開關電源應用——POS機的電源設計

開關電源應用——POS機的電源設計,開關電源,應用,利用,運用,pos,電源,設計 Switchmode Power Supply Reference ManualTMForwardEvery new electronic product, except those that are battery powered, requires converting off-line 115Vac or 230Vac power to some dc voltage for powering the electronics. The availability of design and application information and highly integrated semiconductor control ICs for switching power suppliers allows the designer to complete this portion of the system design quickly and easily. Whether you are an experienced power supply designer, designing your first switching power supply or responsible for a make or buy decision for power supplies, the variety of information in the SWITCHMODE Power Supplies Reference Manual and Design Guide TMshould prove useful.ON Semiconductor has been a key supplier of semiconductor products for switching power supplies since we introduced bipolar power transistors and rectifiers designed specifically for switching power supplies in the mid-70s.We identified these as SWITCHMODE Products. A TMswitching power supply designed using ON Semiconductor components can rightfully be called a SWITCHMODE power supply or SMPS.This brochure contains useful background information on switching power supplies for those who want to have more meaningful discussions and are not necessarily experts on power supplies. It also provides real SMPS examples, and identifies several application notes and additional design resources available from ON Semiconductor, as well as helpful books available from various publishers and useful web sites for those who are experts and want to increase their expertise. An extensive list and brief description of analog ICs, power transistors, rectifiers and other discrete components available from ON Semiconductor for designing a SMPS are also provided. This includes our newest GREENLINE, Easy Switcher and very high voltage ICs(VHVICs), as well as high efficiency HDTMOS and HVTMOS power FETs, and a wide choice of discrete products in surface mount packages.For the latest updates and additional information on analog and discrete products for power supply and power management applications, please visit our website(http://onsemi.com).What Everyone Should Know About Switching Power SuppliesIntroductionEfficient 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 semiconductor, magnetic and passive technologies make the switching power supply an ever more popular choice in the power conversion arena today.This Guide is designed to give the prospective designer an overview of all the issues involved in designing switchmode power supplies. It describes the basic operation of the more popular topologies of switching power supplies, their relevant parameters, provides circuit design tips, and information on how to select the most appropriate semiconductor and passive components. This Guide lists the ON Semiconductor components expressly built for use in switching power supplies.Linear versus Switching Power SuppliesHistorically, the linear regulator was the primary method of creating a regulated output voltage. It operates by reducing a higher input voltage down to the lower 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 30 to 50 loaddrpIV*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 heat sink actually makes the linear regulator uneconomical above 10 watts for small applications. Below that point, however, they are cost effective in step-down applications.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 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 suppliers downward which makes it an attractive choice for output powers greater than 10 watts or where multiple outputs are desired.Basic ConvertersForward-Mode Converter FundamentalsThe most elementary forward-mode converter is the Buck or Step-down Converter which can be seen in Figure 1. Its operation can be seen as having two distinct time period which occur when the series power switch is on and off. When the power switch is on, the input voltage is connected to the input of the inductor. The output of the inductor is the output voltage, and the rectifier is back-biased. During this period, since there is a constant voltage source connected across the inductor, the inductor current begins to linearly ramp upward which described by: i =( - )*t)(ONL)(inV)(out/L　 .)(onDuring the “on” period,　energy is being stored within the core material of the inductor in the form of flux.　There is sufficient energy stored to carry the requirements of the load during the next off period.The next period is the “off” period of the power switch. When the power switch turns off, the input voltage of the inductor flies below ground and is clamped at one diode drop below ground by the catch diode. Current now begins to flow through the catch diode thus maintaining the load current loop. This removes the stored energy from the inductor. The inductor current during this time is: i =( - )* /L.)(ofL)(utVD)(oftThis period ends when the power switch is once again turned on.Regulation is accomplished by varying the on-to-off duty cycle of the power switch. The relationship which approximately describes its operation is: ＝§* ,where § is the duty )(outV)(incycle(§＝t /（t ＋ ）).)(on)()(oftThe buck converter is capable of kilowatts of output power, but suffers from one serious shortcoming which would occur if the power switch were to fail short-circuited, the input power source is connected directly to the load circuitry with usually produces catastrophic results. To avoid this situation, a crowbar is placed across the output. A crowbar is a latching SCR which is fired when the output is sensed as entering an overvoltage condition. The buck converter should only be used for board-level regulation.Flyback or Boost-mode Converter FundamentalsThe most elementary flyback-mode converter is the Boost or Step-up Converter. Its schematic can be seen in Figure 2. Its operation can also be broken into two distinct periods where the power switch is on and off. When the power switch turns on, the input voltage source is placed directly across the inductor. This causes the current to begin linearly ramping upwards from zero and is described by: ＝( * )/L.)onLi)inV)(otOnce again, energy is being stored within the core material.The amount of energy stored during each cycle times the frequency of operation must be higher than the power demands of the load or, ＝0.5*L* .stoPoutOPKfI?*2The power switch then turns off and the inductor voltage flys back above the input voltage and is clamped by the rectifier at the output voltage. The current then begins to linearly ramp downward until the energy within the core is completely depleted. Its waveform which is shown in Figure 3 is determined by: .LtViofinoutfL/*)()(??The boost converter should also be only used for board-level regulation.Common TopologiesA 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: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 power semiconductors.The first choice that faces the designer is whether to have input to output transformer isolation. Non-isolated topologies should also be used where the possibility of a failure does not connect the input power source to the fragile load circuitry. Transformer isolation should be used in all other situations. Associated with that is the need for multiple output voltages. Transformers provide an easy method for adding additional output voltage to the switching power supply. The companies building their own power systems are leaning toward transformer isolation in as many power supplies as possible since it prevents a domino effect during failure conditions.The remainder of the factors involve how much stress the power semiconductors are being subjected to. Table 1 shows the differences between the various topologies used within switching power supplies. Figure 4 illustrates where the transformer-isolated topologies are typically used within the power industry at various power and voltage levels. At reduced DC input voltages and at higher powers, the peak currents that must be sustained by the power switch grow higher which then affects the stress they must endure. The various areas show which topology best fits within that range of input voltage and output power that exhibits the least amount of stress on the power semiconductors.