Application Note Davis Wen AN005 – March 2014 Analysis of Buck Converter Efficiency Abstract The synchronous buck circuit is wildly used to provide non-isolated power for low voltage and high current supply to system chip. To realize the power loss of synchronous buck converter and to improve efficiency is important for power designer. The application note introduces the analysis of buck converter efficiency and realizes major power component loss in synchronous buck converter. 1. Buck converter power loss analysis To realize the power loss in converters is important for converter design optimization. Figure1 shows the general single phase synchronous buck converter circuit. The major power losses in synchronous buck converter circuit are listed as bellow : A : Power semiconductor loss B : Inductor loss C : Driver loss D : PCB trace loss Figure 1. Synchronous buck converter AN005 © 2014 Richtek Technology Corporation 1 Analysis of Buck Converter Efficiency 2. Power loss calculation Low-power loss and highly efficient synchronous buck converters are in great demand for advanced micro-processors. The application note introduces and provides how to calculate the majority of power losses in a typical synchronous buck converter occur in the following components based on that the converter works in continuous conduction mode (CCM) fixed switching frequency, fixed input voltage and fixed output voltage. A : Power semiconductor loss : HMOS (High-Side MOSFET) summarizes to include : switching on & off and conduction loss. LMOS (Low-Side MOSFET) summarizes to include : conduction, dead-time and reverse recovery charge loss. HMOS switching on loss : VDS Drain VDriver Cgd RHI Gate RG1 Cds RGATE RLO Cgs Source Figure 2. HMOS Driver switching on VGS VDriver VDS IDS VGS VPL VTH t1 QTH QGS t2 QSW t QGD QG Figure 3. HMOS switching on loss area PHS-ON Fsw VDS IDS t1 t 2 Fsw VIN Io 2 THS-ON IG-ON AN005 THS-ON 2 QSW IG,ON VDriver VPL RHI RGATE RG1 © 2014 Richtek Technology Corporation 2 Analysis of Buck Converter Efficiency HMOS conduction loss : The conduction loss of high-side MOSFET is determined by the on-resistances of the MOSFET and the transistor RMS current. HS IHG IOUT L + + ILG Gate Signal VIN RLoad COUT LS _ _ Figure 4. HMOS conduction on HG LG IL IHG ILG DT TD1 TD2 DT TD1 TD2 Figure 5. HMOS conduction on period 2 PCON_HS Irms,HG Rds(on),HS 2 Iripple 2 Where Irms,HG D IOUT 12 AN005 © 2014 Richtek Technology Corporation 3 Analysis of Buck Converter Efficiency LMOS conduction loss : HS IHG IOUT L + + ILG Gate Signal VIN RLoad COUT _ _ Figure 6. LMOS conduction on HG LG IL IHG ILG DT TD1 TD2 DT TD1 TD2 Figure 7. LMOS conduction on period 2 PCON_LS Irms,LG Rds(on),LS 2 Iripple 2 Where Irms,LG (1-D) IOUT 12 AN005 © 2014 Richtek Technology Corporation 4 Analysis of Buck Converter Efficiency LMOS dead time body diode loss : Dead-time loss is induced by LMOS body diode conduction during dead-times. HS IHG IOUT L + + ILG Gate Signal VIN RLoad COUT _ _ Figure 8. LMOS body diode conduction on HG LG IL IHG ILG DT TD1 TD2 DT TD1 TD2 : Figure 9. LMOS body diode conduction on period PDeadtime VSD IL TD2 IL TD1 Fsw Iripple Iripple VSD IOUT TD2 IOUT 2 TD1 Fsw 2 AN005 © 2014 Richtek Technology Corporation 5 Analysis of Buck Converter Efficiency LMOS reverse recovery charge loss : Figure 10. LMOS body diode reverse recovery period Prr Qrr VDD Fsw Qrr VIN Fsw AN005 © 2014 Richtek Technology Corporation 6 Analysis of Buck Converter Efficiency B : Inductor DC & AC loss Inductor DC loss : HS IL IHG + IOUT + L RL_DCR ILG Gate Signal VIN LS RLoad COUT _ _ Figure 11. Current through inductor path HG LG IL IHG ILG DT TD1 TD2 D TD1 TD2 Figure 12. Inductor current path period 2 Pcopper Irms,L RL_DCR 2 Where Irms,L IOUT AN005 2 Iripple 12 © 2014 Richtek Technology Corporation 7 Analysis of Buck Converter Efficiency Inductor core loss : Inductor core losses are major caused by an alternating magnetic field in the core material. The losses are a function of the operating frequency and the total magnetic flux swing. The core loss may vary from one magnetic material to another. HG LG ΔI IL IHG ILG DT T D1 T D2 DT T D1 T D2 Figure 13. Inductor ripple current Figure 14. Core loss curve The calculated and/or measured core loss is often directly provided by the inductor supplier. If not, a formula can be used to calculate the core loss as bellow : X Y PL C FSW Bmax Ve Bmax L ΔI N Ae The PL is the power loss (mW), Fsw : operating frequency B : peak flux desity in Gauss Ve : effective core volume The