Power Electronics - MOSFET

Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is a type of transistor used to switch electronic signals. It has four terminals namely; source (S), Drain (D), Gate (G) and Body (B).The MOSFET’s body is normally connected to the terminal of the source(S), which results in three-terminal device similar to other field effect transistors (FET). Since these two main terminals are usually interconnected via short circuit, only three terminals are visible in electrical diagrams.

It is the most common device in circuits that are both digital and analogue. Compared to the regular transistor, a MOSFET needs low current (less than one mill-ampere) to switch ON. At the same time, it delivers a high current load of more than 50 Amperes.

Operation of a MOSFET

MOSFET has a thin layer of silicon dioxide, which acts as the plate of a capacitor. The isolation of the controlling gate raises the resistance of the MOSFET to extremely high levels (almost infinite).

The gate terminal is barred from the primary current pathway; thus, no current leaks into the gate.

MOSFETs exist in two main forms −

• Depletion state − This requires the gate-source voltage (V_GB) to switch the component OFF. When the gate is at zero (V_GB) the device is usually ON, therefore, it functions as a load resistor for given logic circuits. For loading devices with N-type depletion, 3V is the threshold voltage where the device is switched OFF by switching the gate at negative 3V.
• Enhancement state − The gate-source voltage (V_GB) is required in this state to switch the component ON. When the gate is at zero (V_GB) the device is usually OFF and can be switched ON by ensuring the gate voltage is higher than the source voltage.

Symbol and Basic Construction

Where, D − Drain; G − Gate; S − Source; and Sub − Substrate

Power Electronics - IGBT

The insulated gate bipolar transistor (IGBT) is a semiconductor device with three terminals and is used mainly as an electronic switch. It is characterized by fast switching and high efficiency, which makes it a necessary component in modern appliances such as lamp ballasts, electric cars and variable frequency drives (VFDs).

Its ability to turn on and off, rapidly, makes it applicable in amplifiers to process complex wave-patterns with pulse width modulation. IGBT combines the characteristics of MOSFETs and BJTs to attain high current and low saturation voltage capacity respectively. It integrates an isolated gate using FET (Field effect transistor) to obtain a control input.

IGBT Symbol

The amplification of an IGBT is computed by the ratio of its output signal to its input signal. In conventional BJTs, the degree of gain (β) is equal to the ratio of its output current to the input current.

IGBT has a very low value of ON state resistance (RON) than a MOSFET. This implies that the voltage drop (I²R) across the bipolar for a particular switching operation is very low. The forward blocking action of the IGBT is similar to that of a MOSFET.

When an IGBT is used as controlled switch in a static state, its current and voltage ratings equal to that of BJT. On the contrary, the isolated gate in IGBT makes it easier to drive BJT charges and hence less power is required.

IGBT is switched ON or OFF based on whether its gate terminal has been activated or deactivated. A constant positive potential difference across the gate and the emitter maintains the IGBT in the ON state. When the input signal is removed, the IGBT is turned OFF.

IGBT Principle of Operation

IGBT requires only a small voltage to maintain conduction in the device unlike in BJT. The IGBT is a unidirectional device, that is, it can only switch ON in the forward direction. This means current flows from the collector to the emitter unlike in MOSFETs, which are bi-directional.

Applications of IGBT

The IGBT is used in medium to ultra-high power applications, for example traction motor. In large IGBT, it is possible to handle high current in the range of hundred amperes and blocking voltages of up to 6kv.

IGBTs are also used in power electronic devices such as converters, inverters and other appliances where the need for solid state switching is necessary. Bipolars are available with high current and voltage. However, their switching speeds are low. On the contrary, MOSFETs have high switching speeds although they are expensive.

Power Electronics - Switching Devices

A power electronic switching device is a combination of active switchable power semiconductor drivers that have been integrated into one. The main characteristics of the switch are determined by internal correlation of functions and interactions of its integrated system. The figure below shows how a power electronic switch system works.

The external circuit of the above diagram is usually held at a high potential relative to the control unit. Inductive transmitters are used to support the required potential difference between the two interfaces.

Features of switch selection :

• Fast
• Low power gate
• High breakdown voltage
• Low loss during off
• Cheap
• 
  

Power Electronics - Introduction

Power Electronics refers to the process of controlling the flow of current and voltage and converting it to a form that is suitable for user loads. The most desirable power electronic system is one efficiency and reliability is close to 100%. Basic block diagram is as below.

