The diverse fields of application and topologies of power supplies for technology and medical uses lead to very different requirements on power semiconductors in terms of blocking voltage, forward current and switching characteristics.
Inductive heating for processes such as melting, annealing, tempering, soldering, forging, vaporisation or semiconductor production requires supplies at a power level ranging from around 1 kW to 20 MW, with pulse frequencies of between 100 Hz and 3 MHz at different voltage levels.
Depending on the area of application, power supplies for plasma plants use pure DC current or pulsed DC current in the range of up to a few tens of kHz. The output voltage range is between 300 V and several kV, with power levels between 1 kW and several hundred kW. Major applications include surface coatings, e.g. of solar cells, architectural glass and metal.
In laser technology, the power supply requirements are determined by the principle and performance of the light source. A current source for controlling a laser diode must generate pulses of 30 V, 50A with a pulse frequency of between 200 kHz and 1MHz, for example.
Medical diagnostic systems
State-of-the-art medical diagnostic systems such as MRI, CT and X-ray equipment require a lot of energy. There are particularly demanding requirements in terms of the reliability and switching characteristics of the power semiconductors. The high voltage of 120 kV, for example, required by a CT unit, is generated from 700 V to 800 V DC using a resonant inverter switching at 100 kHz. As the switching rate increases, the diagnostic result improves, while at the same time the weight and size of the equipment decreases.
Topologies for arc welding and resistance welding
Selected standard topologies are described below by way of example, together with the most important requirements for power transistors, thyristors and diodes in power supplies for arc welding and resistance welding.
Electric welding is a classic application of power semiconductors in power supplies for electro technology, which need to deliver a high direct current with low output voltage. Depending on the process, there are different requirements in terms of residual ripple and control precision of the welding current, and hence in terms of the properties of the power semiconductors used. As welding is, in almost all cases, a duty cycle process with high power surges, there are usually very precise requirements in terms of the load cycle stability of the power semiconductor.
Welding power supplies for arc welding
The simplest topology for a welding power supply for arc welding comprises a single- or three-phase power transformer and a single- (B2) or three-phase (B6) welding rectifier with choke connected downstream. Current control is only possible if the rectifier uses thyristors or if an additional AC switch is connected upstream of the transformer. Control dynamics and current ripple are specified by the line frequency.
It is possible to achieve significantly improved welding results together with improved efficiency, smaller dimensions and lower weight using input- or secondary-side pulsed welding power supplies, for which different topologies are applied. Here the energy flow is controlled by IGBTs or power MOSFETs of discrete or modular construction, which are subject to requirements such as high load cycle stability and low losses. As a result, they can be switched with a minimum of cooling complexity at pulse frequencies of between 10 kHz and 100 kHz. Welding inverters convert power drawn from the 230 V or 400 V mains into welding current via a buffering DC bus voltage circuit.
Secondary-side pulsed welding current source (chopper)
As for welding rectifiers, first the output voltage of the power transformer is rectified, then it is smoothed by DC link capacitors and, as a result, is decoupled from the line frequency. Current control is carried out by the output-side DC chopper, which typically comprises a power MOSFET module with freewheeling diode. The properties of the MOSFETs, with a maximum of 200 V reverse recovery voltage, allow for high pulse frequencies (40 Hz to 100 kHz), which enables good control dynamics and very low ripple in the welding current, up to around 1,000 A @ 125 ms turn-on time.
Primary pulsed welding power supplies (welding inverter)
Here, too, the mains voltage is rectified using an uncontrolled B6 bridge. However, the bridge is connected directly to the mains, so the power transformer, and its significant material, weight and volume requirements, can be dispensed with. The DC is smoothed by DC link capacitors. A single-phase inverter made from two IGBT half-bridge modules then converts the DC voltage into AC voltage with a pulse frequency of between 20 kHz and 30 kHz. The voltage is then adapted to the welding process by the transformer connected downstream – which is significantly smaller, lighter and cheaper than a power transformer, owing to the high switching rate.
Welding current sources for resistance welding
In the case of resistance welding, e.g. spot welding, the overlapping individual sheets, which are 0.5 to 3.0 mm thick, are pressed together with electrodes, brought to melting temperature by a brief current surge, and thus welded. Today, medium frequency resistance welding with output frequencies of around 1 kHz is used virtually to the exclusion of all other resistance welding processes.
The topological differences as compared to arc welding result, first and foremost, from the requirements of the significantly higher welding current – up to 120,000 A @ 10% on-time. The primary advantage over 50 Hz welding is again the substantially lower weight of the transformer, e.g. if the welding equipment is fitted onto robotic arms, as in the automotive industry.