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RD0803

Lamp Preheating

The lamp must be sufficiently preheated before ignition. The correct preheat current

can be determined from published data or from International Rectifiers Ballast Designer

software.

The preheat time can be set by adjusting the value of CPH. As a general rule the lamp

filament should glow red before ignition. If preheat is insufficient the ballast is likely to

shutdown during ignition because the output inductor will be unable to operate at the

high current required. The number of turns in the auxiliary cathode windings of the

output inductor LRES should be chosen to provide sufficient preheat. In designs for

ballasts with integral lamps the shutdown pin can be grounded so that the inductor may

saturate without shutting down the circuit.

The lamp filament (Cathode) resistance over the range of dimming levels should be

between 3 and 5.5 times the resistance when cold. A simple method for determining

the hot resistance is to first connect one cathode to a DC power supply via an ammeter

and slowly increase the voltage from zero, noting the current at 1V intervals. This

should be done until the cathode can be seen to be glowing red. When this occurs the

voltage should not be increased further in order to prevent possible cathode damage.

The resistance can then be calculated for each voltage and hence the acceptable

voltage range can be found to comply with the 3 to 5.5 times cold resistance, which can

be easily measured with a digital multi-meter (DMM).

Then when the ballast is being run a true RMS digital voltmeter can be connected

across one cathode and the voltage can be observed at maximum and minimum

brightness. The cathode voltage increases as the ballast is dimmed. The values of

CH1 and CH2 will control how much it increases by, reducing the capacitance will

reduce the amount by which the voltage rises. The values should be chosen to prevent

the voltage exceeding the upper limit at minimum output.

It is important to consider that using additional windings on the inductor to provide

cathode heating means that power is now being transferred through the core and

consequently the core losses will increase and hence the core operating temperature.

The core will reach its highest operating temperature when the ballast is running at

minimum brightness.