1、 外文文献及译文 Using Power Semiconductors in Switched Mode Topologies The electric energy is not normally used in the form in which it was produced or distributed. Practically all electronic systems require some form of energy conversion. A device that transfers electric energy from a given source to a gi
2、ven load using electronic circuits is referred to as Power Supply. A typical application of a power supply unit (PSU) is convert utility AC voltage into regulated DC voltage required for electronic equipment. Nowadays in PSU the energy flow is controlled with semiconductors that are continuously swi
3、tching on an off with high frequency. Such devices are referred to as switch mode power supplies or SMPS. They offer greater efficiency compared with linear supplies because a switch can control energy flow with low losses: when a switch is off, it blocks the flow of voltage and current, can be rela
4、tively low in both states. SMPS are also smaller in size and lighter in weight due to the reduced size of passive components and lower heat generation. The industry tended towardminiaturization, advancements in semiconductor technology, as well as various energy efficiency regulations have made swit
5、ch the dominant type of PSRacross practically the full spectrum of applications. Most of the switchers manufactured today for AC input applications include a PFC front end. Power supplies in the mid70s. We identified these as SWITCHMODE products. A switching power supply designed using ON Semiconduc
6、tor components can rightfully be called a SWITCH MODE power supply or SMPS. In general, SMPS can be classified into four types according to the input and output waveforms: AC in, DC out: rectifier, off-line converter input stage DC in, DC out: voltage converter, or current converter, or DC to DC con
7、verter AC in, AC out: frequency changer, cycloconverter, transformer DC in, AC out: inverter For many years the world of power supply design has seen a gradual movement away from the use of linear power supplies to the more practical switched mode power supply (S.M.P.S.). The linear power supply con
8、tains a mains transformer and a dissipative series regulator. This means the supply has extremely large and heavy 50/60 Hz transformers, and also very poor power conversion efficiencies, both serious drawbacks. Typical efficiencies of 30% are standard for a linear. This compares with efficiencies of
9、 between 70% and 80%, currently available using S.M.P.S. designs. Furthermore, by employing high switching frequencies, the sizes of the power transformer and associated filtering components in the S.M.P.S. are dramatically reduced in comparison to the linear. For example, an S.M.P.S. operating at 2
10、0 kHz produces a 4 times reduction in component size, and this increases to about 8 times at 100 kHz and above. This means an S.M.P.S. design can produce very compact and lightweight supplies. This is now an essential requirement for the majority of electronic systems. The supply must slot into an e
11、ver shrinking space left for it by electronic system designers. At the heart of the converter is the high frequency inverter section, where the input supply is chopped at very high frequencies (20 to200kHz using present technologies) then filtered and smoothed to produce dc outputs. The circuit conf
12、iguration which determines how the power is transferred is called the TOPOLOGY of the S.M.P.S., and is an extremely important part of the design process. The topology consists of an arrangement of transformer, inductors, capacitors and power semiconductors. Regulation of the output to provide a stab
13、ilized DC supplyis carried out by the control / feedback block. Generally,most S.M.P.S. systems operate on a fixed frequency pulsewidth modulation basis, where the duration of the on timeof the drive to the power switch is varied on a cycle by cyclebasis. This compensates for changes in the input su
14、pplyand output load. The output voltage is compared to anaccurate reference supply, and the error voltage producedby the comparator is used by dedicated control logic toterminate the drive pulse to the main power switch/switchesat the correct instance. Correctly designed, this will providea very sta
15、ble AC output supply. In most applications, the S.M.P.S. topology contains apower transformer. This provides isolation, voltage scalingthrough the turns ratio, and the ability to provide multipleoutputs. However, there are non-isolated topologies(without transformers) such as the buck and the boostc
16、onverters, where the power processing is achieved byinductive energy transfer alone. All of the more complexarrangements are based on these non-isolated types. ON Semiconductor has been a key supplier of semiconductor products for switching power supplies since we introduced bipolar power transistor
17、s and rectifiers designed specifically for switching Efficient conversion of electrical power is becoming a primary concern to companies and to society as a whole. Switching power supplies offer not only higher efficiencies but also offer greater flexibility to the designer. Recent advances in semic
18、onductor, magnetic and passive technologies make the switching power supply an ever more popular choice in the power conversion arena today. Historically, the linear regulator was the primary method of creating a regulated output voltage. It operates by reducing a higher input voltage down to the lo
19、wer output voltage by linearly controlling the conductivity of a series pass power device in response to changes in its load. This results in a large voltage being placed across the pass unit with the load current flowing through it. This headroom loss causes the linear regulator to only be 30 to 50
20、 percent efficient. That means that for each watt delivered to the load, at least a watt has to be dissipated in heat. The cost of the heatsink actually makes the linear regulator uneconomical above 10 watts for small applications. Below that point, however, they are cost effective in step-down appl
21、ications. The switching regulator operates the power devices in the full-on and cutoff states. This then results in either large currents being passed through the power devices with a low “on” voltage or no current flowing with high voltage across the device. This results in a much lower power being
22、 dissipated within the supply. The average switching power supply exhibits efficiencies of between 70 to 90 percent, regardless of the input voltage. Higher levels of integration have driven the cost of switching power supplies downward which makes it an attractive choice for output powers greater t
23、han 10 watts or where multiple outputs are desired. A topology is the arrangement of the power devices and their magnetic elements. Each topology has its own merits within certain applications. Some of the factors which determine the suitability of a particular topology to a certain application are:
24、 1) Is the topology electrically isolated from the input to the output or not. 2) How much of the input voltage is placed across the inductor or transformer. 3) What is the peak current flowing through the power semiconductors. 4) Are multiple outputs required. 5) How much voltage appears across the powersemiconductors. The first choice that faces the designer is whether to have input to output transformer isolation. Non-isolated switching power supplies are typically used for
