1Motorola Applications Data

Niveau: Supérieur, Licence, Bac+1
1Motorola Applications Data Prepared by: Michael BAIRANZADE Power Semiconductors Application Engineer MOTOROLA – SPS – TOULOUSE INTRODUCTION The self oscillant circuit, commonly used in the low cost half bridge converter, is prone to thermal runaway when the fluo- rescent lamp does not strike. As a consequence, either the switches are over sized to sustain such a fault condition, or the circuit includes a safety network to avoid this risk. Although several schematics are usable to perform such a function, the one described in this paper is easy to implement and does not influence the normal operation of the converter. PROBLEM DESCRIPTION The schematic diagram of the evaluation board given Fig- ure 1 is built around a standard half bridge self oscillant con- verter to feed the lamp, together with a Power Factor Correction circuit in the front end. This topology makes profit of the RLC series resonant net- work. It can indefinitely sustain the open load condition (i.e. broken filament) since there are neither a current flow nor volt- age spikes in the circuit under this mode. When the lamp runs in steady state, the current is limited es- sentially by the impedance of the series inductor L1 and, thanks to the free wheeling diodes connected collector to emit- ter, there are no voltage spikes across the power transistors. The operation of the ballast is more complex during the start–up sequence, when the circuit operates close to the res- onance built with L1/C11/C12/R18, yielding large peak collec- tor current

• clamp diode


• series inductor


• diode dc


• improved high


• start–up clamp


• safety circuit


• dc current