Application Note Selection of CoolMOS™ Power Handling Capability

Application Note, V1.2, Jan. 2002
CoolMOS
TM
AN-CoolMOS-03
How to Select the Right CoolMOS and
its Power Handling Capability
Power Management & Supply
N e v e r
s t o p
t h i n k i n g .
How to Select the Right CoolMOSTM and
its Power Handling Capability
Revision History:
2002-01
Previous Version:
Page
V1.2
V1.1
Subjects (major changes since last revision)
Document’s layout has been changed: 2002-Sep.
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Edition 2002-01
Published by Infineon Technologies AG,
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© Infineon Technologies AG 2002.
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How to Select the Right CoolMOSTM and
its Power Handling Capability
Table of Contents
Page
1
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3
Selecting the Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4
How to Choose the Voltage Rating of the MOSFET? . . . . . . . . . . . . . . . 6
5
How to Select the Operating Junction Temperature? . . . . . . . . . . . . . . 8
6
How to Choose the Current Rating of the MOSFET? . . . . . . . . . . . . . . . 8
7
How to Choose the Right Rds(on)? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8
How to Calculate the Maximum Power Losses for a Specific Junction
Temperature? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9
How to Calculate the Power Losses? . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10
10.1
10.2
Conduction Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Discontinuous Conduction Mode Converter . . . . . . . . . . . . . . . . . . . . . . . 14
Continuous Conduction Mode Converter . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11
11.1
11.2
Switching Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Discontinuous Conduction Mode Converter . . . . . . . . . . . . . . . . . . . . . . . 25
Continuous and Discontinuous Conduction Mode Converter . . . . . . . . . . 26
12
12.1
12.2
Total Power Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Discontinuous Conduction Mode Converter . . . . . . . . . . . . . . . . . . . . . . . 27
Continuos Current Mode Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
13
13.1
13.2
Calculation of Peak Pulse Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Discontinuous Conduction Mode Converter . . . . . . . . . . . . . . . . . . . . . . . 31
Continuous Conduction Mode Converter . . . . . . . . . . . . . . . . . . . . . . . . . . 33
14
14.1
14.2
Maximum Output Power Capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Discontinuous Conduction Mode Converter . . . . . . . . . . . . . . . . . . . . . . . 34
Continuous Conduction Mode Converter . . . . . . . . . . . . . . . . . . . . . . . . . . 35
15
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Application Note
3
V1.2, 2002-01
How to Select the Right CoolMOS and
its Power Handling Capability
1
AN-CoolMOS-03
Abstract
This application note is focusing on the selection of the high voltage MOSFET for
Switched Mode Power Supply (SMPS). It shows a mathematical way to select the
MOSFET with the intention to accelerate the design cycle of a SMPS.
Iteration process is introduced to evaluate as well the maximum allowable power
dissipation as the conduction and switching power losses of the MOSFET. Maximum
peak pulse current for discontinuous and continuous conduction mode converters has
been calculated. To simplify the first iteration, charts of the output power handling
capability versed the switching frequency have been shown.
2
Introduction
In the last couple of years the design cycle duration of SMPS in electronic applications
becomes more and more important. Many applications need well controlled voltage
containing features like low power dissipation for standby mode and a high efficiency
during normal mode. These features make great demands on the power supply itself
which can be satisfied by SMPS, like it is done actually for instance in most PC, charger
or TV-set. Due to this wide range of applications the requirements for the SMPS cover a
extensive array of features.
Some applications require one output voltage of the SMPS, others need more than one,
some SMPS have a higher output voltage than the input voltage, others vice versa. One
of the most important parameter for the selection of SMPS is the output power. To have
a solution for each of the different applications, different topologies have been developed
to achieve an optimum cost/performance ratio.
Figure 1 shows the most popular topologies for different output power classes.
Application Note
4
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Introduction
Converter Type
Phase Shifted
ZVT Bridge
Half Bridge Converter
Two Transistor
Forward Converter
Synchronous Rectification
Single Transistor
Forward Converter
PowerFactor
Factor Correction
Power
Correction
Flyback Converter
1 50
100
200
300 400
600
1000
Output Power, [W att]
Figure 1
SMPS-Topologies Versed Output Power
The topologies mentioned in Figure 1 can be operated in continuous conduction mode
and in discontinuous conduction mode. The main difference between these both families
are the shape of the current. Discontinuous conduction mode has a triangular shaped
current, whereas for the continuous conduction mode a trapezoid shaped current is
typical.
These topologies lead to different requirements for the MOSFETs, like
•
•
•
•
packages
drain-source voltage rating
drain current rating
on state resistance.
Application Note
5
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Selecting the Package
3
Selecting the Package
The selection of the MOSFET package mainly depends on following parameters.
Power dissipation/ Power losses of the MOSFET has a great impact on selection of the
cooling
package.
SMD packages can be used for lower power dissipation:
DPAK for approximately 0.5 W (depending on pad size)
D2PAK for approximately 1 W (depending on pad size)
The through hole packages like TO-220 and especially TO-247 with
attached heat sink and forced cooling can dissipate much more
power.
Creepage
distance
The creepage distance between legs of the package should
correspond to the voltage requirements in the given application
field.
Space/volume
The size of the package can be also influence by the available
space/volume/height in a given case of SMPS/lamp ballast. For
examples, notebook adapters use I2Pak in order to reduce the
height of the device.
Cost
Smaller packages are usually less expensive, than the bigger ones.
Also SMD mounting technology can be more cost-effective during
the manufacturing process. Fully isolated package helps to reduce
the cost of heat sink assembly by skipping the manufacturing step
of putting the isolation pad between package and heat sink.
4
How to Choose the Voltage Rating of the MOSFET?
The avalanche breakdown voltage of CoolMOS is slightly higher than it's voltage rating
due to typical safety margin. The voltage rating is defined at room temperature.
Breakdown voltage of CoolMOS has strong positive temperature coefficient as it can be
seen in Figure 2.
Application Note
6
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
How to Choose the Voltage Rating of the MOSFET?
Figure 2
Dependence of the Breakdown Voltage on the Junction Temperature
of 600 V CoolMOS and 800 V CoolMOS
Breakdown voltage at typical operation temperature of 100 - 120°C is approx. 7% higher
than rated voltage.
