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Functional Description

The circuit is centered around the IR51H420 Ballast Driver Hybrid which contains the IR2151 Ballast

Driver IC and two 500 volt size 2 HEXFET’s in a half bridge configuration. With a 120 volt AC line

input (AC1-N), the voltage is rectified and doubled to provide a bus voltage of approximately 300

volts. With a 220 volt AC line input (AC1-AC2), the voltage is rectified but not doubled and again

provides a bus voltage of approximately 300 volts.The start up resistor R2 is sized such that it can

supply enough current to start the oscillator in the IR51H420 but not enough to cause the shunt

clamp to regulate and maintain constant oscillation. With this constraint the power dissipation in

resistor R2 is low enough so that a 1/4 watt unit will suffice. A charge pump circuit, consisting of

capacitor C10 and diodes D5 and D6, is used so that when the IR51H420 begins to oscillate, the

charge pump circuit supplies the current to increase the voltage on Vcc to cause the shunt clamp to

regulate. If the lamp is removed from the circuit there is no longer a path for the charge pump

capacitor C10. This causes the voltage at Vcc of the IR51H420 to begin falling. When the voltage at

Vcc of the IR51H420 falls below the negative undervoltage lockout threshold the oscillator stops

switching. At this point the voltage will begin to rise again and when the voltage reaches the positive

undervoltage lockout threshold the IC again begins to oscillate. If there is no lamp installed in the

circuit there will be no path for the charge pump circuit to supply current and the voltage at Vcc will

again fall below the negative undervoltage lockout threshold. The circuit will continue this sequence

indefinitely until the power is removed or a lamp is reinserted into the circuit. If a lamp is reinserted

into the circuit, the lamp will light.

To provide long life and to insure soft-starting of the lamp, the cathodes must be pre-heated so that

their hot resistance is approximately three to four times that of the cold resistance value. This is

performed by using a three step start-up sequence; the three steps being three oscillator frequency

settings. The oscillator is started at a frequency well above the resonant point of the LC circuit

formed by inductor L1 and capacitor C9. This is done to insure that the initial voltage applied across

the lamp is below the strike potential. The second frequency step, below step 1, was chosen to

provide a current through the cathodes large enough to heat them in the pre-heat time while also

maintaining the voltage across the lamp below the strike potential. The third step is to move the

oscillator to the final running frequency. At this point the voltage across the lamp becomes large

enough to strike the arc and the resonant point of the circuit shifts lower and the current in the lamp

is limited by the inductor L1.

The frequency shifting is accomplished by switching out different capacitors used to program the

oscillator frequency. The capacitors are switched out by shorting them with MOSFET’s which are

timed to turn on at different times. The pre-heat frequency is determined by the following formula:

)

6

)(

5

)(

6

(

4

.

1

6

5

C

C

R

C

C

f

ph

+

=