www.irf.com RD-0602 16
During the Preheat mode the voltage across resistor RCS is not measured. However, at the end of
Preheat mode (the beginning of the Ignition Ramp mode) the hard-switching and over-current detec-
tion are enabled. During RUN mode, If at any time thereafter the voltage magnitude across resistor
RCS rises above the over-current threshold (1.3V), a lamp fault condition is signaled and the half-
bridge output MOSFETs’, (MHS and MLS) are turned off and the ballast goes into Fault mode. During
Ignition, a lamp fault condition is signaled only after 10 cycles to avoid triggering this protection in the
case of a current transient that can happen during normal ignition. An over-current condition can
occur if the lamp fails to ignite or the lamp is broken (an open circuit cathode or broken lamp).
Figure 7 shows the voltage across resistor RCS and the voltage appearing across the lamp when the
ballast detects a failure to ignite the lamp and goes into Fault mode. Figure 8 shows the voltage
appearing across the lamp during the tail end of the Preheat mode and the Ignition Ramp mode for a
failure of the lamp to ignite condition. If a cathode is broken (open circuit) the half-bridge output
hard-switches and each time the low side MOSFET (MHS) is turned on a large current pulse occurs
and thus a large voltage pulse occurs across resistor RCS signaling a fault, Figure 9 shows this
hard-switching condition. Figure 10 shows the lamp voltage during the Preheat mode and beginning
of Ignition Ramp mode for this hard-switching condition when the lamp fault condition is detected.
The ballast will remain in Fault mode until either the line voltage is reset or a lamp replacement is
performed.
Figure 7: Failure of lamp to ignite condition
(lamp filaments good): Upper trace: voltage
across RCS, Lower trace: lamp voltage
Figure 8: Failure of lamp to ignite condition
(lamp filaments good): Lamp voltage during
end of Preheat and Ignition Ramp modes
