Numerous and frequently-updated resource results are available from this WorldCat. Please choose whether or not you want other users to be able to see on your profile that this library is a favorite of yours. Finding libraries that hold this item You may have already requested this item. Please select Ok if you would like to proceed with this request anyway.
|Published (Last):||11 December 2010|
|PDF File Size:||15.80 Mb|
|ePub File Size:||14.53 Mb|
|Price:||Free* [*Free Regsitration Required]|
Tadeu Monteiro flag Denunciar. It is found almost exclusively in power electronics applications. Turns off only when current becomes zero.
Ratings from 10A up to more than A, and from V up to 6 kV. The SCR is found almost exclusively in power electronics applications, and is the most common member of the thyristor family.
Can substitute for BJTs in applications where power ratings must be very high. The ratings approach those of SCRs, and the speeds are similar as well. Used in inverters rated above about kW. Ratings from 2 to 50A and to V. Used in lamp dimmers, home appliances, and hand tools. Not as rugged as many other device types, but very convenient for many ac applications. Much faster than conventional GTOs, and easier to use.
They act only as switches. Some of the features of the most common power semicon- ductors are listed in Table 1. The table shows a wide variety of speeds and rating levels. As a rule, faster speeds apply to lower ratings. For each device type, cost tends to increase both for faster devices and for devices with higher power-handling capacity. Conducting direction and blocking behavior are fundamen- tally tied to the device type, and these basic characteristics constrain the choice of device for a given conversion func- tion.
Consider again a diode. It carries current in only one direction and always blocks current in the other. Ideally, the diode exhibits no forward voltage drop or off-state leakage cur- rent.
Although it lacks many features of an ideal switch, the ideal diode is an important switching device. Other real devices operate with polarity limits on current and voltage and have corresponding ideal counterparts. The ideal diode is one example of a restricted switch. It therefore represents a forward-conducting reverse-blocking FCRB restricted switch, and operates in one quadrant on a graph of device cur- rent vs. Other restricted switches require a third gate terminal to determine their state.
Consider the polarity possibilities given in Table 1. The quadrant operation shown in the table indicates polarities. For example, the current in a diode will be positive when on and the voltage will be negative when off. This means diode operation is restricted to the single quadrant compris- ing the upper vertical current axis and the left horizontal voltage axis. The other combinations appear in the table. These symbols are used infrequently, but are valu- able for showing the polarity behavior of switching devices.
A circuit drawn with restricted switches represents an idealized power converter. Restricted switch concepts guide the selection of devices.
For example, consider an inverter intended to deliver ac load current from a dc voltage source. A switch matrix built to perform this function must be able to manipulate ac current and dc voltage.
Regardless of the physical arrangement of the matrix, we would expect bidirectional-conducting forward- blocking switches to be useful for this conversion. This is a correct result: modern inverters operating from dc voltage sources are built with FETs, or with IGBTs arranged with reverse-parallel diodes. As new power devices are introduced to the market, it is straightforward to determine what types of converters will use them.
Each element of this matrix, called a switching function, shows whether the corresponding physical device is on or off.
Switching functions are discrete-valued functions of time, and control of switching devices can be represented with them. Figure 1.
It is periodic, with period T , representing the most likely repetitive switch action in a power converter. For convenience, it is drawn on a relative time scale that begins at 0 and draws out the square 12 P. In many converters, the switching function is generated as an actual control voltage signal that might drive the gate of either a MOSFET or some other semiconductor switching device.
The timing of switch action is the only alternative for control of a power converter. Since switch action can be represented with a discrete-valued switching function, timing can be rep- resented within the switching function framework. Based on Fig. The duty ratio, D, is the fraction of time during which the switch is on. For control purposes, the pulse width can be adjusted to achieve a desired result.
We can term this adjustment process pulse-width modulation PWM , perhaps the most important process for implementing control in power converters. For control purposes, frequency can be adjusted. In other converters,.
Tadeu Monteiro flag Denunciar. This displacement current can initiate switching similar to an externally applied gate current. The thyristor is latched and, for SCRs, cannot be returned to a blocking mode by using the gate terminal. This inter- rupts the base current to the pnp transistor, leaving the pnp open-base, causing thyristor turn-off.
Muhammad H. Rashid
Power electronics : circuits, devices, and applications