STEVAL-ILL013V1 Datasheet, PDF (39/43 Pages) STMicroelectronics – 80 W offline LED driver with PFC

Design calculation
UM0670
Equation 10
l_{R2 MS} = D ×
l_{A2 VR}
+
l_--P2---P--
12
= 0.2 1 + -⁰---^.--⁸---²- = 0.21
12
Power MOSFET conduction loss is:
Equation 11
P_CON = l_{R2 MS} × R_{DS(ON)(70° C)} = 0.21 × 0.756 = 0.159W
Where the power MOSFET chosen is the STF9NM50N (see datasheet Section 11:
References and related materials: 3.) and its R_DS(on) for 70 °C is:
Equation 12
R_{DS(ON)(° C)} = R_{DS(ON)(° C)} × 1.35= 0.56 × 1.35= 0.756Ω
Power MOSFET switching losses can be approximately calculated (turn OFF time was
measured 120 ns - see Figure 19):
Equation 13
P_SW=
V_IN × I_MAX × t_{OFF SW} × f
-------------------------------------------------------------------
2
=
4_----0_---0_-----×_------1_---._--4_-----×_-----1_----2_---0_-----×_------1_---0_----^–_--⁹_----×_------5_---0_-----×_------1_---0_----³_-
2
=
1.68W
The total power loss on the power MOSFET is 1.839 W, so the heat sink can be calculated
from following equation:
Equation 14
P_TOT=
T_{JMAX MOSFET} – T_A
-----------------------------------------------------------
R_thJC + R_thCH + R_thHA
And maximum heat sink-to-ambient resistance is:
Equation 15
R_thHA
<
T_{JMAX MOSFET} – T_A
------------------------------------------------------
P_TOT
–
R_thJC
–
R_thCH=
7_----0_{-----–----}3_---0_{-- – 5 – 0.5=}
1.839
16.25° C ⁄
W
The heat sink used in the power MOSFET on the STEVAL-ILL013V1 has a thermal
resistance of 13.5 °C / W, and therefore this heat sink is optimized for this design.
The last power component remaining to be calculated is the power diode. The diode
conducts during the OFF time, and therefore its average current is:
Equation 16
I_AVR
D⁼
(1 – D) ×
-^I-^M-----^A---^X-----+-----^I-^M-----^I-^N--
2
=
(1 – 0.2) ×
1_----._-4_-----+2-----0_---._--6_{-- =}
0.8A
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Doc ID 15327 Rev 2