AN-9050 Datasheet, PDF (3/7 Pages) Fairchild Semiconductor – Power Loss Calculation

APP NOTE NUMBER
APPLICATION NOTE
Calculation Example of Power Loss
Figure 4. Normalized Module Ploss vs. Vout
FDMF6704 Evaluation Board
The FDMF6704 evaluation board is used for FDMF6704
electrical characterization test. The board specifications and
structure are shown in Table 3.
Dimension
15 x 15 cm
No. of layers 4 layers
Layer sequence TOP-GND-PWR-BOT
Total thickness 1.6 mm
TOP and BOT 1.5 oz (1 oz base + 0.5 oz plating)
GND and PWR 1 oz
Table 3. Evaluation Board Spec. and Structure
Table 4 shows reference test condition of evaluation board.
VIN
12 V
VCIN & VDRV 5 V
VOUT
1.3 V
PWM HI/LO 5 V/0 V
FSW
350 kHz
IOUT
0~35 A, 5 A step
Soaking time 5 minutes
LOUT
440 nH/0.32 mOhms/35 A
Snubber
Not used
All power loss graphs in the FDMF6704 datasheet are
generated from measured evaluation board test data. System
designer can calculate module power loss with normalized
power loss graphs, and estimate performance of the module
and application. Examples below show how to calculate
module power loss with normalized power loss graphs in
datasheet.
z Example 1
1. Define design parameters
Vin=12 V
Vcin & Vdrv=5 V
Vout=1.5 V
Iout=30 A
Fsw=600 kHz
Inductor=440 nH
2. Calculate each steps
- Find reference design parameter values and
module power loss with Figure 3 Æ 6.8 W with 12
Vin, 5 Vcin, 1.3 Vout, 30 A, 350 kHz and 440 nH
- Find normalized value of module power loss with
Figure 4 when Vout is 1.5 V Æ 1.04
- Multiply 6.8 W by 1.04 Æ 7.072 W
- Find normalized value of module power loss with
Figure 5 when Fsw is 600 kHz Æ 1.12
- Multiply 7.072 W by 1.12. Æ 7.921 W
- The calculated module power loss is 7.921 W.
Air flow
Not used
Heat sink
Not used
Ambient Temp. 25 C
Table 4. Evaluation Board Reference Test Condition
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 7/14/09
Figure 5. Normalized Module Ploss vs. Fsw
In this example 1, two design parameters, such as Vout and
Fsw, are changed from reference values. The calculated
module power loss shows under 0.5 % error compared to
the real lab test data. Table 5 shows the module power loss
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