1、 1 外文文献 Switched-mode power supply A switched-mode power supply (also switching-mode power supply, SMPS, or simply switcher) is an electronic power supply unit (PSU) that incorporates a switching regulator. While a linear regulator maintains the desired output voltage by dissipating excess power in
2、a pass power transistor, the switched-mode power supply switches a power transistor between saturation (full on) and cutoff (completely off) with a variable duty cycle whose average is the desired output voltage. It switches at a much-higher frequency (tens to hundreds of kHz) than that of the AC li
3、ne (mains), which means that the transformer that it feeds can be much smaller than one connected directly to the line/mains. Switching creates a rectangular waveform that typically goes to the primary of the transformer； typically several secondaries feed rectifiers, series inductors, and filter ca
4、pacitors to provide various DC outputs with low ripple. The main advantage of this method is greater efficiency because the switching transistor dissipates little power in the saturated state and the off state compared to the semiconducting state (active region). Other advantages include smaller siz
5、e and lighter weight (from the elimination of low frequency transformers which have a high weight) and lower heat generation due to higher efficiency. Disadvantages include greater complexity, the generation of high amplitude, high frequency energy that the low-pass filter must block to avoid electr
6、omagnetic interference (EMI), and a ripple voltage at the switching frequency and the harmonic frequencies thereof. A note about terminology Although the term power supply has been in use since radios were first powered from the line/mains, that does not mean that it is a source of power, in the sen
7、se that a battery provides power. It is simply a device that (usually) accepts commercial AC power and provides one or more DC outputs. It would be more correctly referred to as a power converter, but long usage has established the term. Classification SMPS can be classified into four types accordin
8、g 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 converter 2 AC in, AC out: frequency changer, cycloconverter, transformer DC in, AC out: inverter Input rectifier stage If the SMPS has an
9、 AC input, then the first stage is to convert the input to DC. This is called rectification. The rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically. This is a feature of larger supplies to permit operation from nominally 120
10、 volt or 240 volt supplies. The rectifier produces an unregulated DC voltage which is then sent to a large filter capacitor. The current drawn from the mains supply by this rectifier circuit occurs in short pulses around the AC voltage peaks. These pulses have significant high frequency energy which
11、 reduces the power factor. Special control techniques can be employed by the following SMPS to force the average input current to follow the sinusoidal shape of the AC input voltage thus the designer should try correcting the power factor. An SMPS with a DC input does not require this stage. An SMPS
12、 designed for AC input can often be run from a DC supply (for 230V AC this would be 330V DC), as the DC passes through the rectifier stage unchanged. Its however advisable to consult the manual before trying this, though most supplies are quite capable of such operation even though nothing is mentio
13、ned in the documentation. However, this type of use may be harmful to the rectifier stage as it will only utilize half of diodes in the rectifier for the full load. This may result in overheating of these components, and cause them to fail prematurely. If an input range switch is used, the rectifier
14、 stage is usually configured to operate as a voltage doubler when operating on the low voltage (120 VAC) range and as a straight rectifier when operating on the high voltage (240 VAC) range. If an input range switch is not used, then a full-wave rectifier is usually used and the downstream inverter
15、stage is simply designed to be flexible enough to accept the wide range of dc voltages that will be produced by the rectifier stage. In higher-power SMPSs, some form of automatic range switching may be used. Inverter stage The inverter stage converts DC, whether directly from the input or from the r
16、ectifier stage described above, to AC by running it through a power oscillator, whose output transformer is very small with few windings at a frequency of tens or hundreds of kilohertz (kHz). The frequency is usually chosen to be above 20 kHz, to make it inaudible to humans. The output voltage is op
17、tically coupled to the input and thus 3 very tightly controlled. The switching is implemented as a multistage (to achieve high gain) MOSFET amplifier. MOSFETs are a type of transistor with a low on-resistance and a high current-handling capacity. Since only the last stage has a large duty cycle, pre
18、vious stages can be implemented by bipolar transistors leading to roughly the same efficiency. The second last stage needs to be of a complementary design, where one transistor charges the last MOSFET and another one discharges the MOSFET. A design using a resistor would run idle most of the time an
19、d reduce efficiency. All earlier stages do not weight into efficiency because power decreases by a factor of 10 for every stage (going backwards) and thus the earlier stages are responsible for at most 1% of the efficiency. This section refers to the block marked Chopper in the block diagram. Voltag
20、e converter and output rectifier If the output is required to be isolated from the input, as is usually the case in mains power supplies, the inverted AC is used to drive the primary winding of a high-frequency transformer. This converts the voltage up or down to the required output level on its sec
21、ondary winding. The output transformer in the block diagram serves this purpose. If a DC output is required, the AC output from the transformer is rectified. For output voltages above ten volts or so, ordinary silicon diodes are commonly used. For lower voltages, Schottky diodes are commonly used as
22、 the rectifier elements； they have the advantages of faster recovery times than silicon diodes (allowing low-loss operation at higher frequencies) and a lower voltage drop when conducting. For even lower output voltages, MOSFETs may be used as synchronous rectifiers； compared to Schottky diodes, the
23、se have even lower conducting state voltage drops. The rectified output is then smoothed by a filter consisting of inductors and capacitors. For higher switching frequencies, components with lower capacitance and inductance are needed. Simpler, non-isolated power supplies contain an inductor instead
24、 of a transformer. This type includes boost converters, buck converters, and the so called buck-boost 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 prop
25、ortion 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
