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Fig 9. Ballast Operating Points for IRPLDIM3

The operating points are as follows:

Preheat Frequency

56.4 kHz

Ignition Frequency

52.3 kHz

Resonance Frequency

43.8 kHz

Run Frequency

54.3 kHz

Table 3: Practical Operating Points. Lamp type: TL5/28 W, Line Input Voltage: 90-305 VAC

In order to guarantee that the ballast is always capable of producing a high enough output voltage

to ignite the lamp, the minimum frequency set by CT and RFMIN should be set close to the resonance

frequency. If the minimum frequency is set too far above the resonance frequency, then during ignition the

frequency will ramp down to the minimum frequency and remain there indefinitely. The ballast would not

be able to operate correctly as it would never reach a low enough frequency to enable the ignition

regulation to function, and would never produce enough voltage at the output, to ignite the lamp. In this

design CT is 1 nF and RFMIN is 15 k

giving a minimum frequency of approximately 40 kHz and a dead

time of 1.5 µs, which are acceptable values.

The value of RPH sets the preheat frequency and has been selected at 36 k . Since the minimum

frequency is now below the run frequency, in order to prevent over driving the lamp, it is important to set

the maximum lamp output. The operating range is defined by the values of RLS, R18 and R19. RLS is

chosen to be as large as possible, to produce a sufficient feedback voltage, without dissipating too much

power. A value of 22

has been calculated to produce an average feedback voltage of 1.5 V when the

lamp current is at its maximum value of 140 mA (RMS), with a power dissipation of 100 mW.