STEVAL-ILL013V1 Datasheet, PDF (37/43 Pages) STMicroelectronics – 80 W offline LED driver with PFC

Design calculation
Appendix A Design calculation
UM0670
The aim of this section is to demonstrate how the components for the modified buck
converter are calculated. Design calculation follows precisely the equations used in
application note AN2928 (Section 11: References and related materials: 2.). Therefore,
please refer to this application note for more information.
A.1
Design specifications for a modified buck convertor
● V_IN = 400 V
● V_LED = 80 V
● I_AVR = 1 A
● I_MAX = 1.4 A
● I_MIN = 0.6 A
● f = 50 kHz
● T_A = 30 °C
● T_{JMAX_MOSFET} = 70 °C
Modified buck converter working with duty cycle (output LED current is 1 A):
Equation 1
D = ^V-----^L---^E---^D-- = --⁸---⁰---- = 0.2
V_IN 400
Calculated OFF time for selected switching frequency of 50 kHz is:
Equation 2
t_OFF
=
(_---1_{-----–----}D_-----)
f
=
(_---1_{-----–----}0_----._-2_----)
50000
=
16μs
Now, the FOT network should be calculated. First, resistor R203 is selected:
R₂₀₃ = 3900 Ω
Two capacitors in parallel (C204 and C206) are used for the 1 A output LED current (jumper
JP3 is connected) and their size is determined using the following equation:
Equation 3
C₂₀₄ | |
C₂₀₆
=
--------^t--^O----^F----^F---------
2.1 × R₂₀₃
=
--¹---⁶-----^×------¹---⁰----^–--⁶---
2.1 × 3900
=
1.95nF
Therefore the capacitors C204 and C206 have the following size:
C₂₀₄ = 390 pF
C₂₀₆ = 1.5 nF
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Doc ID 15327 Rev 2