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Figure 4: Lamp filament current during Startup,

Preheat, Ignition Ramp and Run (500mA/div)

Figure 5: Lamp filament voltage during Startup,

Preheat, Ignition Ramp and Run

Figure 6 shows a plot of the half-bridge oscillation frequency as a function of time for all of the normal

modes of operation: Preheat mode, Ignition Ramp mode and Run mode. As shown in Figure 6 there

is an initial startup frequency that is much higher than the steady state Preheat mode frequency that

lasts for only a short duration. Components CSTART and RSTART are used to program this initial

startup frequency. This is done to insure that the initial voltage appearing across the lamp at the

startup of oscillation does not exceed the minimum lamp ignition voltage. If, at the initiation of oscil-

lation of the half-bridge, the voltage across the lamp is large enough, a visible flash of the lamp

occurs which should be avoided. This in effect is a cold strike of the lamp which could shorten the life

of the lamp. An oscillograph of the lamp voltage at startup is shown in Figures 7 and 8 (next page).

Figure 7 shows the lamp voltage without the high initial startup frequency while Figure 8 shows the

lamp voltage with the high initial frequency startup.

f

osc

t

preheat

ignition

run

f

Preheat

f

Run

f

Ignition

Figure 6: Oscillator frequency versus time, Normal operating conditions