Advanced Switches
Boost PFC Efficiency
By Wonsuk Choi and Sungmo Young, Application Engineers,
Fairchild Semiconductor, Bucheon, Korea
While improved MOSFET technology can reduce switching losses in CCM PFC stages, even
greater reductions in MOSFET switching losses
are achieved using SiC technology for the boost
diode.
T
he power factor is a numerical parameter used to
improvements are combined with a SiC Schottky diode
measure the quality of input power supplied to
having a low reverse-recovery charge (QRR). The resulting
ac-dc converters. Recently, power factor correcimproved performance in a 400-W CCM PFC application,
tion (PFC) requirements, such as regulation IEC
compared with the performance of a typical Si-diode/Pla61000-4-3, have been strongly imposed on many
nar-MOSFET approach, indicates the merit of using the
systems, reflecting a growing trend in the ac-dc power-supply
combined device.
market. To meet these demands, designers can use passive
and active PFC design techniques that comply with line
Device Requirements for CCM PFC
harmonics standards for power-supply systems.
PFC in CCM has many advantages compared to DCM
One approach is to use passive PFC as a low-cost soluowing to continuous current on the boost inductor. This
tion, but this requires a heavy and bulky LC filter. Active
advantage is more obvious in high-power designs, because
PFC is widely used to reduce the size and weight of the
current filtered by the EMI filter is much lower than in
system’s filter inductor. Therefore, increasing efficiency and
DCM or CRM modes. In typical applications, the power
power density are key design factors affecting this approach.
losses are usually determined by the switching losses in the
Continuous conduction mode (CCM) boost PFC is the
MOSFET, which are actually caused by the reverse-recovery
preferred active topology for high-power ac-dc converters.
characteristics of the discrete boost diode. This large source
Unlike discontinuous conduction mode (DCM) or critical
of switching loss depends on operating current and diode
conduction mode (CRM), CCM PFC results in low ripple
temperature. These factors lead to increased power dissipacurrent, simplifies EMI-filter design and
maintains stability at light loads. As a
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result, CCM PFC is commonly used not
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only in server/telecom power supplies,
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but also in power supplies for flat-panel
displays.
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According to the trend in power-con�
verter PFC designs of greatly improving
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power densities, designers must reduce
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system losses and decrease overall sys��
tem size and weight, either by increasing
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switching frequency or integrating active
components.
A new MOSFET/diode combination
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can achieve higher efficiency and reduced
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switching loss in CCM PFC controller
designs accomplished through low RDS(ON) Fig. 1. In the basic PFC circuit operated in CCM, the reverse-recovery current of the boost diode
and fast-switching in the MOSFET. These significantly contributes to the switching losses of the MOSFET.
Power Electronics Technology June 2007
42
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PFC EFFICIENCY
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At time t1, the stored charge in the vicinity of the p-n junction
is depleted. Some negative diode current continues to flow,
removing any remaining stored minority charge, while depositing charge to reform the depletion layer. At time t2, this
current is essentially zero, and the diode operates in steady
state under reverse-biased condition.[1] These power losses,
caused by the Si diode reverse-recovery behavior, limit the
efficiency and switching frequency of a CCM PFC.
The main concern in CCM PFC is reducing conduction
and switching losses of the MOSFET and the boost diode.
To design a high-performance CCM PFC with smaller size
and higher operating frequency, the MOSFET requirements
are as follows: low RDS(ON) to reduce conduction loss; low
CGD to reduce switching loss; low QG to reduce gate-drive
power; and low thermal resistance. The requirements for
the boost diode in the same CCM PFC are as follows: low
tRR to reduce MOSFET turn-on loss, and low QRR to reduce
diode switching loss; low VF to reduce conduction loss; soft
reverse-recovery characteristics to reduce EMI; and low
thermal resistance.
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MOSFET Comparisons
Fig. 3 shows the cross-section of Fairchild Semiconductor’s SuperFET 600-V MOSFET, which uses charge-balanced
technology (right), together with a conventional planar
MOSFET (left). One immediately noticeable difference is
the deep p-type pillar in the body of the SuperFET device.
