Patent ID: 6043612
Filing Date: 2000-03-28
Classification: H05B,Y10S

Abstract:
An electronic ballast (1) for operation of low-pressure gas discharge lamps (2), comprising:a DC voltage source (3) which serves to supply current to at least one gas discharge lamp (2), which has two filaments (22,25) as electrodes;at least one half bridge (6), which is connected to the DC voltage source (3) and supplies an AC voltage at an output terminal (16), wherein said output terminal (16) is connected to the at least one gas discharge lamp (2) via coupling means (L1, C4);a driving circuit (11), which is provided for the half bridge (6), is connected via control terminals (12, 13) to the half bridge (6) and drives the latter at a settable frequency, and which has a supply voltage input connected to a supply voltage (Vcc);wherein when the supply voltage exceeds a threshold value (UVLO) the driving circuit (11) adopts an active operating mode in which it drives the half bridge (6) at a given frequency, andwherein when the supply voltage (Vcc) falls below the threshold value (UVLO) the driving circuit (11) adopts a passive operating mode in which the half bridge (6) blocks;a first controlled switch (32) with a self-holding characteristic which is connected to the supply voltage (Vcc) against frame (4), in order to reduce said voltage below the threshold value (UVLO) when it is closed,wherein the first controlled switch (32) is connected in series with a circuit (33) generating an essentially fixed potential offset when the switch (32) is closed, and is closed by a first monitoring circuit (27) which detects an impermissible state at the at least one gas discharge lamp (2), with the result that the supply voltage (Vcc) is lowered to below the threshold value (UVLO) but not to zero, andwherein there is connected against frame (4) to the supply voltage (Vcc) a second controlled switch (34), which has a non-conducting and an at least limitedly conducting state and which is connected to a second monitoring circuit (35), which detects a flow of current through at least one filament (25) of the at least one gas discharge lamp (2), in such a way that the switch (43) lowers the supply voltage (Vcc) of the driving circuit (11) further than the first controlled switch (32) when no flow of current through the filament (25) is detected.