Efficiency Improvements Using 10 Volt Schottky Diodes

AND8083/D
Efficiency Improvements
Using 10 Volt
Schottky Diodes
Prepared by: Jim Spangler, Larry Hayes, Ron Perina
ON Semiconductor
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APPLICATION NOTE
ABSTRACT
This application note describes the advantages realized by
using 10 volt Schottky diodes in a variety of applications.
The major advantage of using a lower voltage Schottky is the
reduced forward voltage drop. This lower drop results in a
significant increase in efficiency. In addition, one can affect
a substantial cost advantage of using a single Schottky diode
as opposed to the use of a power FET as a synchronous
rectifier.
The typical forward voltage drop of various low voltage
Schottky diodes is shown in Table 1. The data is taken from
the data sheet of each device [1, 2, 3, 4, 5, 6, 7] at 25°C. A
comparison can be made between a 10 volt device and a 20
or 30 volt device. Three surface mount packages are shown,
POWERMITE, SMA, and SMC. The forward voltage
drop is presented at five test currents in order to make
comparisons.
1. Notice the lower forward voltage drop of a 10 volt
device compared to a 20 or 30 volt device.
2. Also notice that in order to obtain a smaller
forward voltage drop, a higher current device can
be chosen. For example, the 4 Amp device,
MBRS410LT3 has a smaller forward voltage drop
than the 1 Amp or 2 Amp devices.
All Schottky diodes have a negative temperature
coefficient of approximately 1 mV/°C. The forward voltage
drop can be as much as 0.100 volts lower at 125°C when
compared to the voltage drop at 25°C.
Lower Forward Voltage Drop
Figure 1 shows the relative improvement in forward
voltage drop, Vf, which is made possible by the new 10 volt
Schottky technology. The comparison is made to industry
typical Schottky diodes using traditional low voltage
processes.
It is clear that such a significant improvement can have
major impact on the efficiency of many circuit applications.
VF Improvement
ON Semiconductor
Industry
Typical
Figure 1. VF Comparison
 Semiconductor Components Industries, LLC, 2002
March, 2002 – Rev. 0
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Publication Order Number:
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Table 1. Forward Voltage for Various Schottky Diodes as a Function of Forward Current, all values are at 25C
MBRM110L
1 A 10 V
MBRM120E
1A 20 V
MBRM120L
1 A 20 V
MBRA210L
2 A 10 V
MBRA130L
1 A 30 V
MBRS410L
4 A 10 V
MBRS320
3 A 20 V
Amps
POWERMITE
POWERMITE
POWERMITE
SMA
SMA
SMC
SMC
0.5
0.310
0.470
0.360
0.270
0.320
0.220
0.300
1.0
0.335
0.490
0.380
0.290
0.360
0.240
0.340
2.0
0.370
0.520
0.415
0.310
0.420
0.260
0.415
3.0
0.390
0.540
0.430
0.325
0.460
0.270
0.450
4.0
0.415
0.570
0.460
0.335
0.500
0.280
0.500
OR’ing Diode Applications
Power–Up Sequence Protection
In the application shown in Figure 2, a low forward
voltage drop part is desired. In this example, it is more cost
effective to use a single diode than to use a synchronous
rectifier constructed by using a power MOSFET. This can be
useful where multiple power supplies are needed for
redundancy to improve the mean–time–between–failure
(MTBF) for a power supply system.
The current rating of the OR’ing device is often much
higher than the current flowing through the device used. As
an example, a MBRS410L might be used for applications
where only 1.0 Amp of load current is need for the load. This
is used only to keep the energy loss to a minimum. The
forward voltage drop is approximately 0.24 volts for the
MBRS410L at room temperature, as shown by Table 1
above.
If one of the power supplies is a battery, the forward
voltage drop is important. The lower the forward voltage
drop, the greater the amount of energy available for use as
a standby power supply.
Some microprocessors (MPUs) require multiple power
supply voltages like that shown in Figure 3. The reason for
the different voltages is because there are different sections
within the MPU. The main processor may operate at 1.8
volts; the external memory or memory transfer section may
operate at 3.3 volts. There may be external peripherals such
as sensors that need to operate at 5.0 volts. Because of this
a power–up sequence may be required.
The main purpose of the power–up sequence is to prevent
SCR latch up within the MPU. The various sections of the
MPU must always be higher than other sections in order to
prevent this SCR latching current from flowing. This means
the 5.0 V supply must not be less than 2.7 volts (3.3 – 0.6)
when the 3.3 volt supply is operational. The same holds for
the 3.3 volt, its lower limit is 1.2 volts (1.8 – 0.6) when the
1.8 volt supply is functioning.
If there is a problem with the 5.0 volt and 3.3 volt sections,
there may be a requirement to keep both of these (3.3 V and
5.0 V) above the 1.2 volt level to prevent damage and in
addition preventing a SCR latch up condition. A Schottky
diode will be required for each supply where the forward
voltage drop would be less than 0.30 to 0.35 volts. The
diagram is shown in Figure 3. A perfect part for this type of
application is the MBRA210L, which is in a SMA package.
