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Simple Dimming Circuit for Fluorescent Lamp

IP.com Disclosure Number: IPCOM000121464D
Original Publication Date: 1991-Sep-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 3 page(s) / 78K

Publishing Venue

IBM

Related People

Nakano, M: AUTHOR

Abstract

This article describes a simple dimming circuit for a hot cathode fluorescent lamp.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 70% of the total text.

Simple Dimming Circuit for Fluorescent Lamp

      This article describes a simple dimming circuit for a hot
cathode fluorescent lamp.

      Recently a back light for a liquid crystal display (LCD)
requires high luminance with a dimming function.  A hot cathode
fluorescent lamp (HFL) can be used for the back light in this
application.

      An HFL needs to keep filaments at a high temperature about
1000oK (Kelvin) for a stable discharge.  In a high brightness
condition, filaments heat themselves by the energy of discharge. On
the other side, in deep dimming condition, the discharging current is
so small that discharging energy is not enough to maintain filaments
at a high temperature.  Therefore, a forced heating circuit is
necessary.

      A conventional HFL inverter needs another inverter not to
discharge but to heat filaments.  This extra inverter works in the
deep dimming condition.

      The dimming effect is obtained by changing the ratio of
discharge-on time and discharge-off time.  Discharge ON/OFF frequency
should be designed between the inverter's oscillation frequency
(20-50 KHz) and flicker-free frequency (exceeding about 100 Hz).

      Fig. 1 shows the  circuit disclosed to control discharge
current of fluorescent lamp which consists of a heating circuit for
filaments.  F.L. is a hot cathode fluorescent lamp.  C (or L) is a
ballast capacitor (or inductor) to keep constant discharge current of
HFL with high voltage source.  S...