Optocouplers, also known as opto-isolators, are components that transfer electrical signals between two isolated circuits by using infrared light. As an isolator, an optocoupler can prevent high voltages from affecting the side of the circuit receiving the signal. Transferring signals over a light barrier by using an infrared light-emitting diode and a light-sensitive product, such as a phototransistor, is the main structure of an optocoupler. On the first page, datasheets provide the main product description, its features, suggested areas of applications, ordering information, and agency approvals, as shown in Figure 1 for the VO617A optocoupler with phototransistor output. Following pages provide key technical specifications, operating conditions, and graphs showing the behavior of the product.
Optocoupler devices are renowned for their high reliability in the areas of isolation and safety. The safety and insulation ratings table serves as a quick reference for all key parameters the device is qualified for. The number of safety agency approvals may vary from product to product, even according to available product options. All agency certificates are available on our website at the specific product page.
Optocouplers can be stacked in parallel so that a single controlled signal, driving the infrared emitter side, may provide two separated and isolated output types. When stacking multiple couplers, the current sharing needs some consideration. The emitter infrared diode has a negative temperature coefficient, thus even when the ambient temperature is equal for all emitters, any emitter with a slightly higher junction temperature will be drawing more current then the rest. When multiple optocouplers are required in a stacked configuration, a constant current source should be considered to provide a constant forward current.
Because optocouplers are provided in a large selection of CTR groups (binning), it is advantageous to have a method for establishing a factor that can provide a quick cross comparison between all available CTR groups. We use normalization scaling to accomplish just that. Normalization is the scaling of data to a nominal condition and it is mostly done at 25 C and the coupler-specific forward current, as the graph in Figure 15 shows (NCTR at Tamb = 25 C and IF = 5 mA is 1.0).
The frequency cut-off graph of Figure 16 provides information regarding the highest effective frequency of a small AC signal that can be transmitted through the optocoupler. It is actually the frequency at which the output voltage reaches half the amplitude, which is defined at -3dB. The different curves show different load resistances.
The phase-angle sweep across the operating frequency for a given collector-emitter voltage (VCE) and load resistance (RL) provides a quick phase-angle reference for popular optocoupler applications such as SMPS (switched mode power supply),which transfers power from a source switching between low-dissipation states and minimizes the wasted energy, where the optocoupler is generally used for the feedback loop.
Datasheets also provide dynamic characteristic information, as is the case with phototransistor switching under specific conditions such as collector current (IC ) and collector-emitter voltage (VCE ), providing a sweep across load resistance (RL ). The graph in Figure 18 provides a quick reference for the tendencies of turn-on (ton ) and turn-off (toff ) switching time in microsecond units. This information, together with the data in the Figure 7 table, can provide a more accurate estimate on total switching times. If a base connection is available in the optocoupler, it can be used to adjust the switching time performance.
For new designers, an optocoupler sounds like it might have little to do with electronics, but they are important devices for providing isolation between different circuit blocks. An optocoupler IC integrates optical elements that act like a simple switch. They are easy to bring into different circuits and nicely provide isolation between circuit blocks running at different voltages. They are also ideal for use in feedback loops between different circuit blocks, especially where isolation is required. Some optocouplers are also designed to provide switching at high data rates. Here are some options for optocoupler ICs you can quickly import into your next electrical or electro-optical system.
Very simply, an optocoupler integrates an infrared LED alongside a photodetector (usually a phototransistor) and acts like an optical switch. When the LED receives an input signal, the LED turns on and supplies photons to the base of the phototransistor. This then turns on the phototransistor, allowing current to flow through a connected circuit. The LED may be running at a different level than the internal phototransistor, which allows some isolation between these two signal levels. This is one way to conduct a low voltage signal into a high voltage circuit block without using an amplifier.
Triac: An optocoupler IC with a triac as the detector is used in systems that require high output voltage/current. They have slow response speed and are best for high voltage DC systems that require high current output.
Silicon-controlled rectifier (SCR): These optocouplers also provide high gain, similar to a triac. However, they are also quite slow and are also best used for moderately high voltage/current DC systems.
Photodiode: An optocoupler with a photodiode as the detector is common in systems that need fast switching. These components can be used when the LED is switched with a stream of digital pulses or with an AC signal. A photodiode will provide a very low output-to-input current transfer ratio compared to a typical phototransistor IC.
LED forward voltage and trigger current. This tells you how you need to power your input LED to ensure it turns on and provides the desired switching behavior. In optocouplers designed to be switched with a square wave or PWM signal, the peak forward current required to trigger the switch depends on the pulse width of the signal in the ON state. Shorter pulses require larger peak signal current to force triggering.
Output-to-input current ratio. This tells you the current transfer between each end of the optocoupler. Note that this is dependent on the absolute maximum collector-emitter voltage for a phototransistor optocoupler.
The FODM611 optocoupler IC from ON Semiconductor is a single-channel optocoupler rated for up to 10 Mbps data rate (NRZ, 100 ns propagation delay). This device outputs at 5 V while offering high common mode transient noise immunity, making it ideal in industrial networks (CAN, RS485, and DeviceNet systems) or low-speed automotive systems. Switching is triggered by a photodiode connected to a buffer (see below).
The PS2802-4 quad-channel optocoupler from Renesas uses a Darlington pair phototransistor to provide high output-to-input current ratio ranging from 2 to 20 (up to 40 V rated collector-emitter voltage). This component provides 4 channels in parallel, making it useful in power management systems requiring isolation between a variety of voltages. Dark current in this component is as low as 400 nA, so very little power is wasted between switching events in a high power system. This component is also available as a single-channel variant (PS2802-1, see below).
A variety of systems can benefit from using optocoupler ICs for isolation, and you can find the components you need for your next system with the component search and filtration features from Octopart. 1e1e36bf2d