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Texas Instruments Incorporated
Power Management
Efficiency of synchronous versus
nonsynchronous buck converters
By Rich Nowakowski, Power Management Product Marketing
and Ning Tang, Systems Engineer, SWIFT DC/DC Converters
Choosing the right DC/DC converter for an application can
be a daunting challenge. Not only are there many available
on the market, the designer has a myriad of trade-offs to
consider. Typical power-supply issues are size, efficiency,
cost, temperature, accuracy, and transient response. The
need to meet ENERGY STAR ® specifications or green-
mode criteria has made energy efficiency a growing
concern. Designers want to improve efficiency without
increasing cost, especially in a high-volume consumer
electronics application where reducing power consump-
tion by one watt can save megawatts from the grid. The
semiconductor industry has recently developed low-cost
DC/DC converters that employ synchronous rectification
and that are thought to be more efficient than nonsyn-
chronous DC/DC converters. This article will compare the
efficiency, size, and cost trade-offs of synchronous and
nonsynchronous converters used in consumer electronics
under various operating conditions. It will be shown that
synchronous buck converters are not always more efficient.
Typical application
To demonstrate the subtle differences between the two
converter topologies, a typical point-of-load application was
chosen. Many low-cost consumer applications use a 12-V
rail that accepts power from an unregulated wall adapter
oranoff-linepowersupply.Outputvoltagesusuallyrange
from 1 to 3.3 V, with output currents under 3 A. The Texas
Instruments devices in Table 1 were chosen to compare
actual efficiency measurements under various output-
current and output-voltage conditions. The rated output
current, which is the level of output current each device
is marketed to deliver, was taken directly from the data
sheets (see References 1 and 2).
Figure 1. Synchronous and nonsynchronous
buck circuits
V IN
Q1
Switch Node
Control
L1
V OUT
Q2
D1
C1
Control
Q2 Integrated with
Synchronous Converter
alow-sideMOSFET(Q2)isused.Inanonsynchronous-
buck topology, a power diode (D1) is used. In a synchro-
nous converter, such as the TPS54325, the low-side power
MOSFETisintegratedintothedevice.Themainadvantage
of a synchronous rectifier is that the voltage drop across
thelow-sideMOSFETcanbelowerthanthevoltagedrop
across the power diode of the nonsynchronous converter.
If there is no change in current level, a lower voltage drop
translates into less power dissipation and higher efficiency.
Choosing the power diode
Nonsynchronous converters are designed to operate with
an external power diode (D1). The three key specifications
a designer needs to consider when choosing a power diode
are the reverse voltage, the forward voltage drop, and the
forward current. First, the rated reverse voltage must be
at least 2 V higher than the maximum voltage at the switch
node. Second, the forward voltage drop should be small for
higher efficiency. Third, the peak-current rating must be
greater than the maximum output current plus one-half the
peak-to-peak inductor current. When the duty cycle is low
(i.e., at low output voltages), D1 operates as a catch diode
thatconductsmorecurrentthanthehigh-sideMOSFET.
A fourth consideration is to make sure the package of the
diode chosen can handle the power dissipation. The diode
chosen for the TPS54331 was the B340A, which has a
reverse voltage rating of 40 V, a forward voltage drop of
0.5 V, and a forward current rating of 3 A.
Table 1. Device comparison
INPUT
VOLTAGE RANGE
(V)
PART
NUMBER
RATED I OUT
(A)
TOPOLOGY
TPS54325
Synchronous buck
4.5 to 18
3
TPS54331
Nonsynchronous buck
4.5 to 28
3
Basic operation
A typical block diagram for a step-down (buck) regulator is
shown in Figure 1. The main components are Q1, which is
thehigh-sidepowerMOSFET;L1,thepowerinductor;and
C1, the output capacitor. For a synchronous-buck topology,
15
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4Q 2009
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Power Management
Texas Instruments Incorporated
The TPS54325 does not need a power diode, since a 70-mΩlow-sideMOSFETisintegratedintothechip.Theintegrated
MOSFETsavesspace;butthecomplexityofthecontrolcircuitrymustbeincreasedtoensurethatbothMOSFETsdonot
conduct simultaneously, which would result in a direct path from the input to ground. Any cross conduction would result in
lower efficiency and could even overload and damage the system.
Efficiency calculations
To calculate the efficiency of a DC/DC converter, the total power dissipation needs to be computed. The key contributors
to the power dissipation for a DC/DC converter in continuous conduction mode (CCM) are the high- and low-side switching
losses and the IC’s quiescent-current loss. The formulas for these losses are as follows:
V
V
2
OUT
IN
(1)
P
=
I
×
R
×
Conduction
_
HS OUT Son
(
)
(2)
PVV
=× ×
0.(
t
+
t
)
×
f
SW
IN OUT
Rise all
W
P
=×
V
I
(3)
Quiescent
INq
Equations 1 through 3 apply to both the synchronous and the nonsynchronous converter in CCM. However, the losses in
thelow-sideMOSFETforthesynchronousbuckconverter(Equation4)andinthelow-sidepowerdiode(P D1 ) for the non-
synchronous buck converter (Equation 5) need to be included:
V
V
OUT
IN
2
(4)
P
=
I
×
R
×−
1
+×
(
2
t
×× × )
f
I
V
Conduction LS
_
OUT Son
(
)
Del
ay
SW OUT wd
Low-Side MOSFET
Body Diode
V
V
OUT
IN
(5)
PV
=
× ×−
I
1
DDFwd UT
1
1
_
In Equation 4, the first component represents the con-
ductionlossinthelow-sideMOSFET,andthesecondcom-
ponent represents the conduction loss in the body diode.
