[Technology Report]
Semiconductors Spark Advances In Welding Power
Better silicon and packaging for IGBTs and diodes produce smaller, more-efficient high-current supplies for welding equipment.
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 (VF) 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 (QRR) characteristics. In other words, reverse recovery time (tRR) 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 VF but have low tRR 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.
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