EFS

7/14

Present solutions work with a unidirectional

switch, like MOS transistors or GTO (Gate Turn

Off thyristor). As a starter is a bi-directional switch,

it is necessary to use a rectifier bridge (4 diodes of

1500V ). More, 2 or 3 diodes in series with the

GTO are required to get the necessary switch off

effect, and the whole is controlled with an analog

timer built around a small SCR.

The interest of a bi-directional switch arises itself:

a drastic reduction of the number of components,

and of course, a reduction of power losses (only 1

forward voltage instead of 5).

A lamp requires a minimum energy level to be ignited, but this energy depends especially on the lamp

temperature. The lower the temperature is, the more energetic the lamp strike is.

This energy stored in the ballast is directly proportional to the Switched Off current

IE

LI

SO

SO

(

.

)

=

2

2

.

temperature. Results based on experiments show that it is necessary to switch off a current of 350 mA to

strike a 58W tube at -30

°C (with a voltage amplitude clamped at 1200V and starting aid).

for all the temperature range. Unfortunately, the solution is not so simple to implement because the energy

level at ambient or warm temperature would be much important: the lamp would be ignited, but the lamp

lifetime would be shortened. This is why one of the innovations of the ASD

™ is to modulate the striking

energy versus temperature (see feature hereafter).

3/ LAMP IGNITION FEATURE:

level decreases slowly.

GTO

SCR

1500V

Conventional discrete circuit (minimal version)

A.S.D.

™ FEATURE

I.S.O.

350mA

200mA

Switched off current

versus ambient temperature