VK05CFL STMicroelectronics, VK05CFL Datasheet - Page 5

• Manufacturer Part Number: VK05CFL
• Description: Electronic Driver For Cfl Application
• Manufacturer: STMicroelectronics

Functional description
When the circuit is supplied, the capacitor C8 is charged by the resistor R2 till the voltage across it
reaches the internal diac threshold value (~ 30V). The low side switch is turned ON and consequently
current will flow from the HV rail to ground through the path formed by C3//C2, C4 and Lp (in case that the
pre-heating network is not present: PTC and C13 are not connected). The voltage drop on Lp is
“transferred” to the two secondary windings (wound in opposition) in order to confirm the ON state for the
low side device and the OFF state for the high side device. As soon as the low side device switches ON,
the capacitor C8 is discharged to ground by an internal HV diode to avoid diac restart.
In this preliminary phase the tube is OFF and the circuit will oscillate at the Lp-C4 series with (C3//C2)
resonance frequency
As this frequency is higher than the steady-state one, the two devices will switch ON-OFF at this
frequency, as the voltage on the two secondary windings falls below the voltage needed to keep the
device on.
As soon as the tube is ignited the resonance frequency is reduced (Lp-C3//C2) and the circuit will work
at the steady-state frequency fixed by the two capacitors C5 and C6.
It is possible to calculate the steady-state frequency by these formulae:
Considering the VK05CFL board: R=12K ; C5=C6=1.2nF; t
the working frequency will be: f
In figure 2 and figure 3, the start-up phase without preheating is reported, while in figure 4 the main
waveforms in steady-state are shown.
Figure 2: Start-up phase
f
1
-- - T
2
f
T
st up
on
=
–
=
-- -
T
1
=
T
=
R C
on
---------------------------- -
2
+
L c C 4
t
1
storage
ln
5
-- -
2
+
we can neglect C3//C2
t
dv
dt
35KHz.
I
(R = internal impedance)
device
midpoint
storage
400nsec
; C7=680pF
t
(dv)/(dt)
VK05CFL
800nsec
5/14
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