AN827 Datasheet, PDF (5/20 Pages) STMicroelectronics – A 500W HIGH POWER FACTOR WITH THE L4981A

AN827 APPLICATION NOTE
Where:
C_oss = 650pF is the mosfet drain capacitance at 25V
C_ext = 100pF is the equivalent stray capacitance of the layout and external parts
The estimated crossover switching losses (turnon and turn-off) are calculated as follows:
P_crossover = V_out · I_Qrms · f_sw · t_cr + Prec = 400 · 5.42 · 80k · 40ns + 1.5 = 8.43W
Where:
t_cr is the crossover time
P_rec is the boost diode recovery power loss contribution
To reduce the turn-off losses in the mosfet, an RCD turn-off snubber has been employed. The capacitor value
is calculated as follows:
C11 = -^I-^Q----¹-_∆--^p--_V-^k----_o^â‹---_u^t--^r-_t-ⁱ-^s---^e- = ⁸----^.-⁹----²--₄---^â‹₀---⁴⁰---⁰----ⁿ----^s- = 892pF
Therefore, use C11 = 820pF, 1000VDC rating
The resistors, R23-24, must dissipate the energy stored in the snubber capacitor upon turn-on of the power
mosfet. The capacitor must fully discharge during the switching cycle.
The time constant of the RC combination is determined as follows:
R ≤ ₁--¹--₀-- ⋅ _f--_s---_w-----_â‹-¹--_C-----₁---₁-- = 1524
The power dissipated in the resistors, R23-24, is calculated as follows:
P_diss = ¹²-- C11 ⋅ V_out2 ⋅ f_sw = ¹²-- ⋅ 820pF ⋅ 400² ⋅ 80k = 5.25W
Therefore, use R23 = R24 = 510Ω, 3W rating.
The power mosfet chosen is the STMicoelectonics Part Number STW20NA50.
This part has a BV_dss = 500V, R_DSon = 0.27Ω, and is in a TO-247 package. In order to keep the junction tem-
perature at a safe level, the mosfet is attached to an AAVID Heatsink Part Number 61085 with a thermal resis-
tance of 3.0°C/W. This will keep the mosfet junction temperature at a safe level at worst case conditions, low-
line input voltage (88V) and full load (500W).
The thermal resistance of the heatsink may need to decrease depending upon the ambient temperature, type
of enclosure (vented or non-vented) and the method of cooling (natural or forced convection).
Boost Diode
The main criteria for the selection of the boost diode, D2, include the repetitive peak reverse breakdown voltage
(V_rrm), average forward current (I_fave), reverse recovery time (t_rr) and thermal considerations.
Voltage Rating:
The voltage rating of the boost diode is determined by the same equation as for the power mosfet. The value
chosen is V_rrm = 600V.
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