AN2708 datasheet(14/42 Pages) STMICROELECTRONICS

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High-frequency ballast

AN2708

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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 VIN, 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

Value

Input DC bus voltage: Vdc

400 V

Preheat current: Iph

0.6 A

Preheat time: Tph

1 sec

Max preheat voltage: Vphmax

300 Vpk

Ignition voltage: Vign

800 Vpk

Running lamp power: Prun

34 W

Running lamp voltage: Vrun

144 Vpk

Expected efficiency:

95%

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