The MBRA210L exhibits a typical forward voltage drop of
0.27 volts at 1.0 Amp, as shown in Table 1 above.
OR’ing Diodes
A
B
5.0 V
Power
Supply
A
Power
Supply
B
Power
Supply
C
Power
Supply
Load
3.3 V
MPU
1.8 V
Figure 2. OR’ing Diode Application
Figure 3. Power–Up Sequence Protection
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Reverse Battery
Protect Diode
Diodes for Reverse Battery Protection
Battery Pack
Another application where a low voltage diode is often
desired is reverse battery protection. This is shown in
Figure 4. In many consumer type products there can be 3, 4,
or 6 AA, or AAA batteries in series.
In order to prevent damage to the electronic circuits inside
the product, a series diode, like a 1N4004 [8], is used. This
device has a typical forward voltage drop of 0.9 volts at 1.0
Amps.
If the batteries are alkaline, the terminal cell voltage can
between 1.4 volts and 1.65 volts depending upon the state of
charge and current draw. Using a 1.5 volt cell, the ratio of
0.9/1.5 indicates that 60 percent of one cell’s voltage is used
to overcome the protection diode. In a typical application
where there are 4 alkaline cells, the ratio of forward diode
voltage to battery voltage is between 13.64 and 16 percent:
(0.9/(4*1.65) = 0.1364 or 13.64%) or (0.9/(4*1.40) = 0.16
or 16%)
Instead of using a 1N4004 diode, substitute a 1 Amp low
voltage Schottky in a surface mount POWERMITE package
or a 2 Amp in a surface mount SMA package. The burden
voltage of the MBRM110L is only 0.335 volts, which is a
62.8 percent reduction, (0.9 – 0.335/0.9 = 0.628) over a
1N4004 diode. Using the MBRA210L at 0.290 volts, which
is in a SMA package, the percentage gain is even higher at
67.8 percent, (0.9 – 0.29/0.9 = 0.678) over the industry
standard 1N4004 rectifier.
The burden voltage is very critical if the batteries are
nickel cadmium (NiCd) or nickel metal hydride (NiMH).
There are many cordless phones that have three batteries in
series. The use of a MBRA210L can provide protection and
not be a major burden on the battery voltage needed for
phone operation.
The forward voltage drop can be reduced further by using
the 4 Amp Schottky, (MBRS410L), which has a forward
voltage drop of only 0.240 volts at 1.0 Amp.
+
–
+
–
+
–
+
–
Product
Figure 4. Reverse Battery Protection
DC to DC Converters
Another application where the use of low voltage
Schottky diodes improves efficiency is the DC to DC
converter circuit. In portable type products where a multiple
cell NiCd, NiMH, or a dual cell lithium ion (Li+)
rechargeable battery pack is used as a power source, a buck
regulator can produce output voltages such as 1.5 volts, 2.2
volts, or even 3.3 volts. A buck regulator can be very
efficient. An alternative method of obtaining 1.8, 2.2 or 3.3
volts at the output is to use a linear regulator, but this is very
inefficient. When implementing the CS51411 as a buck
regulator and using different Schottky rectifiers, it can be
shown that the lower the forward voltage drop of the catch
diode, the greater the efficiency of the buck regulator. Such
a schematic is shown in Figure 5.
Figure 5 is a modified schematic shown in the data sheet
[9], and demo board, CS51411DEMO/D [10]. Several
changes were made to the demo board circuit to improve
circuit operation. First, the 1N4148 diode was replaced with
a MBRM130L. This was done to lower the line–conducted
noise, electromagnetic interference (EMI). The 1N4148 is a
signal diode that has a reverse recovery time. The recovery
time is a source of EMI noise between 10 MHz and 30 MHz
in many switching power supplies.
Vin
MBRM120E
10 F
16 Vdc
0.1 F
2
0.1 F
1
CS51411
8
6
15 H
3
330 pF
10V
Schottky
7
0.1
F
205 K
127 K
Figure 5. CS51411 Buck Regulator Circuit
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220 F
Eout
6.3 VDC
AND8083/D
Once the duty cycle or duty factor, , is determined; the
loss calculation and efficiency calculations can be made. An
assumption is made that this is a continuous duty inductor
current system. The load current has an effective DC value
and is constant. The i terms are very small and not
considered for a first order approximation. The efficiency is
calculated by using Equation 9.
To show where the losses are in a buck regulator, Figure
6 is used to write equations. These equations are solved for
the effect of the forward voltage drop, Vf, on the efficiency
of the system.
Using Figure 6, two voltage loop equations are written
using Kirchoff’s voltage law. The first loop is written during
the switch conduction time, t1. The second loop equation is
written during the diode conduction time. These equations
are found in various power electronic test books and
application notes [11, 12, 12].
