电力电子外文翻译.doc

毕业设计（外文翻译）

题 目 电路与功率二极管器件
系 （院） 自动化系
专 业 电气工程与自动化
学生姓名 陈芬
学 号 2007090403
指导教师 刘应乾
职 称 助 教

2011年 6月15日
15

Electrical Networks and Power Semiconductor Devices
Electrical Networks
An electrical circuit or network is composed of elements such as resistors, inductors, and capacitors connected together in some manner. If the network contains no energy sources, such as batteries or electrical generators, it is known as a passive network. On the other hand, if one or more energy sources are present, the resultant combination is an active network. In studying the behavior of an electrical network, we are interested in determining the voltages and currents that exist within the circuit. Since a network is composed of passive circuit elements, we must first define the electrical characteristics of these elements.
In the case of a resistor, the voltage-current relationship is given by Ohm’s law, which states that the voltage across the resistor is equal to the current though the resistor multiplied by the value of the resistance. Mathematically, this is expressed as
(1-1A-1)
where u=voltage,V; i=current, A; R=resistance, Ω.
The voltage across a pure inductor is defined by Faraday’s law, which states that the voltage across the inductor is proportional to the rate of change with time of the current through the inductor. Thus we have
(1-1A-2)
Where =rate of change of current, ; L=inductance, H.
The voltage developed across a capacitor is proportional to the electric change q accumulating on the plates of the capacitor. Since the accumulation of charge may be expressed as the summation, or integral, of the charge increments dq, we have the equation
(1-1A-3)
where the capacitance C is the proportionality constant relating voltage and charge By definition, current equals the rate of change of charge with time and is expressed as i= . Thus an increment of charge dq is equal to the current multiplied by the corresponding time increment, or dq=idt. Eq.(1-1A-3) may then be written as
(1-1A-4)
where C= capacitance, F.
Active electrical devices involve the conversion of energy to electrical form. For example, the electrical energy in a battery is derived from its stored chemical energy. The electrical energy of a generator is a result of the mechanical energy of the rotating armature.
Active electrical elements occur in two basic forms: voltage sources and current sources. In their ideal form, voltage sources generate a constant voltage independent of the current drawn from the source. The aforementioned battery and generator are regarded as voltage sources since their voltage is essentially constant with load. On the other hand, current sources produce a current whose magnitude is independent of the load connected to the source. Although current sources are not as familiar in practic, the concept does find wide use in representing an amplifying device, such as the transistor, by means of an equivalent electrical circuit.
A common method of analyzing an electrical network is mesh or loop analysis. The fundamental law that is applied in this method is Kirchhoff’s first law, which states that the algebraic sum of the voltages around a closed loop is 0, or, in any closed loop, the sum of the voltage rises must equal the sum of the voltage drops. Mesh analysis consists of assuming that currents-termed loop currents-flow in each loop of a network, algebraically summing the voltage drops around each loop, and setting each sum equal to 0.

