AN993 Datasheet, PDF (21/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

AN993
5
Design tips
Design tips
5.1
Inductance and capacitor evaluation
To design an application with the L6574, a preliminary evaluation of the components can be
done by fixing the lamp type and its electrical characteristics only.
This evaluation is an "iterative" process because some assumptions have to be made that
need to be checked at the end of the process. Figure 19 shows a simplified schematic of the
load.
Figure 19. Simplified schematic of the lamp
L
L
r
lamp
Cb
C
rr
C
r
Cb
AM01327v1
In Figure 19 r is the lamp filament resistance, rr is the operating lamp equivalent resistance
when the lamp is off or during preheating. rr is an open circuit.
First of all, one has to evaluate a proper inductance value.
L has to provide the right current value to the lamp when it is already ignited and is working
("choke" inductance). Therefore, L depends on the current required by the lamp, that is to
say on the lamp’s operating wattage and voltage, on the operating frequency f_min, but also
on the voltage across L and the lamp (V_b). The greater the V_b variation, the greater the
inductance required to give a constant current to the lamp. V_b variations are due to the high-
voltage bus variations and to the ripple on the half battery capacitor (C_b). The ripple
depends on the size of C_b and a proper hypothesis has to be done on it to estimate the
variation of V_b. Taking into account all this data and hypothesis, one can assume that during
an operating condition most of the current flows into the lamp, not into C, and all the power
delivered to the system is delivered to the lamp. V_lamp is the operating voltage across the
lamp and P_lamp is the operating lamp wattage, so a good approximation to conduct the
choke inductance is:
Equation 16
The second step is the evaluation of the capacitance across the lamp (C).
Doc ID 5656 Rev 10
21/27