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THE L6569: A NEW HIGH VOLTAGE IC DRIVER FOR
ELECTRONIC LAMP BALLAST
by G. Calabrese and T. Castagnet
INTRODUCTION
Electronic lamp ballasts are now popular in both
consumer and industrial lighting. They offer power
saving, flicker free operation and reduced sizes.
Improvements to the light control and cost reduc-
tion of the ballast will broaden their market accep-
tance.
Today designers focus on reducing the cost of the
ballast, but also work to add features to the bal-
last like saving energy by dimming the light, or in-
creasing the life time with better preheat and pro-
tections. Such requirements have contributed to
the development of dedicated high voltage con-
trollers like the L6569, which are able to drive the
floating transistor of a symmetric half bridge in-
verter. This device is a simple, monolithic oscilla-
tor-half bridge driver that allows quick design of
the ballast.
HIGH VOLTAGE IC DRIVERS IN BALLAST AP-
PLICATIONS
The voltage fed half bridge
Voltage fed series resonant half bridge inverters
are currently used for Compact Fluorescent Lamp
ballasts (CFL), for Halogen Lamp transformers,
and for many European Tube Lamp (TL) ballasts.
This simple converter is preferred for new de-
signs, because it minimizes the off state voltage
of the power transistors to the peak line voltage,
and requires only one resonant choke. In addition
this choke protects the half bridge against short
circuits across lamp terminals. However overheat-
ing and overcurrent occur during open load op-
eration. The inverter robustness must be im-
proved, or some protections are required.
The half bridge inverter operates in Zero Voltage
Switching (ZVS) resonant mode [1], to reduce the
transistor switching losses and the electromag-
netic interference generated by the output wiring
and the lamp.
Fully integrated ballast controllers
By varying the switching frequency, the half
bridge inverter is able to modulate the lamp
power. However most current designs use a sin-
gle frequency with a saturable pulse transformer
(see fig. 1) to drive the transistors. This type of
design has a higher component count, a higher
tolerance on the switching frequency, and it can-
not adjust the lamp power.
The only way to design a cost effective, compact
and smart control of the lamp is to use a dedi-
cated I.C. that is able to drive the upper transistor
of an symmetric half bridge inverter. Such control-
lers require a high voltage capability for the float-
ing transistor driver [2]. MOSFETs are preferred
over Bipolar transistors as power switches be-
cause their gate driver requires a lower supply
current and a smaller silicon size [3].
February 2003
®
Figure 1: CFL series resonant half bridge inverter.
2
µ
s/dv ; 50 V/dv ; 0.1 A/dv
I
D
V
DS
RF
LV G
GND
GND
GND
Figure 2: Current and voltage of the STD3NA50
MOSFETs when driven in ZVS with
the L6569.
AN880
APPLICATION NOTE
1/14
