AN827 Datasheet, PDF (4/20 Pages) STMicroelectronics – A 500W HIGH POWER FACTOR WITH THE L4981A

AN827 APPLICATION NOTE
V_cap > V_out + ∆V_ripple + V_margin = 400 + 8 + 40 = 448V
Where: V_out is the nominal regulated DC output voltage
∆V_ripple is the ac voltage superimposed on the regulated DC output voltage
∆V_margin is the allowance for tolerances in V_out and additional margin before OVP intervention
The capacitor chosen has a voltage rating of 450VDC. The overvoltage trip level of Pin 3 (OVP) must be set
below 450VDC.
Power Mosfet
The power mosfet, Q1, is used as the active switch due to its high frequency capability, ability to be driven di-
rectly from the controller and availability. The main criteria for its selection include the drain to source breakdown
voltage (BVdss), delivered power and temperature considerations.
Voltage Rating:
The power mosfet has to sustain the maximum boosted output dc voltage according to the following equation:
BV_dss > V_out + ∆V_ripple + V_margin = 400 + 8 + 40 = 448V
The power mosfet chosen has a BV_dss of 500V.
Power Rating:
The main parameters to consider are Rdson and the thermal characteristics of the package and heatsink. The
main losses in the power mosfet are the conduction and switching losses. The switching losses can be sepa-
rated into two quantities, capacitive and crossover losses. The switching losses are dependent on the mosfet
current di/dt. The maximum conduction (on-state) power losses can be calculated according to the following
equations:
I_Qmsmax
=
----------------^P----^o---^u---^t--------------- ⋅
η ⋅ 2 V_{irms min}
2
–
-¹---⁶-----^â‹----₃---²-_π----^⋅â‹---^VV----ⁱ-_or--^m-_u---_ts-----^m----ⁱ--ⁿ-
=
-----^5--0-----0--^.---^9--0--------
2 ⋅ 88
⋅
2 – ¹----⁶³----_πâ‹-----_â‹---²⁴---₀--^â‹--₀-⁸---⁸--
I_Qrmsmax = 5.42A
P_onmax = I_Qrms ²max · R_{(DS)on max} = 5.42² · 0.54 = 15.86W
Where:
I_Qrmsmax is the max. power mosfet rms current
V_irmsmin is the min. specified rms input voltage
R_(DS)on typ. = 0.27Ω at 25°C at 10A, V_GS = 10V
R(_DS)on max = 0.54Ω at 100°C
The capacitive switching losses at turn-on are calculated as follows:
P_{capaci tance}
=
3.3
⋅
 ^{C o s s}
⋅
V_o
u
1.5
t
+
12_{-- Cext}
⋅
V_o
2_
ut _
⋅ f_sw
=
2W
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