Semiconductors Spark Advances In Welding Power

The application of high-frequen-cy switch-mode power-supply (SMPS) technology in welding equipment has yielded many of the same benefits associated with SMPSs in other, nonindustrial designs. By migrating from line-frequency operated supplies to high-frequency SMPSs, designers have been able to increase power efficiencies while reducing supply size and weight. As a consequence, SMPSs—commonly referred to as inverter power supplies—are turning up in a variety of welder types, including tungsten inert gas, metal arc, metal inert gas, electrical resistance, and plasma cutting machines.

Naturally, the better efficiency and size of an SMPS come at a price, namely a more complex design in comparison to a line-frequency equivalent. Moreover, the design of a welding power supply is complicated by the nonlinear load presented by the current arc. Consequently, the welding supply requires a more sophisticated control scheme than other types of switching supplies.

Output current and power requirements are other differentiators. For welding equipment, average current requirements are typically specified in hundreds of amperes. With arc voltages somewhere in the vicinity of 30 V in the case of stick welders, this leads to output power levels of several kilowatts or higher. These requirements, in turn, lead to high voltage and current ratings for SMPS components, and packaging designs that must address thermal-management needs.

Despite these challenges, SMPS approaches provide an evolutionary path toward higher efficiency, smaller component size, tighter functional integration with smarter control and protection schemes, and better manufacturability, as well as lower costs. The ongoing improvement in power semiconductor dies and packaging makes these gains attainable.

A high-frequency SMPS for welding typically consists of an input rectification stage, a switching or inverter stage, a high-frequency transformer, and an output rectification stage (Fig. 1). The switching stage is commonly built using IGBTs, but it may also be constructed with MOSFETs or diodes. In addition to these power blocks, a number of control functions, such as pulse-width modulation (PWM), gate drive, and soft start, must be implemented.

The input configuration of rectifiers will vary depending on whether the input is a single- or three-phase ac. An additional switching element may be inserted between the input rectifiers and the switching transistors when power-factor correction is required (Fig. 2). Within the switch stage itself, designers can opt for a full-bridge configuration of four transistor-diode pairs, or a dual-forward design consisting of two transistors and two diodes (Fig. 3). The latter approach simplifies control but sacrifices efficiency.

The configuration of the output stage will depend on the power requirements of the specific welding technology. Stick welders, formally known as shielded metal arc welders, require constant-current supplies. Other welder types may require constant voltage, a combination of constant voltage and current, or some form of pulsed output. Consequently, the components that follow the output rectifiers will vary. For example, the rectifiers may be followed by an inductor when the welder requires a constant dc current for welding steel or copper materials. But when a pulsed dc output is required for aluminum, the output inductor might be replaced by a second inverter stage (Fig. 2, again).

The semiconductor components that are selected or developed for the supply's different power sections must be optimized for different characteristics. Input rectifiers must be able to withstand line surges and should have low forward-voltage drops (V_F) to minimize their conduction losses.

Intended to operate at frequencies of up to about 100 kHz (a frequency limitation imposed mainly by the transformer), the high-voltage transistors applied in the switch stage need low switching losses. They must also be paired up with free-wheeling diodes that have the best reverse-recovery charge (Q_RR) characteristics. In other words, reverse recovery time (t_RR) needs to be as low as possible. Naturally, the importance of this parameter also depends on the actual switching frequency. Therefore, working conditions of the transistors will determine the degree of speed and "softness" required of the free-wheeling diodes. Meanwhile, rectifiers used in the output stage, where conduction losses dominate, must exhibit low V_F but have low t_RR too, though not as low as the free-wheeling diodes.

The current and voltage ratings necessary for semiconductors in the switching and rectifier stages vary directly with the output current requirements for machines in both the single-phase- and three-phase-input categories. A comparison of stick-welding machines found in the U.S. and Europe depicts the key current and voltage requirements for machines of varying output currents (Tables 1 and 2). Notice that the 200-A output current level seems to divide single-phase and three-phase welders. International Rectifier gathered this data and found no differences between single-phase machines when comparing those marketed in the U.S. to those sold in Europe.

