www.irf.com 7
IRPLMB1E
fmin (VCO > 4.6V), the non-ZVS and crest-factor pro-
tection will be activated and the frequency will in-
crease again to try and maintain ZVS. The frequency
will sweep back through resonance (from the ca-
pacitive side) and the crest-factor protection will shut-
down the IC on the first event when the inductor satu-
rates to a level where the crest factor exceeds 3 (see
Fig. 7.1).
Fig. 7.2 shows pin LO, pin VS and the current in the
resonant inductor during shutdown, with a shorter
time scale. The final shortened pulse of LO just be-
fore shutdown (Fig. 7.2) occurs due to the internal
1us blank time of the crest-factor detection during
each turn-on rising edge of LO (to provide immunity
to noise and transients).
Fig. 7.1:
4 is the current in the resonant inductor, 2 is the lamp
voltage, 3 is the voltage in pin VCO
Fig. 7.2:
4 is the current in the resonant inductor, 2 is pin VS (HB
Voltage), 3 is pin LO
7.2. Open Filaments Protection
The open filament protection relies on the non-ZVS
circuit of the IR2520D, enabled when pin VCO
reaches 4.6V. Should an open filament lamp fault
occur, hard-switching will occur at the half-bridge and
the non-ZVS circuit inside the IR2520 will detect this
condition, increase the frequency each cycle and shut
down when VCO decreases below 1V; both gate
driver outputs will be latched ‘low’. This will prevent
hard-switching and damaging of the half-bridge
MOSFETs.
Fig. 7.3 shows the pin VCO and pin VS at the shut-
down with open filament. As you can see, at startup
pin VCO charges from 0V up to 4.6V, at 4.6V the
non-ZVS circuit is enabled, CVCO discharges and
the frequency increases. When the voltage on pin
VCO decreases below 1V we have latched shutdown.
