AN2640 Datasheet, PDF (1/36 Pages) STMicroelectronics – Intelligent multipower digital ballast for fluorescent lamps

AN2640
Application note
Intelligent multipower digital ballast for
fluorescent lamps
Introduction
Fluorescent lamps are highly popular due to their luminous efficiency, long life and color
rendering. These lamps need external circuitry to compensate for their negative resistance
characteristic. This circuitry is called "ballast". The simplest ballast is a magnetic inductor
connected in series at the lamp. The electronic ballast with respect to the magnetic one
offers the following advantages:
■ Better efficiency
■ Increased lamp life
■ Lightweight with smaller dimensions
■ Better lamp power control
For these reasons, in the last years there has been a shift in the market towards the use of
electronic ballasts with dedicated drivers and controllers. Today, thanks to microcontrollers,
it is possible to add intelligence into the circuit. Instead of having a dedicated circuit for each
lamp with a single ballast it is possible to drive many different lamp groups. This application
note describes an electronic ballast that is able to recognize lamps within the T5 fluorescent
family such as 24 W, 39 W, 54 W and 80 W. It consists of two main blocks:
■ A boost converter (Power Factor Controller PFC) working in transition mode (fixed T_ON
and variable frequency)
■ An inverter in half-bridge configuration working in zero voltage switching
Both ballast and PFC stages are controlled by the ST7FLIT19B that offers its entire signal to
the L6382D5 which provides the right voltage and current levels for the Power MOSFET.
This system after tube recognition sets the right parameter and drives the lamp correctly.
Figure 1 shows the ballast block diagram.
Figure 1. Ballast block diagram
January 2008
Rev 1
1/36
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