Power Electronics

Components Brave Higher Operating Temperatures

High ambient temperatures are a factor in numerous automotive, industrial and telecom applications for power electronics. Those high ambients, together with requirements for high component-level power dissipation, are driving suppliers to extend the operating-temperature range of their discrete devices. A sampling of recently introduced resistors, varistors, diodes and optocouplers illustrates how some newer parts are better equipped to take the heat.

When it comes to passive-component performance, resistors appear to offer the highest operating-temperature capability. An example is the CCW Series of axial wirewound resistors, which were introduced in May by TT electronics' IRC Wire and Film Technologies Division. Rated from 0.5 W to 9 W, these resistors handle temperatures as high as 275°C, thanks to their use of a ceramic core.

“The high temperature rating, along with a withstanding voltage rating up to 1000 V, makes these resistors ideal for commercial power supply and industrial applications,” says David Winkler, product manager for IRC's Wire and Film Technologies Division. With this series, resistance ranges from 0.1 Ω to 1.5 kΩ, tolerances are ±2%, ±5% and ±10%, and TCRs are as low as ±300-ppm/°C at values of 20 Ω or less.

Though not in the same high-temperature ballpark, inductors such as BI Technologies' HM88 series are also capable of very-high-temperature operation. Aimed at automotive applications, this just-introduced series of surface-mount high-frequency inductors owes its ability to operate up to 180°C to the use of better core materials, epoxies and wire.

This core is constructed from an advanced metal-powder-core material, which allows the inductor to operate at high frequencies (300 kHz and higher) with low core losses and high saturation levels. The wire is a flat-ribbon wire that provides low DCR. In addition, the part's shielded construction and small footprint make it suitable for space-constrained point-of-load power applications.

Some semiconductor devices can work at temperatures nearly as high as these passives. Because of its high thermal conductivity, silicon carbide enables the development of rectifiers that can operate up to junction temperatures of 175°C. An example is the CSD08060, an 8-A, zero-recovery rectifier introduced last month by Cree.

However, some silicon devices can operate at similarly high junction temperatures. Consider STMicroelectronics' SMA6J series of 600-W peak-pulse-power clamping diodes, which are capable of operating at a junction temperature of up to 175°C. Introduced in October, these transient-voltage suppressors are said to be the industry's first 600-W 10/1000-µs devices in the compact SMA package. With a 9-mm2 footprint, the SMA package saves 50% of the pc-board area required by the SMB package commonly used by suppressors with this power capability.

Because of its 175°C TJ max, the SMA6J affords greater design margins than the usual 150°C devices when used in hot environments such as switch-mode power supplies (SMPSs). The leakage current is specified at 85°C as well as at 25°C (0.2 µA at 25°C). According to the vendor, the use of planar technology to fabricate these diodes results in their low leakage current, which in turn enables the company to specify the complete SMA6J range for operation up to 175°C.

For nonsilicon forms of transient protection, temperature limits can be much lower than those of diodes. According to Stackpole Electronics, multilayer varistors have been limited until now to operating at temperatures below 125°C, with some devices only capable of 85°C. These limits present problems in some automotive applications. In response, Stackpole has developed its ZVY and AVY series of multilayer low-voltage automotive varistors. Announced last month, these varistors operate at temperatures up to 150°C — a capability that is attributed to a proprietary mixture of ceramic powders.

The ZVY and AVY series varistors are specifically designed for 12-V, 24-V and 42-V automotive systems, and are characterized for high maximum energy handling and high load dump energy. Both varistor series are available in surface-mount or through-hole versions. For the ZVY series, SMD devices range from 0603 through 1206 and leaded devices range from 5-mm size to 20-mm size equivalent. DC operating voltages range from 3-V to 125-V, and the ZVY is capable of handling up to 7.4-J standard.

For optocouplers, which contain two die, temperature limits have traditionally been much lower than the 150 TJ max associated with other semiconductor devices. Two recently introduced families of optocouplers extend the operating temperature range for these devices from the existing 100°C limit to 110°C.

One benefit of this change is it gives the designer flexibility when placing the optocoupler on the pc board. So, while optocouplers have typically been located at the edge of the board to keep them away from heat-generating devices, an optocoupler with a higher temperature limit can be placed at a spot that yields better electrical performance. Placing these devices in the center of the board allows traces that route signals to and from the optocouplers to be shortened.

