Application information TN22
8/14 DocID3768 Rev 5
2.3 Ignition step
When C2 is charged above the SCR triggering gate voltage (V
GT
), the SCR switches on.
The voltage across the TN22 gate to cathode junction is fixed to a negative value, which
allows a proper TN22 switch-off, with a high holding current (I
H
) level.
When the current reaches I
H
, the TN22 switches off and the ballast inductor generates a
high voltage pulse across the tube (see Figure 13). This over-voltage is clamped by the
TN22 to a value fixed by the breakdown voltage (V
BR
). A 1200 V to 1500 V level is
necessary to ensure a correct ignition of the fluorescent tubes.
Figure 14. Typical high voltage pulse of an electronic starter circuit
(P0130AA SCR, TN22)
If the lamp is not ignited after the first pulse, the starter circuit starts a new ignition
sequence. The pulse is regenerated until ignition of the tube lamp. If the lamp is not ignited
after several attempts, the starter circuit can automatically stop the ignition sequence.
Figure 15. Repetitive ignitions sequence
2.4 Lighting state
When the lamp is ignited, the capacitor C2 is discharged through the resistors R2, R3 and
R4. The voltage across the lamp remains lower than the D6 clamping voltage (V
CL
),
avoiding the triggering of the TN22. The starter circuit remains in stand-by mode.
I
T
(50 mA/div)
V
S
(200 V/div)
V
BR
1300 V≈
I
H
225 mA≈
f
BURST
= 50 Hz
V
S
(500 V/div)
I
T
(500 mA /div)
Mains voltage
Pulse
