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VK05CFL

Secondary Filter Design

The design of RC network applied on the sec pin of both devices has to be done taking into account the

following considerations:

1) The sec filtered voltage must reach the device ON threshold at the end of the negative dV/dt and before

the end of the freewheeling diode conduction in order to avoid hard switching or switching ON delay.

device supply voltage.

A good choice for time constant (

τ=RC) is in the range:1.5 µs ÷ 3.3 µs.

The resistor value has chosen in relation to the power dissipated on it during the start-up phase, the worst

condition is verified when the preheating is used.

Tube pre-heating

By using the VK05CFL, the tube pre-heating can be done with the classical solution with PTC (see

application schematic in figure 1) or with a more reliable low voltage network (see figure 5b). The pre-

heating low voltage network allows to obtain an optimum pre-heating avoiding the overstress on the PTC

thus improving the ballast reliability and the lamp life-time.

Figure 5a: Pre-heating phase with PTC

APPLICATION BOARD

Please note that this demo can be used for Europe (230Vrms) market as well as for USA (110Vrms)

market.

In order to use the demoboard for Europe market the following modification must be done: electrolytic

capacitors C1 and C12 must be replaced with only one electrolytic capacitor Cx = 3,3

µF/400V connected

with the positive pin on the D1 catode and the negative pin on the D3 anode. Also different power range

CFL can be driven by using this demoboard; on the left side of the component list reported below you find

component values able to drive CFL in the power range 5W to 15W, the component values written in

brackets in the table on the right are referred to the power range 15W to 23W.

sec

VK05CFL

Low side

osc

VK05CFL

High side

+

10

µ

39K

1.2n

1M

+

10

µ

39K

1.2n

1M

sec

osc

Figure 5b: Pre-heating low voltage network