AN2654 Datasheet, PDF (24/26 Pages) STMicroelectronics – CCFL backlight half-bridge topology based on L6574 and STD7NS20

Resonant tank design
AN2654
never operates in the capacitive mode which means f-running should be always higher than
f_op in any case.
As we know, the CCFL shows negative impedance characteristic during startup. The ballast
capacitor C_b is used to compensate this characteristic and stabilize the control loop. The
value is normally determined experientially to provide 1.5 times impedance as the lamp
impedance at switching frequency, thus C_b is determined with regards to the specification of
the lamp.
Once the f_ig, f_op, f-running and C_b have been fixed, it is possible to obtain the L value and C_p
by Equation 3 and 5. Then by inputting the value of L and C_p into Equation 4, Vs is
determined. After that, Equation 1 gives the turn ratio N of the design.
Unlike the Vs, transformer secondary output voltage is frequency dependent. Considering
the simplified model in Figure 32, the V_lamp/V_second transfer function can be written as
follows:
Equation 13
---^V----^l--^a---^m----^p----
V _second
=
-------------------¹-------------------
1 + -_j-_ω-----_C----_b---^1R-----_l-_a---_m----_p-
To ensure the ignition of CCFL, the maximum striking voltage must be taken into account.
Making R_lamp → ∞ in Equation 6, the maximum secondary voltage is equal to lamp striking
voltage. Then the secondary number of turns N_sec can be determined by:
Equation 14
N _sec
= -----²-----^â‹---^V----^s---^e----^c--------–----^m------^a---^x--
2π ⋅ f ⋅ B_sat ⋅ A_e
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