Reliability tests are done at rated voltage, especially the HTRB (High Temperature
Reversed Bias). These test results are used as input information for calculation of
acceleration factors in different reliability models. In order to achieve high forecasted
reliability the maximum operating voltage should be lower than the one used in HTRB.
Another criteria for selecting the voltage rating of the MOSFET are the overvoltage
spikes. During turn off transient the voltage on the drain can reach much higher values
as in steady state due to parasitic inductances in the circuit.
All these criteria should be considered during the selection of the MOSFET voltage
rating. The maxi-mum steady state voltage during turn off should not exceed 70 to 90%
of the rated voltage. These derating values has been achieved by the years of
experience.
Application Note
7
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
How to Select the Operating Junction Temperature?
5
How to Select the Operating Junction Temperature?
Similar reliability rules can be applied for the selection of operating junction temperature
of the MOSFET. The operating junction temperature should not exceed the maximum
value specified in the datasheet. Pushing the operating temperature to the maximum is
not reasonable. The majority of reliability test are done at maximum junction
temperature, especially the HTRB (High Temperature Reversed Bias) and HTGS (High
Temperature Gate Source). These test results are used as input information for
calculation of acceleration factors in different reliability models. In order to achieve high
forecasted reliability the maximum operating temperature should be lower than the
maximum one. For example, reducing the junction temperature by 30°C will improve the
MTBF (Mean Time Between Failure) of the CoolMOS by an order of magnitude.
On the other hand, the on-state resistance of the MOSFET increases with the junction
temperature. It leads to increase in conduction power losses.
For these reasons the derating factors of 70-90% of the maximum junction temperature
are recommended.
6
How to Choose the Current Rating of the MOSFET?
In the majority of SMPS applications the MOSFET is not being stressed up to it's
maximum current rating due to poor cooling conditions. Designer prefers to take an
advantage of low on-state resistance of the MOSFET in order to reduce the power
losses. Usually MOSFETs with selected low Rds(on) have higher current rating then
needed in the application. Nevertheless, it is useful to check the Safe Operating Area of
the selected MOSFET.
On the other hand, gate-source voltage should be high enough to completely turn on the
MOSFET. The transistor should be able to carry the maximum pulse current in converter
under all conditions. Especially during start up or short circuit on the output of SMPS the
supply voltage for the control IC can fall close to under voltage lockout limit. Some
modern control IC have the under voltage lock out of approximately 7 V. The gate-source
voltage of MOSFET can be less than 5 V, if we consider the voltage drops on the output
stage of the control IC and on the current sense resistor in source path of transistor.
MOSFET should be able to carry the required current without increase of drain-source
voltage at given gate-source voltage. The transfer characteristic from the datasheet can
be used in order to prove it Figure 3. If the MOSFET does not meet the requirements,
another transistor with higher current rating should be selected.
Application Note
8
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
How to Choose the Right Rds(on)?
Figure 3
7
Transfer Characteristic
How to Choose the Right Rds(on)?
The most complicated selection is to choose the correct on state resistance of the
device. The limit for the on state resistance is the maximum allowable power dissipation
of the application and the maximum junction temperature of the MOSFET.
The power losses of the MOSFET can be divided into the conduction losses and the
switching losses. The conduction losses are easy to calculate due to a constant on state
resistance of the MOSFET and a well defined drain current.
Problems occur by calculating the switching losses of the MOSFET. These losses
strongly depend on parasitic parameters of the circuit. This application note is based on
measurements of the switching energy versed the drain current in a test setup, which
means, that it is not one to one transferable to other applications due to different
parasitics. Test setup of SMPS is still necessary for the designer, but with this application
note it is possible to save iterations for the design process.
The intention of this application note is to relieve the development of a SMPS by
selecting an optimum MOSFET fulfilling the requirements of the application. To achieve
this the maximum allowable power dissipation of the application with consideration of
heat sink, topology etc. is compared with power losses of the MOSFET itself.
Application Note
9
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
How to Choose the Right Rds(on)?
Designing the SMPS is a complicated process that requires many iterations. One thing
the designer knows for sure is the output power of the SMPS. In most case the choice
of topology is also done. It is usually based on the output power and the output voltage
level.
Next step might be to fix the switching frequency. This is the point where the problem
starts. Let us assume that the switching frequency is fixed to some value, which
corresponds to other design criteria like EMI noise or magnetic losses, but is not related
to the power losses in the MOSFET.
We will also assume, that the space for heat sink is known. That means we can estimate
the thermal resistance of the heat sink RthCA.
Application Note
10
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
How to Choose the Right Rds(on)?
Step 1
Max power dissipation
for specific junction
temperature P max
Step 2
Required R ds (ON)
@ Pmax
Step 3
Select CoolMOS
with lower R ds (ON)
type = N
Step 4
Calculate total power
losses @ type = N
Ptot @ N
Step 5
Calculate possible
power dissipation
@ type = N
Pmax @ N
Step 6
No
Ptot (N) < Pmax @ N
Step 3
Yes
Select CoolMOS with
higher R ds (ON)
type = N+1
Step 4
Calculate total power
losses @ type = N+1
Ptot @ N+1
Step 5
Calculate possible
power disspation
@ type = N+1
Pmax @ N+1
Step 6
Yes
Ptot @ N+1
<
Pmax @ N+1
No
Choose N- type
Application Note
Step 1: Calculate the maximum power dissipation for a specific
junction temperature
First step would be to calculate how much power losses can be allowed for
the defined heat sink and specific junction temperature.
Step 2: Calculate the required Rds(ON) satisfying the maximum power
losses from Step 1
At this point we already know the value of the maximum allowable power
dissipation. We also know the topology and the drain current waveform. By
using the drain current waveform we can calculate the value of Rds(ON), that
will satisfy the maximum power losses. For the first iteration we will use only
conduction power losses because we do not know the MOSFET type yet.
The switching losses depend strongly on the particular MOSFET type. For
this reason we will skip the switching losses for the first iteration, but we will
check it in the following steps.
Step 3: Select the CoolMOS type with Rds(ON) defined in Step 2
In this step we will select the CoolMOS transistor type, that has the required
on state resistance calculated in Step 2. Please note to use not a room
temperature value, but a value at a higher junction tem-perature specific for
particular design (usually between 110°C and 120°C). It should be double
of a room temperature value.
Step 4: Calculate the total power losses for the selected Cool-MOS
type in Step 3
Now we have enough information to calculate the total power losses for the
selected CoolMOS transistor under particular operating conditions. As we
now know the exact transistor type, we are able to calculate the switching
power losses for a given switching frequency.
