VK05CFLTR-E Datasheet VK05CFLTR-E | P4-P6

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VK05CFL

APPLICATION DESCRIPTION

Technology Overview

The VK05CFL is made by using STMicroelectronics proprietary VIPower M3-3 technology. This

technology allows the integration in the same chip both of the control part and the power stage. The power

stage is the “Emitter Switching”. It is made by putting in cascode configuration a bipolar high voltage

darlington with a low voltage MOSFET. This configuration provides a good trade-off between the bipolars

low ON drop with high breakdown voltage in OFF state, and the MOSFETS high switching speed. The

maximum theoretical working frequency is in the range of 300KHz.

Circuit description

The electrical scheme of the VK05CFL used as a self-oscillating converter to drive fluorescent tubes is

shown in Fig. 1.

Figure 1: Application schematic

This topology does not require the saturable transformer to set the working frequency. Two secondary

windings are wound on the main ballast choke Lp. These windings have two functions:1) to trigger the ON

state and 2) to provide the power supply to the device. A good trade-off for the ratio between the primary

winding Lp and the two secondary windings is 10:1; in order to minimize the power dissipated on the

resistors R4 - R5 and to guarantee sufficient voltage to supply the device.

The steady-state working frequency is set by the two capacitor C5 and C6. They are charged by a current

cap

≈300µA. When the voltage on the capacitor reaches an internal fixed value the power stage is turned

OFF. By choosing the same value for C5 and C6 the circuit will work with a duty-cycle of 50%. During the

start-up, as the resonance frequency is higher than the steady-state frequency, the secondary voltage

falls lower than the device sustain voltage before the capacitor C5-6 is charged, switching OFF the device.

For this reason the circuit can work at different frequencies during the start-up and steady-state phases.

The resistor R2 and the capacitor C8 are needed to bias the internal diac in the low side device in order

to start-up the system. In the high side device the diac pin must be connected to the midpoint. R1 is the

pull-up resistor and C7 is the snubber capacitor.

Input filtering is realized by R4-C10 and R5-C11. It is necessary to have a proper supply voltage on the

input pin.

L1s

L2s

VK05CFL

Tube

C7 C3

C10

C11

VK05CFL

Bridge

Input Filter

C4 C13

PTC

di ac

sec

osc

Sour ce

Col lect or

di ac

sec

osc

Sour ce

Collector

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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 ≈(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:

(R = internal impedance)

Considering the VK05CFL board: R=12KΩ; C5=C6=1.2nF; t

storage

≈400nsec; C7=680pF⇒t

(dv)/(dt)

≈800nsec;

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

st up–

2π L

⋅

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

---

ln⋅⋅=

---

storage

dv()dt()⁄

++=

---

midpoint

device

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VK05CFL

Figure 3: Start-up phase

From figure 4 it can be observed that the value of secondary voltage decreases when the lamp current

increases. This happens because increasing the value of the current flowing through the tube, increase

the drop on it, consequently decreasing the voltage on the ballast inductor Lp and thereby decreasing also

the secondary voltage.

By inserting the filters (R4-C10; R5-C11) between the two secondary windings and the devices, it is

possible to guarantee a higher voltage on the input pin of the devices for longer time compared to the

secondary signal. In this way it is possible to extend the use of the VK05CFL to all the power range eg.5W

– 23W.

Figure 4: Steady state waveforms

midpoint

diac

midpoint

L2s

sec

lam

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VK05CFLTR-E

Mfr. #:

Buy VK05CFLTR-E

Manufacturer:

STMicroelectronics

Description:

Gate Drivers ELECTRONIC DRIVER

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VK05CFLTR-E