WOUT
WOUT WLOSSES
(eq. 9)
2 R
WLOSSES VSIO VfIO(1 ) IO
C (eq. 10)
Switch
Rcoil
Inductor
Diode
Vin
Output Vout
Capacitor
Load
Rswitch
Controller
VS IORDS ON
(eq. 12)
VOIO
VOIO VSIO VfIO(1 ) IORCIO
During the switch ‘On–time’ or t1 period.
(eq. 13)
VO
VO VS Vf(1 ) IORC
(eq. 1)
(eq. 2)
t2 = Off–time or diode conduction time
Vf = Forward voltage drop of the diode
In some text, Vd is used in place of Vf to describe the
forward voltage drop of the diode.
Equating the two equations using Li as the common item
results in Equation 3, 4 and 5.
0.90
3.3 Vout 10 V Schottky
0.88
0.86
3.3 Vout 30 V Schottky
(eq. 3)
(VO VRC Vf)t2 Li
(eq. 4)
EFFICIENCY
0.84
(VIN VS VRS VO)t1 Li
(VIN VS VRS VO)t1 (VO Vf VRC)t2
(eq. 6)
t1
1
t1 t2
(eq. 7)
V VRC Vf
O
VIN VS Vf
(eq. 8)
1.8 Vout 10 V Schottky
0.82
0.80
0.78
1.8 Vout 30 V Schottky
0.76
0.74
0.72
0.70
(eq. 5)
t1
duty cycle
t1 t2
(eq. 14)
The last term, IORC, is small and can be eliminated for a
first order approximation. When is plotted for various
outputs and for various diodes, the efficiency depends upon
several factors, switch voltage (VS) and diode voltage (Vd).
To show the effect of the diode voltage drop, the efficiency
is plotted using the CS51411, for two outputs, 1.8 and 3.3
volts. This efficiency plot is shown in Figure 7.
The output rectifier is changed from a 20 or 30 volt
Schottky to a 10 volt Schottky for each output. Taking only
one input voltage point, 7 volts, a comparison is made
between the use of the 10 volt Schottky diode to that of the
30 volt Schottky. The 10 volt Schottky produces 5 percent
improvement in efficiency when the output is 1.8 volts. The
improvement is just over 2 percent with a 3.3 volt output is
needed.
VS = Saturation voltage of the switch
VRC= Voltage due to inductor resistance,
Load current X Rcoil
VIN= Input voltage
VO = Output voltage under regulation
t1 = On–time of the power switch
i = Change in inductor current
During the switch ‘Off–time’ or t2 period.
0 –L i VRC VO Vf
t2
(eq. 11)
The load current term, IO, cancels for a first order
approximation that results in Equation 14.
Figure 6. Simple Buck Regulator
VIN VS VRS L i VO
t1
WOUT VOIO
0.68
5
6
7
8
9
INPUT SUPPLY VOLTAGE
Figure 7. Buck Regulator Circuit Efficiency Using
CS51411 with Various Schottky Diodes
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References
10. CS51411DEMO/D, “Demonstration Note for
CS51411/3 5.0 V–16 V to 3.3 V/1.0 A Buck
Regulator”, Demonstration Note,
ON Semiconductor.
11. AN920/D, “Theory and Applications of the
MC34063 and A78S40 Switching Regulator
Control Circuits”, Application Note, ON
Semiconductor.
12. M. Rashid, Power Electronics, Circuits, Devices,
and Applications, 2nd Ed. 1988, 1993, Prentice
Hall, Englewood Cliff, NJ 07632, ISBN
0–13–678996–X
13. Mohan, Undeland, Robbins, Power Electronics:
Converter, Applications, and Design,
1st & 2nd Ed., 1989, 1995, John Wiley & Son,
NY. ISBN 0–471–61342–8
1. MBRM110L/D,“Surface Mount Schottky Power
Rectifier”, Data Sheet, ON Semiconductor.
2. MBRM120L/D, “Surface Mount Schottky Power
Rectifier”, Data Sheet, ON Semiconductor.
3. MBRM120E/D, “Surface Mount Schottky Power
Rectifier”, Data Sheet, ON Semiconductor.
4. MBRA210LT3/D, “Surface Mount Schottky
Power Rectifier”, Data Sheet, ON Semiconductor.
5. MBRA130LT3/D, “Surface Mount Schottky
Power Rectifier”, Data Sheet, ON Semiconductor.
6. MBRS410LT3/D, “Surface Mount Schottky Power
Rectifier”, Data Sheet, ON Semiconductor.
7. MBRS340T3/D, “Surface Mount Schottky Power
Rectifier”, Data Sheet, ON Semiconductor.
8. 1N4001/D, “Axial Lead Standard Recovery
Rectifiers”, Data Sheet, ON Semiconductor.
9. CS51411/D, “1.5 A, 260 kHz and 520 kHz, Low
Voltage Buck Regulators with External Bias or
Synchronization Capability”, Data Sheet, ON
Semiconductor.
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Notes
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Notes
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