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INTRODUCTIONpioneering
The industry drive toward smaller, lighter and more efficient electronics has led to the development of the Switch Mode Power Supply (SMPS). There are veral topologies commonly ud to implement SMPS.
This application note, which is the first of a two-part ries, explains the basics of different SMPS topologies. Applications of different topologies and their pros and cons are also discusd in detail. This application note will guide the ur to lect an appropriate topology for a given application, while providing uful information regarding lection of electrical and electronic components for a given SMPS design.
WHY SMPS?
The main idea behind a switch mode power supply can easily be understood from the conceptual explanation of a DC-to-DC converter, as shown in Figure1. The load, R L, needs to be supplied with a constant voltage, V OUT, which is derived from a primary voltage source, V IN. As shown in Figure1, th
e output voltage V OUT can be regulated by varying the ries resistor (R S) or the shunt current (I S).
When V OUT is controlled by varying I S and keeping R S constant, power loss inside the converter occurs. This type of converter is known as shunt-controlled regulator. The power loss inside the converter is given by Equation1. Plea note that the power loss cannot be eliminated even if I S becomes zero.
FIGURE 1:DC-DC CONVERTER EQUATION 1:SHUNT-CONTROLLED
REGULATOR POWER LOSS However, if we control the output voltage V OUT by varying R S and keeping I S zero, the ideal power loss inside the converter can be calculated as shown in Equation2.
EQUATION 2:SERIES-CONTROLLED
REGULATOR POWER LOSS This type of converter is known as a ries-controlled regulator. The ideal power loss in this converter depends on the value of the ries resistance, R S, which is required to control the output voltage, V OUT, and the load current, I OUT. If the value of R S is either zero or infinite, the ideal power loss inside the converter should be zero. This feature of a rie
s-controlled regulator becomes the ed idea of SMPS, where the conversion loss can be minimized, which results in maximized efficiency.
In SMPS, the ries element, R S, is replaced by a miconductor switch, which offers very low resistance at the ON state (minimizing conduction loss), and very high resistance at the OFF state (blocking the conduction). A low-pass filter using non-dissipative passive components such as inductors and capacitors is placed after the miconductor switch, to provide constant DC output voltage.
The miconductor switches ud to implement switch mode power supplies are continuously switched on and off at high frequencies (50 kHz to veral MHz), to transfer electrical energy from the input to the output through the passive components. The output voltage is controlled by varying the duty cycle, frequency or pha of the miconductor devices’ transition periods. As the size of the passive components is inverly proportional to the switching frequency, a high switching frequency results in smaller sizes for magnetics and capacitors.
While the high frequency switching offers the designer a huge advantage for increasing the power density, it adds power loss inside the converter and introduces additional electrical noi.
Author:Mohammad Kamil
Microchip Technology Inc.
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Switch Mode Power Supply (SMPS) Topologies (Part I)
© 2007 Microchip Technology Inc.DS01114A -page 1
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DS01114A -page 2© 2007 Microchip Technology Inc.
SELECTION OF SMPS TOPOLOGIES
There are veral topologies commonly ud to implement SMPS. Any topology can be made to work for any specification; however, each topology has its own unique features, which make it best suited for a certain application. To lect the best topology for a given specification, it is esntial to know the basic operation, advantages, drawbacks, complexity and the area of usage of a particular topology. The following factors help while lecting an appropriate topology: a)Is the output voltage higher or lower than the whole range of the input voltage?b)How many outputs are required?
c)Is input to output dielectric isolation required?d)Is the input/output voltage very high?e)Is the input/output current very high?
f)
What is the maximum voltage applied across the transformer primary and what is the maximum duty cycle?
Factor (a) determines whether the power supply topology should be buck, boost or buck-boost type.Factors (b) and (c) determine whether or not the power supply topology should have a transformer. Reliability of the power supply depends on the lection of a proper topology on the basis of factors (d), (e) and (f).
Buck Converter
A buck converter, as its name implies, can only produce lower average output voltage than the input voltage. The basic schematic with the switching waveforms of a buck converter is shown in Figure 2.In a buck converter, a switch (Q 1) is placed in ries with the input voltage source V IN . The input source V IN feeds the output through the switch and a low-pass filter, implemented with an inductor and a capacitor. In a steady state of operation, when the switch is ON for a period of T ON , the input provides energy to the output as well as to the inductor (L). During the T ON period, the inductor current flows through the switch and the difference of voltages between V IN and V OUT is applied to the inductor in the forward direction, as shown in Figure 2 (C). Therefore, the inductor current I L ris linearly from its prent value I L 1 to I L 2, as shown in Figure 2 (E).
During the T OFF period, when the switch is OFF, the inductor current continues to flow in the same
direction, as the stored energy within the inductor continues to supply the load current. The diode D1completes the inductor current path during the Q 1 OFF period (T OFF ); thus, it is called a freewheeling diode.During this T OFF period, the output voltage V OUT is applied across the inductor in the rever direction, as shown in Figure 2(C). Therefore, the inductor current decreas from its prent value I L 2 to I L 1, as shown in Figure 2(E).
© 2007 Microchip Technology Inc.DS01114A -page 3
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FIGURE 2:
BUCK CONVERTER
CONTINUOUS CONDUCTION MODE
网上字典The inductor current is continuous and never reaches zero during one switching period (T S ); therefore, this mode of operation is known as Continuous Conduction mode. In Continuous Conduction mode, the relation between the output and input voltage is given by Equation 3, where D is known as the duty cycle, which is given by Equation 4.