Step 5: Recalculate the maximum power dissipation for a selected
CoolMOS type
With the available junction to case thermal resistance of the selected
CoolMOS device it is possible to make the calculation of maximum
allowable power dissipation more precisely. This step can help to skip one
of the iteration what will be explained in Step 6.
Step 6: Compare the total power losses calculated in Step 4 for the
selected CoolMOS type with maximum allowable power dissipation
from Step 5
At this point it is necessary to compare the total power losses calculated in
Step 4 with the maximum allowable power dissipation resulting from the
defined junction temperature and heat sink (Step 5).
If the total power losses from Step 4 are lower than the maximum allowable
power dissipation (Step 1), then the selected CoolMOS type meets the
requirements. We did find the right type.
As a further optimization it could be possible to check if the next CoolMOS
type with higher Rds(ON) will do the same job. Repeat Steps 4, 5 and 6 with
this new selection.
In case of total power losses from Step 4 are higher than the maxi-mum
allowable power dissipation (Step 1), select the next type from the
CoolMOS family with lower on state resistance and repeat the Steps 4, 5
and 6. Another possibility would be to adjust the heat sink.
The right type is found, when the next CoolMOS with higher Rds(ON) does not
meet the requirements.
11
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
How to Calculate the Maximum Power Losses for a Specific
Step 1
Calculate the maximum power dissipation for a specific junction
temperature
First step would be to calculate how much power losses can be allowed for the defined
heat sink and specific junction temperature.
8
How to Calculate the Maximum Power Losses for a
Specific Junction Temperature?
This section explains how to calculate the maximum allowable power dissipation in the
CoolMOS for a specific junction temperature using the datasheet parameters.
Input information:
TJ(max) - maximum junction temperature
TA - ambient temperature
RthJC - assumed thermal resistance junction to case for the specific device
RthCA - thermal resistance case to ambient
Solution:
The actual junction temperature:
TJ
éëPmax⋅ ( RthJC + RthCA )ùû + TA
[1]
then the maximum allowable power dissipation:
T J ( max) − TA
Ptot
[2]
RthJC + RthCA
Example for Step 1:
We assume that the thermal resistance of the heat sink is
RthCA = 40 K/W.
Actually we have to consider the thermal resistance junction to case of the MOSFET
also, but we did not select the MOSFET type yet. Let us assume some value for the
thermal resistance of MOSFET just for the first iteration. Let us say it will be the value of
second smallest CoolMOS device with a thermal resistance of 5 K/W:
Application Note
12
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
How to Calculate the Power Losses?
RthJC = 5 K/W.
The maximum allowed junction temperature in our case will be very conservative value:
TJ = 110°C.
The ambient temperature is:
TA = 70°C
Now using the equation 2 we can calculate the maximum allowable power dissipation in
our case:
P max
Step 2
TJ
R thJC
TA
R thCA
110 °C
5
K
W
70 °C
40
0.889 W
K
W
Calculate the required Rds(ON) satisfying the maximum power losses
from Step 1
At this point we already know the value of the maximum allowable power dissipation.
We also know the topology and the drain current waveform. Using the drain current
waveform we can calculate the value of Rds(on), that will satisfy the maximum power
losses. For the first iteration we will use only conduction power losses because we did
not know the MOSFET type yet. The switching losses depend strongly on the
particular MOSFET type. For this reason we will skip the switching losses for the first
iteration, but we will check it in the following steps.
9
How to Calculate the Power Losses?
This section demonstrates how to calculate the power losses in the CoolMOS from the
actual circuit using the datasheet parameters.
10
Conduction Losses
Input information:
D - duty cycle
ton - turn on time
f - switching frequency
Vin(dc) - input DC voltage
Rds(ON) - drain to source on-state resistance
Application Note
13
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Conduction Losses
Solution:
Conduction energy losses can be obtained by
t on
E cond
t on
v ds ( t ) .i d ( t ) d t
Rds( ON ) .i d ( t ) d t
2
0
[3]
0
where, vds(t) is the on-state voltage drop and id(t) is the drain current waveform after turn
on. Then conduction power losses:
P cond E cond .f
[4]
The turn on resistance of a MOSFET depends on its junction temperature. The on-state
resistance at defined junction temperature can be calculated as:
Rds( ON ) T J
Rds( ON ) ( 25 °C ) . 1
TJ
α
25 °C
100
The temperature factor a is for all CoolMOS transistors 0.8.
10.1
Discontinuous Conduction Mode Converter
The drain current waveform:
Mathematical expression:
id = i peak ⋅
ipeak
0
t
ton
ton
t on
E cond
0
v ds ( t ) .i d ( t ) d t
t on
Rds( ON ) .i d ( t ) d t
2
0
1
2
P cond E cond .f .Rds( ON ) .i peak .D
3
Application Note
1.
2
Rds( ON ) .i peak .t on
3
[5]
[6]
14
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Conduction Losses
10.2
Continuous Conduction Mode Converter
The drain current waveform:
Mathematical expression:
id = imin +
ipeak
imin
i peak − imin
ton
⋅t
In order to simplify the equations we will
assume, that:
0
i min K min .i peak
ton
This means that the minimum drain current
is a fixed percentage of the peak drain
current.
t on
Rds( ON ) .i d ( t ) d t
2
E cond
0
1.
3
1
2
P cond E cond .f .Rds( ON ) .D. i min
3
2
i min
i peak .i min
i peak .i min
i peak
2
i peak .Rds( ON ) .t on [7]
2
[8]
Example for Step 2 (discontinuous conduction mode converter):
We can find how to calculate the Rds(ON) using equation 6:
Rds( ON )
3 .P cond
2
i peak .D
In our design we will use the following operating parameters:
ipeak = 2.4 A (peak drain current),
D = 0.21 (duty cycle).
From the example for Step 1 we have
Pmax = 0.889 W.
Now we can calculate the required on state resistance at TJ = 110°C:
Rds( ON )
3 .P cond
2.
3 .0.889 W
2.
2.205 Ω
i peak D ( 2.4 A ) 0.21
Application Note
15
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Conduction Losses
As we can see the required on state resistance of CoolMOS for satisfying the maximum
power dissipation is slightly above 2 Ω.
For a selection of required CoolMOS we need the on state resistance at a junction
temperature of 25°C.
