Application Note OptiMOS™ CoolMOS™ Optimal Solutions Suitable for DCDC Converter

Opti MOS™ and Co ol M O S™
The Opt im al S olut io ns Su it abl e f or D C/ D C
Con ver ter Used in H V D C S ys tem
Application Note
About this document
This application note analyze two cascaded solutions: Buck followed by LLC and Boost followed by LLC.
Both suitable for wide input range DC/DC converters used in ICT HVDC systems. The optimal solution in
terms of power loss, occupation of PCB and MOSFETs costs is being presented.
Scope and purpose
Guide the users select proper solution and power MOSFETs in DC/DC converter of ICT HVDC system.
Intended audience
Experienced power supply designers who want to optimize their solution and power MOSFETs selection.
Table of Contents
About this document ................................................................................................................... 1
Table of Contents ........................................................................................................................ 1
1
Introduction ............................................................................................................... 2
2
DC/DC Converter ......................................................................................................... 5
3
3.1
3.2
3.3
Solutions analysis ....................................................................................................... 8
Buck converter .................................................................................................................................... 8
Boost converter ................................................................................................................................. 11
LLC converter .................................................................................................................................... 13
4
4.1
4.2
Inductors and transformer ......................................................................................... 17
Inductors............................................................................................................................................ 17
LLC transformers ............................................................................................................................... 18
5
5.1
5.2
5.3
Conclusion ............................................................................................................... 20
Occupations of PCB by MOSFETs and magnetic elements .............................................................. 20
Power loss of MOSFETs and magnetic elements ............................................................................. 20
Cost of MOSFETs................................................................................................................................ 23
6
Reference ................................................................................................................. 25
Revision History ........................................................................................................................ 25
1
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Introduction
1
Introduction
A need for improved and more efficiency power distribution for mission critical application is a result of
continuous increases in global power consumption and a shift in power profile of modern loads[1]. About
1.3% of world electricity energy is taken over by equipment in data center and the number is about 2% in
United States. The higher efficiency power distribution in data center is meaningful for reducing the
consumption of resources and reducing CO2 emission; at the same time the higher efficiency power
distribution in data center means IT companies or telecom carriers can reduce CAPEX and OPEX, which in
return is beneficial to IT consumers.
Most traditional data centers use alternative current (AC) power system as shown in the Figure 1: threephase AC voltage main power is handled by UPS (uninterruptible power supply) firstly, a better voltage
waveform and backup power provided by battery group are obtained. The output single phase or threephase AC voltage is distributed by PDU (power distribution unit) to each cabinet or rack, where servers or
other IT equipment is installed. The power supplies in a server rectify AC input voltage to 12 VDC and then
convert it to a much lower voltage such as 1 VDC, which is ultimately consumed by CPUs or other ICs.
Figure 1
AC power supply system in data centers
It is easy to be found that there are two redundant parts in the AC power distribution system, one is the
inverter (DC/AC) in UPS and the other is the rectifier (AC/DC) in sever, the redundant parts decrease the
power system efficiency and reliability, also increase the cost of equipment. There are other shortcomings in
AC power system such as the difficulty in UPS paralleling, large area occupation.
HVDC system is promoted at the end of last century to cope with these problems, the term ”higher voltage
DC (HVDC)” is used to identify voltage in an information and communication technology equipment(ICTE)
space that are higher than 200 VDC and lower than 600 VDC[2]. Figure 2 illustrates the architecture of HVDC
system. Compared with Figure 2, UPS is replaced by HVDC rectifier whose output voltage is a higher DC
value compared to 48 VDC, the DC voltage is then distributed by PDU to power supplies in severs, and the
followed conversions units are same with those of traditional power system.
Figure 2
HVDC power supply system in data centers
The HVDC system is much simpler and is composed of less power conversion parts compared to UPS
showed in Figure 1, therefore higher efficiency, higher reliability and lower cost can be obtained.
Application Note
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Introduction
Figure 3 shows the advantages of HVDC system compared with traditional power distribution system in data
center provided by HP[3].
Figure 3
Advantage of HVDC power system over traditional power system
The output voltage of HVDC rectifier is specified differently in different countries, Figure 4 shows some of
world-wide DC deployments [4]. 380 VDC HVDC system is widely adopted in many counties and groups, in
China, however, an additional much lower voltage classs, 240 VDC is implemented in many data centers. As
as result, associated standards, which specify the requirements for HVDC rectifier, PDU, protections,
components and so on, have been published in some countries and international group such as China and
Europe Union and ITU (International Telecommunication Union).
Application Note
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Introduction
Figure 4
World-wide HVDC deployment
In the following, in section 2, the basic information of DC/DC converter of HVDC power system is introduced
and the configuration and working conditions of proposed DC/DC converter solutions that are discussed
further in the following sections are presented. The detailed calculations of Buck, Boost and LLC converters