The major contribution (>90%) of the low RDS(ON) provided
by the SuperFET comes from the n-drift layer. The effect of
the deep p-type pillars is to confine the electric field in the
lightly doped epitaxial region of the MOSFET. The resistivity of the n-epi layer is then dramatically reduced compared
to that of conventional planar MOSFET, while maintaining
the same breakdown voltage. Lowering RDS(ON) of the highvoltage MOSFET allows the device to achieve a die shrink
about 35% compared with a conventional planar MOSFET
rated for the same drain current.
A MOSFET’s switching characteristics are a function
of its parasitic capacitances. For example, the small active
area of the high-voltage SuperFET leads directly to a small
input capacitance, and therefore to a low gate charge. This
results in a very short turn-on delay time and a low drive
power requirement. When comparing the capacitances of a
SuperFET to those of a planar MOSFET, the value of CGD
decreases rapidly as VDS approaches 10 V (in the case of
the SuperFET), while smaller output capacitance decreases
discharge losses at the turn-on switching transient. Because
this technology is designed to withstand both high-speed
voltage (dv/dt) and current (di/dt) switching transients,
these devices operate reliably at higher frequencies, having
a figure of merit (FOM) that is one-third that of a similarly
rated planar device due to the reduced resistance.
An advantage of using a SuperFET is that its low on-resistance diminishes power losses. This allows designers to eliminate the need for expensive cooling systems and to reduce
the size of heatsinks. Its low gate charge also makes it easier
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Fig. 2. The waveforms of a CCM PFC stage reveal how switching losses
occur in pulses when non-zero voltages and currents, including reverserecovery currents from the boost diode, overlap.
tion in the diode and MOSFET, which can significantly affect
the converter’s performance.
Fig. 1 and Fig. 2 show the operating modes of CCM PFC,
including current and voltage waveforms, to illustrate the
low QRR requirement for a PFC diode. Initially, the diode
D1 conducts the input current with some amount of stored
minority charge present in the diode. During the turn-on
switching transition, the MOSFET M1 turns on while the
diode D1 turns off. A large inrush current flows through
the MOSFET, including D1’s reverse-recovery and discharge
current, in addition to the rectified input current. The rate
of change of the current is typically limited by M1’s package inductance and other stray inductances present in the
external circuit. The area within the negative portion of the
diode-current waveforms is the reverse-recovery charge QRR,
while the interval length (t0 to t2) is the reverse-recovery
time tRR. During the interval defined by t0 < t < t1, the diode
remains forward-biased, so the MOSFET voltage is VOUT + VF.
Power Electronics Technology June 2007
44
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PFC EFFICIENCY
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Fig. 3. The cross-section of Fairchild Semiconductor’s SuperFET 600-V MOSFET, which uses charge-balanced technology (right), together with a
conventional planar MOSFET (left).
and more efficient to drive at high frequency. These characteristics reduce the overall power losses in the system.
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Diode Comparisons
Si Schottky diodes are typically used for low- or middlevoltage applications of less than 300 V, because they exhibit
very low switching losses and a positive temperature coefficient, while maintaining leakage current and forward-voltage
drop within acceptable levels. However, this type of diode
is not ideal for high-voltage applications, where the leakage
current and the forward-voltage drop can be much higher.
By comparison, SiC Schottky diodes are more attractive for
high-voltage designs. This is because the electric field of
silicon carbide at breakdown is 10 times higher than that of
silicon. Furthermore, the wide bandgap of SiC allows higher
operating temperatures [2]. Additionally, a SiC Schottky diode
has no reverse-recovery current during switching transition
because it has no excessive minority carriers. Although it does
have displacement current from parasitic junction capacitances, this is negligible. Therefore, SiC Schottky diodes offer
greater efficiency than Si diodes in CCM PFC application
due to these superior reverse-recovery characteristics, which
are independent of temperature and forward conduction
characteristics of the device [3-7].
Fig. 4 shows the reverse recovery of a SiC Schottky diode
compared with Si diodes. In this example, fast-recovery Si
diodes from Fairchild are classified into three types according to tRR and VF, with the Stealth diode having the fastest
reverse-recovery characteristics and the ultrafast device having the lowest VF value. By performing reverse-recovery tests
at 25°C, Si diodes exhibit a large amount of reverse-recovery
current while a SiC Schottky diode only has displacement
current through the capacitor formed by the reverse-biased
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Fig. 4. The reverse-recovery time of a SiC diode is virtually nonexistent
compared to that of Si diodes, as revealed in this test where the forwardbiased current is 6 A, and the reverse voltage is 400 V.