Rds( on ) T J
Rds( ON ) ( 25 °C )
1
α
TJ
2.205 Ω
25 °C
1
0.8
110 °C
25 °C
1.12 Ω
100
100
For later calculations we will make the simplification, that on state resistance at 25°C is
half of the resistance at junction temperature.
Using this information we can select a type from the CoolMOS family.
Application Note
16
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Conduction Losses
Step 3
Select the CoolMOS type with Rds(on) defined in Step 2
In this step we will select the CoolMOS transistor type, that has the required on state
resistance calculated in Step 2. Please note to use not a room temperature value, but
the value at a higher junction temperature specific for particular design (usually
between 110°C and 120°C). It is approximately double of a room temperature value.
This selection based only on conduction losses. The switching losses were not
considered in this first iteration.
Example for Step 3 (discontinuous conduction mode converter):
From our previous calculation the required on state resistance is
Rds(on) =1.12 Ω
at TJ = 25°C.
Let us take a look on the CoolMOS product family.
SOT-223
6.0 Ω
0.8 A
TO-252
(D-PAK)
TO-251
(I-PAK)
TO-220 SMD
(D²-PAK)
TO-220
TO-220
FullPAK
TO-262
I²-PAK
TO-247
SPN01N60S5
SPD01N60S5
SPU01N60S5
3.0 Ω
1.9 A
SPN02N60C31
SPN02N60S5
SPD02N60C3
SPD02N60S5
SPU02N60C31
SPU02N60S5
SPB02N60C3
SPB02N60S5
SPP02N60C3
SPP02N60S5
1.4 Ω
3.2 A
SPN03N60C31
SPN03N60S5
SPD03N60C3
SPD03N60S5
SPU03N60C31
SPU03N60S5
SPB03N60C3
SPB03N60S5
SPP03N60C3
SPP03N60S5
0.95 Ω
4.5 A
SPN04N60C31
SPN04N60C2
SPN04N60S5
SPD04N60C3
SPD04N60C2
SPD04N60S5
SPU04N60C31
SPU04N60C2
SPU04N60S5
SPB04N60C3
SPB04N60C2
SPB04N60S5
SPP04N60C3
SPP04N60C2
SPP04N60S5
SPA04N60C3
SPA04N60C2
SPD07N60C3
SPD07N60C2
SPD07N60S5
SPU07N60C31
SPU07N60C2
SPU07N60S5
SPB07N60C3
SPB07N60C2
SPB07N60S5
SPP07N60C3
SPP07N60C2
SPP07N60S5
SPA07N60C3
SPA07N60C2
SPI07N60C3
SPI07N60S5
0.38 Ω
11 A
SPB11N60C3
SPB11N60C2
SPB11N60S5
SPP11N60C3
SPP11N60C2
SPP11N60S5
SPA11N60C3
SPA11N60C2
SPI11N60C3
SPI11N60S5
0.19 Ω
20 A
SPB20N60C3
SPB20N60C2
SPB20N60S5
SPP20N60C3
SPP20N60C2
SPP20N60S5
SPA20N60C3
SPA20N60C2
0.6 Ω
7.3 A
0.07 Ω
47 A
1
Figure 4
available on request, 2available in Q2 2002
600 V Product Family
The SPP04N60C3 seems to be a good choice.
Application Note
17
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Switching Losses
Step 4
Calculate the total power losses for the selected CoolMOS type
in Step 3
Now we have enough information to calculate the total power losses for the selected
CoolMOS transistor under particular operating conditions. As we now know the exact
transistor type, we are able to calculate the switching power losses for a given
switching frequency.
11
Switching Losses
Input information:
f - switching frequency
Vds(on) - DC voltage between drain and source before the start of the turn-on transition
Vds(off) - DC voltage between drain and source after the end of the turn-off transition
Rgate - gate resistor
Eon - energy losses during turn-on transition
Eoff - energy losses during turn-off transition
Rgate(test) - gate resistance during measurement of Eon and Eoff
Vds(test) - drain to source voltage during measurement of Eon and Eoff
Solution:
Switching energy losses occur due to simultaneous presence of significant drain-source
voltage and drain current during each transient from turn off state into turn on state and
vice versa.
Turn on switching energy losses can be obtained by
td( on )
tr
E on
v ds ( t ) .i d ( t ) d t
[9]
0
Turn off switching energy losses is expressed as
t d ( off)
E off
tf
v ds ( t ) .i d ( t ) d t
[10]
0
Application Note
18
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Switching Losses
Total switching power losses are:
Psw = ( Eon + Eoff ) ⋅ f
[11]
Next charts will demonstrate the parameters, which influence the switching behavior of
MOSFET and correspondingly the values of switching energy losses. The information
shown is based on the investigations of a usual boost converter driven in the double
pulse measurement mode. Please note that the turn on transient strongly depends on
the used commutated diode and not on the MOSFET itself in case of non-triangle current
waveforms. The power losses are given mostly by the characteristics of the commutated
diode.
Both Eon and Eoff strongly depend on the value of drain current Figure 5. Eon losses are
dominated by the used commutated diode, not by MOSFET. We should take the power
losses value at the corresponding drain current value in particular case. The curve can
be interpolated with a second order polynome in order to simplify the calculations.
0.05
0.045
0.04
0.035
0.03
0.025
0.02
0.015
0.01
0.005
0
Eon
Poly. (Eon)
Poly. (Eoff)
0.025
2
y = 0.0001x + 0.0003x + 0.0021
0.02
0.015
0.01
2
y = 0.0004x - 0.0012x + 0.0066
Eon [mJ]
Eoff [mJ]
Eoff
0.005
0
0
2
4
6
8
10
12
Id [A]
Figure 5
Switching Energy Losses vs. Drain Current
(SPP11N60C3, SDP06S60, Rgate = 6.8 Ω, Vds = 380 V, TJ = 125°C)
Application Note
19
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Switching Losses
This information can be found in CoolMOS datasheets. It can be easily implemented
directly into calculation.
E on i d
E on i d
[12]
E off i d
E off i d
[13]
Figure 6 demonstrates the switching energy losses versed external gate resistor. The
recharging speed of CoolMOS capacitance can be controlled by Rgate. It influences the
switching time, and correspondingly the switching losses. The curves are almost linear.