are shown in section 3. Section 4 compares the inductors and transformers of Buck, Boost and LLC
converters in copper loss and magnetic loss. The conclusions of comparisons of different DC/DC converter
solutions are shown in section 5, which is intended as an aid for power electonics engineers to choose
suitable solutions when designing DC/DC converter of HVDC power system.
Application Note
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Used in HVDC System
DC/DC Converter
2
DC/DC Converter
As illustrated in the Figure 2, DC/DC converter in a server is connected to the output of HVDC rectifier
through PDU, the input voltage of the converter is equivalent to the output voltage of HVDC rectifier
excluding the voltage drops caused by PDU and conduction lines. Normal value of output voltage of HVDC
rectifier is showed in Figure 4 in world-wide range, which ranges from 240 VDC to 400 VDC. In China, two kinds
of HVDC voltage class, 240 VDC and 336 VDC, are piloted in some data centers in recent years. Actually 336 VDC
HVDC system in China is termed as 380 VDC system in Europe or America, though there are some differences
in detailed specifications. The output voltages and numbers of VRLA (Valve Regulated Lead Acid) battery of
240 VDC and 336 VDC HVDC rectifier are listed in the Table 1 and Table 2 according to Chinese standard and
company specification [5][6].
Table 1
Main specifications of 240 VDC HVDC rectifier
HVDC voltage class Normal rectifier
output voltage
Range of rectifier
output voltage
Input range of
DC/DC converter
Numbers of 2 VDC
VRLA battery
240 VDC
204 VDC ~ 288 VDC
192 VDC ~ 288 VDC
120
Table 2
268 VDC
Main specifications of 336V HVDC rectifier
HVDC voltage class
Normal rectifier output
voltage
Input range of DC/DC
converter
Numbers of 2 VDC
VRLA battery
336 VDC
378 VDC
300 VDC ~ 400 VDC
168
Table 1 and Table 2 list the numbers of VRLA, which is very important because it plays a decisive role in the
output voltage variety range of HVDC rectifier. The input voltage range of DC/DC converter is about 100 V for
the two HVDC systems as showed in Table 1 and Table 2, and more challenging for many energy conversion
companies is that they have to develop the HVDC DC/DC converter that can be compatible to both HVDC
systems for cost down and short time to market. In this application the input voltage will be from 190 VDC to
400 VDC. It is very hard to obtain a high conversion efficiency using single topology for such a wide input
range during the load range with acceptable cost. Therefore cascaded topology is promoted, on the one
hand to increase the efficiency during the whole input and load range, on the other hand to simplify the
control strategy. In this application note two cascaded topology solutions- Buck and Boost followed by LLCare analyzed in a 1200 W(12 V/100 A) server DC/DC converter that suitable for Chinese 240 V VDC and 336 VDC
HVDC system.
Buck and Boost are selected for its simplification in architecture and control strategy and well researched.
Certainly other topologies such as Buckboost can also be implemented in cascaded topology for its
possibility to realize ZVS, flexible output voltage and other merits, but the cost of the cascaded topology will
be higher than Buck or Boost so it isn’t feasible to be applied. LLC is welcome for its excellent performance
in high efficiency during the load range and low cost compared with other resonant converters such as ZVS
full-bridge PWM converter. The configuration of the cascaded topology can be Buck/Boost followed by LLC
or LLC followed by Buck/Boost, the former, as shown in Figure 5and Figure 6, is arbitrarily chosen in this
paper because the analysis method and procedure are same for both configurations. One converter (for
example Buck) of cascaded configuration should be closed-loop control to get a high-precision output
voltage over the input and output range, the other converter (for example LLC) can be closed-loop, semiclosed-loop or open-loop control, thus the whole control strategy of cascaded topology converter is simpler
than that of single topology converter.
Application Note
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
DC/DC Converter
Q3
Q1
Lr
L_buck
DC
Q2
C_buck
Co
Q4
Cr
Figure 5
Q5
Rload
Q6
The cascaded topology of Buck+LLC
Q3
L_boost
Lr
DC
Q1
Q2
Co
Q4
Rload
C_boost
Cr
Figure 6
Q5
Q6
The cascaded topology of Boost+LLC
The comparison of different cascaded topologies is implemented in a converter that output is 12 V/1200 W,
the analysis is based on the same loss for each MOSFET of the converter, TO-220 package MOSFETs are
chosen from Infineon Technologies for Buck, Boost and primary-side of LLC converter switches and
SuperSO8 package MOSFETs are selected from Infineon Technologies for secondary-side of LLC converter
switches for its low power loss and high thermal dissipation capability. While half-bridge LLC converter is
selected and is controlled to make sure that the resonant converters always work at resonant frequency fres,
the two resonant frequencies are set to be equivalent to each other for the two solutions.
In order to make it easier to comparison, the inductors of Buck and Boost converter perform at continuous
mode and the max. ripple currents through Buck inductor and Boost inductor are assigned to 25% of its
average value.
Some other presettable working conditions of cascaded topologies are:
(1) Working frequency of Buck/Boost:
(2) Resonant frequency of LLC:
Application Note
_ = 80
(1)
_ = 130
(2)
6
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Used in HVDC System
DC/DC Converter
The two working frequencies are chosen arbitrarily differently in case of resonating at high frequencies
between front and end converters in a cascaded solution and causing system instability and severe EMI
noise.
(3) The calculation and comparisons between two solutions will be made at five input voltage points
according to Table 1 and Table 2, those are: 190 VDC (min. input voltage), 268 VDC (normal input
voltage for 240 V HVDC system), 300 VDC (max input voltage for 240 VDC HVDC system and min input
voltage for 336 V HVDC system), 378 VDC (normal input voltage for 336 VDC HVDC system), 400 VDC
(max. input voltage).
(4) The comparison between two solutions will be made at three kinds of output load: 20% load (light
load); 50% load (medium load) and 100% load (full load).
In the following parts, the performance of Buck converter, Boost converter and two LLC converters following
Buck and Boost respectively are analyzed according to these conditions.
Application Note
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Solutions analysis
3
Solutions analysis
3.1
Buck converter
The illustration of Buck converter is shown in Figure 7, Q1 and Q2 are high-side and low-side MOSFETs
respectively and L_buck is the Buck inductor.
Q1
DC
L_buck
Q2
Figure 7
R_buck
C_buck
Topology of Buck converter
The output voltage of buck converter is arbitrarily assigned to 150 VDC ,
_ = 150 
(3)
It is supposed that efficiency of LLC is 98%,
η = 0.98
(4)
Then the output current (also the average current through the inductor) of Buck converter at full load is:

100
− = η ∗
= 0.98∗150 = 8.2
(5)

The inductor of Buck converter is calculated as:
 ≥
−
− ∗−
−

∗(1−)
= −
∗
−
Where D is the duty cycle ratio and for Buck converter:

 = −

(6)
_
(7)

As the range of input voltage of Buck converter (− ) is from 190 VDC to 400 VDC, the duty cycle of the
Buck converter is from 0.375 to 0.79 according to equation (3) and (7).
So the minimum inductance of Buck converter inductor is
− =
− ∗(1−min)
− ∗_
=
− ∗(1−0.375)
0.25∗− ∗80
≈ 574 uH
(8)
The inductance of the Buck inductor is assigned to 720uH, that is
(9)
 = 720 
Concerning the material and manufacture errors of inductor (generally ±20%), then the current ripple
through the inductor and the minimum and maximum current through the inductor can be obtained
according to:
Application Note
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Solutions analysis
ΔIL_buck =
− ∗(1−)
 ∗
(10)
_ = − − 0.5 ∗ _
(11)
_ = − + 0.5 ∗ _
(12)
The results from equation (9)~(12) are listed in Table 3.
Table 3
Ripple, min. and max. current through Buck inductor
Vin_1=190 VDC
Vin2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
ΔIL_buck [A]
0.548
1.147
1.302
1.571
1.628
[email protected]%load [A]
1.359
1.059
0.982
0.847
0.819
[email protected]%load [A]
1.907
2.206
2.284
2.418
2.446
[email protected]%load [A]
3.808
3.508
3.431
3.296
3.268
[email protected]%load [A]
4.356
4.655
4.733
4.867
4.895
[email protected]%load [A]
7.889
7.59
7.512
7.378
7.349
[email protected]%load [A]
8.437
8.737
8.814
8.949
8.977
It is feasible to suppose that max. power loss of high-side MOSFET of buck converter is about 6W concerning
the package of MOSFETs and cooling condition for ICT servers. The power losses of high-side MOSFET of
Buck converter include switching loss, conduction loss and Coss loss.
The switching loss can be calculated as [7]:
+
 =  ∗  ∗  ∗ (
)
(13)
2
Where tri and tfu are the current rise time and voltage fall time during turn on and turn off respectively.
Conduction loss is calculated as:
 = _ℎ 2 ∗ ()
(14)
Where RDS(on) is the conduction resistor of MOSFET at some junction temperature such as 120°C, _ℎ is
the rms current through the high-side MOSFETs of Buck converter and is calculated as:
1
−ℎ 2 = 3 ∗  ∗ ( 2 +  2 +  ∗  )
(15)
As the current through the inductor is illustrated by Figure 8.
Figure 8
Waveform illustration of high-side MOSFET of Buck converter
Coss loss can be calculated as:
 =  ∗ _
(16)
However, when we calculate the RDS(on) suitable for the application Coss loss is omitted for it is related to the
specific MOSFET.
Application Note
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Solutions analysis
It is supposed that:
 =  = 10 ∗ 10−9
(17)
Then, we can get the different RDS(on) of high-side MOSFET at different input voltage that can meet the loss
requirement according to:

−
() =  
(18)
2
−ℎ
From equation (13) to (18), the RDS(on) can be obtained, as shown in the following table
Table 4
RDS(on) [Ω]
RDS(on) of high-side MOSFET of Buck converter suitable for the application
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
0.09
0.114
0.121
0.133
0.135
From Table 4, it is known that the RDS(on) of the high-side MOSFET suitable for the application should be
smaller than 0.09Ω
The maximum total power loss of low-side MOSFET of Buck converter is also supposed to be 6 W, the power
loss of low-side MOSFET includes conduction loss caused by MOSFET and by body diode, which can be
calculated
__ = _ 2 ∗ ()
__ =  ∗ − ∗  ∗ 
Where _ is the rms current through the low-side MOSFETs of Buck converter, and:
1
_ 2 = 3 ∗ (1 − ) ∗ ( 2 +  2 +  ∗  )
(19)
(20)
(21)
− is the forward voltage drop of body diode of low-side MOSFET,  is the dead time between the
high-side and low-side driving signal.
It is supposed that:
(22)
(23)
− = 1
 = 200 ∗ 10−9 
Then we can get the different RDS(on) of low-side MOSFET at different input voltage that can meet the loss
requirement according to equation (19)~(23)
The results are shown in the following table:
Table 5
RDS(on) of low-side MOSFET of Buck converter suitable for the application
RDS(on) [Ω]
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
0.418
0.2
0.176
0.146
0.14
From Table 5 we can know that the RDS(on) of low-side MOSFET of Buck converter suitable for the application
should be smaller than 0.14Ω.
The max. voltage MOSFETs have to endure for Buck converter is the max. input voltage, which is 400 VDC for
the application. Considering 80% or more voltage derating and voltage spike produced by parasitic
parameters, the voltage class of 600V or 650V CoolMOSTM is suitable for the application. Therefore, two
paralleled IPP60R099P6 and two paralleled IPP65R150CFD are chosen for high-side and low-side MOSFETs
respectively concerning RDS(on) for high-side and low-side MOSFETs and excellent body diode requirement for
low side MOSFETs.
For IPP60R099P6 used as high side switch of Buck converter, RDS(on) is 0.165Ω at 120°C junction temperature
and Eoss (Energy of Coss) is about 4 uJ[8]; for IPP65R150CFD used as low side switch of Buck converter, RDS(on)
is 0.29Ω at 120°C junction temperature, and forward voltage drop of body diode is about 0.8V[9]. The power
Application Note
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Solutions analysis
loss of each MOSFET can be calculated at 120°Cjunction temperature of MOSFET at different input voltage
and output load after the MOSFETs are selected, the results are shown from Table 6 to Table 8.
Table 6
Each MOSFET loss of Buck converter at 20% load at different input voltage
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
High-side MOS[W]
0.532
0.559
0.574
0.614
0.626
Low-side MOS[W]
0.062
0.109
0.123
0.146
0.152
Table 7
Each MOSFET loss of Buck converter at 50% load at different input voltage
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
High-side MOS[W]
1.174
1.145
1.156
1.213
1.234
Low-side MOS[W]
0.307
0.588
0.661
0.79
0.817
Table 8
Each MOSFET loss of Buck converter at 100% load at different input voltage
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin-5=400 VDC
High-side MOS[W]
3.111
2.736
2.677
2.648
2.66
Low-side MOS[W]
1.122
2.235
2.525
3.028
3.134
3.2
Boost converter
The illustration of Boost converter is shown in Figure 9, Q1 is the main switch MOSFET and Q2 is the diode
MOSFET, L_boost is the Boost inductor.
L_boost
DC
Q1
Q2
R_boost
C_boost
Figure 9
Topology of Boost converter
The output voltage of Boost converter is arbitrarily assigned to 460 VDC, it is supposed that efficiency of
Boost converter and its followed LLC converter are both 98%, then average input current of the Boost
converter is:

