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Switch Mode Power Supply (SMPS) Topologies

AN1114. Switch Mode Power Supply (SMPS) Topologies (Part I). EQUATION 1: SHUNT-CONTROLLED. Author: Mohammad Kamil REGULATOR Power LOSS. Microchip Technology Inc. 2. P LOSS = V OUT I S + ( I OUT + I S ) R S. INTRODUCTION. However, if we control the output voltage VOUT by The industry drive toward smaller, lighter and more varying RS and keeping IS zero, the ideal Power loss efficient electronics has led to the development of the inside the converter can be calculated as shown in Switch Mode Power Supply (SMPS). There are several Equation 2. Topologies commonly used to implement SMPS. This application note, which is the first of a two-part EQUATION 2: SERIES-CONTROLLED. series, explains the basics of different SMPS. REGULATOR Power LOSS. Topologies . Applications of different Topologies and their pros and cons are also discussed in detail. This 2. RS. application note will guide the user to select an P LOSS = V IN -------------------------2- ( RS + RL ).

Sep 10, 2007 · 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 losses inside the converter and introduces additional electrical noise. Author: Mohammad Kamil

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Transcription of Switch Mode Power Supply (SMPS) Topologies

1 AN1114. Switch Mode Power Supply (SMPS) Topologies (Part I). EQUATION 1: SHUNT-CONTROLLED. Author: Mohammad Kamil REGULATOR Power LOSS. Microchip Technology Inc. 2. P LOSS = V OUT I S + ( I OUT + I S ) R S. INTRODUCTION. However, if we control the output voltage VOUT by The industry drive toward smaller, lighter and more varying RS and keeping IS zero, the ideal Power loss efficient electronics has led to the development of the inside the converter can be calculated as shown in Switch Mode Power Supply (SMPS). There are several Equation 2. Topologies commonly used to implement SMPS. This application note, which is the first of a two-part EQUATION 2: SERIES-CONTROLLED. series, explains the basics of different SMPS. REGULATOR Power LOSS. Topologies . Applications of different Topologies and their pros and cons are also discussed in detail. This 2. RS. application note will guide the user to select an P LOSS = V IN -------------------------2- ( RS + RL ).

2 Appropriate topology for a given application, while providing useful information regarding selection of electrical and electronic components for a given SMPS This type of converter is known as a series-controlled design. regulator. The ideal Power loss in this converter depends on the value of the series resistance, RS, which is required to control the output voltage, VOUT, WHY SMPS? and the load current, IOUT. If the value of RS is either zero or infinite, the ideal Power loss inside the The main idea behind a Switch mode Power Supply can converter should be zero. This feature of a easily be understood from the conceptual explanation series-controlled regulator becomes the seed idea of of a DC-to-DC converter, as shown in Figure 1. The SMPS, where the conversion loss can be minimized, load, RL, needs to be supplied with a constant voltage, which results in maximized efficiency.

3 VOUT, which is derived from a primary voltage source, VIN. As shown in Figure 1, the output voltage VOUT can In SMPS, the series element, RS, is replaced by a be regulated by varying the series resistor (RS) or the semiconductor Switch , which offers very low resistance shunt current (IS). at the ON state (minimizing conduction loss), and very high resistance at the OFF state (blocking the When VOUT is controlled by varying IS and keeping RS. conduction). A low-pass filter using non-dissipative constant, Power loss inside the converter occurs. This passive components such as inductors and capacitors type of converter is known as shunt-controlled is placed after the semiconductor Switch , to provide regulator. The Power loss inside the converter is given constant DC output voltage. by Equation 1. Please note that the Power loss cannot be eliminated even if IS becomes zero.

4 The semiconductor switches used to implement Switch mode Power supplies are continuously switched on and FIGURE 1: DC-DC CONVERTER off at high frequencies (50 kHz to several MHz), to transfer electrical energy from the input to the output RS IOUT through the passive components. The output voltage is controlled by varying the duty cycle, frequency or phase of the semiconductor devices' transition periods. As the size of the passive components is inversely proportional to the switching frequency, a high RL VOUT. VIN IS 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 losses inside the converter and introduces additional electrical noise. 2007 Microchip Technology Inc. DS01114A -page 1. AN1114. SELECTION OF SMPS Topologies Buck Converter There are several Topologies commonly used to A buck converter, as its name implies, can only implement SMPS.