Eon
Eoff
Linear (Eoff)
Linear (Eon)
0.25
y = 0.0031x + 0.0176
E [mJ]
0.2
0.15
0.1
y = 0.002x + 0.0082
0.05
0
0
20
40
60
80
Rg [Ohm]
Figure 6
Switching Energy Losses vs. Gate Resistor
(SPP11N60C3, SDP06S60, Id = 11 A, Vds = 380 V, TJ = 125°C)
Application Note
20
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Switching Losses
The CoolMOS datasheet includes this chart. If your particular design does have another
gate resistor as given in the datasheet you should implement a corrective factor into the
equations of switching losses as following:
Eon ( i d , Rgate)
Eoff ( i d , Rgate)
Eon ( i d ) ⋅
Eoff ( i d ) ⋅
Eon ( Rgate)
Eon ( Rgate( test) )
Eoff ( Rgate)
Eoff ( Rgate( test) )
Eon ( i d ) ⋅ CF on ( Rgate)
[14]
Eoff ( i d ) ⋅ CF off ( Rgate)
[15]
We include corrective factors to maintain the clarity for following calculations.
CF on ( Rgate)
CF off ( Rgate)
Eon ( Rgate)
[16]
Eon ( Rgate( test) )
Eoff ( Rgate)
[17]
Eoff ( Rgate( test) )
The drain to source voltage across the CoolMOS also has an effect on switching
behavior. The dependence of switching losses on the drain-source voltage is almost
linear Figure 7.
Application Note
21
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Switching Losses
0.06
Eon [mJ]
y = 0.0001x + 0.0028
Eoff [mJ]
0.05
Linear (Eoff [mJ])
Linear (Eon [mJ])
E [mJ]
0.04
0.03
0.02
y = 6E-05x - 0.0017
0.01
0
0
100
200
300
400
500
Vds [V]
Figure 7
Switching Energy Losses vs. Drain-Source Voltage
(SPP11N60C3, SDP06S60, Id = 11 A, Rgate = 6.8 Ω, TJ = 125°C)
This information is not included in the datasheet due to the linearity of this dependence.
We are still able to calculate the corrective factor for the switching losses, if the particular
design's voltage differs from the datasheet parameters:
Eon ( i d , V ds( on ) )
Eoff ( i d , V ds( off ) )
Eon ( i d ) ⋅
Eon ( V ds( on ) )
Eon ( V ds( test) )
Eoff ( i d ) ⋅
Application Note
Eoff ( V ds( off ) )
Eoff ( V ds( test) )
−5
Eon ( id ) ⋅
6 ⋅ 10 ⋅
mJ
−3
⋅ V ds( on ) − 1.7 ⋅ 10 mJ
V
Eon ( i d ) ⋅ CF on ( VDS ( on ) )
[18]
0.021 mJ
−4
Eoff ( i d ) ⋅
10
⋅
mJ
⋅ V ds( off ) + 2.8 ⋅ 10
V
−3
Eoff ( i d ) ⋅ CF off ( VDS ( off ) )
[19]
0.043 mJ
22
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Switching Losses
We include corrective factors to maintain the clarity for following calculations.
CFon ( VDS ( on ) )
CFoff ( VDS ( off ) )
Eon ( V ds( on ) )
−5
6 ⋅ 10 ⋅
mJ
−3
⋅ V ds( on ) − 1.7 ⋅ 10 mJ
[20]
V
Eon ( V ds( test) )
0.021 mJ
−4
10 ⋅
Eoff ( V ds( off ) )
mJ
⋅ V ds( off ) + 2.8 ⋅ 10
−3
[21]
V
Eoff ( V ds( test) )
0.043 mJ
Summarizing the switching energy depends on the peak current, the drain to source
voltage and the gate resistance. To calculate the switching energy, the charts of the
datasheet are the basis plus the correction factors for gate resistance and drain to
source voltage.
Eon ( i d , V ds( on ) , Rg )
Eoff ( i d , V ds( off ) , Rgate)
Eon ( i d ) ⋅
Eon ( V ds( on ) )
Eon ( Rgate)
⋅
Eon ( V ds( test) ) Eon ( Rgate( test) )
Eoff ( i d ) ⋅
Eoff ( V ds( off ) )
⋅
Eoff ( Rgate)
Eoff ( V ds( test) ) Eoff ( Rgate( test) )
Eon ( i d ) ⋅ CF on ( VDS ( on ) ) ⋅ CF on ( Rgate)
Eoff ( i d ) ⋅ CF off ( VDS ( off ) ) ⋅ CF off ( Rgate)
[22]
[23]
As it can be seen in Figure 8, the dependence of turn off energy losses on the gatesource voltage is negligibly low. Turn on behavior is dominated by the commutated
diode, not by the CoolMOS. We will skip both dependencies in order to simplify the
analysis.
Application Note
23
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Switching Losses
0.06
0.05
y = -0.0004x + 0.0485
E [mJ]
0.04
0.03
0.02
Eon [mJ]
y = -0.0076x + 0.1256
Eoff [mJ]
0.01
Linear (Eoff [mJ])
Linear (Eon [mJ])
0
9
10
11
12
13
14
15
16
Vgs [V]
Figure 8
Switching Energy Losses vs. Gate-Source Voltage
(SPP11N60C3, SDP06S60, Id = 11 A, Rgate = 6.8 Ω, Vds = 380 V, TJ = 125°C)
Application Note
24
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Switching Losses
11.1
Discontinuous Conduction Mode Converter
The drain current waveform:
Mathematical expression:
id = i peak ⋅
ipeak
0
t
ton
ton
Turn-on energy losses is negligible:
[24]
E on 0
Turn-off energy losses
Eoff ( i peak , V ds( off ) , Rg )
Eoff ( i peak) ⋅ CF off ( VDS ( off ) ) ⋅ CF off ( Rgate)
[25]
Since the switching energy is proportional to Vds and Rg, the result is scaled by the ratio
of the actual circuit voltage and gate resistance to the test voltage and the test gate
resistance in the datasheet.
Total switching power losses:
Psw
(Eon + Eoff )⋅ f
Application Note
Eoff ( i peak) ⋅ CF off ( VDS ( off ) ) ⋅ CF off ( Rgate) ⋅ f
25
[26]
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Total Power Losses
11.2
Continuous and Discontinuous Conduction Mode Converter
The drain current waveform:
Mathematical expression:
id = imin + ⋅
ipeak
imin
i peak − imin
ton
⋅t
In order to simplify the equations we will
assume, that:
0
i min K min .i peak
ton
This means that the minimum drain current
is a fixed percentage of the peak drain
current.
