Power switches


Diodes are purely static power switching devices and are used extensively with thyristor and transistor power schemes. Transistors are relatively cheaper and easy to handle compared to thyristors. The latter are more expensive and more complex as noted below. This text deals with the application of such devices for the control of induction motors, which can now be employed to perform variable duties through its stepless speed control by close monitoring of load requirements during a particular process or while performing a specific duty cycle. The controls are assisted by microprocessor-based, open- or closed-loop control techniques, which can sense and monitor many variables such as speed, flow of material, temperature, pressure or parameters important for a process or a duty cycle. With these techniques, it is possible to achieve any level of automation. Open-loop systems are used where high accuracy of controls and feedback is not so important and closed-loop where a high degree of accuracy of control is essential. With solid-state technology it is now possible to utilize a conventional machine to perform a variable duty. Transistors so far have been developed to handle currents up to 2000 A and voltages up to 1200 V and are utilized for low-capacity power requirements. Thyristors have been developed up to 3000 A and voltages up to 10 kV and are employed for large power requirements such as HV d.c. transmission and static VAr controls. With the variety of such devices and their number of combinations, it is possible to achieve any required output variation in  [c.112]

Use of a snubber circuit across a power switching  [c.132]

Motor-operated interrupting devices are employed when the system requires remote-controlled power switching, as for an auto-reclosing scheme. The electrical interlocking schemes remain generally the same, as discussed earlier, but with an additional circuit for the motor spring charging mechanism and the closing coil of the interrupter. Brief details of the electrical closing features are as follows.  [c.382]

Standard for control cables Power switching equipment Panel boards  [c.399]

What is the peak current through the power switches  [c.28]

What is the maximum operating voltage across the power switches  [c.28]

The higher the maximum voltage the power switches experience, the greater the likelihood that they will exceed their safe operating areas (SOA). Voltage spikes are very common within switching power supplies, and the opportunity of these spikes exceeding the avalanche voltage rating of the power switch becomes more likely. For transformer isolated topologies, the industry has settled into certain topologies that they use within the different ranges of applications. This is shown in Figure 3-8.  [c.29]

Selecting the power switches and rectifiers  [c.35]

Each topology has predictable voltage and current stresses for the power switches and rectifiers. These estimates have about a 90 percent confidence factor. Selecting the power devices at this stage in the design cycle can save precious time later in the program by not having to wait for parts. Table 3-2 contains equations that may be conservative in nature, but will work in the application.  [c.35]

Power switches (refer to Sections 3.4 and 3.7)  [c.124]

The second significant area is the power switch driver circuit. For bipolar power transistors used as power switches, the base drive current must be greater than the peak drain current divided by the gain (fife) of the transistor. A typical gain of a power transistor is between 5 and t5, which means, for example, a peak current of tO A would require a base current of between 0.66 and 2 A. The base-emitter has a drive voltage of 0.7 V and if this current is not derived from a voltage very close to this voltage then a significant loss will result.  [c.139]

The disadvantage of the quasi-resonant converter compared to the newer lossless snubber and active clamp techniques in addition to the basic PWM converters, is the voltage or current stresses placed upon the power components. The peak voltage or current values that exist within quasi-resonant converters can be two to three times higher than in PWM converters. This forces the designer to use higher-rated power switches and rectifiers which may not have as good conduction characteristics.  [c.151]

Because current or voltage waveforms within QR converters are sinusoidal, the peak values are higher than those equivalent parameters found in PWM switching power supplies where the waveforms are typically rectangular or trapezoidal. Qne can expect the peak values to be about f.5 or more times higher than PWM topologies. The ZCS QR supplies present a high current stress upon the power switches, and the ZVS QR supplies present a high voltage  [c.155]

Since full-bridge converters occupy the highest output power region of the topologies, their power switch losses pose a particularly significant problem. In the normal PWM full-bridge converter, the power switches in opposing corners are switched simultaneously. This practice completely releases the primary winding from any low impedance ac ground. This results in large spikes and ringing caused by the leakage inductance of the primary and any residual magnetization inductance. Historically, this noise can only be reduced by the use of a lossy snubber network.  [c.161]