_ = η ∗η  ∗V
LLC
Application Note
11
boost
in
(24)
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Solutions analysis
The ripple current through the Boost inductor is
Δ_ =
− ∗∗(1−)
 ∗
(25)

_
(26)
Where D is the duty cycle ratio and for Boost converter
 =1−
So the maximum ripple current through the inductor appears when D is equal to 0.5 according to equation
(25), then the input voltage when D=0.5 is
_0.5 = _ ∗ (1 − 0.5) = 230
(27)
And the average input current when D=0.5 is obtained according to equation (24) and (27)

1000
_0.5= η ∗η  ∗V
= 0.98∗.098∗230 = 5.433
(28)
LLC
in_D0.5
boost
The inductance of Boost inductor is calculated as:
− ∗∗(1−)
 = ΔI
∗
_
(29)
_
Therefore the minimum inductance of the boost inductor is obtained when D is equal to 0.5:
460∗0.5∗0.5
 = 0.25∗5.433∗80∗103 = 1.06 ∗ 10−4
(30)
Concerning the material and manufacture errors of inductor (generally ±20%) the inductance of Boost
inductor is set to 1.33 mH, that is
 = 1.33 
(31)
Then the current ripple through the inductor and the minimum and maximum current through the inductor
can be obtained according to equation (25) and equation (11) and equation (12)
Table 9
Ripple, min. and max. current through Boost inductor at different input voltage
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
ΔIL_boost[A]
1.048
1.051
0.981
0.633
0.49
IL_boost,[email protected]%load[A]
0.791
0.407
0.343
0.344
0.38
IL_boost,[email protected]%load[A]
1.839
1.458
1.323
0.978
0.87
IL_boost,[email protected]%load[A]
2.764
1.805
1.592
1.336
1.317
IL_boost,[email protected]%load[A]
3.812
2.857
2.573
1.969
1.807
IL_boost,[email protected]%load[A]
6.052
4.137
3.675
2.989
2.897
IL_boost,[email protected]%load[A]
7.1
5.188
4.655
3.622
3.369
Similar to high-side MOSFET of buck converter, the power loss of main switch MOSFET(Q1 in Figure 9) of
Boost converter include switching loss, conduction loss and Coss loss, and calculation of these power loss
are same to those of high-side MOSFET of Buck converter(equation (13)~(16)). Also it is feasible to suppose
that max. power loss of main switch MOSFET of Boost converter is about 6W, then we can get the RDS(on) of
the MOSFET suitable for the application, as shown in the Table 10.
Table 10
RDS(on) of main switch MOSFET of Boost converter suitable for the application
RDS(on) [Ω]
Vin1=190 VDC
Vin2=268 VDC
Vin3=300 VDC
Vin4=378 VDC
Vin5=400 VDC
0.141
0.469
0.735
2.445
3.8
From Table 10 we know that the RDS(on) of the main switch MOSFET suitable for the application should be
smaller than 0.141 Ω.
Application Note
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Solutions analysis
For boost diode MOSFET (Q2 in Figure 9), the power loss include conduction loss of MOSFET and its body
diode, the calculation of the two loss is also same with that of low-side MOSFET of Buck converter (equation
(19)~(21)), the assumptions of equation (22) and (23) of low-side MOSFET are applicable to the calculation of
boost diode MOSFET, then we can get the RDS(on) of the boost diode MOSFET suitable for the application, as
shown in the Table 11.
Table 11
RDS(on) of diode MOSFET of Boost converter suitable for the application
RDS(on) [Ω]
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
0.329
0.465
0.521
2.445
3.8
The max. voltage MOSFETs have to endure for Boost converter is the max. output voltage, which is 460 VDC
for the application. Considering 80% or more voltage derating and voltage spike produced by parasitic
parameters, the voltage class of 600V or 650V CoolMOSTM is suitable for the application. As a result, two
paralleled IPP60R125P6 and two paralleled IPP65R310CFD are chosen for main switch and diode MOSFETs
respectively concerning RDS(on) for main switch and diode MOSFETs and excellent body diode requirement for
diode MOSFET.
The power loss of each MOSFET then can be calculated. For main switch MOSFET RDS(on) of IPP60R125P6 at
120°C junction temperature is 0.22Ω and Eoss at 460V is about 8.3uC[10]. For diode MOSFET, RDS(on) of
IPP65R310CFD at 120°C junction temperature is 0.58Ω and forward voltage drop of the body diode is 0.8V,
Eoss at 460 VDC is about 4 uC[11]. Then the power loss of each MOSFET is obtained at different input voltage
and output load.
Table 12
Each MOSFET loss of Boost converter at 20% load at different input voltage
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
Main switch MOS[W]
0.965
0.786
0.779
0.769
0.767
Diode MOS[W]
0.446
0.413
0.404
0.385
0.38
Table 13
Each MOSFET loss of Boost converter at 50% load at different input voltage
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
Main switch MOS[W]
1.621
1.041
0.998
0.941
0.932
Diode MOS[W]
1.015
0.817
0.764
0.671
0.65
Table 14
Each MOSFET loss of Boost converter at 100% load at different input voltage
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
Main switch MOS[W]
3.273
1.665
1.497
1.271
1.234
Diode MOS[W]
3
2.224
2.021
1.668
1.593
3.3
LLC converter
The topology of half-bridge LLC converter with synchronous rectification following Buck/Boost converter is
illustrated in Figure 10.
Application Note
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Used in HVDC System
Solutions analysis
Q3
Lr
DC
Co
Q4
Cr
Figure 10
Q5
Rload
Q6
Topology of half-bridge LLC converter
As mentioned in the second part, the main specifications of LLC converter include:
(1) Output voltage is 12 VDC and output full load is 100 A
_ = 12 , _ = 100
(2) Resonant frequency (also working frequency) is 130 kHz, time of turn off is 20nS.
 = 130, _ = 20 ∗ 10−9 
(32)
(33)
The LLC will always work at resonant frequency to get a high efficiency.
For LLC converter following Buck converter, input voltage is 150 VDC, so the expected turn ratio of primary
side to secondary side of transformer is
_ =
_
2∗
150
(34)
= 2∗12 = 6.25
The final turn ratio of transformer of LLC following Buck is determined as 6, that is
_ = 6
For LLC converter following Boost converter, input voltage is 460Vdc, then the expected turn ratio is
_
460
_ = 2∗
= 2∗12 = 19.167
_
The final turn ratio of transformer of LLC following Boost is determined as 19, that is
_ = 19
It is supposed that magnetic cores of the two LLC transformers are same, which means the effective
magnetic area and air gap are same. Another critical presupposed condition is that winding number of
secondary side transformer is 1(total winding number is 2 for center-taped winding) for the two
transformers, which is realized in many server power supplies to reduce the secondary-side winding
conduction loss, so the ratio of primary winding inductance of two transformers is

