AN827 Datasheet, PDF (6/20 Pages) STMicroelectronics – A 500W HIGH POWER FACTOR WITH THE L4981A

AN827 APPLICATION NOTE
Current Rating:
The power losses in the boost diode consist of the conduction and switching losses. The switching losses are
a function of the reverse recovery ime (t_rr) and output voltage (V_out) . The switching losses are negligible com-
pared to the conduction losses if a suitable ultra fast recovery diode is chosen. The conduction power losses
can be calculated as follows:
I_{o ut} = ^PV-----^oo---^uu--^tt = ⁵⁴----⁰⁰---⁰⁰-- = 1.25A
I_Drms
=
--------------^P----ⁱ-ⁿ---------------
2 V_{in rms min}
1_----6_-----⋅_---3----2_{-⋅---π-}⋅_---V_{⋅---V-i-n---o--r-um---t-s----m----i-n- = 3.24A}
P_cond
=
V_to
⋅
I_out
+
I_D
rm
2
s
⋅
Rd
=
1.15
⋅
1.25
+
3.24²
⋅
0.043
= 1.89W
Where:
V_to = 1.15V is the threshold voltage of the diode R_d = 0.043W is the diode differential resistance
The diode must sustain the average output current and also keep the power losses to a minimum in order to
keep the diode junction temperature within acceptable limits. The switching losses can be significantly reduced
if an ultra-fast diode is employed. Since this circuit operates in the continuous current mode, the mosfet has to
recover the boost diode minority carrier charge at turn-on.
Thus, a diode with a small reverse recover time, t_rr, must be used. This circuit employs the STMicroelectronics
Turboswitch Diode Part Number STTA806D. This part offers the best solution for the continuous current mode
operation due to its very fast reverse recovery time, 25ns typical. This part has a breakdown voltage rating (V_rrm)
of 600V, average forward current rating (I_fave) of 8A and reverse recovery time (t_rr) of 25ns.
The diode is attached to the same heatsink as the power mosfet, Q1. The STTA806D is non-isolated thus re-
quiring a thermal insulator with good heat transfer characteristics. The STTA806DI is an isolated package and
can be attached directly to the heatsink. Silicone thermal grease may be applied to improve the thermal contact
between the diode and heatsink.
Boost Inductor
The boost inductor, T1, design starts with defining the minimum inductance value, L, to limit the high frequency
current ripple, ∆I_L. The next step is to define the number of turns, air gap length, ferrite core geometry, size and
type for the specified power level. Finally, the wire size and type are determined.
In the continuous mode approach, the acceptable current ripple factor, K_r, can be considered between 10% to
35%. For this design, the maximum specified current ripple factor is 23%. The maximum current ripple occurs
when the peak of the input voltage is equal to Vout/2.
∆I_Lmax -₄----_â‹--^V-_f--_S--^o--_W-^u---^t--_â‹---_L- = ₄-----_â‹---₈----₀---_k-⁴---⁰-_â‹---⁰⁰---_.--₅----_m-----_H--- = 2.50A
Occurs at V_inpk = V_out/2 = 200V; V_inrms = 141V
∆I_L = -^V---ⁱ--ⁿ---^p-_V--^k--_o-⁽--_u^V---_t-^o--_â‹-^u--_f-^t-_s--–-_w----^V_â‹---_L-ⁱ-ⁿ---^p---^k---⁾-
For all other input voltages
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