High-voltage MOSFETs handle the heat

Vince Biancomano

eeProductCenter (09/07/2005 2:49 PM ET)

Systems designers are fast coming to realize that a discrete MOSFET’s performance is more and more measured by how well it performs in just one essential category: an ever tighter operating environment. In the last reckoning, high-density systems imply an elevated temperature. But for designers in the competitive MOSFET market, one of the few in power that is growing rapidly despite tepid economic conditions, building thermally superior higher voltage and higher current devices isn't usually a challenge for too long. Indeed, vendors continue to match their progress, and with seemingly greater ease, for each new generation of devices when it comes to higher cell and packaging density, lower on-resistance and better figure-of-merit, parameters which are becoming an even more critical means to an end in higher-voltage, higher-current systems.

A snapshot of recent activity exemplifies the trend towards better thermal performance. At the lower end of the voltage range suited to high-current power applications, Siliconix says its newest portfolio of 40- and 60-volt n-channel MOSFETs, with 10 devices in several product families (Si74xx, Si79xx, SUD, and SUM) for industrial and automotive applications, are the industry’s first with a high threshold voltage yet a low on-resistance (see www.eeproductcenter.com, articleID=161600407). In this case, it’s nominally 3.4 volts, and as low as 2.7 milliohms, respectively. In meeting the goal for these MOSFETs, which are suited to high-current, high-temperature applications with inductive loads, Siliconix minimizes the variation in threshold despite large increases in operating temperature, which when high enough shifts the traditional MOSFET’s threshold to near zero and initiates a spontaneous turn-on. Designers addressed this issue in the past by introducing a negative bias to the circuit, at the expense of additional components, complexity, and overall cost.

Zoning in on 12-volt switchmode supplies as well as low-voltage motor applications, International Rectifier’s 75- and 100-volt IRF2907ZS-7PPbF devices can deliver up to 160 amps, about 45 percent higher than competing solutions according to the company (see www.eeproductcenter.com, articleID=170100115). Contributing to the MOSFETs’ power handling capability is their low RDS (3.8 milliohms), which is reported to be at least 16 percent lower than similar MOSFETs.

For high-voltage applications in the military sector, IR’s R6 line of radiation-hardened MOSFETs, in 13 versions rated from 100 to 250 volts, has 40 percent lower on-resistance than competing devices in critical power supply applications (see www.eeproductcenter.com, article ID=168601851). These devices are optimized for 24- and 48-volt systems associated with launch vehicles and satellites.

STMicroelectronics newest 500- and 600-volt MOSFETs in their MDMesh family cuts on-resistance by 40 percent to boost efficiency in high-voltage switchmode power supplies (see www.eeproductcenter.com, articleID=164302135). These second-generation devices, suited to power supply applications from 90 watts to 1 kW, tout an innovate drain structure that reportedly improves the devices’ dV/dt and avalanche characteristics. The devices’ overlaid horizontal source strips are said to ensure superior control of the device’s internal gate resistance and intrinsic capacitances. The new devices will also handle higher currents for a given VGS voltage.

For high-voltage TV power supplies and ballast lighting, Toshiba America’s TK15A60S is a 15-amp, 600-volt device that boasts a fourfold improvement in figure of merit (the product of RDS and gate-charge, with lower numbers being best). Using its new Deep Trench technology employing a “super junction structure” said to be the first of its kind, these devices claim a 40 percent reduction in RDS over conventional MOSFETs, and according to the company allows device performance to break the so-called “theoretical limit of silicon" (see www.eeproductcenter.com, articleID=160401293).

Another device that touts the breaking of the theoretical limit of silicon, Infineon’s first two members of its CoolMOS CS Server power transistor family, 600-volt devices designed specifically for computer servers and high-power-density applications such as telecom equipment, reportedly sets new records for the lowest on-resistance and highest switching speed for devices in this application (see www.eeproductcenter.com, articleID=159903015). These MOSFETs claim an RDS of just 99 milliohms in a TO-220 package, and 45 milliohms in TO-247package, coupled with the industry’s fastest switching speed of 150 V/ns. The company says its 1-kW reference design, using its 99-milliohm MOSFET rated at 31 amps continuous at +25°C, boosts overall supply efficiency by 1.5 percent compared to designs using two “standard” 250-milliohm MOSFETs placed in parallel.