6
_2_ = ( _ )2 = (19)2 = 0.1
_
(35)
(36)
(37)
(38)
If the inductance of primary side transformer of LLC following Buck converter is assigned to 80µH, that is
__ = 80
(39)
Then according to equation (38) the winding inductance of primary side transformer of LLC following Boost
converter is

80
_ =  __ = 0.1 = 800
(40)
_2_
Application Note
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Used in HVDC System
Solutions analysis
So the peak values of magnetizing currents of two transformers are

150
_ = 4 _
= 4∗130∗103 ∗80∗10−6 = 3.606
 __
_
_ = 4
 _
460
= 4∗130∗103 ∗800∗10−6 = 1.103
(41)
(42)
As it is known that the rms current of primary side of LLC converter can be calculated as
__ = 4
1

√ 4
√2 ∗

∗ 
2 ∗
2

2
+ 4π2
(43)
Where N is the turn ratio of primary-side winding to secondary-side winding and Lm is the inductance of
primary-side winding,  is the resistor of output load and can be calculated as

 =  _
(44)
_
The rms current of primary-side LLC following Buck converter at different load can be obtained according to
equation (43) and (44), as shown in the Table 15.
Table 15
Primary-side rms current of LLC converter following Buck converter at different load
rms current[A]
20% load
50% load
100% load
3.899
9.337
18.552
For LLC following Boost converter, the rms current of primary-side transformer at different load is
Table 16
Primary-side rms current of LLC converter following Boost converter at different load
20% load
50% load
100% load
rms current[A]
1.231
2.948
5.859
The power loss of MOSFET used in LLC converter includes conduction loss and turn off loss, the conduction
loss is calculated as:
_ = _ 2 ∗ ()
(45)
Where − is the rms current through MOSFET, for LLC converter
1
− = 2 __
(46)
√
When LLC converter performs at its resonant frequency, turn off loss can be calculated as:
1
_ =  ∗  ∗ _ ∗ 
2
(47)
Where Vin is the input voltage of LLC(output voltage of Buck or Boost converter),  is the peak current of
magnetic inductance of transformer as calculated in equation (41) or (42), _ is the turn off time during
which VDS (voltage between Drain and Source pin of MOSFET) of MOSFET increase from zero to Vin.
We suppose that the power loss of high-side or low-side MOSFETs of LLC is 5W, the RDS(on) of high-side or lowside MOSFETs can be calculated
() =
 −_
 2
(48)
The max. RDS(on) of the MOSFETs used in the two LLC converters then can be obtained according to equation
(33), (45)~(48):
()_max _ = 0.025
(49)
()_max _ = 0.253
50)
Where RDS(on)_max_buckLLC and RDS(on)_max_boostLLC are the max. MOSFET RDS(on) of the LLC converter following Buck
and Boost respectively.
The max. voltage MOSFETs have to endure for LLC converter following Buck converter is 150 VDC in the
application. Considering 80% or more voltage derating and voltage spike produced by parasitic parameters,
the voltage class of 250 V OptiMOSTM is suitable for the application. However, the max. voltage MOSFETs
Application Note
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Solutions analysis
have to endure for LLC converter following Boost converter is 460 VDC in the application. Considering 80% or
more voltage derating and voltage spike produced by parasitic parameters, the voltage class of 600 V or
650 V CoolMOSTM is suitable for the application. Therefore two paralleled IPP220N25NFD and two paralleled
IPP65R190CFD are chose for LLCs following Buck converter and Boost converter respectively concerning
power loss and excellent body diode requirement. RDS(on) is 0.0388Ω and 0.58Ωat 120°C for IPP220N25NFDand
IPP65R190CFD respectively[12][13], then the power loss of each MOSFET using IPP220N25NFD and
IPP65R190CFD for LLCs following Buck and Boost converter can be obtained according to equation
(45)~(47), as shown in the following table.
Table 17
Power loss of each MOSFET in the two LLC converters
Power loss of each MOSFET of primary side of
LLC following Buck[W]
20% load
50% load
100% load
0.777
1.126
2.372
Power loss of each MOSFET of primary side of
0.728
1.051
2.204
LLC following Boost[W]
For secondary-side MOSFETs of the two LLC converters, the rms currents through any branch of
synchronous rectifier (Q5 or Q6 in fig.10) of the two LLC converters are same and can be calculated
1
 = 4 ∗
∗π

