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EFFICIENT SAFETY CIRCUIT FOR ELECTRONIC BALLAST

IP.com Disclosure Number: IPCOM000008580D
Original Publication Date: 1998-Mar-01
Included in the Prior Art Database: 2002-Jun-25
Document File: 4 page(s) / 195K

Publishing Venue

Motorola

Related People

Michael Bairanzade: AUTHOR

Abstract

In electronic ballast circuits for fluorescent lamps, the self oscillant circuit, commonly used in the low cost half bridge converter, is prone to thermal runaway when the fluorescent lamp does not strike. As a consequence, either the switches are oversized to sustain such a fault condition, or the circuit includes a safety network to avoid this risk. Although several solutions are known which perform such a function, the one described in this paper is easy to implement and does not influence the normal operation of the converter.

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MOTOROLA Technical Developments

EFFICIENT SAFETY CIRCUIT FOR ELECTRONIC BALLAST

by Michael Bairanzade

INTRODUCTION

  In electronic ballast circuits for fluorescent lamps, the self oscillant circuit, commonly used in the low cost half bridge converter, is prone to thermal runaway when the fluorescent lamp does not strike. As a consequence, either the switches are oversized to sustain such a fault condition, or the circuit includes a safety network to avoid this risk. Although several solutions are known which 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

  Typically electronic lamp ballast circuits com- prise a standard half bridge self oscillant converter to feed the lamp, together with a Power Factor Correction circuit in the front end. This topology makes profit of a RLC series resonant network. Such an arrangement can indefinitely sustain the open load condition (i.e. broken filament) since there is neither current flow nor voltage spikes in the circuit under this mode. When the lamp runs in steady state, the current is limited essentially by the impedance of an inductor in series with the lamp and, thanks to the free wheeling diodes, connected collector to emitter, 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 resonance of the RLC network, yielding large peak collector current and high voltage across the lamp. Usually, the lamp strikes rapidly, depending upon the temperature and the peak voltage applied across the electrodes. A typical four foot long tube needs 800V to strike, with a preheating time of around 500ms for the filaments. However, at the end of life, or under worst case conditions (low line voltage, negative ambient temperature, etc.), the

lamp may not strike and the circuit will continuously operate in the start-up mode, yielding maximum losses in the power transistors. Such level of losses generates heat which, unless the devices are heavily heatsunk, will increase the die temperature above the maximum rating in a few seconds. At this moment, the transistors are exposed to a high thermal runaway risk and TO220 packaged parts may blow up in less than two minutes. This time is shorter for smaller packages like the DPAK or the T092.

  To prevent such damage, the designer can either oversize the power transistors and the associated heatsinks (but this is costly) or include a safety network to switch off the converter when the fault condition is sensed. On top of that, local regulations may make such protection mandatory to avoid the risk of severe burns or fire to the module.

DESCRIPTION OF THE SOLUTION

  The schematic given in Figure I, partially reproduced in Figure 2, includes a safety circuit built with R8, DlO, Q4, and a sense network Cl6, D5,C10,R17,R16andDll.

  Basically, the stri...