My Understanding of the Power Electronics.docx

1、My Understanding of the Power ElectronicsMy Understanding of the Power ElectronicsEea2 zhangdi(001013)Abstract This paper describes what I have learned and understood about Power Electronics after this semester. More words are concentrated on my understanding, so that the characters and the points i

2、mportant in common circuit analysis are emphasized instead of the complex calculation and analysis of the concrete circuits. I exceeded my utmost to state the foundation and basic conceptions of some chapters in my own words instead of just repeating the book or the PPT, such as the DC-DC converter

3、and so on.Key word: Power Electronics, DeviceIntroductionElectric power is used in almost every part of modern home and society. An efficient use of power, therefore, is not only tremendously advantageous but has become essential. To meet the demand, power electronics came into beings and developed

4、very quickly in the 20th century, especially during the last 20 years, with the advancement of power semiconductor. It can be said that power electronics is just the application of solid-state electronics for the control and conversion of electric power. But much different from the usual electronics

5、, power electronics is characterized by basing primarily on the switching of the power semiconductor devices, with small power loss occurring. In practice, power electronics are usually used in converters. According to the forms of the power transferred, converters can be divided into four kinds, na

6、mely ac-ac, dc-dc, ac-dc and dc-ac. Each of them has unique applications:AC-DC: a)dc power supplies for computers and electronic equipment b)Battery charging systemc)High voltage dc(HVDC) transmission converters which is more economical than the counterpartDC-DC:a)switch-mode power supplies b)DC mot

7、or drives c)as interfaces between the dc systems of different voltage levelDC-AC:a)the battery-operated power systems such as the Uninterruptible Power Supplied (UPS) for hospitals b)AC motor drivesAC-AC:a)variable speed drives for appliances and tools b)four-quadrants PWM drives for traction c)stee

8、l mill roll drivesMany others are not listed one after another and sometimes a combination of them is required. It is easy to conclude that power electronics have already found an important position in modern technology, and are used in various places including in our rooms. Thanks to power electron

9、ics, the electricity needed to run the things we use everyday is processed, with maximum efficiency, smallest size and minimal weight. Recent advances in semiconductor switching devices have contributed very significantly to research and reengineering in the field of modern power electronics and it

10、will continue, so we can say that power electronics will become increasingly popular and used more widely in the future.Power DevicesTo control the energy flow, the basic function of the power converter, the Power Devices act as switches. Not the same as an ideal switch, with non-zero voltage drop w

11、hen turned on and non-zero leakage current when turned off, the product of the voltage and current of the power devices is not identically zero at all times, thus producing power loss. During switching period, the ratio of the change of the current and voltage should also be limited to some level to

12、 avoid the devices permanent damage, for example, an extensive rating of may cause a hot point and if the voltage changes too fast, a high displacement current may be imposed on the device, damaging the device as a result.Diode, SCR, GTO, BJT, MOSFET, and IGBT are the common power devices. The chara

13、cters of them will be discussed separately.Among all the static switching devices used in power electronics, the power diode is the simplest. It is also the only uncontrolled one and the only that can not sustain a forward voltage. The voltage and current it can withstand are up to 500V and 500A and

14、 the switching time is about 100limited by the recovery time shown in fugure1. To increase the switch frequency, Fast Recovery Diode must be employed.Figure1Thyristors are usually three-terminal devices with four layers of alternating p and n types. All thyristors types are controlled in switching o

15、n, but the silicon-controlled rectifier (SCR) is not controllable when turned off. With a pulse firing current, fast rising and sufficient length, SCR is turned on, and after that, the gate signal can be removed, leaving the forward-conduction remained, which is an import distinction between thyriso

16、rs and other types of power electronic devices.Gate-turn-off thyristor (GTO) is another three-terminal power semiconductor devices belonging to the thyristor family. They also belong to a group of power semiconductor devices that have the ability to be fully controlled on and off via the control ter