Turn-on energy losses:
Eon ( i min , V ds( on ) , Rg )
Eon ( i min ) ⋅ CF on ( VDS ( on ) ) ⋅ CF on ( Rgate)
[27]
Since the switching energy is proportional to voltage, the result is scaled by the ratio of
the actual circuit voltage to the test voltage in the datasheet.
Turn-off energy losses:
Eoff ( i peak , V ds( off ) , Rgate)
Eoff ( i peak) ⋅ CF off ( VDS ( off ) ) ⋅ CF off ( Rgate)
[28]
Since the switching energy is proportional to voltage, the result is scaled by the ratio of
the actual circuit voltage to the test voltage in the datasheet.
Total switching power losses:
Psw
12
(Eon + Eoff )⋅ f
é( Eon ( imin ) ⋅ CFon ( VDS ( on ) ) ⋅ CFon ( Rgate) ) ...
ê+ ( E ( i ) ⋅ CF ( V ( off ) ) ⋅ CF ( R ) )
off DS
off gate
ë off peak
ù⋅ f
ú
û
[29]
Total Power Losses
Total power losses for periodical signal can be calculated as the sum of conduction
losses and switching losses:
P tot P cond
Application Note
[30]
P sw
26
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Total Power Losses
12.1
Discontinuous Conduction Mode Converter
1
Ptot
⋅ Rds( ON ) ⋅ ipeak ⋅ D + Eoff ( ipeak) ⋅ CFoff ( VDS ( off ) ) ⋅ CFoff ( Rgate) ⋅ f
[31]
2
3
12.2
Continuos Current Mode Converter
(
⋅ Rds( ON ) ⋅ D⋅ i min + i peak⋅ i min + i peak
)
...
3
+ éë( Eon ( i min ) ⋅ CFon ( VDS ( on ) ) ⋅ CF on ( Rgate) ) + Eoff ( i peak) ⋅ CFoff ( VDS ( off ) ) ⋅ CFoff ( Rgate)ùû ⋅ f
1
Ptot
2
2
[32]
Example for Step 4 (discontinuous conduction mode converter):
Our discontinuous conduction mode converter has following operating conditions:
ipeak = 2.4 A (peak drain current),
D = 0.21 (duty cycle),
f = 60 kHz (switching frequency),
Rgate = 12 Ω (gate resistance)
Vds(on) = 380 V (DC voltage between drain and source before the start of the turn-on
transition, the bulk capacitor voltage),
Vds(off) = 480 V (DC voltage between drain and source after the end of the turn-off
transition, the bulk capacitor voltage plus the flyback voltage).
The selected CoolMOS SPP04N60C3 in Step 3 has approximately 1.9 Ω on state
resistance at junction temperature of 110°C: Rds(ON) = 1.9 Ω. The turn off energy losses
are Eoff (ipeak) = 6 µJ at ipeak = 2.4 A and Rgate = 18 Ω, this information can be found in the
datasheet of CoolMOS.
Due to the dependence of switching energy to drain source voltage and gate resistance,
this energy must be calculated for the particular conditions of SMPS. With the charts
given in Figure 6 and Figure 7 this calculation can be easily made. The results of this
calculation are valid for an 11 A device but to calculate the switching losses for our
example you have to use the ratio between the energies of SMPS and the energies
determined in the test circuit. This ratio is the same for all CoolMOS devices.
−4
CFoff ( Vds( off ) )
10 ⋅
mJ
⋅ V ds( off ) + 2.8 ⋅ 10
−3
−4
10
V
Application Note
mJ
⋅ 480 V + 2.8 ⋅ 10
−3
V
1.181
0.043 mJ
CF off ( Rgate)
⋅
Eoff ( Rgate)
Eoff ( Rgate( test) )
0.043 mJ
Eoff ( 12 Ω )
Eoff ( 18 Ω )
27
4.9uJ
0.731
6.7uJ
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Total Power Losses
Using this information we can calculate the total power losses:
1
Ptot
⋅ 1.9 Ω ⋅ ( 2.4 A) ⋅ 0.21 + 6uJ ⋅ 1.181 ⋅ 0.731 ⋅ 60 ⋅ kHz
2
1.077 W
3
Now we know the total power losses of SPP04N60C3 in this particular design.
Step 5
Recalculate the maximum power dissipation for a selected CoolMOS
type
With the available junction to case thermal resistance of selected CoolMOS device it
is possible to make the calculation of maximum allowable power dissipation more
precisely. This step can help to skip one of the iterations what will be explained in
Step 6.
Example for Step 5:
The thermal resistance of the heat sink including the isolation material remains the same
RthCA = 40 K/W.
The thermal resistance junction to case of the SPP04N60C3 is:
RthJC = 2.5 K/W.
The maximum allowed junction temperature in our case is:
TJ = 110°C.
The ambient temperature is:
TA = 70°C
Now using the Equation [2] we can calculate the maximum allowable power dissipation
in our case more precise:
P max
TJ
R thJC
TA
R thCA
110 °C
2.5
K
W
70 °C
40
K
0.941 W
W
As we can see the maximum allowable power dissipation for SPP04N60C3 is 0.941 W.
Application Note
28
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Total Power Losses
Step 6
Compare the total power losses calculated in Step 4 for a selected
CoolMOS type with maximum allowable power dissipation from Step 5
At this point it is necessary to compare the total power losses calculated in step 4 with
the maxi-mum allowable power dissipation resulting from the defined junction
temperature and heat sink (Step 5).
If the total power losses from step 4 are lower than the maximum allowable power
dissipation (Step 1), then the selected CoolMOS type meets the requirements. We did
find the right type. As a further optimization it could be possible to check if the next
CoolMOS type with higher Rds(ON) will do the same job. Repeat Step 4, Step 5 and
Step 6 with this new selection.
In case of total power losses from Step 4 are higher than the maximum allowable
power dissipation (Step 1), select the next type from the CoolMOS family with lower
on state resistance and repeat the Step 4, Step 5 and Step 6. Another possibility
would be to adjust the heat sink.
The right type is found, when the next CoolMOS with higher Rds(ON) does not meet the
requirements.