According to the company, that's because single-phase welders in the U.S. are generally connected to two phases of the three-phase line, or they employ a doubler circuit on the input. The situation is slightly different, however, for three-phase welders.

In the U.S., the three-phase welders are likely to be running off of a 220-V line in much of the country. But, the ac input may be as low as 208 V in California, or as high as 480 V in some industrial environments. Meanwhile, in Europe, three-phase ac inputs vary from 380 to 400 V. As a result, current demands for the switching transistors are generally significantly higher for the welders targeted in the U.S..

Given the high current levels and high switching frequencies involved, packaging becomes extremely important. It impacts thermal performance, determining how much power can be dissipated by a die of a given size, as well as electrical performance. In particular, parasitics associated with semiconductor packaging will affect switching losses. Consequently, device layout becomes critical for its effect on these same thermal and electrical parameters.

Beyond the thermal and electrical considerations, the choice of semiconductor packaging will play a role in determining overall power-supply size, reliability, manufacturability, and cost. In selecting components for the switching stage, designers may choose discrete components or modular, multichip alternatives. The latter are now being promoted by semiconductor vendors, such as International Rectifier. These companies are developing modular solutions not only as a means to improve supply performance and manufacturability, but also as building blocks on the path to greater functional integration.

According to Carlo M. Ciaramelletti, product marketing manager for the High-Power Products and Systems Business Unit at International Rectifier, trends in the North American welding markets indicate that welding manufacturers are changing their designs from discrete switching devices to IGBT multichip modules (MCMs) despite the price premium on the modules. At a cost of $30 to $150 per unit, depending on device type and configuration, the power modules are significantly more expensive than the discrete devices they're meant to replace. That price is being justified, though, by the reduction in production costs for the welding equipment when modules are employed instead of discrete devices.

The module eliminates some labor-intensive pc-board assembly, including the additional heatsinks, power control, and safety devices necessary in the discrete design. Ciaramelletti notes, "In some welding applications, designers need modules to handle 150 to 200 A. They can parallel discrete devices, but you can get better efficiencies with a module and a cable." The MCM approach can eliminate, or at least alleviate, the need for paralleling of switching devices, while allowing the use of heatsinks that are more compact. The modular approach is said to offer higher reliability than discrete designs, especially for the lower input voltages.

Currently, International Rectifier is developing the dies for high-voltage MOSFETs, IGBTs, and free-wheeling diodes that will be incorporated into switch-stage modules in full-bridge, dual-forward, and half-bridge configurations in the MTPA package. The list of dies being developed for these modules includes:

• 500-V MOSFETs with R_DS(ON) as low as 95 mΩ, configured with or without a fast body diode;
• 600-V WARP IGBTs rated at either 6 or 10 A_AVG;
• 900-V WARP IGBTs rated at 8 A_AVG;
• 1200-V nonpunch-through IGBTs rated at 10 or 20 A_AVG.

These MOSFETs and IGBTs may be used in combination with one of two free-wheeling diodes also under development. These are the hyperfast platinum 600-V fast-recovery epitaxial diode (FRED) and the 1200-V HEXFRED. In the full-bridge and dual-forward converter modules, the MOSFETs will permit switching speeds as high as 100 kHz.

In the half-bridge modules, up to 80-kHz switching speeds will be possible. Plus, the half-bridge modules will allow paralleling of up to two die per transistor. Furthermore, the company is developing a 1200-V 50-A nonpunch-through IGBT for application as a single die in the half-bridge configuration.

When designing these modules, International Rectifier will focus on layout optimization that relies on the extraction of parasitic parameters for direct bondable copper, wire bonds, and terminals. Another aspect of the module design is simulation and optimization of the module in the machine, prior to building the actual prototypes and characterization. These modules are essentially custom, but they might ultimately be released as standard catalog items.

Combining the company's switch-stage modules with its existing line of input rectifiers creates a more modular high-frequency design. But ultimately, the company hopes to raise the level of integration further. "We propose to integrate some of the control functions into our switch module," Ciaramelletti says. Depending on the available space in the design, it might be desirable to integrate the input rectifier with the switch stage as well.

