The future of Power Electronics

The future of Power Electronics is expected move in the following directions:

• The future will demand Integrated Systems for electronic power processing. A more multi-disciplinary approach is needed for new achievements in integration, packaging, reliability and cost reduction.
• Intelligent control and energy management will come easily. Thermal and passive component integration is equally important.
• Large penetration of power electronics into power systems, mainly in distributed generation.
• Large-scale use of power electronics in automotive applications. This largely depends on political decisions as well as technology advances.
• Advances in high current, higher voltage devices will have a major impact on traction applications.
• Emerging applications in commercial / residential areas: HVAC, Induction cooking, lighting, computer power etc.
• The cost of using power electronics will depend on factors like: • Recycling • Standards regarding EMI • Intelligence / protection • Modular converters • Reliability considerations • Thermal engineering • Electromagnetic Integration
• Silicon carbide is an important future development in power semi-conductors.
• Distributed generation and power quality are important future considerations.
• For very high-power applications, modularization will be expanding, provided the system cost, efficiency, flexibility and EMC are acceptable.
• For medium-power applications, the total system integration still leaves a lot of room for improvements in cost, power modules, control and sensing, passives, reliability and performance.
• Fast energy storage is required in numerous applications. At present, super capacitors seem to be the most promising solution for energies upto 5 kWh. In the next ten years, all other energy storage options will continue to be considered.
• Applications in power transmission include HVDC converter stations, Flexible AC Transmission Systems (FACTS).
• Applications in power distribution include DC-DC converters, Dynamic filters, frequency conversion and Custom Power Devices.

The on-going development of interconnection standards and regulations will present both market opportunities and technology challenges for the power electronics industry. Future research and development efforts will need to focus on improving efficiency and reliability, communication and interface, thermal management, reduce parts and points of failure, packaging and bringing down cost.

The range of applications continue to expand in areas such as power supplies to motion control, factory automation, transportation, energy storage, multi-megawatt industrial drives, and electric power transmission/ distribution.

Power electronics....?

Power electronics is the technology associated with the efficient conversion, control and conditioning of electric power by static means from its available input form into the desired electrical output form.

Power Electronics deals with the study of
• Power semiconductor devices
- their physics, characteristics, drive requirements and their protection for optimum utilisation of their capacities,
• Power converter topologies involving them,
• Control strategies of the converters,
• Digital, analogue and microelectronics involved,
• Capacitive and magnetic energy storage elements,
• Rotating and static electrical devices,
• Quality of waveforms generated,
• Electro Magnetic and Radio Frequency Interference,
• Thermal Management

The Insulated Gate Bipolar Transistor (IGBT)

IGBT is a voltage controlled four-layer device with the advantages of the MOSFET driver and the Bipolar Main terminal. IGBTs can be classified as punch-through (PT) and non-punch-through (NPT) structures. In the punch-through IGBT, a better trade-off between the forward voltage drop and turn-off time can be achieved. Punch-through IGBTs are available up to about 1200 V. NPT IGBTs of up to about 4 KV have been reported in literature and they are more robust than PT IGBTs particularly under short circuit conditions. However they have a higher forward voltage drop than the PT IGBTs. Its switching times can be controlled by suitably shaping the drive signal. This gives the IGBT a number of advantages: it does not require protective circuits, it can be connected in parallel without difficulty, and series connection is possible without dv/dt snubbers. The IGBT is presently one of the most popular device in view of its wide ratings, switching speed of about 100 KHz a easy voltage drive and a square Safe Operating Area devoid of a Second Breakdown region.

Power Semiconductor devices-history(brief)

Power electronics and converters utilizing them made a head start when the first device the Silicon Controlled Rectifier was proposed by Bell Labs and commercially produced by General Electric in the earlier fifties. The Mercury Arc Rectifiers were well in use by that time and the robust and compact SCR first started replacing it in the rectifiers and cycloconverters.

The necessity arose of extending the application of the SCR beyond the line-commutated mode of action, which called for external measures to circumvent its turn-off incapability via its control terminals. Various turn-off schemes were proposed and their classification was suggested but it became increasingly obvious that a device with turn-off capability was desirable, which would permit it a wider application. The turn-off networks and aids were impractical at higher powers.

The Bipolar transistor, which had by the sixties been developed to handle a few tens of amperes and block a few hundred volts, arrived as the first competitor to the SCR. It is superior to the SCR in its turn-off capability, which could be exercised via its control terminals. This permitted the replacement of the SCR in all forced-commutated inverters and choppers. However, the gain (power) of the SCR is a few decades superior to that of the Bipolar transistor and the high base currents required to switch the Bipolar spawned the Darlington. Three or more stage Darlingtons are available as a single chip complete with accessories for its convenient drive. Higher operating frequencies were obtainable with a discrete Bipolars compared to the 'fast' inverter-grade SCRs permitting reduction of filter components. But the Darlington's operating frequency had to be reduced to permit a sequential turn-off of the drivers and the main transistor.

Further, the incapability of the Bipolar to block reverse voltages restricted its use.The Power MOSFET burst into the scene commercially near the end seventies. This device also represents the first successful marriage between modern integrated circuit and discrete power semiconductor manufacturing technologies. Its voltage drive capability – giving it again a higher gain, the ease of its paralleling and most importantly the much higher operating frequencies reaching upto a few MHz saw it replacing the Bipolar also at the sub-10 KW range mainly for SMPS type of applications.

Extension of VLSI manufacturing facilities for the MOSFET reduced its price vis-à-vis the Bipolar also. However, being a majority carrier device its on-state voltage is dictated by the RDS(ON) of the device, which in turn is proportional to about V2.3DSS rating of the MOSFET. Consequently, high-voltage MOSFETS are not commercially viable.

Improvements were being tried out on the SCR regarding its turn-off capability mostly by reducing the turn-on gain. Different versions of the Gate-turn-off device, the Gate turn-off Thyristor (GTO), were proposed by various manufacturers - each advocating their own symbol for the device. The requirement for an extremely high turn-off control current via the gate and the comparatively higher cost of the device restricted its application only to inverters rated above a few hundred KVA.The lookout for a more efficient, cheap, fast and robust turn-off-able device proceeded in different directions with MOS drives for both the basic thysistor and the Bipolar.

The Insulated Gate Bipolar Transistor (IGBT) – basically a MOSFET driven Bipolar from its terminal characteristics has been a successful proposition with devices being made available at about 4 KV and 4 KA. Its switching frequency of about 25 KHz and ease of connection and drive saw it totally removing the Bipolar from practically all applications. Industrially, only the MOSFET has been able to continue in the sub – 10 KVA range primarily because of its high switching frequency.

The IGBT has also pushed up the GTO to applications above 2-5 MVA.Subsequent developments in converter topologies – especially the three-level inverter permitted use of the IGBT in converters of 5 MVA range. However at ratings above that the GTO based converters had some space. Only SCR based converters are possible at the highest range where line-commutated or load-commutated converters were the only solution. The surge current, the peak repetition voltage and I2t ratings are applicable only to the thyristors making them more robust, specially thermally, than the transistors of all varieties.

Presently there are few hybrid devices and Intelligent Power Modules (IPM) are marketed by some manufacturers. The IPMs have already gathered wide acceptance. The 4500 V, 1200 A IEGT (injection-enhanced gate transistor) or the 6000 V, 3500 A IGCT (Integrated Gate Commutated Thyristors) which are promising at the higher power ranges. However these new devices must prove themselves before they are accepted by the industry at large.