Switching-aid Circuits with Energy Recovery

Passive turn-on and turn-off snubber circuits for the IGBT transistor, the GCT and the GTOthyristor have been considered in chapter 7. These snubber circuits modify the device I-V switching trajectory and in so doing reduce the device transient losses. Snubber circuit action involves temporary energy stored in either an inductor or capacitor. In resetting these passive components it is usual to dissipate the stored energy in a resistor as heat. At high frequencies  these losses (being proportional to frequency) may become a limiting factor because of the difficulties  associated with equipment cooling. Instead of dissipating the switching-aid circuitstored energy, it may be viable to recover the energy back into the dc supply or into the load, or both. Two classifications of energy recovery circuits exist, either passive or active. A  passive recovery  circuit involves only passive components such as  L  and  C  while  active recovery techniques involve extra switching devices, as in a switched-mode power supply, smps.

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Soft Magnetic Materials

Soft magnetic materials are used extensively in power electronic circuits, as voltage and current transformers, saturable reactors, magnetic amplifiers, inductors, and chokes. These magnetic devices may be required to operate at only 50/60 Hz, or at frequencies down to dc or over 1 MHz.

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Series and Parallel Device Operation and Protection

 This chapter considers various areas of power device application that are often overlooked, or at best, underestimated. Such areas include parallel and series device utilisation, over-current and overvoltage protection, radio frequency interference (rfi) noise, filtering, and interactive noise effects.

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resistors

Power resistors (≥1W) are used extensively in power electronic circuits, either as a pure dissipative element, or to provide a current limiting path for charging/discharging currents. These energy transfer paths may be either inductive or non-inductive. Resistors are used for the following non-inductive resistance applications.

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Protecting Diodes, Transistors, and Thyristors1

 

All power switching devices attain better switching performance if some form of switching aid circuit, called snubber, is employed.  Snubber activation may be either passive or active which involves extra power switches.  Only passive snubbers, which  are based on passive electrical components, are considered in this chapter, while active snubbers are considered in Chapter 9.  Fundamentally, the MOSFET and  IGBT do not require switching aid circuits, but circuit imperfections, such as stray inductance and diode recovery, can necessitate the need for some form of switch snubber protection.

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Power Switching and their Static Electrical Characteristics

 
There is a vast proliferation of power switching semiconductor devices, each offering various features, attributes, and limitations. The principal device families of concern in the power switching semiconductor range are the diode, transistor, and thyristor. Each family category has numerous different members. The basic characteristics of the three families and a range of their members, both uni-polar and bipolar carrier types, will be presented.

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Power Inverters

Inversion is the conversion of dc power to ac  power at a desired output voltage or current and
frequency. A static semiconductor inverter circuit performs this electrical energy inverting transformation. The terms voltage-fed and current-fed are used in connection with the output from inverter circuits. A  voltage-source inverter (VSI)  is one in which the dc input voltage is essentially constant and independent of the load current drawn. The inverter specifies the load voltage while the drawn current shape is dictated by the load. 

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Load, Switch, and Commutation Considerations

 
Power switching devices are employed for controlling inductive, resistive or capacitive loads. Inductive loads include electrical machines, transformers, solenoids, and relays. High-current in-rush occurs with loads such as incandescent lamps, pulse-forming networks, snubbers, and motors. Incandescent lamps are essentially resistive, but the cold resistive in-rush current during turn-on is 12 to 18 times the steady-state current. This turn-on surge presents special  switch-on problems. Capacitive loads, such as fluorescent lighting, also present high-current in-rush at turn-on. Electromechanical loads, such as  shakers, present loads that vary between capacitive and inductive over their operating frequency range. 

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Electrical Ratings of Power Semiconductor Switching

Semiconductor device characteristics and ratings are  primarily concerned with electrical and thermal properties. The thermal properties and cooling design aspects are similar for all power switching semiconductor devices. A common, unified thermal design approach is applicable since manufacturers use the concept of a semiconductor device being thermally represented by one  virtual junction. This virtual junction is considered as the point source of all losses, which comprise on-state and off-state losses as well as switch-on and switch-off losses and any control input loss.

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Driving Transistors and Thyristors

The thyristor, being a multiple (three) bipolar junction device, is essentially a current-controlled device. As illustrated in figure 7.1a, a  current must be supplied between the gate and cathode terminals to produce cathode injection, hence anode current flow, provided the anode is forward biased. The magnitude of gate drive current determines the delay time and the anode current rise time. In gate commutated thyristors, a negative gate current must be produced, themagnitude determining the turn-off delay time and anode current fall time.

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DC Choppers

A  dc chopper is a dc-to-dc voltage converter. It is a static switching electrical appliance that in one electrical conversion, changes an input fixed dc voltage to an adjustable dc output voltage without inductive or capacitive intermediate energy storage. The name chopper is connected with the fact that the output voltage is a ‘chopped up’ quasi-rectangular version of the input dc voltage. In chapters 11 and 12, thyristor devices were used in conjunction with an ac supply that forces thyristor turn-off at ac supply current reversal. This form of thyristor natural commutation, which is illustrated in figure 13.1a, is termed line or source commutation.

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Cooling of Power Switching Semiconductor

Semiconductor power losses are dissipated in the form of heat, which must be transferred away from the switching junction, if efficient switching is to be maintained. The reliability and life expectancy of any power semiconductor are directly related to the maximum device junction temperature experienced. It is therefore essential that the thermal design determine accurately the maximum junction temperature from the device power dissipation. Every 10°C junction temperature decrease, doubles device lifetime.

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Capacitors

which is just to name a few uses of capacitors in electrical power applications. In each application, the capacitor is subjected to stresses, such as high temperature,  dv/dt  or high ripple current, which must be taken into account in the design and selection process. To make the correct capacitor selection it is necessary to consider various capacitor types, their construction, features, and uses.

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Basic Semiconductor Physics

Power electronic circuits utilise power  semiconductor switching devices which  ideally  present infinite resistance when off, zero resistance when on, and switch instantaneously between those two states.  It is necessary for the power electronics engineer to have a general appreciation of the semiconductor physics aspects applicable to power switching devices so as to be able to understand the vocabulary and the non-ideal device electrical phenomena.  To this end, it is only necessary to attempt a qualitative description of switching devices and the relation between their geometry, material parameters, and physical operating mechanisms.

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AC Voltage Regulators

AC voltage regulators have a constant voltage ac supply input and incorporate semiconductor switches which vary the rms voltage impressed across the ac load. These regulators generally fall into the category of naturally commutating converters since their thyristor switches are naturally commutated by the alternating supply. This converter turn-off process is termed line commutation. The regulator output current, hence supply current, may be discontinuous or non-sinusoidal and as a consequence input power factor correction and harmonic reduction are usually necessary, particularly at low output voltage levels (relative to the input ac voltage magnitude).  A feature of direction conversion of ac to ac is the absence of any intermediate energy stage, such as a capacitive dc link or energy storage inductor.  Therefore ac to ac converters are potentially more efficient but usually involve a larger number of switching devices and output is lost if the input supply is temporarily lost.  There are three basic ac regulator categories, depending on the relationship between the input supply frequency  fs, which is usually assumed single frequency sinusoidal, possibly multi-phased, and the output frequency fo.  Without the use of transformers (or boost inductors), the output voltage rms magnitude VOrms is less than or equal to the input voltage rms magnitude Vs , Vo rms ≤ Vs . •  output frequency increased,   fo > fs, for example, the matrix converter •  output frequency decreased,  fo < fs, for example, the cycloconverter  •  output frequency fundamental = supply frequency,  fo = fs,for example, a phase controller

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