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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

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

Lp

C8

C7

C3

C2

C5

C6

R2

R1

C10

C11

R4

R5

VK05CFL

Bridge

+

Input Filter

C4

C13

PTC

diac

sec

osc

Source

Collector

diac

sec

osc

Source

Collector

1