Pumps, compressors, turbines, drivers, and auxiliary machinery should be designed to provide reliable, rugged performance. Pump selection and performance depend on the capacity required and tlie nature of Uie fluids involved. Remotely controlled power switches and shutoff valves are necessary to control fluid flow during an emergency. The inlets for air compressors should be strategically located to prevent the intake of hazardous materials.  [c.495]

RCT are designed to successfully solve a whole number of tasks in nuclear power when testing fuel elements, in aviation and space industry when testing construction materials, nozzles and engine units, turbine blades and parts, in electromechanical industry-cables switching elements, electric motors in defense sphere- charges, equipment in prospecting for research of rock distribution and detection of precious stones in samples.  [c.598]

The device operates as described below. After switching on the power source, CB begins to be charged from Ac. CS provides direct charging current of 0.1 A that makes it possible not to exceed Ac permissible discharging current value as well as to minimize CB charging time till approximately 10s.  [c.651]

In order to prevent information loss in case of switching off the general power source, IC WM is supplying through the supervisor MAX 691 from accumulator of 3.6 V and 60 mA h which is only connected with WM in case of supply has stopped. In normal conditions the above accumulator is being charged from power source of -t 5 V.  [c.651]

Ultrasonic waves are generated by a Q-switched Nd Yag laser operating at the wavelength of 1.064 /xm with a half-width pulse duration of 15 ns, while a Mach-Zehnder heterodyne interferometer is used for the detection of ultrasound (Fig. 1). The probe, with a large bandwidth 20 kHz - 30 MHz, only measures the out-of-plane displacements with a sensitivity of about 10 run/ /Hz on a mirror-like surface [2]. The laser beam is focused on the surface of the sample by a spherical or a cylindrical lens to form a circular spot or a line source, respectively. The optical power density is adjusted to avoid any damage ensuring a non-destructive testing (thermoelastic regime). Mechanical displacements drive the movement of the sample according to two directions, which is, as the data processing, entirely controlled by computer. The ultrasonic images are visualized (B-scan views) thanks to the softwares developed by our laboratory.  [c.694]

Figure Bl.14.9. Imaging pulse sequence including flow and/or diflfiision encoding. Gradient pulses before and after the inversion pulse are supplemented in any of the spatial dimensions of the standard spin-echo imaging sequence. Motion weighting is achieved by switching a strong gradient pulse pair G, (see solid black line). The steady-state distribution of flow (coherent motion) as well as diffusion (spatially Figure Bl.14.9. Imaging pulse sequence including flow and/or diflfiision encoding. Gradient pulses before and after the inversion pulse are supplemented in any of the spatial dimensions of the standard spin-echo imaging sequence. Motion weighting is achieved by switching a strong gradient pulse pair G, (see solid black line). The steady-state distribution of flow (coherent motion) as well as diffusion (spatially
Pulsed, or time-domain, EPR spectrometers have also been developed at higher frequencies up to 140 GHz [55. 56]. They are generally low-power units with characteristically long pulse lengths (typically 50 ns for a n/2-pulse) due to tire limited MW powers available at millimetre wavelengths and the lack of fast-switching  [c.1586]

The most useful direct modulation teclmique is the current gain switching of semiconductor laser devices. This technique is unique to semiconductor sources, nearly all other lasers are modulated externally. In tliese devices tire excitation current of tire laser is modulated, resulting in modulated gain and tlierefore modulated output power. A detailed analysis of tliis process is found in [27]. Simply put, an oscillating current of tire fonn  [c.2872]