5 Any topology can be made to work produce lower average output voltage than the input for any specification; however, each topology has its voltage. The basic schematic with the switching own unique features, which make it best suited for a waveforms of a buck converter is shown in Figure 2. certain application. To select the best topology for a In a buck converter, a Switch (Q1) is placed in series given specification, it is essential to know the basic with the input voltage source VIN. The input source VIN. operation, advantages, drawbacks, complexity and the feeds the output through the Switch and a low-pass area of usage of a particular topology. The following filter, implemented with an inductor and a capacitor. factors help while selecting an appropriate topology: In a steady state of operation, when the Switch is ON for a) Is the output voltage higher or lower than the a period of TON, the input provides energy to the output whole range of the input voltage?

6 As well as to the inductor (L). During the TON period, the b) How many outputs are required? inductor current flows through the Switch and the c) Is input to output dielectric isolation required? difference of voltages between VIN and VOUT is applied d) Is the input/output voltage very high? to the inductor in the forward direction, as shown in Figure 2 (C). Therefore, the inductor current IL rises e) Is the input/output current very high? linearly from its present value IL1 to IL2, as shown in f) What is the maximum voltage applied across the Figure 2 (E). transformer primary and what is the maximum duty cycle? During the TOFF period, when the Switch is OFF, the inductor current continues to flow in the same Factor (a) determines whether the Power Supply direction, as the stored energy within the inductor topology should be buck, boost or buck-boost type. continues to Supply the load current.

7 The diode D1. Factors (b) and (c) determine whether or not the Power completes the inductor current path during the Q1 OFF. Supply topology should have a transformer. Reliability period (TOFF); thus, it is called a freewheeling diode. of the Power Supply depends on the selection of a During this TOFF period, the output voltage VOUT is proper topology on the basis of factors (d), (e) and (f). applied across the inductor in the reverse direction, as shown in Figure 2 (C). Therefore, the inductor current decreases from its present value IL2 to IL1, as shown in Figure 2 (E). DS01114A -page 2 2007 Microchip Technology Inc. AN1114. FIGURE 2: BUCK CONVERTER. IIN. Q1. L IOUT. (A) VIN. + IL - D1 VOUT. (B) Q1 GATE. t (C) VL VIN - VOUT. t -VOUT. (VIN - VOUT)/L. (D) IIN. t -VOUT/L. IL2. (E) IL. IL1. t (A) = Buck converter (B) = Gate pulse of MOSFET Q1. (C) = Voltage across the Inductor L.

8 (D) = Input current IIN. (E) = Inductor current IL. CONTINUOUS CONDUCTION MODE EQUATION 4: DUTY CYCLE. The inductor current is continuous and never reaches zero during one switching period (TS); therefore, this T ON. D = --------- - mode of operation is known as Continuous Conduction TS. mode. In Continuous Conduction mode, the relation between the output and input voltage is given by where: Equation 3, where D is known as the duty cycle, which TON = ON Period is given by Equation 4. TS = switching Period EQUATION 3: BUCK CONVERTER VOUT/VIN. RELATIONSHIP If the output to input voltage ratio is less than , it is always advisable to go for a two-stage buck converter, which means to step down the voltage in two buck V OUT = D V IN. operations. Although the buck converter can be either continuous or discontinuous, its input current is always discontinuous, as shown in Figure 2 (D).

9 This results in a larger electromagnetic interference (EMI) filter than the other Topologies . 2007 Microchip Technology Inc. DS01114A -page 3. AN1114. CURRENT MODE CONTROL of operation (inductor current reaches zero in one switching cycle). This may happen if the buck converter While designing a buck converter, there is always a inductor is designed for a medium load, but needs to trade-off between the inductor and the capacitor size operate at no load and/or a light load. In this case, the selection. output voltage may fall below the regulation limit, if the A larger inductor value means numerous turns to the synchronous MOSFET is not switched off immediately magnetic core, but less ripple current (<10% of full load after the inductor reaches zero. current) is seen by the output capacitor; therefore, the loss in the inductor increases. Also, less ripple current MULTIPHASE SYNCHRONOUS BUCK.

10 Makes current mode control almost impossible to CONVERTER. implement (refer to Method of Control for details on It is almost impractical to design a single synchronous current mode control techniques). Therefore, poor load buck converter to deliver more than 35 amps load transient response can be observed in the converter. current at a low output voltage. If the load current A smaller inductor value increases ripple current. This requirement is more than 35-40 amps, more than one makes implementation of current mode control easier, converter is connected in parallel to deliver the load. and as a result, the load transient response of the To optimize the input and output capacitors, all the converter improves. However, high ripple current parallel converters operate on the same time base and needs a low Equivalent Series Resistor (ESR) output each converter starts switching after a fixed time/phase capacitor to meet the peak-to-peak output voltage from the previous one.


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