EQUATION 3:
BUCK CONVERTER V OUT /V IN RELATIONSHIP
EQUATION 4:DUTY CYCLE
If the output to input voltage ratio is less than 0.1, it is always advisable to go for a two-stage buck converter,which means to step down the voltage in two buck operations. Although the buck converter can be either continuous or discontinuous, its input current is always discontinuous, as shown in Figure 2 (D). This results in a larger electromagnetic interference (EMI) filter than the other topologies.
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DS01114A -page 4© 2007 Microchip Technology Inc.
CURRENT MODE CONTROL
While designing a buck converter, there is always a trade-off between the inductor and the capacitor size lection.
A larger inductor value means numerous turns to the magnetic core, but less ripple current (<10% of full load current) is en by the output capacitor; therefore, the loss in the inductor increas. Also, less ripple current makes current mode control almost impossible to implement (refer to “Method of
Control” for details on current mode control techniques). Therefore, poor load transient respon can be obrved in the converter. A smaller inductor value increas ripple current. This makes implementation of current mode control easier,and as a result, the load transient respon of the converter improves. However, high ripple current needs a low Equivalent Series Resistor (ESR) output capacitor to meet the peak-to-peak output voltage ripple requirement. Generally, to implement the current mode control, the ripple current at the inductor should be at least 30% of the full load current.
FEED-FORWARD CONTROL
In a buck converter, the effect of input voltage variation on the output voltage can be minimized by implementing input voltage feed-forward control. It is easy to implement feed-forward control when using a digital controller with input voltage n, compared to using an analog control method. In the feed-forward control method, the digital controller starts taking the appropriate adaptive action as soon as any change is detected in the input voltage, before the change in input can actually affect the output parameters.
SYNCHRONOUS BUCK CONVERTER
When the output current requirement is high, the excessive power loss inside the freewheeling diode D1,limits the minimum output voltage that can be achieved. To reduce the loss at high current and to achieve lower output voltage, the freewheeling diode is replaced by a MOSFET with a very low ON state resistance R DSON . This MOSFET is turned on and off synchronously with the buck MOSFET. Therefore, this topology is known as a synchronous buck converter. A gate drive signal, which is the complement of the buck switch gate drive signal, is required for this synchronous MOSFET.
A MOSFET can conduct in either direction; which means the synchronous MOSFET should be turned off immediately if the current in the inductor reaches zero becau of a light load. Otherwi, the direction of the inductor current will rever (after reaching zero)becau of the output LC resonance. In such a scenario, the synchronous MOSFET acts as a load to the output capacitor, and dissipates energy in the R DSON (ON state resistance) of the MOSFET, resulting in an increa in power loss during discontinuous mode
of operation (inductor current reaches zero in one switching cycle). This may happen if the buck converter inductor is designed for a medium load, but needs to operate at no load and/or a light load. In this ca, the output voltage may fall below the regulation limit, if the synchronous MOSFET is not switched off immediately after the inductor reaches zero.
MULTIPHASE SYNCHRONOUS BUCK CONVERTER
It is almost impractical to design a single synchronous buck converter to deliver more than 35 amps load current at a low output voltage. If the load current requirement is more than 35-40 amps, more than one converter is connected in parallel to deliver the load. To optimize the input and output capacitors, all the parallel converters operate on the same time ba and each converter starts switching after a fixed time/pha from the previous one. This type of converter is called a multipha synchronous buck converter. Figure 3shows the multipha synchronous buck converter with a gate pul timing relation of each leg and the input current drawn by the converter. The fixed time/pha is given by Time period/n or 300/n , where “n ”is the number of the converter connected in parallel. The design of input and output capacitors is bad on the switching frequency of each converter multiplied by the number of parallel converters. The ripple current en by the output capacitor reduces by “n” times. As shown in Figure 3 (E), the input current drawn by a multipha synchronous buck converter is continuous with less ripple current as compared to a single converter shown in Figure 2 (D). Therefore, a smaller input capacitor meets the design requirement in ca of a multipha synchronous buck converter.
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© 2007 Microchip Technology Inc.DS01114A -page 5
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DS01114A -page 6© 2007 Microchip Technology Inc.
日语字典>optical是什么意思Boost Converter
A boost converter, as its name implies, can only produce a higher output average voltage than the input voltage. The basic schematic with the switching waveform of a boost converter is shown in Figure 4. In a boost converter, an inductor (L) is placed in ries with the input voltage source V IN . The input source feeds the output through the inductor and the diode D 1.In the steady state of operation, when the switch Q 1 is ON for a period of T ON , the input provides energy to the inductor.
惊喜用英文怎么说
During the T ON period, inductor current (I L ) flows through the switch and the input voltage V IN is applied to the inductor in the forward direction, as shown in Figure 4 (C). Therefore, the inductor current ris linearly from its prent value I L 1 to I L 2, as shown in Figure 4 (D). During this T ON period, the output load current I OUT is supplied from the output capacitor C O .The output capacitor value should be large enough to supply the load current for the time period T ON with the minimum specified droop in the output voltage.
During the T OFF period when the switch is OFF, the inductor current continues to flow in the same direction as the stored energy with the inductor, and the input source V IN supplies energy to the loa
上海儿童英语哪个好d. The diode D 1completes the inductor current path through the output capacitor during the Q 1 OFF period (T OFF ). During this T OFF period, the inductor current flows through the diode and the difference of voltages between V IN and V OUT is applied to the inductor in the rever direction,as shown in Figure 4 (C). Therefore, the inductor current decreas from the prent value I L 2 to I L 1, as shown in Figure 4 (D).
CONTINUOUS CONDUCTION MODE
As shown in Figure 4 (D), the inductor current is continuous and never reaches zero during one switching cycle (T S ); therefore, this method is known as Continuous Conduction mode, which is the relation between output and input voltage, as shown in Equation 5.