The computer time required for a molecular dynamics simulation grows with the square of the number of atoms in the system, because of the non-bonded interactions defined in the potential energy function (Eq. (32)). They absolutely dominate the time necessary for performing a single energy evaluation and therefore the whole simulation. The easiest way to speed up calculations is to reduce the number of non-bonded interactions by the introduction of so-called cutoffs. They can be ap-phed to the van der Waals and electrostatic interactions by simply defining a maximum distance at which two atoms are allowed to interact through space. If the distance is greater than this, the atom pair is not considered when calculating the non-bonded interactions. Several cutoff schemes have been introduced, from a simple sphere to switched or shifted cutoffs, which all aim to reduce the distortions in the transition region that are possibly destabilizing the simulation.  [c.362]

The elution order of solutes in HPLC is governed by polarity. In a normal-phase separation the least polar solute spends proportionally less time in the polar stationary phase and is the first solute to elute from the column. Retention times are controlled by selecting the mobile phase, with a less polar mobile phase leading to longer retention times. If, for example, a separation is poor because the solutes are eluting too quickly, switching to a less polar mobile phase leads to longer retention times and more opportunity for an acceptable separation. When two solutes are adequately resolved, switching to a more polar mobile phase may provide an acceptable separation with a shorter analysis time. In a reverse-phase separation the order of elution is reversed, with the most polar solute being the first to elute. Increasing the polarity of the mobile phase leads to longer retention times, whereas shorter retention times require a mobile phase of lower polarity.  [c.580]

If the first pair of switches is examined, one is off and the other on, and the result of touching each must be a resulting on (off-on and on-off, giving a total of on). For the other pair, exactly the opposite sequence is present but the net result is on. As far as the machine is concerned, the result is on, on, which in binary code is 11 and in decimal code is 3, the correct answer. Therefore, to get the machine to add in binary, it is necessary to have a switch for each power of two that we want. The number 2 is 64 (decimal) and, to represent any number up to 63, we must have seven switches (seven flip-flop circuits), viz., 2, 2, 2, 2, 2 , and zero. In computer jargon, these  [c.306]

As the voltage is increased, intermittent discharges occur because the discharge loses energy to its surroundings. Eventually the discharge becomes self-sustaining and, by maintaining a constant current flow, the discharge continues and light is emitted until the power is switched off.  [c.388]

These are the basic transistors (triodes) and are illustrated in Figure 6.15. They are unidirectional and controllable and are capable of handling large currents and high voltages and also possess high switching speeds (faster than thyristors). However, they require a high base current due to the high voltage drop across the device, which causes a high loss and dissipation of heat. This adverse feature of their characteristics renders them unsuitable as power switching devices for efficient power conversion. Therefore they are generally used as electronic control devices rather than power devices in electronic control circuits and arc not produced at higher ratings.  [c.112]

The amount of voltage appearing across the primary of the transformer is indicative of how much peak current is flowing through the power switches. Switching power supplies are constant power circuits. That is, the lower the primary voltage, the higher the peak currents in order to provide the needed output power. For power transistors and MOSFETk in TO-220 packages and smaller, a maximum peak current limit of 20A is recommended. Above 20A, the failure modes of the power switches become very erratic and the power devices are difficult to protect. By using another topology, the peak current can be reduced.  [c.29]

T e flyback topology (see Figure 3-f2) is the favorite below fOO to f50W because of its low parts count (hence cost) and intrinsically better efficiency. But because its peak currents are much higher than the forward-mode converters, it reaches the SOA limits of the power switches at a relatively low output power. Between an output power of f50 and 500 W the half-bridge (see Figure 3-f5) becomes the favorite. The parts cost more but they are still reasonable. The half-bridge converter only places one-half of the input voltage across the primary winding and therefore exliibits fairly high peak currents. It therefore is only used to 500 W or less. Above 500 W and into many kilowatts, the full-bridge (see Figure 3-16) topology is used. This requires four power switches, two of which have floating drive circuits, and is the most costly to implement, but at these output power levels the added cost is necessary. The push-pull (see Figure 3-14) can also be used in this region, but it suffers from a potentially severe failure mode called core imbalance. This is where the flux within the transformer will operate non-symmetrically about a zero balance point. This will cause the  [c.29]

When the topology has multiple power switches, then multiply the Pioss(ckt) by  [c.36]

