AN2590 Datasheet, PDF (9/17 Pages) STMicroelectronics – 20 W dimmable CFL ballast solution

AN2590
Board description
As a consequence of that, we have the following.
Equation 2
t'
=
---^V-----ⁱ-ⁿ-----
4Vc1
Ts
Equation 3
Ipk
=
--^T----^s----
4L1
•
⎛
⎝
1
–
2^---V-V^----i-cn^---1^{--⎠⎞}
•
Vin
● Step 2: The input ac current (Iac) equals half of the average inductor current due to two
divider capacitors, so,
Equation 4
Iac
=
⎛
⎝
1_--
2
i_{L1⎠⎞}
a
v
=
e
14_{-- Ipk=}
----^T----^s-----
16L1
•
⎝⎛^{1 –} ₂^---V-_V^----i-_cn^---₁^--⎠⎞
•
Vin
● Step 3: The input power Pin is determined by,
Equation 5
π
∫ ^{Pin = 1-}_π^{- Vac • Iacdθ}
0
Where Vac is the input voltage. With the above three steps, the values of L1 and Ipk can be
found if the input/output conditions and switching frequency are given.
● Step 4: For the half bridge resonant circuit, the equivalent resistance of the lamp is
given by:
Equation 6
R_lamp
=
V_{-----r--m----s--,----l-a---m-----p-}
I_{rms, lamp}
Where V_rms,lamp is the rated lamp voltage, I_rms,lamp is the rated lamp current. With V_rms,lamp
and I_rms,lamp and the input voltage of the half bridge (Vc1), the results of Lr & Cr are easily
obtained.
● Step 5: MOSFET selection. In order to select an adequate MOSFET parameters
V_(BR)DSS, I_D and R_DSON are important. The V_(BR)DSS should be selected according to
Vc1 with a certain safe margin. In a compact application, the thermal issue becomes
very critical. Thus I_D and R_DSON must be selected with a safe margin also.
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