基于sg3525設(shè)計單相正弦波spwm逆變電源外文翻譯-文庫吧在線文庫
【正文】 d power supplies contain an inductor instead of a transformer. This type includes boost converters, buck converters, and the so called buckboost converters. These belong to the simplest class of single input, single output converters which utilize one inductor and one active switch. The buck converter reduces the input voltage in direct proportion to the ratio of conductive time to the total switching period, called the duty cycle. For example an ideal buck converter with a 10 V input operating at a 50% duty cycle will produce an average output voltage of 5 V. A feedback control loop is employed to regulate the output voltage by varying the duty cycle to pensate for variations in input voltage. The output voltage of a boost converter is always greater than the input voltage and the buckboost output voltage is inverted but can be greater than, equal to, or less than the magnitude of its input voltage. There are many variations and extensions to this class of converters but these three form the basis of almost all isolated and nonisolated DC to DC converters. By adding a second inductor the ?uk and SEPIC converters can be implemented, or, by adding additional active switches, various bridge converters can be realised. Other types of SMPSs use a capacitordiode voltage multiplier instead of inductors and transformers. These are mostly used for generating high voltages at low currents (CockcroftWalton generator). The low voltage variant is called charge pump.RegulationA feedback circuit monitors the output voltage and pares it with a reference voltage, which is set manually or electronically to the desired output. If there is an error in the output voltage, the feedback circuit pensates by adjusting the timing with which the MOSFETs are switched on and off. This part of the power supply is called the switching regulator. The Chopper controller shown in the block diagram serves this purpose. Depending on design/safety requirements, the controller may or may not contain an isolation mechanism (such as optocouplers) to isolate it from the DC output. Switching supplies in puters, TVs and VCRs have these optocouplers to tightly control the output voltage.Openloop regulators do not have a feedback circuit. Instead, they rely on feeding a constant voltage to the input of the transformer or inductor, and assume that the output will be correct. Regulated designs pensate for the parasitic capacitance of the transformer or coil. Monopolar designs also pensate for the magnetic hysteresis of the core.The feedback circuit needs power to run before it can generate power, so an additional nonswitching powersupply for standby is added.Transformer designSMPS transformers run at high frequency. Most of the cost savings (and space savings) in offline power supplies e from the fact that a high frequency transformer is much smaller than the 50/60 Hz transformers formerly used.There are several differences in the design of transformers for 50 Hz vs 500 kHz. Firstly a low frequency transformer usually transfers energy through its core (soft iron), while the (usually ferrite) core of a high frequency transformer limits leakage. Since the waveforms in a SMPS are generally high speed (PWM square waves), the wiring must be capable of supporting high harmonics of the base frequency due to the skin effect, which is a major source of power loss.Power factorSimple offline switched mode power supplies incorporate a simple full wave rectifier connected to a large energy storing capacitor. Such SMPSs draw current from the AC line in short pulses when the mains instantaneous voltage exceeds the voltage across this capacitor. During the remaining portion of the AC cycle the capacitor provides energy to the power supply.As a result, the input current of such basic switched mode power supplies has high harmonic content and relatively low power factor. This
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