AN2870 datasheet(4/41 Pages) STMICROELECTRONICS

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Introduction

AN2870

4/41

Introduction

1.1

Typical configuration

Typical fluorescent lamp electronic ballasts are composed by (Figure 1):

●

An input PFC section, if input power is greater than 25 W, usually a TM PFC converter,

that generates a DC output voltage and absorbs power from mains with very high

Power Factor (typically 0.95 or grater) and very low THD (mandatory less than 10%).

●

A high frequency half bridge driver, fed by the PFC output, with internal or external

oscillator, a timer and various protections in order to drive correctly the lamp, to avoid to

deliver an excessive power to the lamp and to detect any malfunction of the lamp

(broken lamp, broken cathode or lamp absence)

●

An output resonant stage, realized by reactive components (capacitors and inductors),

that, together with the half bridge driver, optimizes the power delivered to the lamp (one

or more) during all working conditions (preheating, ignition and run mode).

1.2

Lamp requirements

Fluorescent lamp, during its normal operation, has to be supplied by means of alternative

and controlled current. In order to reduce the size of the ballast and increase the light

efficiency of the lamp a frequency greater than 20 kHz is typically used. A half bridge quasi

resonant inverter (series-parallel converter Figure 1) is used to obtain sinusoidal current into

the lamp and to reduce the power dissipation of the half bridge switches, in fact zero voltage

switching is achieved.

Lamp current and lamp voltage during normal operation are reported in lamp documentation

and are to be considered as design specification. Moreover, a well preheated lamp ignites at

a lower voltage; this implies a longer lamp life and a greater number of ignitions. The

efficiency of the preheating is mainly related with the total energy delivered to the cathode

(reported on lamp documentation), and then it depends on the time available for this

operation: keeping constant the preheating energy, longer is the preheating time, smaller is

the instantaneous power delivered to the cathode. During the preheating operation the

voltage across the lamp must be kept below a specified value in order to avoid unwanted

ignitions (when these happen, the lamp experiences multiple re-strike and dissipates large

amounts of power).

There are many ways to deliver power to the cathode, but the most used are two:

Current controlled preheating: the cathodes are interposed between the choke and the

resonant capacitor so they experience the same current of the resonant LC circuitry. An

efficient preheating is obtained controlling this current and the time of preheating. The

advantages of this method are the cheapness and easiness of design; it has also some

disadvantages, namely the difficulty of keeping low the lamp voltage during preheating

and the fact that during steady state the cathode experiences the sum of the lamp

current and of the resonant capacitor current.

Voltage controlled preheating: the current into the cathodes is generated by auxiliary

windings coupled with ballast choke or driven by an auxiliary oscillator. This implies

that, in any case, the design of the preheating circuitry is somewhat independent from

the design of the LC circuitry, even if it requires a lot of external components. This

method is then more efficient, but is cheaper and more difficult to design.

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