Example for Step 6 (discontinuous conduction mode converter):
As we calculated in Step 4 the SPP04N60C3 has total power losses of 1.077 W in this
particular design. The maximum allowable power dissipation from Step 5 is 0.941 W.
This means the SPP04N60C3 does not meet the requirements of this particular design.
Now we have two possibilities -selecting the next CoolMOS with lower on state
resistance or adjusting the heat sink.
Example for CoolMOS type with lower Rds(ON) (discontinuous conduction mode
converter):
Let us first select the next CoolMOS with lower Rds(ON) and repeat the Step 4, Step 5 and
Step 6.
Step 3 (second iteration):
We choose SPP07N60C3.
Step 4 (second iteration):
Our discontinuous conduction mode converter has following operating conditions:
ipeak = 2.4 A (peak drain current)
D = 0.21 (duty cycle)
f = 60 kHz (switching frequency)
Rgate=12 Ω (gate resistance)
Application Note
29
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Total Power Losses
Vds(on) = 380 V (DC voltage between drain and source before the start of the turn-on
transition, the bulk capacitor voltage)
Vds(off) = 480 V (DC voltage between drain and source after the end of the turn-off
transition, the bulk capacitor voltage plus the flyback voltage)
The CoolMOS SPP07N60C3 selected in Step 3 has approximately 1.2 Ω on state
resistance at junction temperature of 110°C: Rds(ON) = 1.2 Ω. The turn off energy losses
is Eoff (ipeak) = 7 µJ at ipeak = 2.4 A and Rgate = 12 Ω, this information can be found in the
datasheet of CoolMOS.
The dependency of switching energy versed drain source voltage and gate resistance
must be calculated with the formulas [17] and [21] for both correction factors. The
correction factor for the gate resistance is 1, due to the condition of same used Rgate for
the test and for this particular SMPS.
Using all this information we can calculate the total power losses:
1
Ptot
⋅ 1.2 Ω ⋅ ( 2.4 A) ⋅ 0.21 + 7uJ ⋅ 1.181 ⋅ 60 kHz
2
0.98 W
3
Step 5 (second iteration):
The thermal resistance of the heat sink including the isolation material remains the same
RthCA = 40 K/W.
The thermal resistance junction to case of the SPP07N60C3 is:
RthJC = 1.5 K/W.
The maximum allowed junction temperature in our case is:
TJ = 110°C.
The ambient temperature is:
TA = 70°C
Now using the Equation [2] we can calculate the maximum allowable power dissipation
in this case more precise:
P max
TJ
R thJC
TA
110 °C
R thCA
1.5
K
W
70 °C
40
K
0.964 W
W
Step 6 (second iteration):
As we can see, the total power losses from Step 4 are higher than the maximum
allowable power dissipation (Step 5). The SPP07N60C3 is still not the right choice.
Application Note
30
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Calculation of Peak Pulse Current
Let us now try another possibility. We will keep the SPP07N60C3, but we will slightly
improve the thermal resistance of our heat sink by 3 K/W. Now we can calculate the
maximum allowable power dissipation for this new heat sink (Step 5).
Step 5 (third iteration):
The thermal resistance of the heat sink including the isolation material is
RthCA = 37 K/W.
All other parameters will remain the same.
P max
TJ
R thJC
TA
R thCA
110 °C
1.5
K
W
70 °C
37
K
1.039 W
W
Step 6 (third iteration):
Let us compare the total power losses from Step 4 (second iteration) and the maximum
allowable power dissipation calculated in Step 5 (third iteration). At this point we did
achieve an optimum between selected CoolMOS type and adjusted heat sink. The
selection of MOSFET is done.
13
Calculation of Peak Pulse Current
As shown in the previous section, the selection of the right MOSFET type is a
complicated approach, which requires many iterations. In order to reduce the number of
iterations, it is useful to select the optimal CoolMOS type for the first iteration. This
section shows how to calculate the peak pulse current for the particular CoolMOS type
and presents useful charts for preselection.
Combining the Equation [2] and [25], the information about the peak pulse current can
be obtained.
13.1
Discontinuous Conduction Mode Converter
From the condition that the total power losses should be lower than the maximum
allowable power dissipation:
P tot P max
[33]
and correspondingly:
1
⋅ Rds( ON ) ⋅ i peak ⋅ D + Eoff ( i peak) ⋅ CF off ( VDS ( off ) ) ⋅ CF off ( Rgate) ⋅ f ≤
2
3
Application Note
31
TJ − TA
RthJC + RthCA
[34]
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Calculation of Peak Pulse Current
The relation between peak current and switching frequency can be expressed as:
TJ − TA
f ≤
RthJC + RthCA
−
1
⋅ Rds( ON ) ⋅ ipeak ⋅ D
2
[35]
3
CFoff ( VDS ( off ) ) ⋅ CF off ( Rgate) ⋅ Eoff ( ipeak)
Now let us calculate what peak current can be handled by each particular CoolMOS C3
type, depending on the switching frequency. Due to the complexity of the equations and
dependencies we will use the numeric solution. Next figure shows the peak drain current
in a discontinuous conduction mode converter with following operating conditions:
D = 0.21
duty cycle
Vdc(in) = 380 V
bulk capacitor voltage
Vr = 100 V
reflected voltage
TJ = 110°C
junction - temperature
TA = 70°C
ambient - temperature
η = 0.8
efficiency
12
10
8
i peak [A]
Linear (SPP02N60C3
Poly. (SPP03N60C3)
Poly. (SPP04N60C3)
6
Poly. (SPP07N60C3)
Poly. (SPP11N60C3)
Poly. (SPP20N60C3)
4
2
0
50
100
150
200
250
300
f [kHz]
Figure 9
Peak Current Handling Capability in Discontinuous Conduction
Mode Converter (external heat sink thermal resistance is 10 K/W)
Application Note
32
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Calculation of Peak Pulse Current
13.2
Continuous Conduction Mode Converter
From the condition that the total power losses should be lower than the maximum
allowable power dissipation
P tot P max
[36]
and correspondingly:
1
(
⋅ Rds( ON ) ⋅ D⋅ imin + i peak⋅ imin + ipeak
2
) ...
