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VK05CFL

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

we can neglect C3//C2

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

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:

(R = internal impedance)

Considering the VK05CFL board: R=12K

Ω; C5=C6=1.2nF; t

the working frequency will be: f

≈35KHz.

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

st

up

–

1

2

π L

c

C

4

⋅

-----------------------------

=

T

on

RC

5

2

---

ln

⋅⋅

=

1

2

---TT

on

t

storage

t

dv

() dt

()

⁄

++

=

f

1

T

---

=

midpoint