The photocoupler is coming of age as Matthias Diephaus, Senior Manager of Opto Semiconductors at Toshiba Electronics Europe explores here with advancing technology solutions to keep apace with market demands.
Photocouplers are today the solution of choice for a wide range of applications in which designers need to establish electrical isolation between one electronic circuit and another. With all the familiar advantages of solid state technology, they have rapidly displaced more traditional solutions based on electromechanical or wound components, and are enabling rapid development of new industrial and consumer products that would have been impractical just a few years ago.
Like all devices, photocouplers continue to develop and improve. The market is demanding increased performance in terms of switching speed, isolation voltage, and power consumption. Designers also require miniaturisation and integration, allowing them to improve the feature set of their end products while at the same time reducing costs.
The exact evolution path for these components is very much dependent on the type of application. Motor controllers, power supply electronics, factory automation, data communications and general-purpose switching-type applications all have their distinctive requirements.
One significant change over the last several years has been the emergence of a number of new application areas. Clean energy installations – primarily solar and wind – use inverters that require photocouplers. These have their own particular requirements; as do other emerging markets such as automotive, security, smart metering, and home appliances such as induction cookers.
A parallel consideration to the application area is the exact role of the device in the end system, for instance, an intrinsic part of the photocoupler’s function may be to toggle between on- and off- states – a job that might in the past have been performed by a switch or relay. In such circuit functions, the focus for the designer will probably be on parameters such as switching speed and input threshold.
In contrast, an IGBT driver can be considered to be more like a signal conversion device, allowing the control of high currents and voltages by logic level input signals. Key specifications therefore include voltage rating, current transfer characteristic and channel count.
In almost every case, the photocoupler is also used to provide a safety and protection function. It prevents power circuits from damaging sensitive electronics; reduces the transmission of electrical noise between the two parts of the circuit; and keeps users safe from injury.
Looked at from this point of view, the relevant device-specific features are isolation voltage, creepage and clearance. In general, international standards for these quantities only move in one direction, with specifications becoming more exacting with every passing year.
Other evolutionary pressures are common across the entire range of application and function. As with all of today’s electronic devices, there is a continuing quest to improve energy efficiency. As noted, there is also a drive towards miniaturisation in order to reduce system cost and allow more features to be crammed into the same space. Hand-in-hand with the latter trend goes the move to integrate more functionality in each component.
To provide even greater space savings, surface mount packaging is becoming increasingly common.
Design pressures in action
A photocoupler such as Toshiba’s TLP352 high-current gate drive photo-IC exemplifies many of these trends. Designed to directly drive mid-capacity IGBTs or power MOSFETs, the device is intended for use in AC servo amplifiers, industrial control, domestic solar power systems, digital home appliances and induction heating products. It is ultra-compact yet integrates an entire gate drive subsystem into a single package while conforming to industry standards for creepage, clearance and isolation voltage.
As the photocoupler offers a low maximum delay time and a high degree of timing consistency between devices, it allows engineers to design inverter circuits with excellent energy efficiency. The device itself features a low maximum input current of 5mA, reducing system power consumption.
An even smaller high-speed photocoupler is also available that combines GaAlAs LEDs with high-gain photodetectors in an SO6 surface mount package.
It is primarily intended for use in robotics and motion control applications and features a maximum switching time dispersion of only 400ns, making it suitable for the control of intelligent power modules (IPMs).
The TLP104 offers isolation voltage rating of 3750V, satisfying the safety requirements of international standards. It is integrated physically, as well as electrically, with an internal Faraday shield conferring minimum common mode transient immunity of ±15kV/µs.
The two solutions are both good illustrations of many of the design trends in the photocoupler market. They also exemplify another change: the move towards extended temperature range. With operation from -40°C to 125°C, the two devices can be used in highly challenging environments.
While such a specification has traditionally been associated with industrial usage, it is becoming increasingly common in mainstream designs.
Modern consumer goods often employ tight PCB spacings that make it difficult to remove heat and therefore require operation at higher temperatures. In addition, applications such as induction heating control inherently take place at elevated temperatures.
All of these advancing capabilities extend the range of application of this fast-developing category of products. With their long service life, full feature set and ease of design-in, photocouplers will continue to lead the way in applications that require isolation for some time into the future.
