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1、Application Report SLPA009AJune 2011Revised July 2011 Power Loss Calculation With Common Source Inductance Consideration for Synchronous Buck Converters David Jauregui, Bo Wang, and Rengang Chen.PMP - Power Stage ABSTRACT The synchronous buck converter is a widely used topology in low-voltage, high-
2、current applications. Low-power loss and highly efficient synchronous buck converters are in great demand for advanced microprocessors of the future. Good understanding of power losses in a synchronous buck converter is critical for improving converter performance. Common source inductance (CSI) is
3、the inductance shared by the main current path and the gate driver loop in a converter and carries the drain-source current and the gate-charging current. A high-side (HS) FET CSI has significant impact on converter performance, especially switching power loss: the greater the HS CSI, the greater th
4、e switching loss. This application report analyzes MOSFET-related power losses in a synchronous buck converter. The effects of HS CSI are also discussed and quantified. This document introduces the Texas Instruments NexFET Power Block, which uses advanced device and package techniques to minimize CS
5、I and thus reduce system power loss. To simply design efforts, a complementary design calculator (SLPC015) is available. Contents 1Introduction2 2Conduction Loss2 3Switching Loss3 4Power Loss Calculation Assumption.8 5Power Loss Calculation Examples.8 6Summary.11 Appendix AEstimation of Common Sourc
6、e Inductances13 Appendix BList of Symbols15 List of Figures 1Schematic of Synchronous Buck Converter.2 2Ideal Waveform at Device Turnon3 3Schematic for Converter With CSI Consideration4 4Ideal Waveforms at Device Turnoff.6 5Diode Reverse-Recovery Waveforms7 6Schematic for Converter 1 Discrete Soluti
7、on.9 7Schematic for Converter 2 TI Power Block Solution.9 8MOSFETs Power Loss in a Synchronous Buck Converter10 9Power Loss Calculation Comparisons With/Without CSI Consideration.11 List of Tables 1Device Parameters.9 2Power Loss Calculation Example for Synchronous Buck Converters10 3HS FET Switchin
8、g Loss Calculation With CSI Consideration.11 4Typical Values of Common Source Inductances.13 NexFET is a trademark of Texas Instruments. 1SLPA009AJune 2011Revised July 2011Power Loss Calculation With Common Source Inductance Consideration for Synchronous Buck Converters Submit Documentation Feedback
9、 Copyright 2011, Texas Instruments Incorporated 2 cond(HS)ds(ON)HSRMS(HS) P= RI 2 cond(LS)ds(ON)LSRMS(LS) P= RI ? ? ? =+-+- ? ? ? 22 rippleripplerippleripple RMS(HS)OUTOUTOUTOUT IIII D IIIII 32222 I 1Introduction The synchronous buck converter is one of the most popular topologies for todays low-vol
10、tage regulators. As transistor counts in a single processor continue to increase, various challenges arise for low-voltage, high-current voltage regulator design. High efficiency is one of the most critical requirements for improving power density. Good understanding of power loss in synchronous buc
11、k converters is important for converter design optimization. Power loss of a synchronous buck converter includes several parts: MOSFET loss, inductor loss, printed-circuit board (PCB) loss, etc. Among these, MOSFET loss is complicated and significant. This application report only discusses the MOSFE
12、T loss in synchronous buck converters. Figure 1. Schematic of Synchronous Buck Converter Figure 1 shows the schematic of a typical synchronous buck converter. MOSFET-related power loss in a synchronous buck converter is composed of conduction loss and switching loss. Conduction loss is the sum of hi
13、gh side (HS) and low side (LS) FETs conduction loss. This loss is independent with switching frequency. Switching loss consists of HS FET switching loss, LS FET switching loss, gate drive loss, LS body diode loss, and FETs output capacitance loss. Switching loss goes up linearly with increasing swit
14、ching frequency. Not all calculations for power losses are straightforward. Specifically, MOSFET switching loss is determined by the transition time and influenced by several parameters in the gate drive loop. Common source inductance (CSI) is one of the most important parameters. CSI is the inducta
15、nce shared by the main-current path and the gate-driver loop and carries the drain-source current and the gate-charging current. Any voltage induced on CSI changes the effective gate-source voltage of the MOSFET. Because of the importance of gate-source voltage on the switching performance of power
16、MOSFETs, CSI has a significant impact on the system performance, especially on HS FET switching loss. This application report first analyzes the MOSFET-related power loss in a synchronous buck converter and then presents the equations for power loss calculations. The impacts of CSI on converter power loss also is discussed and quantified in this document. 2Conduction Loss The conduction loss is determined by
