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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.