01-12-2012, 04:12 PM
Filaments Detection Circuit
1Filaments.pdf (Size: 256.67 KB / Downloads: 40)
INTRODUCTION
One of the requirements of electronic ballasts is the filament failure / lamp
removal protection. When either filaments fails or the lamp is removed, the
ballast has to shut down to prevent a non-desirable condition, such as non-ZVS
or inductor saturation, from occurring extensively. These conditions may damage
some components in the ballast and may cause safety issues.
An electronic ballast designed using IR2520D has the filament failure / lamp
removal protection; however, it does not have an auto-restart feature where the
lamp is automatically restarted after the lamp exchange. There are other ICs in
the market that come with an auto-restart feature, but they are limited to lower
filament failure only, and for some cases, this is not adequate. This Application
Note suggests a circuit to detect and auto-restart either filament failures. The
proposed circuit is designed around the IR2520D to show that both, upper and
lower, filaments failure can be successfully detected, and the lamp is autorestarted.
ELECTRONIC BALLAST USING IR2520D
The schematic does not include the EMI filter and the rectifier stage since they
are not important in the discussion about the protection. The supply voltage VCC
for the IR2520D is mainly derived from the current supplied from the VBUS
through the resistor RSUPPLY. The charge pump, containing CSNUB, DCP1
and DCP2, provides auxiliary supply to VCC.
The open filament protection relies on the non-ZVS circuit of the IR2520D,
enabled in the RUN mode when pin VCO reaches 4.8V (See State Diagram,
page 3). When one of the filaments failed, hard-switching will occur at the halfbridge
and the non-ZVS circuit inside the IR2520D will detect this condition,
increase the frequency each cycle and go into Fault Mode when VCO decreases
below 0.82V.
The IR2520D, however, does not have an auto-restart feature that will allow the
lamp to restart automatically after the lamp exchange. After VCO decreases
below 0.82V, the IC is latched in the Fault mode since VCC is still kept above
VCCUV- by the current supplied from RSUPPLY. In order to restart the lamp,
VCC must be recycled below and back above the UVLO threshold by turning off
the power to the ballast and turning it back on.
BALLASTS WITH AUTO-RESTART FEATURE
There exist electronic ballast ICs in the market that have the auto-restart feature.
These ICs use the lower filament of the lamp to detect lamp removal. The lower
filament, when intact, will pull down one pin of the IC. When the filament fails or
the lamp is removed, this pin will be pulled-up by a resistor, signaling the IC to
shut down and go into the UVLO mode.
CONCLUSIONS
One of the requirements of the electronic ballast is the filament failure / lamp
removal protection. An electronic ballast designed using IR2520D has the
filament failure / lamp removal protection; however, it does not have an autorestart
feature where the lamp is automatically restarted after the lamp exchange.
Other ICs in the market are able to detect either filament failure, but only autorestart
when the lower filament is back intact. In this Application Note, a circuit to
detect either filament failures and to auto-restart after either filament back intact
is proposed and is successfully evaluated. An additional section that describes
how to detect all filaments in the dual lamp series configuration is also given.