A power electronic system converts electrical energy from one form to another and ensures the following is achieved −

• • Maximum efficiency
  • Maximum reliability
  • Maximum availability
  • Minimum cost
  • Least weight
  • Small size
  
  

Smart grid : An overview

A smart grid is a modernized electrical grid that uses information and communications technology to gather and act on information, such as information about the behaviors of suppliers and consumers, in an automated fashion to improve the efficiency, reliability, economics, and sustainability of the production and distribution of electricity. Electric power generated by wind turbines and hydroelectric turbines by using induction generators can cause variances in the frequency at which power is generated. Power electronic devices are utilized in these systems to convert the generated ac voltages into high-voltage direct current (HVDC). The HVDC power can be more easily converted into three phase power that is coherent with the power associated to the existing power grid. Through these devices, the power delivered by these systems is cleaner and has a higher associated power factor. Wind power systems optimum torque is obtained either through a gearbox or direct drive technologies that can reduce the size of the power electronics device. Electric power can be generated through photovoltaic cells by using power electronic devices. The produced power is usually then transformed by solar inverters. Inverters are divided into three different types: central, module-integrated and string. Central converters can be connected either in parallel or in series on the DC side of the system. For photovoltaic "farms", a single central converter is used for the entire system. Module-integrated converters are connected in series on either the DC or AC side. Normally several modules are used within a photovoltaic system, since the system requires these converters on both DC and AC terminals. A string converter is used in a system that utilizes photovoltaic cells that are facing different directions. It is used to convert the power generated to each string, or line, in which the photovoltaic cells are interacting.

Power electronics application in a laptop

A lithium battery powers the system and several dc-dc converters change the battery voltage into the voltages required by the loads. A buck converter produces low voltage dc  required by the microprocessor. A  boost converter increases the battery voltage to the level needed by the disk drive. An inverter produces high voltage high frequency ac to drive the lamps that light the display. A transformer isolation converts the ac line voltage into dc to charge the battery. The  converter switching frequencies are in the range of several 100 KHz which leads to the substantial reduction in weight and size of the reactive elements. Power management is used to control sleep modes in which power consumption is reduced and battery life is extended.

Renewable energy : What to Expect!

Energy is essential to our society to ensure our quality of life and to underpin all other elements of our economy. Renewable energy technologies offer the promise of clean, abundant energy gathered

from self-renewing resources such as the sun, wind, earth, and plants. Virtually all regions of the world have renewable resources of one type or another. Currently  most of renewable energy  comes from hydro power and traditional biomass sources. Wind, solar, biomass, and geothermal technologies are cost-effective today in an increasing number of markets, and are making important steps to broader commercialization. Each of the renewable energy technologies is in a different stage of research, development, and commercialization, and all have differences in current and future expected costs, current industrial base, resource availability, and potential impact on greenhouse gas emissions. The technical status, cost, and applications of major renewable energy technologies and implications for increased adoption of renewable is being reviewed.

From the dawn of human civilization to about 100 years ago, the sources of energy used by mankind were predominantly human and animal muscle and wood, with lesser amounts of solar, wind, hydro, and geothermal. With the discovery of oil, the development of natural gas fields, and the widespread distribution of electricity from coal-powered central power plants, fossil fuels became the predominant sources of energy in the United States and the world. Is there another major transition ahead for energy? Can the renewable resources that sustained early civilization be harnessed with enough efficiency and availability and at a cost to meet a significant portion of the much higher energy needs of today’s society? Although there are always risks in predictions, the convergence

of some of today’s trends suggests interesting possibilities. In summary, today’s converging trends related to renewable energy include:

• relevant scientific discoveries and engineering progress;

• emergence of a new economic structure in the electricity sector;

• trends toward decentralization and modularity;

• explosive advances in computers, information, and telecommunications;

• globalization of markets;

• persistent, pervasive support for a clean environment;

• limited future availability of inexpensive fossil fuels.

Do these converging trends mean that we are on the brink of a new energy transition from fossil fuels to renewables? Several independent entities have recently developed scenarios indicating that renewable energy will play a major role in the energy mix for the world, with increasing impacts beginning as early as 2000–2010 and major impacts by 2050. In both of the Shell International scenarios, energy contributions from conventional energy resources begin to level off in 20–30 years, with petroleum consumption actually decreasing . According to these scenarios, the increases in world energy demand will be supplied by renewable energy technologies, which will provide 30%–50% of world energy by 2050.