specific value of C, X and Y are core loss parameters for each material AN005 © 2014 Richtek Technology Corporation 8 Analysis of Buck Converter Efficiency C: Gate driver loss : The gate driver loss is straightforward given by MOSFET driver to charge /discharge total HMOS and LMOS Qg. The gate driver loss is depending on MOSFET total gate charge, driver voltage and Fsw. VDS Drain VDriver Cgd RHI Gate RG1 Cds RGATE RLO Cgs Source Figure 15. Driver turns on and off path VGS t3 VDriver VGS t4 VDS VDriver VDS IDS IDS VPL VGS VGS VPL VTH VTH t1 t2 QSW QTH QGS t QSW QGD QGD QG QG Figure 16. MOSFET driver on QTH t QGS Figure 17. MOSFET driver off PDriver PGate(HS) PGate(LS) QG(HS) QG(LS) VDriver Fsw AN005 © 2014 Richtek Technology Corporation 9 Analysis of Buck Converter Efficiency D : PCB loss : Figure 18 could be illustrated as Figure 19 and Figure 20 with Rtr1~Rtr7 with loop1 (HMOS conduction) and loop2 (LMOS conduction) in detail. Figure 18. PCB trace diagram Rtr1 HS L Rtr2 RL Rtr3 IOUT + + Rtr4 Gate Signal VIN RLoad CESR Rtr5 Rtr7 _ COUT LS Rtr6 _ Figure 19. PCB loop1 trace Rtr1 HS L Rtr2 RL Rtr3 IOUT + + Rtr4 Gate Signal VIN Rtr7 _ COUT LS Rtr5 RLoad CESR Rtr6 _ Figure 20. PCB loop2 trace PCB loss PCB loop1 loss + PCB loop2 loss ILoop12 Rtr1 Rtr2 Rtr3 Rtr6 Rtr7 ILoop22 Rtr3 Rtr4 Rtr5 Rtr6 2 Iripple 2 Where ILoop1 D IOUT 12 ILoop2 2 1 D IOUT AN005 2 Iripple 12 © 2014 Richtek Technology Corporation 10 Analysis of Buck Converter Efficiency 3. Power loss measurement and calculation comparison Although the buck converter power loss calculated equations are well introduced and documented. In order to check the accuracy of these power loss equations, Table1 shows the typical buck converter application parameter and Figure 21 illustrates the efficiency comparison between measurement and calculation. Table 1. Buck converter application parameter IC RT8120 VIN 12V Vout 1.2V FSW 300kHz VDD 12V L 1H DCR 1.2m HMOS BSC090N03LS LMOS BSC090N03LS*2 EFFICIENCY vs. LOAD C URR ENT 100 100 97.5 95 EFFICIEN CY (%) 92.5 90 CalculateEFF I.OUT Measure I.LOAD 87.5 85 82.5 80 77.5 75 75 0 0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 I.OUT I.LOAD 25 25 LOA D CURRENT (A) Figure 21. Measurement and calculation of efficiency comparison AN005 © 2014 Richtek Technology Corporation 11 Analysis of Buck Converter Efficiency Figure 22 shows the key component loss in buck converter including HMOS, LMOS, inductor, driver and PCB trace loss. Readers can check what the major loss contributed in each system loading. POW ER LOSS vs. LOAD C URR ENT 3 3 2.7 TOTA L POWER LOSS (W) 2.4 P.LMOS I.OUT 1.8 P.L I.OUT 1.5 P.DRV I.OUT 1.2 PD.PCB I.OUT P.HMOS I.OUT 2.1 0.9 0.6 0.3 0 0 0 2.5 5 7.5 10 0 12.5 15 17.5 20 22.5 I.OUT 25 25 LOA D CURRENT (A) Figure 22. Key component loss in buck converter Figure 23 shows detail component loss in buck converter and illustrates the loss v.s Iout in the curve. HMOS : PHSW (Switching loss) and PHCOD (Conduction loss) LMOS : PLCOD (Conduction loss), PL_DIODE (Dead-time body diode loss) and PRR (Reverse recovery loss) Inductor : PL (Inductor DC & core loss) Driver : PDRV (Gate driver charge loss) PCB : PPCB (PCB trace loss) POW ER LOSS vs. LOAD C URR ENT 2.5 2.5 2.25 P.HSW I.OUT 2 1.75 P.LCOD I.OUT P.L_DIODE I.OUT 1.5 P.RR I.OUT 1.25 P.L I.OUT 1 P.DRV I.OUT 0.75 P.PCB I.OUT TOTA L POWER LOSS (W) P.HCOD I.OUT 0.5 0.25 0 0 0 0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 I.OUT 25 25 LOA D CURRENT (A) Figure 23. Detail power loss in buck converter AN005 © 2014 Richtek Technology Corporation 12 Analysis of Buck Converter Efficiency 4. Conclusion This application document analyzes power loss in synchronous buck converters and presents the detailed calculations for each part of the power loss. The loss calculation also compares with real buck converter measurement and provides the key component loss data to consider how to improve the buck converter efficiency for component and PCB plane consideration. AN005 © 2014 Richtek Technology Corporation 13 Analysis of Buck Converter Efficiency Reference [1] Leon Chen, “Power Loss Analysis for Synchronous Buck Converter”, Application Engineer Dept data, 2013. [2] Nelson Garcia, “Determining Inductor Power Losses”, Coil craft Document 486, 2005. Richtek Technology Corporation st 14F, No. 8, Tai Yuen 1 Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: 886-3-5526789 Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries. AN005 © 2014 Richtek Technology Corporation 14

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