starts. The ballast has to start directly with its

nominal conditions to remove any transient oscil-

lation. Hence the operation runs in ZVS mode

with no spurious lamp ignition. This situation does

not occur with the saturable transformer drive, be-

cause the saturation limits naturally the current by

increasing the frequency.

In the example the resonant capacitors are pre-

set to be compatible with the choke current rise

(see figure 19). The blocking capacitor is pre-

resistors, and the lower Mosfet also discharges

the resonant capacitor to ground (see figure 20).

Therefore the blocking capacitor never goes

the operation is safe in ZVS mode. The L6569 is

here preferred to the L6569A, because the lower

Mosfet is on at power-up.

The lamp removal protection

Used in TL ballast, the lamp removal protection is

frequently also requested in the "plug-in" CFL bal-

last . Depending of the topology and the preheat

mode, the lamp removal behaves as:

- a noload resonant mode when the choke and

the capacitor are still connected to the in-

verter ; a required overcurrent protection in-

creases the frequency to reduce the current;

- an open circuit mode when the lamp filaments

are inserted in the resonant circuit.

When the circuit is open, the choke is not sup-

plied. The MOSFETs turn off slowly generating

bridge cross conduction, and undesirable dissipa-

tion losses (see figure 21). The detection stops

the switching to eliminate the cross conduction.

5

µs/dv ; 50 V/dv ; 0.5 A/dv

GND

GND

IDEAL INITIAL TIME

Figure 19: Waveforms of the choke current and

the capacitor voltages in steady state

preheat.

100 nF

4nF

ON

L6569

VS<UVLO

2.4 mH

2 x 180 k

Ω

Figure 20: Configuration of the resonant network during the initialization of the driver.

100 ns/dv ; 50 V/dv ; 5V/dv ; 1 A/dv

GND

GND

GND

Figure 21: Drain current and voltage STP8NA50

MOSFET operating with noload.

AN880 APPLICATION NOTE

9/14