The current flowing through the body diode is about an
order of magnitude lower than the current flowing through
thelow-sideMOSFETandisnegligibleat2A.
These equations make it evident that there are several
factors influencing full-load efficiency, such as the drain-
to-source resistance, drain-to-source forward voltage, duty
cycle,frequency,andpowerMOSFETriseandfalltimes.
The AC and DC losses of the inductor and the equivalent
series resistance of the output capacitance are similar in
the application, since the same LC filter can be used for
both devices. For a DC/DC converter, the duty cycle is
given, and only the drain-to-source resistance, forward
voltage drop, and switching frequency can be chosen.
Typically,theMOSFETriseandfalltimesarenotstatedin
the data sheet but are important specifications to consider,
since the faster they are, the less power is dissipated. The
trade-off is noisy ringing at the switch node when a power
MOSFETisturnedontooquickly.Start-uptimecanbe
reduced to improve thermal performance so that a less
costly package can be chosen to house the smaller power
MOSFETwithahigherdrain-to-sourceresistance.
Efficiency results at high loads
Two circuits were built with the devices shown in Table 2
so that the efficiencies of the circuits could be compared.
The devices used the same LC filter in the bill of materials.
Even though the two devices had slightly different fixed
switching frequencies, there was not enough impact on
circuit efficiency to alter the conclusion of this demon-
stration. An input voltage of 12 V was chosen, and effi-
ciency measurements were taken by simply varying the
output voltages.
Table 2. Basic device characteristics
PART
NUMBER
HIGH-SIDE R DS(on)
(m Ω )
LOW-SIDE R DS(on)
(m Ω )
FREQUENCY
(kHz)
TPS54325
120
70
700
N/A
(V D1_Fwd = 0.5 V)
TPS54331
80
570
16
High-Performance Analog Products
4Q 2009
Analog Applications Journal
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Texas Instruments Incorporated
Power Management
Figure 2 shows the efficiency of both devices with a
12-V input and a 1.5-V output. The figure clearly shows
that the TPS54325 had higher efficiency at full load. Since
the duty cycle was 12.5%, the power diode of the nonsyn-
chronous solution with the forward voltage drop of 0.5 V
dissipated more energy than the 70-mΩMOSFET,despite
the TPS54325’s high-side drain-to-source resistance.
Figure 3 shows the efficiency of both devices with a 12-V
input and a 2.5-V output. It is evident that the efficiency
of the TPS54331 improved dramatically. In this case, the
duty cycle was 21%, and the two full-load efficiencies
were nearly the same. The power diode of the nonsyn-
chronous device conducted less often, and the high-side
MOSFETwithlowONresistanceconductedmoreoften.
When the dissipation of the low-side power diode was
lower at higher duty cycles, the nonsynchronous solution
became more efficient.
Figure 2. Device efficiencies with 12-V input and 1.5-V output
90
80
70
60
TPS54331
(Nonsynchronous)
50
40
30
TPS54325
(Synchronous)
20
10
0
0.001
0.01
0.1
1
10
Output Current ( A )
Figure 3. Device efficiencies with 12-V input and 2.5-V output
90
80
TPS54331
(Nonsynchronous)
70
60
50
40
TPS54325
(Synchronous)
30
20
10
0
0.001
0.01
0.1
1
10
Output Current (A)
17
Analog Applications Journal
4Q 2009
High-Performance Analog Products
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Power Management
Texas Instruments Incorporated
Figure 4. Inductor-current waveforms in CCM and DCM
Synchronous Buck in CCM
0
Nonsynchronous Buck in DCM
0
Time (t)
Considerations for light-load efficiency
In some applications, the need for light-load efficiency out-
weighs the need for higher full-load efficiency. Nonsynchro-
nous buck converters switch in discontinuous conduction
mode (DCM) at light loads. In the nonsynchronous buck
converter, the inductor current flows in only one direction.
With the synchronous buck converter, current is allowed
to flow in both directions, and power is dissipated when
reverse current flows. Figure 4 illustrates the difference
between inductor-current waveforms generated in CCM
versus those generated in DCM.
The TPS54331 has a pulse-skipping feature called
Eco-mode TM that improves light-load efficiency. This
modeofoperationturnsonthepowerMOSFETlessoften,
resulting in lower switching losses. The differences in
light-load efficiency shown in Figures 2 and 3 are due to
the TPS54331’s Eco-mode feature and its low operating
quiescent current. For more information on Eco-mode,
please see Reference 1.
Cost and space considerations
A synchronous converter with an integrated low-side
MOSFEToffersbenefitssuchasreducedsize,lowerparts
count, and easier design. However, if reducing cost is the
mainobjective,anonsynchronousconverterwithanexter-
nal power diode may be less expensive than a synchronous
buck converter.
Conclusion
Synchronous buck converters have become very popular
recently and are widely available. However, they are not
always more efficient. Nonsynchronous buck converters
may have adequate efficiency at higher duty cycles and
lighter loads. By paying attention to the data-sheet specifi-
cations, especially the drain-to-source resistance and the
quiescent current, the designer can make the best choice
based on the goals of a specific design.
References
For more information related to this article, you can down-
load an Acrobat ® Reader ® file at www-s.ti.com/sc/techlit/
litnumber and replace “ litnumber ” with the TI Lit. # for
the materials listed below.
Document Title
TI Lit. #
1.“4.5-Vto18-V,3-AOutputSynchronous
Step Down Switcher with Integrated FET
(SWIFT™),” TPS54325 Data Sheet .......... slvs932a
2. “3A, 28V Input, Step Down SWIFT™ DC/DC
Converter with Eco-mode™,” TPS54331
Data Sheet .............................. slvs839b
Related Web sites
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