Analysis of Buck Converter Efficiency
Abstract
The synchronous buck circuit is wildly used to provide nonisolated 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.
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
Power loss calculation
Lowpower loss and highly efficient synchronous buck converters are in great demand for advanced microprocessors. 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 (HighSide MOSFET) summarizes to include : switching on & off and conduction loss.
LMOS (LowSide MOSFET) summarizes to include : conduction, deadtime and reverse recovery charge loss.
HMOS switching on loss :
Figure 2. HMOS Driver switching on
Figure 3. HMOS switching on loss area
HMOS conduction loss :
The conduction loss of highside MOSFET is determined by the onresistances of the MOSFET and the transistor RMS current.
Figure 4. HMOS conduction on
Figure 5. HMOS conduction on period
LMOS conduction loss :
Figure 6. LMOS conduction on
Figure 7. LMOS conduction on period
LMOS dead time body diode loss :
Deadtime loss is induced by LMOS body diode conduction during deadtimes.
Figure 8. LMOS body diode conduction on
:
Figure 9. LMOS body diode conduction on period
LMOS reverse recovery charge loss :
Figure 10. LMOS body diode reverse recovery period
B : Inductor DC & AC loss
Inductor DC loss :
Figure 11. Current through inductor path
Figure 12. Inductor current path period
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.
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 :
The PL is the power loss (mW),
Fsw : operating frequency
B : peak flux desity in Gauss
V_{e }: effective core volume
The specific value of C, X and Y are core loss parameters for each material
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.
Figure 15. Driver turns on and off path
Figure 16. MOSFET driver on
Figure 17. MOSFET driver off
D : PCB loss :
Figure 18 could be illustrated as Figure 19 and Figure 20 with R_{tr1}~R_{tr7} with loop1 (HMOS conduction) and loop2 (LMOS conduction) in detail.
Figure 18. PCB trace diagram
Figure 19. PCB loop1 trace
Figure 20. PCB loop2 trace
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

1mH

DCR

1.2mW

HMOS

BSC090N03LS

LMOS

BSC090N03LS*2

Figure 21. Measurement and calculation of efficiency comparison
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.
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 : P_{HSW} (Switching loss) and P_{HCOD} (Conduction loss)
LMOS : P_{LCOD} (Conduction loss), P_{L_DIODE} (Deadtime body diode loss) and P_{RR} (Reverse recovery loss)
Inductor : P_{L }(Inductor DC & core loss)
Driver : P_{DRV} (Gate driver charge loss)
PCB : P_{PCB} (PCB trace loss)
Figure 23. Detail power loss in buck converter
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.