p-n junction. The V-I characteristics for both SiC Schottky
and Si diodes are a function of temperature. At low forward
currents, VF decreases as temperature increases. In this region, the exponential behavior of the current flowing across
the Schottky barrier can be observed. As the forward current
increases, the diode’s bulk resistance dominates the forwardbiased behavior, and VF of the Schottky diode increases as the
temperature increases. The larger bandgap of SiC Schottky
diode results in higher intrinsic carrier concentration and
higher operating junction temperatures. In principle, SiC
Schottky diodes have the potential to operate to 600°C, as
compared to the 150°C maximum junction temperature of
45
Power Electronics Technology June 2007
PFC EFFICIENCY
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Fig. 5. The normalized switching losses of the SuperFET/SiC Schottky
diode combination and the planar-MOSFET/Si diode combination
reveal the substantial gains in efficiency provided by the former
configuration.
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Fig. 6. The efficiency of the SuperFET/SiC Schottky diode combination
vs. the planar MOSFET/Si diode combination in a 400-W CCM PFC
design directly translates into lower temperatures, higher power
densities and reduced heatsink requirements.
Si diodes [8]. The increased operating temperature enables
reductions in the weight, volume, cost and complexity of
thermal management systems.
Additionally, SiC Schottky diodes are suitable for parallel
operation at higher voltages than Si diodes owing to their
positive temperature coefficients. A SiC Schottky diode’s
low QRR reduces not only the switching loss of the diode,
but also the turn-on losses of the MOSFET, resulting in
high-efficiency CCM PFC. This remains true even when the
forward current is higher in the SiC diode than it is in the
Si diode. This superior temperature characteristic of a SiC
Schottky diode makes it possible to reduce the peak drain
current during the MOSFET turn-on transient. It also allows
designers to use of smaller MOSFETs to reduce costs.
SuperFET was greatly reduced. A turn-off loss of 159 µJ was
measured for the planar MOSFET, and 125 µJ was measured
for the SuperFET (a reduction of 34 µJ, or 21%).
The MOSFET waveforms were also measured at turn-on
transition under full load while the input was 110 Vac. There
was a reverse-recovery current of 5.3 A through the Si diode
and the MOSFET (apart from the inductor current), which
was caused by the of Si boost diode. However, there was
only a 1.2-A current caused by the negligible displacement
current of the SiC Schottky diode. Therefore, MOSFET
turn-on losses of 73.8 µJ were measured for the Si diode, and
28.9 µJ were measured for the SiC Schottky diode (a reduction of 44.9 µJ, or 61%).
In the test, the diode waveforms were at diode turn-off
transition under full load and an input voltage of 110 Vac.
There was a large peak reverse-recovery current of 5.3 A,
and a peak reverse-recovery voltage of 500 V for the Si diode. There was a negligible reverse-recovery current, and a
reverse-recovery voltage of 450 V for the SiC Schottky diode
under same conditions. The differences in transient behavior
between the two MOSFET types results in different MOSFET
turn-on losses. The turn-off loss of the SiC Schottky diode
is also reduced about 78% compared to that of the Si diode
due to zero recovery time for the SiC diode.
As indicated in the switching-loss summary shown in
Fig. 5, a SuperFET and SiC Schottky diode combination can
lead to a considerable switching-loss reduction. Compared
to the planar MOSFET, the SuperFET can reduce the turnoff loss by 21%. Compared to the Si fast-recovery diode, the
SiC diode can reduce the turn-on loss by 61%. Also, using a
SiC Schottky diode with a conventional MOSFET instead of
Combined MOSFET/SiC Diode Module
A 400-W CCM PFC test circuit was designed using a
high-voltage SuperFET and a SiC Schottky diode. Specifically, this demonstration compared the switching losses and
efficiency of Fairchild’s 600-V n-channel SuperFET MOSFET
(FCA20N60) and 6-A SiC Schottky diode with a planar
MOSFET (FQA24N50) and Ultrafast Si diode (RURP860).
The test circuit operated at 100 kHz and output voltage
and current was set to 400 V and 1 A, respectively. The gate
resistance of the SuperFET was 12 Ω when switching on, and
9.1 Ω when switching off.