2
 4 ∗
∗ √ 2
2
 ∗
∗
5π2 −48
+
12π4
(52)
1
Because LLC converters work at resonant frequency, the power loss of secondary-side MOSFETs is mainly
composed of conduction loss and is expressed as:
_ = _ 2 ∗ ()
(52)
It is reasonable to choose SuperSO8 package of Infineon Technologies as the secondary-side MOSFET for its
very low () and excellent heat dissipating capability. It is supposed that max. power loss for any branch
of synchronous rectification(Q5 or Q6 in Figure 10) is about 4W. The () of the MOSFET than can meet
the power loss requirement can be obtained according to equation (51) and (52)
() = 
_
_²
=
4
12 2
64 ∗(
)
1 12∗π
5π2 −48
100
( ∗ 12 ∗√
∗
+1)²
4
(80∗10−6 )²∗(130∗103 )² 12π4
100
= 0.65 ∗ 10−3
(53)
The max. voltage MOSFETs have to endure for secondary side of LLC converter is 24 VDC in the application.
Considering 80% or more voltage derating and voltage spike produced by parasitic parameters, the voltage
class of 40 V OptiMOSTM is suitable for the application.
The two paralleled BSC010N04LSI are used in any branch of synchronous rectification, () of the
BSC010N04LS at 120°C junction temperature is about 1.35 mΩ [14], then the power loss of each MOSFETs of
secondary side of LLC converter is calculated according to equation (51) and (52), the results at different
load condition are shown in the Table 18.
Table 18
Power loss of each MOSFETs of secondary side of LLC converter
Power loss of each MOSFET [W]
Application Note
20% load
50% load
100% load
0.084
0.521
2.082
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Used in HVDC System
Inductors and transformer
4
Inductors and transformer
4.1
Inductors
From calculation results in the previous parts, inductor of Buck converter is 720 µH and inductor of Boost
converter is 1.33 mH, it is supposed that we choose the two magnetic cores with same dimension and
material such as ferrite for the two inductors, and the airgap of the two cores are supposed to be same to
each other so the ratio of winding number of Buck inductor to Boost inductor is

720
2_ = √  = √1.33 = 0.736
(54)

As the current through the Buck or Boost inductor is of saw-tooth waveform so the rms current is calculated

_ = √ 
2 +
2
 ∗ +
(55)
3
For different input voltage, the rms current through the Buck inductor is nearly same so we can get the rms
current at different load according to Table 3 and equation (55), results are shown in the following table
Table 19
Rms current through the Buck inductor
20% load
50% load
100% load
rms current through the Buck inductor [A] 1.7
4.1
8.2
Different from Buck inductor, however, the rms current through the Boost inductor is related not only to
output load but also to input voltage, so the rms current through the Boost inductor is obtained in Table 9
and equation (55), results are shown in the Table 20.
Table 20
Rms current through the Boost inductor
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
20% load
1.4 A
1.0 A
0.9 A
0.7 A
0.7 A
50% load
3.3 A
2.4 A
2.1 A
1.7 A
1.6 A
100% load
6.6 A
4. 7 A
4.2 A
3.3 A
3.1 A
It is supposed that the maximum current density of the windings of two inductors is same, therefore the
ratio of cross-section area of windings of Buck inductor to Boost inductor is

8.2
2_ =  __ = 6.6 = 1.242
(56)
__
Then the ratio of DC winding resistance of buck inductor to that of boost inductor is

0.736
2 =  2_ = 1.242 = 0.592
(57)
2_
The ratio of DC copper loss of Buck inductor to that of Boost inductor is calculated as
2

