AN2708 Datasheet, PDF (14/42 Pages) STMicroelectronics – 2X36 W digital dimmable ballast with L6574 and ST7FDALI

High-frequency ballast
AN2708
2.2
Half-bridge inverter and ballast
A voltage fed series resonant half-bridge inverter has been implemented to drive the tubes.
This topology allows to easily operates in zero-voltage switching (ZVS) resonant mode,
heavily reducing the transistor switching losses and the electromagnetic interference. In
addition it guarantees design simplicity and low cost.
A parallel configuration has been chosen for the output stage. The half-bridge inverter
operating conditions and the ballast design have been obtained by assuming, for each lamp,
the following basic model:
Figure 5. Lamp ballast model
To increase the life time of the lamps a current mode preheat was preferred. The preheating
current brings the cathodes to the correct temperature, then a high voltage ignites the lamp
and finally the correct current guarantees the running power. These phases are ensured by
changing the frequency of the input voltage and properly selecting V_IN, L and C. During
preheating and ignition, the lamp is not conducting and the circuit is reduced to a series L-C.
During running, the lamp is conducting and the circuit is an L in series with a parallel R-C. To
determine the optimum values for L and C and to calculate the ballast operating frequencies
the transfer functions for each mode of operation have to be inspected.
The table below shows the parameters and the values for a T8 36 W tube lamp which need
to be known in order to calculate the ballast operating conditions.
Table 7. Lamp parameters
Parameter
Input DC bus voltage: Vdc
Preheat current: Iph
Preheat time: Tph
Max preheat voltage: Vphmax
Ignition voltage: Vign
Running lamp power: Prun
Running lamp voltage: Vrun
Expected efficiency: η
Value
400 V
0.6 A
1 sec
300 Vpk
800 Vpk
34 W
144 Vpk
95%
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