Power Semiconductor Devices
Power semiconductor devices constitute the heart of modern power electronic appartus. They are used in power electronic converters in the form of a matrix of on-off switches. And the switching mode power conversion gives high efficiency.
Today’s power semiconductor devices are almost exclusively based on silicon material and can be classified as follow:
Diode
Thyristor or silicon-controlled rectifier (SCR)
Triac
Gate turn-off thyristor (GTO)
Bipolar junction transistor (BJT or BPT)
Power MOSFET
Static induction transistor (SIT)
Insulated gate bipolar transistor (IGBT)
MOS-controlled thyristor (MCT)
Integrated gate-commutated thyristor (IGCT)
Diodes
Power diodes provide uncontrolled rectification of power and are used in applications such as electroplating, anodizing, battery charging, welding, power supplies (DC and AC), and variable-frequency drives. They are also used in feedback and the freewheeling functions of converters and snubbers. A typical power diode has P-I-N stucture, thatis, it is a P-N junction with a near intrinsic semiconductor layer (I-layer) in the middle to sustain reverse voltage.
In the forward-biased condition, the diode can be represented by a junction offset drop and a series-equivalent resistance.The typical forward conduction drop is 1.0V. This drop will cause conduction loss,and the device must be cooled by the appropriate heat sink to limit the junction temperature. In the reverse-biased condition, a small leakage urrent flows due to minority carries, which gradually increase with voltage. If the reverse voltage exceeds a threshold value, called the breakdown voltage, the device goes through avalanche breakdown, which is when reverse current becomes large and the diode is destoroyed by heating due to large power dissipation in the junction.
Power diodes can be classified as follows:
Standard or slow-recovery diode
Fast-recovery diode
Schottky diode
Thyristors
Thyristors, or silicon-controlled rectifiers (SCRs) have been the traditional workhorses for bulk power conversion and control in industry. The modern era of solid-state power electronics started due to the intorduction of this device in the late 1950s. The term ”thyristor” camefrom its gas tube equivalent, thyratron. Often, it is a family name that includes SCR,taiac, GTO,MCT, and IGCT. Thyristors can be classified as standard, or slow phase-control-type and fast-switching,voltage-fed inverter-type. The inverter-type has recently become obsolete.
Basically, it is a three-junction P-N-P-N device, where P-N-P and N-P-N component transistors are connected in regenerative feedback mode. The device blocks volgate in both the forward and reverse direction (symmetric blocking). When the anode is positive, the device can be trggered into conduction by a short positive gate current pulse; but once the device is conducting, the gate loses its control to turnoff the device. A thyristor can also turn on by excessive anode voltage, its rate of rise (), by a rise in junction temperature, or by light shining on the junctions.
At gate current IG = 0, if forward voltage is applied on the device, there will be a leakage current due to blocking of the middle junction. If the voltage exceeds a critical limit (breakover voltage), the device switchs into conduction. With increasing magnitude of IG, the forward breakover voltage is reduced, and eventually at IG3, the device behaves like a diode with the entire forward bloking region removed. The device will turn on successfully if a minimum current, called a latching current, is maintained. During conduction, if the gate current is zero and the anode current is zero and the anode current falls below a critical limit, called the holding current, the device reverts to the forward blocking state. With reverse voltage, the end P-N junctions of the device become reverse-biased. Modern thyristors are available with very large voltage (several kV) and current (several kA) ratings.
Triacs
A triac has a complex multiple-junction structure, but functionally, it is an intergration of a pair of phase-controlled thyristors connected in inverse-parallel on the same chip. A triac is more economical than a pair of thyristors in anti-parallel and its control is simpler, but its integrated constuction has some disadvantages. The gate current sensitivity of a triac is poorer and the turn-off time is longer due to the minority carrier storage effect. For the same reason, the reapplied rating is lower, thus making it difficult to use with inductive load. A well-designed RC snubber is essential for a triac circuit. Triacs are used in light dimming, heating control, alliance-type motor drives, and solid-state relays with typically HZ supply frequency.
GTOs
A gate turn-off thyristor (GTO), as the name indicates, is basically a thyristor-type device that can be turned on by a small positive gate current pulse, but in addition, has the capability of being turned off by a negative gate current pulse. The turn-off capability of a GTO is due to the diversion of P-N-P collector current by the gate, thus breaking the P-N-P / N-P-N regenerative feedback effect. GTOs are available with asymmetric and symmetric voltage-blocking capabilities, which are used in voltage-fed and current-fed converters,respectively. The turn-off current gain of a GTO, defined as the ratio of anode current prior to turn-off to the negative gate current required for turn-off, is very low, typically 4 or 5. This means that a 6000A GTO requires as high as 1,500A gate current pulse. However, the duration of the pulsed gate current and the corresponding energy associated with it is small and can easily be supplied by low-voltage power MOSFETs. GTOs are used in motor drives, static VAR compensators (SVCs), and AC/DC power supplies with high power ratings. When large-power GTOs became available, they ousted the force-commutated, voltage-fed thyristor inverters.
