Photocouplers retain leading position in isolation technology

In Europe, the greatest demand for photocouplers originates in industrial applications. Of these, factory automation takes the lion's share and the typical uses in this sector tend to be related to motion control in devices such as programmable logic controllers, servos and inverters. Alongside these traditional industrial uses, there is a new market opening up for 'green' applications – solar energy installations and wind turbines use inverters and these generally employ photocouplers. The home appliance market – in particular induction heating – is another significant user of photocouplers and new markets continue to emerge. Among these are automotive, security and smart grid metering. Although requirements vary according to market segment, a number of clear trends can be identified that are driving the developments in photocoupler technology. Board space and cost pressures have forced component miniaturisation in all areas and couplers are no different. Dual in line packages (DIPs) have shrunk – Toshiba's six pin Shrink DIP (SDIP) has a footprint 50% smaller than the 8pin DIP it replaces, but still provides the same performance and safety standards – and many DIPs are giving way to pure surface mount devices (SMDs). But even though package sizes are becoming smaller the demands being placed upon them, in terms of isolation performance (both clearance and creepage), operating temperature ranges and switching speed, continue to rise. Here we will look at a number of device categories that fall under the general heading of photocoupler (or optocoupler). • IC couplers IC couplers, including high speed couplers, IGBT gate driver couplers and IPM driver couplers, are becoming increasingly important. These address applications where more advanced output functions are required, for example, in the area of high speed digital transmission, and have been the focus of many of the recent developments in coupler technology. High speed couplers can be used to transmit data signals in applications such as factory automation equipment and switching power supplies. Key selection criteria include switching speed and threshold input current, but increasing importance is now being placed upon the ability to sustain higher operating temperatures. Toshiba's recently introduced TLP708 ultra compact, high speed inverter type photocoupler features guaranteed extended temperature performance from -40 to 125°C, while complying with the reinforced isolation class requirements of international safety standards. The TLP708 is aimed at home appliances, factory automation and other industrial applications where operation under extreme ambient temperatures and high levels of isolation is required. Despite its small size (9.7 x 4.6 x 4mm), this photocoupler has a minimum isolation voltage rating of 5000V rms. It operates with a maximum switching time of just 75ns, allowing it to support typical data transfer rates of 15Mbit/s. Looking beyond package size, other recent developments have sought to save circuit board real estate by integrating additional functionality within couplers and hence reducing the number of external support components that are required. An example of this is the TLP2095/2098, which is capable of accepting an ac input. This eliminates the need for external rectifier devices, such as bridge diodes, which would otherwise be required to handle an input signal whose polarity may change – as is the case with a programmable logic controller. These devices also demonstrate the desire to minimise power consumption, featuring a maximum input threshold current of ±3mA. • Transistors, triacs and photorelays Photocouplers with a transistor output are used in applications including switches for programmable logic controllers, power supplies and inverters. Users make their selection based on criteria including the rated collector-emitter voltage, current transfer ratio, number of channels and package type. While the latter has become more compact, the demands placed on these smaller packages have risen. In particular, new generation products have featured enhanced isolation performance and extended operating temperature ranges. Despite the fact that Toshiba's TLP284/285 series comes in a mini flat sop4 package measuring 4.4 x 2.6 x 2.1mm, it still offers an isolation voltage of 3.75kV. It is rated for operation at temperatures from -55 to 110°C, with a creepage and clearance distance specified as a minimum of 5mm. Maximum operating insulation voltage is 707V peak, while the highest permissible overvoltage is 6000V peak. And, of course, all new devices must meet the stringent requirements of such standards organisations as BSI, UL and VDE. Triac couplers, which conduct current in both directions, can be used to control an ac load. They are typically used in solid state relays, motors and lighting systems. Here, the trends are for products with higher off state voltages – up to 800V – high impulse noise immunity and, as elsewhere, smaller packages. DIP4 are gradually replacing the widely used DIP6 packages. Photorelays (also known as mosfet output couplers) are being used in an increasingly wide variety of switching applications. They are replacing the incumbent mechanical relays due to an attractive combination of long service life, high reliability, reduced power consumption and smaller board space requirements. Energy efficiency is a high priority for manufacturers of electrical and electronic equipment for both cost and environmental reasons and the low drive power consumption of photorelays (two orders of magnitude less than their mechanical equivalent) is appealing. Add to this the performance difference – photo switching time is only 20% of mechanical relay switching time – and the extended life due to the lack of mechanical contact and it is easy to see why there is a compelling case for moving to photorelays. The characteristics of photorelays vary according to application – measurement instrumentation requires low capacitance and low resistance, while home appliances need higher performance – but space considerations and energy usage always play a part. Consequently, smaller package size and lower trigger currents are being demanded. In a response to this Toshiba's TLP170 photorelay features a maximum led trigger current of just 1mA – one third of its predecessor's specification. What does the future hold? The key to meeting the market's requirements for high isolation performance, superior quality and extended operating temperature ranges for photocouplers is the adoption of new high performance leds. Other isolation devices have appeared using competing technologies, such as magnetic or inductive couplers, and there are claims that these will deliver longer life than photocouplers. However, projected operating lifetime of leds used in photocouplers today already exceeds several 100,000hr under normal operating conditions. Therefore photocouplers are likely to maintain their position of technical superiority for some time to come. Matthias Diephaus is senior manager, opto semiconductors, with Toshiba Electronics Europe.