≤
2
3
+ ( Eon ( imin ) ⋅ CFon ( VDS ( on ) ) ⋅ CFon ( Rgate) + Eoff ( ipeak) ⋅ CFoff ( VDS ( off ) ) ⋅ CFoff ( Rgate) ) ⋅ f
(
f ≤
TJ − TA
RthJC + RthCA
)
1
é TJ − TA
2
2 ù
− ⋅ Rds( ON ) ⋅ D⋅ imin + ipeak⋅ imin + ipeak ú
êR + R
ë thJC thCA 3
û
[37]
[38]
Eon ( imin ) ⋅ CF on ( VDS ( on ) ) ⋅ CF on ( Rgate) + Eoff ( ipeak) ⋅ CFon ( VDS ( on ) ) ⋅ CFon ( Rgate)
Kmin
i min
i peak
[39]
1
Now let us calculate what peak current can be handled by each particular CoolMOS C3
type depending on the switching frequency.
Due to the complexity of the equations and dependencies the numeric solution has being
used. Next figure shows the peak drain current in a continuous conduction mode
converter with following operating conditions:
D = 0.45
duty cycle
Vdc(in) = 380 V
bulk capacitor voltage
TJ = 110°C
junction - temperature
TA = 70°C
ambient - temperature
η = 0.8
efficiency
Kmin = 0.72
factor Imin, Imax
Application Note
33
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Maximum Output Power Capability
6
5
i peak [A]
4
Linear (SPP02N60C3)
Poly. (SPP03N60C3)
Poly. (SPP04N60C3)
Poly. (SPP07N60C3)
3
Poly. (SPP11N60C3)
Poly. (SPP20N60C3)
2
1
0
50
100
150
200
250
300
f [kHz]
Figure 10
Peak Current Handling Capability in Continuous Conduction Mode
Converter (external heat sink thermal resistance is 10 K/W)
14
Maximum Output Power Capability
14.1
Discontinuous Conduction Mode Converter
To make a first selection, which CoolMOS is best for a particular application, it is more
interesting to know the output power capability for each device. But that is not a problem
at all, with the formulas given in the above section because of a linear relation between
the primary peak current and the output power.
Maximum output power:
Pout
η ⋅ Pin
η ⋅ V in ( DC) ⋅ I in ( DC)
1
η ⋅ V in ( DC) ⋅ ⋅ ipeak
2
[40]
where η is efficiency of power converter.
Now let us calculate what output power can be handled by each particular CoolMOS C3
type, depending on the switching frequency. Due to the complexity of the equations and
Application Note
34
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Maximum Output Power Capability
dependencies we will use the numeric solution. Next figure shows the output power in a
discontinuous conduction mode converter with following operating conditions:
D = 0.21
duty cycle
Vdc(in) = 380 V
bulk capacitor voltage
Vr = 100 V
reflected voltage
TJ = 110°C
junction - temperature
TA = 70°C
ambient - temperature
η = 0.8
efficiency
1200
1000
Pout [W]
800
Linear (SPP02N60C3)
Poly. (SPP03N60C3)
Poly. (SPP04N60C3)
600
Poly. (SPP07N60C3)
Poly. (SPP11N60C3)
Poly. (SPP20N60C3)
400
200
0
50
100
150
200
250
300
f [kHz]
Figure 11
14.2
Power Handling Capability in Discontinuous Conduction Mode
Converter (external heat sink thermal resistance is 10 K/W)
Continuous Conduction Mode Converter
For a continuous conduction mode converter exists also a linear relation between the
primary peak current and the output power of the SMPS. We can also use the formulas
for the peak pulse current for continuous conduction mode converter, and multiply them
with a linear transformation factor in order to achieve a diagram, where the ratio between
the power handling capability and the frequency are shown.
Application Note
35
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Maximum Output Power Capability
Maximum output power:
1
P out η .P in η .Vin( DC ) .Iin( DC ) η .Vin( DC ) . . i min
2
i peak .D
[41]
where η is efficiency of power converter.
Now let us calculate what output power can be handled by each particular CoolMOS C3
type depending on the switching frequency. Due to the complexity of the equations and
dependencies we will use the numeric solution. Next figure shows the output power in a
continuous conduction mode converter with following operating conditions:
D = 0.45
duty cycle
Vdc(in) = 380 V
bulk capacitor voltage
TJ = 110°C
junction - temperature
TA = 70°C
ambient - temperature
η = 0.8
efficiency
Kminn = 0.72
factor Imin, Imax
12
10
SPP02N60C3
SPP04N60C3
SPP07N60C3
SPP11N60C3
SPP20N60C3
8
SPP03N60C3
i peak [A]
Linear (SPP02N60C3
Poly. (SPP03N60C3)
Poly. (SPP04N60C3)
6
Poly. (SPP07N60C3)
Poly. (SPP11N60C3)
Poly. (SPP20N60C3)
Linear (SPP02N60C3
4
Poly. (SPP03N60C3)
Poly. (SPP04N60C3)
Poly. (SPP07N60C3)
Poly. (SPP11N60C3)
2
Poly. (SPP20N60C3)
0
50
100
150
200
250
300
f [kHz]
Figure 12
Power Handling Capability in Continuous Conduction Mode
Converter (external heat sink thermal resistance is 10 K/W)
Application Note
36
V1.2, 2002-01
How to Select the Right CoolMOSTM and
its Power Handling Capability
Conclusion
15
Conclusion
Due to different MOSFET technologies from several semiconductor manufacturers and
significant differences in the way the datasheet is done selecting the right MOSFET for
the particular design becomes a complicated task. This application note shows a way to
design in the CoolMOS in continuous and discontinuous conduction mode converters.
The introduced iteration approach is based on the calculation of the power losses in the
transistor itself. The power losses could be divided into two main parts, the conduction
losses and the switching losses. Conduction losses depend on the on state resistance
and are simple to calculate. Whereas the switching losses can be influenced by a lot of
parameters like nonlinear output drain-source capacitance, total gate resistance,
parasitic inductances and capacitances of the circuit layout. These are difficult to handle.
Calculation of the switching losses mentioned in this application note are based on
measured values.
The cooling condition of the system and the heat sink design limits the maximum
allowable power dissipation of the MOSFET in an SMPS. Based on this limitation and on
the calculated power losses it is possible to select an optimal transistor type.
Drain current versus the switching frequency charts, as well as output power versus the
switching frequency charts can be used as a pre-selection of the MOSFET type for
discontinuous and continuous conduction mode converters. It is possible to skip some
initial iterations using these charts.
The described approach helps to reduce the number of experimental iterations and thus
reduce the design cycle time.
Application Note
37
V1.2, 2002-01
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