Voltage and current were measured on both the MOSFET
and the diode to estimate the power losses in these components. Also, the input and output power was measured
to calculate the efficiency of the system. When MOSFET
waveforms at turn-off transition were under full load while
the input was 110 Vac, the switching losses were measured
by the crossover area of VDS and ID. The switching time of the
Power Electronics Technology June 2007
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PFC EFFICIENCY
e-Front runners
a fast-recovery Si diode can reduce the MOSFET’s turn-off
loss by 78% and its turn-on loss by 23%. Efficiency measurement results of different combinations of these devices
are shown in Fig. 6. Based on this figure, it is clear that the
MOSFET/SiC Schottky diode combination achieves significant improvements in efficiency over the entire operational
range. The improvement was even greater at high-current
conditions (low input under full load) where an improvement in efficiency of more than 4% was achieved, compared
to a test circuit using conventional devices under the same
conditions. As demonstrated in this switching-loss analysis,
the major reason for higher efficiency is the reduced turn-on
loss in the MOSFET because of minimized reverse-recovery
charge of the SiC Schottky diode. The end result is increased
power density for the CCM PFC stage.
PETech
“Micro DC-DC Converter
with Integrated Inductor”
References
1. Erickson, Robert W. and Maksimovic, Dragan, “Fundamentals of Power Electronics,” Kluwer Academic Publishers,
Massachusetts 2001, 2nd edition.
2. Friedrichs, P.; Mitlehner, H.; Schorner, R.; Dohnke, K.O.;
Elpelt, R. and Stephani, D., “Application Oriented Unipolar
Switching SiC Devices,” Materials Science Forum, pp.11851190, Vol. 389-393(2002).
3. Bruckmann, M.; Baudelot, E.; Weis, B. and Mitichner, H.,
“Switching Behavior of Diodes Based on new Semiconductor
Materials and Silicon — A Comparative Study,” EPE 1997,
7th European Conference on Power Electronics and Applications, Vol. 1, pp. 513-17, 1997.
4. Neudeck, P.G.; Larkin, D.J.; and Powell, J.A., “Silicon
Carbide High-Temperature Power Rectifiers Fabricated and
Characterized,” NASA Lewis Research Center (USA).
5. Liu, J.; Chen, W.; Zhang, J.; Xu, D. and Lee, F.C., “Evaluation of Power Losses in Different CCM Mode Single-Phase
Boost PFC Converters via a Simulation Tool,” IAS 2001, pp.
2455-2459, Vol. 4.
6. Lu, B.; Dong, W.; Zhao, Q.; Lee, F.C.; “Performance Evaluation of CoolMOSTM and SiC Diode for Single-Phase Power
Factor Correction Applications,” APEC 2003, pp. 651-657,
Vol. 2.
7. Spiazzi, G.; Bum, S.; Citron, M.; Corradin, M. and Pierobon, R., “Performance Evaluation of a Schottky Sic Power
Diode in a Boost PFC Application,” IEEE Transactions on
Power Electronics, Vol. 18, No. 6, November 2003.
8. Shenai, K.; Scott, R.S., and Baliga, B.J., “Optimum Semiconductors for High-Power Electronics,” IEEE Transactions
on Electron Devices, Vol. 43, No. 9, pp. 1811-1823, September
1989.
Buck Converter Circuit
Type Number: FB6831J
Vin=2.7V-5.5V, Vout min=0.8V,
lout max=500mA
Switching frequency=2.5MHz
•
Principle of technology
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Integrated controller, FETs and inductor
IC chip on inductor structure
Inductor with terminal pins
Advantages
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Reduced size/weight
2.95mm x 2.4mm x 1mm/30mg
High efficiency 90%
(Vin=3.6V, Vout=1.8V, lout=200mA)
High reliability
Potential application
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Cellular phone, Digital video camera,
Digital still camera, Portable instruments, etc.
Packages
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SON (10pin)
VIN
2.7V to 5.5V
CIN
4.7uF
2 GND
PVDD 9
3 COP
CE 8
4 CRES
VDD 7
5 IN
RFB1
100k
M 10
1 PGND
VOUT 6
VOUT
1.5V / 500mA
COUT
4.7uF
RFB0
150k
“Step Forward, Raise Value”
Fuji Electric Device Technology
America, Inc.
Piscataway, NJ 08854, U.S.A.
Phone: 972-733-1700
Fax: 732-457-0042
For more Info. Visit
www.fujidevicetech.com
www.powerelectronics.com
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Power Electronics Technology June 2007