17、minal. Unlike the SCR, the latching current is higher and the gate current should be kept continuous. When turned off, a negative voltage less than the breakdown should be applied across the gate-cathode, producing a reverse gate current with a peak value of 1/51/3 of the anode current established i

18、n less than 1s and some obvious power loss occur simultaneously. BJT used to be the traditional components for driving several of these industrial applications. However, IGBT and MOSFET technology have developed so much that BJT are not popular now.At turn-off, the base current should be reduced at

19、a ratio the collector current can follow, so as to avoid a risk of break-down. The history of power electronics emerged as a separate discipline when Power MOSFET was introduced as the first kind of the voltage-controlled power devices. Hence, sample control circuits are required and switching speed

20、 is increased obviously with much less stored charge. Another advantage is its positive temperature coefficient for resistance, so the paralleling of devices will be much simpler. But because the narrow induced conducting channel, the power rating is low. Figure2In order to improve the power devices

21、 performance it is advantageous to have the low on-state resistance of power BJT with an insulated gate input similar to that of a power MOSFET simultaneously. That is the IGBT (Insulated Gate Bipolar Transistor), a device containing the advantage of both the BJT and power MOSMET. Nowadays, some new

22、 devices have been developed closer to the ideal electronic switches, such as MCT (MOS-controlled thyristor) and IPM (Intelligent Power Module). They may substitute some devices mentioned above in close future.In practice, the power devices are selected based on the switching speed and power handlin

23、g capability (voltage blocking and current carrying capability). Thyristors are typically used at the highest energy level in power conducting circuits because they are designed to have the largest current and voltage up to 6000V and 6000A. Many medium-power circuits and particularly low-power circu

24、it generally make use of power MOSFET, or IGBT as the main switching elements because of the considerable ease in controlling them. While power diodes are used throughout all levels of power circuits and systems for component protection and wave shaping.One thing must be accentuated. The voltage and

25、 current of any power devices must be kept within the safe operating area (SOA) at all times. Otherwise, it may be damaged. A typical safe operating area (SOA) for a BJT is shown is figure2 and the counterparts for the other devices are similar to this.Rectifying CircuitsA rectifier is one kind of c

26、onverters which links an AC supply to a DC load. But the DC voltage is not on a constant level, as from a battery, but containing an AC ripple added on the mean level. Converter circuits employing diodes are termed uncontrolled while the incorporation of thyristor results in a controlled converter.

27、The functional difference is that the turn-on of the forward-biased thyristor can be controlled from the gate, whereas the diode can not, so an uncontrollable converter provides a fixed output voltage for a given AC supply. Converter involving both the diode and thyristor is termed half-controlled w

28、hich allows an adjustable output voltage by change the delay angle relative to the supply voltage zero just like the full controlled one. But it can not allow the power flow reverse from the DC load into the supply, called inversion, while the full controlled converter can fulfill it.Considering the

29、 uncontrolled converter can be treated as a special example of the full controlled converter when the delay angle is zero, and a combination of the two kinds synthesizes the half-controlled one, the full-controlled converter is mainly discussed.The AC-DC converter topologies may be broadly classifie

30、d into two categories half-wave and full-wave. In the former one each line of AC supply involves only one rectifying device and all cathodes of devices are connected to a common connection first and get back to an AC supply neutral via the DC load. The current in each AC supply line is unidirectiona

31、l. The full-wave circuits (bridge circuits) are combined by two half-wave circuits in series with current alternating in each AC supply line, so no AC supply neutral is needed.The single-phase half- wave connection shown in figure 1 is very simple but illustrates fundamentals. It can be seen that lo

32、ad current flows not only during the positive part of the supply voltage, but also during a portion of the negative supply voltage. The energy stored in load inductor maintains the current, and the inductors terminal voltage changes so as to overcome the negative supply and keep the diode forward-biased and conducting. If the inductor is large enough the current will be continuous, which is a usual assumption in analyzing other circuits. The diode is a commutating diode serving one or both two functions. One is to prevent negative load voltage; the other is to allow the thyristor to re