In October, Vishay Intertechnology announced its TCLT family of 18 optocouplers in long-body, low-profile 4-pin and 5-pin SOP-6 mini-flat packages. These devices feature an operating temperature range of -40°C to 110°C. They also offer a high isolation voltage (5000 VRMS) that is usually associated with larger DIP packages. These optocouplers are optimized for high-temperature performance in feedback loop, microprocessor system interface and electronic control-system applications in SMPSs for medical equipment, power tools, household products, industrial automation equipment and white goods.

The devices offer the same low 2-mm profile as the industry-standard mini-flat package, but with a longer body for a better creepage and clearance distance of 8-mm minimum.

This month, Everlight Electronics announced a family of 6-pin DIP photo-transistor optocouplers featuring an operating-temperature range of -55°C to 110°C along with low-profile surface-mount packaging (as low as 3.9 mm max). The 21 devices in this family are members of the 4N2X, 4N3X, H11AX and CNY17-X and CNY17F-X series.

The improved thermal stability and insulation of these devices can be attributed to their internal construction and choice of materials (LED and epoxy resins). Rather than having the emitter and detector side by side and covered in a dome with soft gel epoxy, the emitter sits above the detector in what's described as a “double-molded over-under” construction.

The two die are mounted to two lead frames, and both the internal light-transmittable epoxy resin and outer epoxy resin are molded to allow for a very rigid package structure. In addition, the inner and outer resins have similar thermal expansion coefficients, which greatly reduces the chances of delamination, thereby preserving the package's long-term insulation characteristics. This internal construction also allows for direct light transfer between emitter and detector, and combined with a high-efficiency LED, greatly improves CTR consistency over time and temperature.

Targeting industrial and control designs, these devices feature high collector-emitter breakdown voltage (80 V min), high-isolation voltage (5000 VRMS), greater than 7-mm creepage distance and a wide selection of CTR bin ranges.

Table. Power components designed for high-temperature operation.
Component type/supplier/model # Key features Operating temperature range Target applications Unit pricing
Wirewound resistors/TT Electronics' IRC division CCW Series • Axial construction with ceramic core and conformal coating
• 0.5-W to 9-W ratings
• Resistance ranges from 0.1 Ω to 1.5 kΩ.
-55°C to 275°C. Commercial power supplies, industrial applications $0.011 typical in quantities of 50,000
Metal powder core inductors/BI Technologies HM88 Series • High-frequency (300-kHz+) operation with low core losses and high saturation levels
• 1-µH inductance
Up to 180°C Point-of-load power designs, automotive systems, telecom equipment $0.75 typical
Silicon carbide rectifier/Cree CSD08060 • 8-A, 600-V Zero Recovery rectifier
• QC = 30 nC
• T0-220 package
-55°C to 175°C junction temperature Power-factor correction in switch-mode power supplies (SMPSs) Not available
Clamping diodes/STMicroelectronics SMA6J Transil series • 600-W rating in SMA Package
• stand-off voltages from 5 V to 188 V
Up to 175 °C junction temperature SMPSs, hard disk drives, low-leakage portable designs $0.10 in small quantities
Multilayer varistors/Stackpole Electronics ZVY and AVY series • SMT or through-hole versions
• 0603 through 1206 case sizes
• DC operating voltages from 3 V to 125 V
Up to 150 °C 12-V, 24-V and 42-V automotive systems $0.14 to $0.40 (ZVY, SMT devices); $0.30 to $0.75 (AVY, SMT)
Phototransistor optocouplers/Vishay TCLT Family • 2-mm profile
• 5000 VRMS
• 4-pin and 5-pin SOP-6 mini-flat packages
-40°C to 110°C. SMPSs for medical equipment, power tools, industrial automation, white goods $0.20 in 10,000-piece quantities
Phototransistor optocouplers/Everlight Electronics 4N2X, 4N3X, H11AX and CNY17-X, CNY17F-X series • 6-pin DIP packages profiles as low as 3.9 mm
• choice of CTR ranges
-55°C to 110°C. Power supplies, programmable logic controllers, general-purpose switching circuits $0.12 in production quantities
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