AN3400 Datasheet, PDF (1/78 Pages) STMicroelectronics – Analysis and simulation of a BJT complementary pair in a self-oscillating CFL solution

AN3400
Application note
Analysis and simulation of a BJT complementary pair
in a self-oscillating CFL solution
Introduction
The steady-state oscillation of a novel zero-voltages switching (ZVS) clamped-voltage (CV)
self-oscillating resonant driving system for compact fluorescent lamps (CFL), using a
complementary pair of bipolar transistors on the half bridge converter section, is analyzed
and simulated.
One or more auxiliary windings are added on the ballast inductor in series with the lamp in
order to generate the periodic signal to supply the bases of the two complementary devices
connected to each other in a common emitter half bridge topology. In fact, an LC network
filters, with a resonant effect, the voltage generated by the secondary winding of the load
transformer, producing a novel, periodic switching signal to accurately control the bases of
the transistors. The two bipolars are supplied by a unitary control signal so it is not possible
to turn on both devices at once because of their opposing base-emitter junction thresholds.
Self-oscillating operation is divided into eight stages according to the variation over a period
of the driving voltage signal across the filter capacitor at the output of the transformer
secondary windings. Stage-wise circuit analysis shows as the resonant filter action limits the
lamp current and dominates the switching frequency of the ballast in steady-state working
condition.
The half bridge of the power active devices generates a rectangular voltage waveform that
drives an opportunely tuned output circuit composed of a parallel loaded RLC output circuit
of which the R is the steady-state LAMP resistance. For the inverter self-oscillating
condition, the switching frequency is determined by all of the circuit elements related to the
oscillation frequency, such as the resonant tank, gas-discharge lamp, driving circuit, and
switching devices. Depending on the circuit design, its oscillation frequency is typically
around 35 to 48 kHz and can be eventually increased by shortening the storage time of the
bipolars. Cost benefits are achieved by the proposed self-oscillating solution that allows to
drive the CFL lamp eliminating the saturable core auxiliary transformer, placed in the more
traditional standard solution, without sacrificing the performance or reducing the expected
life time of the lamps.
In this paper, general structure and self-oscillating principle are discussed and verified by
laboratory experiment, while analytical results are validated by mathematical simulation
using the Matlab tool.
November 2011
Doc ID 018840 Rev 1
1/78
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