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8. Functional Description

The design of a dimmable ballast for linear lamps requires that the lamp power can be reduced

smoothly to a low output level, less than 5% of nominal lumen output. It is necessary that the lamp output

is able to be held at any level and that there will be no discernable flicker or instability at any level. In order

to accomplish this, a closed loop control scheme is required. The fluorescent lamp represents a complex

load, the impedance of which changes depending on the arc current and temperature of the gasses within

the lamp. Like all discharge lamps, the fluorescent lamp displays a negative resistance behavior, meaning

that as the current in the lamp increases the effective resistance of the lamp will reduce. The lamp

impedance is strongly dependent on the arc current, however the relationship is not linear and temperature

is also a factor.

The IRPLDIM3 ballast as shown in the block diagram of Fig. 1 supplies the lamp through a

resonant output circuit. In this design the lamp power is adjusted by changing the oscillation frequency of

the MOSFET half bridge, which is driven by the IRS2158D. In this system the lamp current against ballast

frequency characteristic of the system exhibits a sharp “knee” characteristic such that as the frequency

increases the lamp current is gradually reduced up to a point at which a small increase of frequency will

result in a large reduction in the lamp current.

Lamp

Current

Ballast Running

Frequency

Ballast / Lamp Operating Characteristic

Fig 6. Lamp Current against Ballast Frequency

Consequently it becomes necessary to include a stabilized closed loop feedback system to control

the lamp current by adjusting the ballast frequency by means of a voltage controlled oscillator (VCO). In

this example the VCO within the IRS2158D is driven by the output of an error amplifier and compensation

network that senses the lamp arc current directly and compares it with a reference.