Power MOSFETs
Unlike the devices discussed so far, a power MOSFET (metal-oxide semiconductor field-effect transistor) is a unipolar, majority carrier, “zero junction”, voltage-controlled device.IF the gate voltage is positive and beyond a threshold value, an N-type conducting channel will be induced that permit current flow by majority carrier (electrons) betwwen the drain and the source. Although the gate impedance is extremely high at steady state, the effective gate-source capacitance will demand a pulse current during turn-on and turn-off. The device has asymmetric voltage-bocking capability, and has an integral body diode, which can carry full current in the reverse direction. The diode is characterized by slow recovery and is often bypassed by an external fast-recovery diode in high-frequency applications.
While the conduction loss of a MOSFET is large for higher voltage devices, its turn-on and turn-off switching times are extremely small, causing low switching loss. The device does not have the minority carrier storage delay problem associated with a bipolar device. Although a MOSFET can be controlled statically by a voltage source, it is normal practice to drive it by a current source dynamically followed by a voltage source to minimize switching delaya. MOSFETs are extremely popular in low-voltage, low-power, and high-frequency (hundreds of kHz) switching applications. Application examlles include switching mode power supplies (SMPS), brushless DC motors (BLDMs), stepper motor drives, and solid-state DC relays.
IGBTs
The introduction of insulated gate bipolar transistors (IGBTs) in the mid-1980s was an important milestone in the history of power semiconductor devices. They are extremely popular devices in power electronics up to medium power (a few kW to a few MW) range and are applied extensively in DC/AC drives and power supply systems. They ousted BJTs in the ipper range, and are currently ousting GTOs in the lower power range. An IGBT is basically a hybrid MOS-gated turn-on/off bipolar transistor that combines the advantages of both a MOSFET and BJT. Its architecture is essentially similar to that of a MOSFET, except an additional P+ layer has been added at the collector over the N+ drain layer of the MOSFET. The device has the high-input impedance of a MOSFET, but BJT-like conduction characteristics. If the gate is positive with respect to the emitter, an N-channel is induced in the P region. This forward-biases the base-emitter junction of the P-N-P transistor, turning it on and causing conductivity modulation of the N- region, which gives a significant reduction of conduction drop over that of a MOSFET. At the on-condition, the diver MOSFET in the equivalent circuit of the IGBT carries most of the total terminal current. The thyristor-like latching action caused by the parasitic N-P-N transistor is prevented by sufficiently reducing the resistivity of the p+ layer and diverting most of the current through the MOSFET. The device is turned off by reducing the gate voltage to zero or negative, which shuts off the conducting channel in the P region. The device has higher current density than that of a BJT or MOSFET. Its input capacitance is significantly less than that of a MOSFET. Also, the ratio of gate-collector capacitance to gate-emitter capacitance is lower, giving an improved Miller feedback effect.
MCTs
An MOS-controlled thyristor (MCT), as the name indicates, is a thyristor-like, trigger-into-conduction hybrid device that can be turned on or off by a short voltage pulse on the MOS gate. The device has a microcell construction, where thousand of microdevices are connected in parallel on the same chip. The cell structure is somewhat complex. The MCT is turned on by a negative voltage pulse at the gate with respect to the anode and is turned off by a positive voltage pulse. The MCT has a thyristor-like P-N-P-N structure, where the P-N-P and N-P-N transistor components are connected in regenerative feedback. However, unlike a thyristor, it has unipolar (or asymmetric) voltage-blocking capability. If the gate of an MCT is negative with repect to the anode, a P-channel is induced in the P-FET, which causes forward-biasing of the N-P-N transistor. This also forward-biases the P-N-P transistor and the device goes into saturation by positive feedback effect. At conduction, the drop is around one volt (like a thyristor). If the gate is positive with respect to the anode, the N-FET will saturate and short-circuit the emitter-base junction of the P-N-Ptransistor. This will break the positive feedback loop for thyristor operation and the device will turn off. The turn-off occurs purely by recombination effect and therefore the tail time of the MCT is somewhat large. The device has a limited SOA, and therefore a snubber circuit is mandatory in an MCT converter. Recently, the device has been promoted for “soft-switched” converter applications, where the SOA is not utilized. In spite of complex geomertry, the current density of an MCT is high compared to a power MOSFET, BJT and IGBT