There are two major types of power switches used today the bipolar power transistor (BJT) and the power MOSFET. The IGBT (integrated gate bipolar transistor) is used in the higher power industrial applications, such as 1 kW power supplies and electronic motor drives. The IBGT has a slower turn-off than does the MOSFET, so it is typically used for switching frequencies of less than 20 kHz.  [c.63]

Many times it is desirable to shut the product off during brownout conditions. These are conditions where the input voltage falls below the minimum specification for normal operation. This condition can cause a voltage-mode controlled power supply to enter a type of latch-up mode where the supply would quickly jump to its maximum duty cycle and would no longer control the output regulation. This could be destructive to the supply and the loads when the input voltage returns to normal levels. Also, at the lower input voltages, the higher peak currents flowing through the power switches can cause them to fail due to overdissipation. To avoid this, a simple voltage comparator sensing the input line is needed as in Figure 3-56.  [c.91]

Depending on the electron and ion temperatures in a plasma, all of the processes mentioned in this section of the encyclopedia may be taking place simultaneously in the plasma [163]. Understandmg, or modelling, the plasma may be quite complicated [164]. Flame chemistry involves charged particles [165]. Most of the early investigations and classifications of electron and ion interactions came about in attempts to understand electric discharges, and these continue today in regard to electric power devices, such as switches and high-intensity lamps [166]. Often the goal is to prevent discharges in the face of high voltages. Military applications involving the earth s ionosphere fimded refined work during and following the Second World War. Newer applications such as gas discharge lasers have driven recent studies of plasma chemistry. The rare-gas halide excimer laser is a marvellous example of plasma chemistry, because the lasing molecule may be fomied in  [c.820]

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation.  [c.1281]

Figure Bl.14.3. Echo-planar imaging (EPI) pulse sequence. In analogy to the pure gradient-echo recalled pulse sequence a series of echoes GE1-GE6 is refocused by altematingly switching between positive and negative readout gradients. During the readout gradient switching a small phase-encoding gradient pulse (blip) is applied. The spatial phase encodmg is hence stepped tluough the acquired echo train. Figure Bl.14.3. Echo-planar imaging (EPI) pulse sequence. In analogy to the pure gradient-echo recalled pulse sequence a series of echoes GE1-GE6 is refocused by altematingly switching between positive and negative readout gradients. During the readout gradient switching a small phase-encoding gradient pulse (blip) is applied. The spatial phase encodmg is hence stepped tluough the acquired echo train.
An alternative method to obtain accurate values of the spin-lattice relaxation time is provided by the TREPR teclmique with gated MW irradiation, also called the MW-switched time integration method (MISTI) [13. 14]. The principle is quite simple. The MW field is switched on witli a variable delay x after the laser flash. The amplitude of the transient signal plotted as a fiinction of x renders tlie decay of the spin-polarized initial magnetization towards its equilibrium value. This method is preferred over the TREPR technique at low MW power (see equation (b 1.15.31)) since the spin system is allowed to relax in the absence of any resonant MW field in a true spin-lattice relaxation process. The experiment is carried out by adding a PfN diode MW switch between the MW source and the circulator (see figure Bl.15.4, and set between a pair of isolators. Since only low levels of MW power are switched (typically less than 1 W), as opposed to those in ESE and FT EPR, the detector need not be protected against high incident power levels.  [c.1566]

Mitosis is characterized [99] by steady elongation (at a velocity v, depending inter alia on the concentration of monomeric tubulin) of the microtubule (polymerized tubulin) filaments which search for, and ultimately mechanically separate freshly replicated DNA prior to cell division, punctuated by their abmpt slirinkage (with velocity v ). This dynamic instability is characterized by length fluctuations of the order of the mean microtubule length, hinting at a phase transition, denote the frequency of switching from growth to slirinkage  [c.2832]


See pages that mention the term Power switches : [c.113]    [c.182]    [c.514]    [c.1440]    [c.1574]    [c.2985]    [c.10]    [c.343]   
Power supply cookbook (2001) -- [ c.101 , c.164 ]