AN880 datasheet(7/14 Pages) STMICROELECTRONICS

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THE LAMP SEEN BY THE ELECTRONIC DE-

SIGNER

The lamp equivalent impedance

The compact fluorescent lamps are specified at

25 kHz (IEC 929). The MOSFETs and the L6569

allow to increase the switching frequency, but the

sensitivity of the lamp to the frequency needs to

be analyzed.

A few samples of the CF18DT/E lamp were

tested by varying the frequency and the current of

the lamp. The figure 14 shows the lamp imped-

ance versus its current as it varies from 0.1A to

0.23A with 5 frequencies from 25 to 150 kHz

(TAMB = 25°C).

From the tests the impedance appears insensitive

to the frequency for such lamps. The specified im-

pedance might be valid for higher frequency op-

eration. The relative lamp light output was meas-

ured as proposed in reference [5]. The light flux

increases slightly in that frequency range, but can

be considered constant.

Obviously the impedance is sensitive to the cur-

rent with a negative coefficient, and the ballast

operates with a non linear impedance [6]. When

current is half the nominal one, the impedance is

2.6 times higher, and the voltage is only 25%

higher (see figure 15).

The preheat

Preheat techniques are used in CFL ballasts to

reduce the ignition lamp voltage. During this

phase the lamp is characterized by a high imped-

ance that forces the electrical conduction through

the preheat filaments. These filaments initially

have a low resistance that will increase by 5 times

during the preheat. The preheat typically lasts

from 400 ms to 1 s, and is achieved by controlling

either the current or the voltage of the filaments.

For a current control the filaments are in series

with the resonant network as shown on figure

16a. When the inverter frequency is constant, a

positive temperature coefficient thermistor (PTC)

in parallel with the lamp achieves the task by ad-

justing both the filament current and the preheat

duration. The board uses a 150

Ω PTC with two

8.2 nF capacitors. The preheat lasts 0.8s and the

filament current is 0.45 Arms. The PTC is a cheap

device, but it is dissipative and works only once at

power-up.

The preheat can be achieved with a filament volt-

age control. The filaments are supplied by two

auxiliary windings of the resonant choke as

shown figure 17a. During the preheat the L6569

frequency is increased, and the choke operates

0.05

0.1

0.15

0.2

0.25

0.3

200

400

600

800

1000

1200

I lamp (A)

R lamp (Ohms)

25 kHz

50 kHz

100 kHz

150 kHz

Figure 14: Variation of the lamp impedance

versus its current for several

switching frequencies.

0.05

0.1

0.15

0.2

0.25

300

600

900

1200

100

150

200

I (A)

R (Ohms)

U (V)

Rlamp

Vlamp

Figure 15: Variation of the average impedance

and voltage of the lamp

I CTL

E=R.I²

(A)

(B)

I CTL

LAMP

Figure 16: Basic preheat current control diagram

(a); preheat filament energy curve (b)

AN880 APPLICATION NOTE

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