocoupler is used in the switching power supply.
Finally, industrial applications require photocouplers in many systems because the majority of automation machines require a programmable logic controller (PLC). The PLC also controls some of the a.c./d.c. loads with a microprocessor, so the photocoupler is used to protect the microprocessor. The number of control nodes in the automation machine determines the numbers of phototransistor couplers that are required.
Designing with Photocouplers
Phototransistor Coupler The transistor coupler is the most commonly used photocoupler. Figure 3 shows the typical circuit of a transistor coupler.
When designing with a transistor coupler, current transfer ratio (CTR) is an important parameter to consider. The current transfer ratio of a photocoupler indicates the rate of the output current (IC) of its phototransistor to a forward input current (IF) flowing through its LED. It is defined by:
Table 1 is an example of CTR specification on the datasheet.
Current Transfer Ratio
IC / IF
Table 1: CTR Specification for Transistor Photocouplers
After the CTR is determined, the design engineer should decide the load resistance RL by considering the latter circuit. When RL is assumed to 2,000 ohms in this case, necessary IC is expressed by:
Vce(Sat) is described in the data sheet. Vcc is determined by the designer.
After Ic is determined, IF current is determined by using Ic-IF curve, Ic-Ta curve, and CTR (Ic) degradation curve in the data sheet. The designer should add some margin into the calculated IF.
After IF is determined, input resistance Rin is calculated by:
VF: Forward voltage of LED and VOL: Output ON voltage of Logic IC.
High-Speed IC Coupler
Figure 4 shows a typical circuit of high-speed IC couplers. Compared to the transistor coupler, which is operated for analog functions, the high-speed IC coupler is easier to design due to its digital characteristics. The following is an example of circuit design for 5-MHz pulse transmission. One important parameter for high-speed IC coupler is the input current threshold (IFH). Table 2 is an example of IFH shown on the datasheet.
"H Level Output --> L Level Output" Input current
IOL=13mA, VE=2V VOL=0.6V
Table 2: Input Current Specification for High-Speed IC Couplers
IFH is defined as the value of the input current when the output node is turned on. Since the value may vary by part, only the maximum is specified on the datasheet. For example, Table 2 guarantees that if the input current is 5 mA, the output node will be turned on. The data has already guaranteed that the temperature ranges from 0°-70°C.
The designer should decide IF by considering LED degradation data Dt on the data sheet. Assuming the life coefficient Dt is 0.89, IF is calculated by:
Likewise, designers should add some margin to IF and obtain the RL using a calculation similar to that used for the transistor couplers.
Phototriac and Photothyristor Coupler
Unlike transistor couplers or high-speed IC couplers, which perform d.c. signal transmission between the input and the output sides, triac couplers can control the a.c. load at the output by a d.c. signal at the input. Because of its a.c. control features, a 400-V coupler is used for a 100-V a.c. load, and a 600-V coupler is used for a 200-V a.c. load. Figure 5 is the example of triac coupler circuit. The typical a.c. load includes solenoid bulbs, a.c. motors, and a heater. Table 3 is an example of the LED trigger current.
Coupled electrical characteristics (Ta=25°C)
Trigger LED Current
Table 3: Input Current Specification for Triac Photocouplers
IFT is defined as the value of the input current when output node is turned on. Since the value may vary by part, only the maximum is specified. For example, Table 3 guarantees that if the input current is 10 mA, the output node will be turned on. The designer should decide IF by considering the IFT-Ta curve and LED degradation data of the data sheet. Assuming temperature coefficient DTa is 1.4 and life coefficient Dt is 0.89, IF is calculated by:
Adding some margin into the calculated IF, the input resistance Rin can be obtained using the same method introduced previously.
The photorelay is a MOSFET-output photocoupler. Figure 6 shows a typical circuit of photorelay. It was named a photorelay because its operation and characteristics are the same as those of a conventional mechanical relay. When transmitting and switching small analog signals, the conventional photocoupler typically distorts the waveform of the signal due to its large offset voltage on the output side. Therefore, a mechanical relay is used in the circuit where small analog signals are controlled. However, the problems associated with mechanical relays, including reliability issues, high power consumption, and lower switching speeds, have created an increasing demand for an equivalent semiconductor device, which led to the development of photorelay devices.
The features of the photorelay when compared with mechanical relays include high reliability, space savings, high speed, low drive power consumption, and noise-free operation. In addition, it also has the advantage of both a.c. and d.c. functions, is free of errors caused by dV/dt, and has a very small leakage current with no offset voltage. Table 4 is an example of the LED trigger current.
Coupled electrical characteristics (Ta=25°C)
Trigger LED Current
Table 4: Input Current Specification for Photorelay
IFT is defined as the value of input current when output node is turned on. Again, since the value may vary by part, only the maximum is specified. The designer should decide IF by considering the IFT-Ta curve and LED degradation data of the data sheet. Assuming temperature coefficient DTa is 1.3 and life coefficient Dt is 0.89, IF is calculated as follows:
Adding some margin into the calculated IF, the input resistance Rin can be determined using the same method introduced previously.
Future Design Trends
As discussed in the article, photocouplers are designed into a wide variety of applications. The current development trend is targeting smaller packages, lower cost, higher reliability, higher operating speed, and lower power consumption, which corresponds with the requirements for electronic devices in the future.
Among the various categories of photocouplers, the applications and the demand for photorelays (MOSFET output) are expected to expand rapidly in the future. Currently, mechanical relays have a large market in various applications such as telecommunication line switches, modems, and testing equipment. Modem applications include PCMCIA cards for personal computers, ADSL modems, set-top boxes, and facsimile machines. Testing applications include memory testers, logic testers, test recorders, and other general equipment. Those applications using mechanical relays will require a photorelay because of the demand for high reliability, small packaging, and low power consumption. Most of the mechanical relays remain strong in the current market because of the cost advantage. Therefore, most photorelay suppliers are working on of the development of new semiconductor technology to drive costs down. The improved cost of the photorelay will strengthen its presence and eventually establish a leading position in the relay market.
About the Author
Christine C. Lee is a senior business development engineer for optoelectronic products at Toshiba America Electronic Components, Inc. She holds a Master's degree in Material Science from National Taiwan University and an MBA from University of California , Irvine.