_2 =  _ 2 ∗ 2
(58)
_
The results of equation (58) at different input voltage and load are shown in the Table 21.
Table 21
The ratio of DC copper loss of Buck inductor to that of Boost inductor
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
20% load
0.873 A
1.711 A
2.113 A
3.493 A
3.493 A
50% load
0.914 A
0.914 A
1.728 A
1.803 A
2.257 A
2.257 A
3.445 A
3.657 A
3.889 A
4.144 A
100% load
Application Note
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Used in HVDC System
Inductors and transformer
The current change (ΔI) through Buck and Boost inductor, as calculated in the Table 3 and Table 9, are
shown again in the Table 22.
Table 22
Current change (ΔI) through Buck and Boost inductor
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
0.548 A
1.147 A
1.302 A
1.571 A
1.628 A
ΔIL-boost[A]
1.048 A
1.051 A
The inductance of inductor is expressed as
0.981 A
0.633 A
0.49 A
ΔIL-buck[A]
Ψ
∗∗
 =  =  
(59)
We choose the same magnetic cores for the two inductors so effective core area (Ae) is same, then the ratio
of magnetic density change of the Buck inductors to Boost inductor is


2 =   ∗   ∗ 


1
(60)
2
The results of equation (60) at different input voltage is obtained through equation(9), equation(31),
equation(60) and Table22, as shown in the following table
Table 23
The ratio of magnetic density change for the Buck and Boost inductor
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
2
0.385
0.803
0.977
1.826
2.445
According to magnetic core loss of Steinmetz equation, the magnetic loss is expressed as
 =  ∗  ∗  ∗  
(61)
Where V is the volume of the magnetic core, K,α,β are constant coefficients, ΔB is the magnetic density
change and fw is the working frequency, as we have selected same magnetic cores for the two inductors V,
K, fw, α and β are same for the two inductors, then the ratio of magnetic loss of the Buck inductor to Boost
inductor can be obtained:
−2 = 2 
(62)
4.2
LLC transformers
We have supposed that the winding numbers of secondary-side transformers are same such as 1(each brach
of synchronous rectification) for the two LLC transformers that following Buck and Boost converter
respectively and the working frequencies are same for the two LLC converters. As a result the voltagesecond products of the two transformers are same, so the magnetic density change ΔB is equivalent to each
other. The two transformers use identic magnetic core (equivalent in volume and Steinmetz coefficients),
therefore the magnetic losses are same for the two transformers.
If it is supposed that the maximum current density of the windings (include primary side and secondary
side) of the two transformers are same, the ratio of cross-section area of primary-side windings of two
transformers is
2__ =
___
___
(63)
As it is known that the rms primary-side current of LLC converter is expressed by equation (43), and at most
times the equation can be simplified as
π

∗ 
√2 ∗
Application Note
__ = 2
(64)
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Used in HVDC System
Inductors and transformer
Thus according to equation (64), the ratio of rms current through primary-side windings of the two
transformers is
__
=
__
_
(65)
_
Then the DC resistance ratio of primary-side windings of two transformers is obtained according to equation
(63) and (65)

1
_2__ =  _ ∗ 
_
2
=
__ 2
(66)
__ 2
Therefore the ratio of primary-side winding copper loss of the two transformers is