Semikron is another company that develops custom MCMs for welding. This company's packaging solutions combine IGBT silicon with thermal protection as well as current and voltage protection. Semikron recently unveiled its SKIM IGBT package, which is intended to integrate devices with 600-, 1200-, and 1700-V ratings.

Available device configurations include the SKIM 3, which was introduced last year, and SKIM 4 and 5, which should both ship in the second quarter of this year. (See www.semikron.com for specifications on these IGBT modules. Select "Products," then "SKIM" to view the datasheet.) The SKIM package can be designed with a choice of ceramic materials for the module's isolating substrate—either aluminum oxide or aluminum nitride. The latter material can be employed when superior thermal performance is needed.

Meanwhile, other vendors are focusing on developing discrete devices. IXYS Corp., a semiconductor vendor, produces MOSFETs for use in SMPSs. Some of these are applied in welding applications. According to Ralph Locher, manager of application engineering at IXYS, customers tend to use discretes in parallel to handle the high current levels rather than opting for a costlier modular solution. "In welding, there's a push to build smaller equipment," Locher says. He believes discretes enable the development of more-compact power supplies for welding.

In its development work, IXYS is trying to reduce gate charge by 40% across the board on all of its MOSFETs. This enhancement in device performance will ease the design of gate-drive circuits.

Another vendor, Intersil, is addressing the welding application by developing advanced IGBTs, free-wheeling diodes, and rectifiers. Two new IGBTs being offered in the high-current ISOTOP package are characterized for 100-kHz switching and a T_J of 125°C, with current ratings of 30 and 40 A under these conditions. These transistors are designated as the HGT1N30N60A4D and HGT1N40N60A4D, respectively.

Such IGBTs will permit reductions in on-state losses and power dissipation, particularly when compared to MOSFET-based designs. This point is illustrated by a case study performed by Intersil that looked at a switched-mode power supply built using size-6-die MOSFETs versus an equivalent circuit built using size-3-die 600-V IGBTs. The transistors were applied in a hard-switched boost converter circuit with 50% duty cycle for conduction. Despite the much larger piece of silicon present in the MOSFET, the IGBT dissipated 25% less power with just a slightly greater rise in junction-to-case temperature. Resulting power densities were 10 to 20 A/cm² for the MOSFET design versus 100 A/cm² for the IGBT design. (For more information, visit Intersil's Web site at www.intersil.com/igbt/SMPS_Thermal.asp.)

Because of their higher efficiencies, IGBTs can simplify thermal design. Nevertheless, even when IGBTs are em-ployed, thermal design still demands consideration early on in the design process, observes Alex Craig, lead marketing engineer at Intersil. Craig claims some designers attempt to solve thermal problems by "throwing a bigger IGBT device at the application." That approach may work simply because the larger component is more thermally conductive. Craig explains that this method does nothing to reduce heat dissipation, however. A better thermal-management design that allows for the use of a smaller transistor ultimately offers a more cost-effective solution.

Better recovery diodes provide one way to boost the inverter's power efficiency. To that end, Intersil also is developing a series of 600-V diodes with reverse recovery times of as low as 25 ns. Called Stealth diodes, these devices are copackaged with the IGBTs applied in PFC circuits, such as those applied in high-frequency welding supplies (Fig. 2, again). The diodes are avalanche energy rated and offer soft-recovery switching at rated current, high di/dt, and junction temperatures of as high as 125°C. The Stealth diode reduces EMI, making it possible to eliminate a snubber circuit in some cases, while allowing faster turn on of the associated IGBT, too. That leads to lower turn-on losses in the transistor.

In addition, the company is developing output rectifiers, which it expects to release in the coming months. Housed in the company's ISOTOP package, these components will carry ratings of 200 to 300 V and up to 150 A.

Improvements in device performance and packaging stand to enhance many aspects of high-frequency power-supply design in welding machines, but the benefits aren't limited to these applications. A variety of SMPS applications, such as high-power UPS systems and telecom power supplies, employ similar inverter topologies. The advent of smaller, more-efficient semiconductor components and modules with greater functional integration will help designers of many switching power supplies to satisfy growing demands for designs that are more compact and more manufacturable.