High-power thyristors with direct lighttriggering and integrated protection functions can be utilized advantageously for applications in which several thyristors are connected in series, because this enables a significant reduction in the number of components necessary for the construction of high-power thyristor converters. This in turn leads to greater reliability and lower fabrication costs. It applies in particular to high-voltage dc (HVDC) transmission, static var compensation (SVC), converters for medium voltage drives, and tocertain pulse power applications.
LIGHT-TRIGGERING AND OVERVOLTAGE PROTECTION
Thyristors in HVDC converter stations have to be protected against several failures that may appear under standard operation. There are three classical failure events thyristors must be protected against :
• Voltage pulses with a too high amplitude (overvoltage),
• Voltage pulses with a too highvoltage ramp dV/dt, and
• Voltage pulses appearing during the forward recovery time.
A reliable protection of the thyristor can only be achieved if the thyristor is safely turned on in case of failure. A promising concept for an integration of protection functions is therefore to utilize the amplifying gate (AG) structure of the thyristor in such a way that in case of failure sufficiently large internal trigger current is generated that turns on the device by means of the AG .
(Figure 1) shows the silicon area of a 8000V/6000A light triggered thyristor including the encapsulation and fiber optic cable. The innermost AG was adjusted such that the photogenerated current provided by the integrated diode triggers the thyristor when illuminated by a 40mW light pulse with a duration of 10μs. The breakdown voltage of the BOD is controlled by the design of the silicon cell structure below the optical gate and its doping concentration.
Integration of a dV/dt protection function is achieved by designing the AG structure and the main cathode area.
Commutation of a thyristor leads to an extraction of the free charge carriers in the semiconductor structure. When large regions of the thyristor have completely recovered and an uncontrolled forward voltage pulse appears, the remaining small region with excess carriers in the main cathode area may be turned on. To avoid this, the thyristor should also be turned on in a controlled way by the AG structure if such a failure occurs. However, an increase of the carrier concentration in the AG region can be achieved by modifying the carrier's lifetime distribution so that it is lower in the main cathode area than in the AG structure.
FUTURE POWER CONVERSION
For inverters such as those used in automation technology or traction, there is an increasing demand for a matched system consisting of the control, intelligentdrivers, and power switches. Key parameters for the power switchesin inverters are ruggedness, on-stateand switching losses, and cost.Emerging developments will focusmore on the indirect contributorsto losses of the power switches ininverter systems.
These topics become more important the higher the load currents are. Key factors for current developments are inherent softness of the IGBTs and free wheeling diodes, as well as enhanced controllability. Further improvement will be driven by optimized cell design and vertical structures, basically advanced field stop.
In talking about the IGBT as the key switch in these application fields in detail, one must say that this technology has seen considerable innovation over the last decade regarding loss reduction as well as chip shrink resulting in more compact and cheaper packages and inverters. Examples of this progress in Si developments in terms of on state voltage reduction and chip shrink are shown in (Figure 2).
Main enablers for these improvements were the implementation of modern IC technology into the IGBT process, e.g. small planar transistor cells and trench transistor cells, and evolutional steps of the vertical structure from PT to NPT to FS concept (Figure 3) by challenging ultra-thin wafer technology.
ULTRA FAST SWITCHING DEVICES WITH SUPERJUNCTION TECHNOLOGIES
System miniaturization is a strong driving factor in power electronics. Power supply is often a dominant concern with regards tominiaturization.
The efficiency of many power supply topologies is basically determined by the device capacities of the switching MOSFETs and, especially for low line conditions, the efficiency is significantly affected by the RDSON of the switching transistor.
For standard MOSFET technology these requirements are limited by the so-called silicon limit. This basic conflict could be resolved with the introduction of superjunction MOSFETs. The on resistance is now only a matter of technology performance and design but no longer subject to silicon limit. (Figure 4) shows the best commercially available standard transistor and CoolMOS C3 in comparison to its respective limits.
(CP is shown the latest development). The diagram also shows the conventional cell structure of a power MOSFET (top end of diagram) and the superjunction cell structure (bottom end of diagram).
Furthermore, this superjunction technology combines the low area specific RDSON with half the total gate charge Qg to achieve an outstanding figure of merit, Qg * RDSON of 5Ω*nC, which is less than one tenth of the standard MOSFET value (Figure 5).
MILESTONE FOR HIGHEFFICIENT POWER SUPPLIES
Since 2001 Infineon has been providing SiC Schottky diodes in voltage classes of 300 and 600V. With the availability of these virtually switching loss-free rectifiers, circuit developers have gained a new degree of freedom. Frequency limitations due to rapid increasing dynamic losses are out of the way when using this device class. What remains are only capacitive losses which are of a smaller magnitude than compared to even the fastest Si bipolar diodes in this voltage range. Meanwhile power supply manufacturers have released other products including PFC stages with 200kHz operation frequency based on this technology. Today, SiC Schottky diodes are mainly used in high-end power supply applications for servers and telecom base stations (>500W), and in less cost sensitive solar cell inverters. The most critical parameter for PFC design-ins is the surge current capability of this diode. This issue is addressed in Infineon's SiC diode generation, which employs a merged pn- Schottky junction (Figure 6). This approach enables the destructive surge current to be increased by afactor of 2 for a given current rating.
SUMMARY
Power semiconductors have always been the key technology driver for advanced power conversion systems. As energy consumption becomes the focus, precise control of energy flow from the source up to the load is given by an improved generation of power semiconductors. Along with this, system miniaturization, life time reliability, and system cost reduction are strongly influencedby the power devices.
We are now more than a decade into the 21st century, and on an ever-accelerating fast track to technological innovation in electronics. The transistor and progression into the IC, or microchip, lit the fuse leading to the explosion of innovations in electronics that is now taking place. Since the wi ...
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