_2_ = ( __ )2 ∗ _2__ = (
__
2



) ∗
2

 2


=1
(67)
We can know from equation (67) the copper loss of primary-side windings of the two transformers are same.
As about copper loss of secondary-side windings, the winding numbers the current density of the windings
and the currents through the windings are same and therefore the copper losses are same.
It is concluded that the power loss of two transformers are same if we choose same magnetic core, current
density for windings and same winding numbers for secondary side.
Application Note
19
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Conclusion
5
Conclusion
We compare the two solutions, Buck followed by LLC and Boost followed by LLC, in three dimensions:
occupation of PCB, power loss and the cost of power MOSFETs
5.1
Occupations of PCB by MOSFETs and magnetic elements
For the solution of Buck followed by LLC, total eight TO-220 package MOSFETs and four SuperSO8-package
MOSFETs are chosen for Buck and its followed LLC converters. The same number and package of MOSFETs
are chosen for Boost and its followed LLC converters, so the occupations of PCB by MOSFETs are same for
the two solutions. However, for the LLC following Buck converter, the input voltage is only 150VDC, the
MOSFETs used in the LLC belongs to the OptiMOSTM series of Infineon Technologies, in which there is more
available SMD package such as SuperSO8, DPAK, therefore for high power density application the solution of
Buck followed by LLC will be predominant.
For the magnetic elements, the magnetic cores and current density of windings are same for inductors and
transformers, so the dimensions of inductors and transformers for the two solutions are nearly same.
5.2
Power loss of MOSFETs and magnetic elements
We can get the power loss of MOSFETs used in Buck, Boost and two LLC converters according to the
previous calculation at different loads and input voltages, as shown in the followed tables.
Table 24
MOSFETs loss of two solutions at 20% load
Stage
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
Buck
1.188 W
1.336 W
1.394 W
1.52 W
1.556 W
Boost
2.828 W
2.408 W
2.372 W
2.31 W
2.296 W
LLC following Buck
3.108 W
LLC following Boost
2.912 W
Table 25
MOSFETs loss of two solutions at 50% load
Stage
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
Buck
2.962 W
3.466 W
3.634 W
4.006 W
4.102 W
Boost
5.28 W
3.724 W
3.534 W
3.226 W
3.166 W
LLC following Buck
4.504 W
LLC following Boost
4.204 W
Table 26
MOSFETs loss of two solutions at 100% load
Stage
Vin_1=190 VDC
Vin_2=268 VDC
Vin_3=300 VDC
Vin_4=378 VDC
Vin_5=400 VDC
Buck
8.466 W
9.942 W
10.404 W
11.352 W
11.588 W
Boost
12.552 W
7.786 W
7.046 W
5.882 W
5.656 W
LLC following Buck
4.744 W
LLC following Boost
4.408 W
Application Note
20
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Conclusion
Based on previous data it is easy to get the comparison of MOSFETs loss in the two solutions (Buck followed
by LLC and Boost followed by LLC) at different load and input voltage, as shown in the followed figures.
At light load such as 20% load, the solution of Buck plus LLC will have a low MOSFETs loss compared with
Boost plus LLC. At medium load such as 50% load, the solution of Buck plus LLC will have a low MOSFETs
loss at low input voltage, with the increase of input voltage, however, the solution of Boost plus LLC will
have a low MOSFETs loss. Similar to medium load, when the load is heavy such as full load, the solution of
Boost plus LLC will have a low MOSFETs loss compared with Buck plus LLC solution.
Figure 11
MOSFETs loss comparison at 20% load
Figure 12
MOSFETs loss comparison at 50% load
Application Note
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Conclusion
Figure 13
MOSFETs loss comparison at 100% load
Next is about the inductor power loss comparison, as we have calculated the DC copper loss ratio of Buck
inductor to Boost inductor as shown in Table 21, the data can be transformed to curve to get a more clear
comparison.
Figure 14
Ratio of DC copper loss of Buck inductor to Boost inductor at different load and input
voltage
As show in the Figure 14, Buck converter will have a higher DC copper loss compared with Boost converter if
the ripple current through the two inductors and current density of windings are same. And the higher input
voltage the higher Buck inductor DC copper loss compared with Boost converter.
We have obtained the ratio of magnetic loss of Buck inductor to that of Boost inductor in equation (62), for
ferrite material such as 3C95 or PC95 that is widely used in SMPS(Switched Mode Power Supply), the range
of  is generally from 2.5 to 3 at about 100 kHz working frequency, so the ratio of magnetic loss of the two
inductors as a function of α is shown in the following figure according to equation (62) and Table (23).
Application Note
22
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Conclusion
Figure 15
Ratio of magnetic loss of Buck inductor to Boost inductor
2_1 in Figure 15 represents the ratio of magnetic loss of Buck inductor to Boost inductor
when input voltage is 190 VDC, and V2 to V5 represents input voltage of 268 VDC, 300 VDC, 378 VDC and 400 VDC
respectively. When input voltage is smaller than 300 V (green, blue and red curve in Figure 15), Buck inductor
will have a smaller magnetic loss compared to Boost inductor, as input voltage increase, however, Buck
inductor will have a larger magnetic loss compared to Boost inductor, the higher input voltage, the higher
Buck inductor magnetic loss compared to Boost inductor if the dimension of the magnetic cores and air gap
are same for the two inductors.
The two solutions will have same loss of transformers of LLC if magnetic core, winding current density and
secondary-side winding number are same.
5.3
Cost of MOSFETs
The cost of MOSFETs can be obtained according to the choice of MOSFETs and price listed in Infineon
website, as shown in the following table
Table 27
Cost of MOSFETs for the two solutions
Buck+LLC
Boost+LLC
Application Note
Part Items
Number(PCS)
Price per part(€)
Total cost(€)
IPP60R099P6
2
2.76
19.96
IPP65R150CFD
2
1.93
IPP220N25NFD
2
3.07
BSC010N04LSI
4
1.11
IPP60R125P6
2
1.91
IPP65R310CFD
2
1.3
23
14.08
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The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Conclusion
Part Items
Number(PCS)
Price per part(€)
IPP65R190CFD
2
1.61
BSC010N04LSI
4
1.11
Total cost(€)
As shown in the Table 27, the cost of MOSFETs of Boost+LLC is smaller than that of Buck+LLC, about 30%
cost down is realized according to the price listed in Infineon Technologies website.
As a conclusion, the solution of Boost followed by LLC is predominant in the application of HVDC DC/DC
converter that suitable for 240 VDC and 336 VDC HVDC system considering medium to high load efficiency and
power MOSFETs cost. On the contrary, solution of Buck followed LLC will be good at high power density and
high efficiency requirement at light load and low input voltage.
Application Note
24
Revision 1.0, 2015-01-26
The Optimal Solutions Suitable for DC/DC Converter
Used in HVDC System
Reference
6
Reference
[1] 380Vdc Architectures for the Modern Data Center, white paper, Emerge Allicance.
[2] The Green Grid, Qualitative Analysis of Power Distribution Configuration for Data Centers (2007)
[3] HVDC Demo Site and Future Trend, Netpower Labs
[4] 380VDC Eco-system Development Present Status and Future Challenges, David E. Geary, David P. Mohr,
David Owen, Maurizio Salato, BJ Sonenberg, Intelec 2013
[5] 240V Direct Current Power Supply System for telecommunications, YD/T 2378-2011
[6] 336V Direct Current Power System, QB-H-008-2012, China Mobile
[7] MOSFET Power Losses Calculation Using the Datasheet Parameters, Dusan Graovac, Marco Purschel,
Andreas Kiep, Application Note, Infineon Technologies co. Ltd.
[8] IPX60R099P6, CoolMOSTM 600V P6 datasheet, www.infineon.com
[9] IPX65R150CFD, CoolMOSTM 650V CFD datasheet, www.infineon.com
[10] IPX60R125P6, CoolMOSTM 600V P6 datasheet, www.infineon.com
[11] IPX65R310CFD, CoolMOSTM 650V CFD datasheet, www.infineon.com
[12] IPP220N25NFD, OptiMOSTM FD power-transistor 250V datasheet, www.infineon.com
[13] IPX65R190CFD, CoolMOSTM 650V CFD datasheet, www.infineon.com
[14] BSC010N04LSI, datasheet, www.infineon.com
Revision History
Major changes since the last revision
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--
Application Note
Description of change
First Release
25
Revision 1.0, 2015-01-26
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Edition 2015-01-26
Published by
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