AN4213 datasheet(24/42 Pages) STMICROELECTRONICS

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DC/AC converter and lamp

AN4213

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DocID023984 Rev 2

4.2

Lamp requirements

4.2.1

Output inductor design

The output inductor should be designed to allow a sufficient peak ignition current without

saturating. This is important as the ballast will be unable to ignite if it cannot deliver sufficient

voltage to the lamp.

The ignition current depends on the type of the lamp being used and must be kept to a

minimum by ensuring that the cathodes are sufficiently preheated. To minimize eddy current

losses in the inductor, multi-stranded wire for the windings should be used. It is important to

have a large enough air gap to produce the highest available peak current before allowing

the inductor to saturate. When the cores are hot, the saturation point and hence the peak

current for the inductor will be lower, therefore a poorly designed inductor may result in the

ballast failing to ignite the lamp during an attempted hot re-strike. The value of the output

inductor is given by following equation:

Equation 43

4.2.2

Lamp preheating

It is essential that the lamp be sufficiently preheated before ignited. In order to achieve the

maximum possible lamp life, it is necessary to heat the cathodes to the correct temperature

before ignition. Accelerated deterioration occurs if lamps are ignited when the cathodes are

either not sufficiently heated or overheated. The preheating of the cathodes allows an easy

strike of the lamp, reducing ignition voltage. During preheating time the lamp is

characterized by high impedance and the current flows through the filament. Its resistance

value is strictly dependent on the lamp model. Typically these filaments show an initially low

value (a few ohms) that will increase by 5 times during the preheating. One method to obtain

the preheating cathodes is to adopt a PTC resistor.

Figure 13 shows its typical connection.

Figure 13. Preheating circuit

run

AM17327v1

PTC

Lres

Cres

C’res

C’’res

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