IR2159S datasheet(14/25 Pages) IRF

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IR21592/IR21593(S)

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Functional Description

Phase Control

To understand phase control, a simplified model

for the ballast output stage is used (Figure 1). The

lamp and filaments are replaced with resistors,

with the lamp inserted between the filament

resistors (R1, R2, R3 and R4).

Rlamp

Vin

Figure 1, Dimming ballast output stage.

During preheat and ignition (Figure 2), the circuit

is a high-Q series LC with a strong input current to

input voltage phase inversion from +90 to -90

degrees at the resonance frequency. For operating

frequencies slightly above resonance and higher,

the phase is fixed at -90 degrees for the duration

of preheat and ignition. During dimming, the circuit

is an L in series with a parallel R and C, with a

weak phase inversion at high lamp power and a

strong phase inversion at low lamp power.

-30

-20

-10

152025

3540

4550

Frequency [kHz]

-100

-50

100

150

200

250

300

350

400

10%

50%

100%

50%

10%

PH/IGN

Figure 2, Typical output stage transfer function for

different lamp power levels.

In the time domain (Figure 3), the input current is

shifted -90 degrees from the input half-bridge

voltage during preheat and ignition, and

somewhere between 0 and -90 degrees after

ignition during running.

Zero phase-shift

corresponds to maximum power.

run

Vin

Iin

ph/ign

Iinrun

ph/ign

Figure 3, Typical ballast output stage waveforms.

When the phase is calculated and plotted versus

lamp power (Figure 4), the result is a linear dimming

curve, even down to ultra-low light levels where

the resistance of the lamp can change by orders

of magnitude.

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