Short circuit protection for multiple lamp LCD backlight ballasts with PWM dimming
A lamp ballast for energizing a lamp has short circuit protection. The lamp ballast contains a transformer that provides an output voltage to the lamp. The transformer has a primary winding, a secondary winding, and a third sense winding. The third sense winding has an output that provides an indication of transformer output voltage. A reference voltage circuit creates a reference voltage that provides an indication of a non-short circuited lamp ballast condition. A comparator compares the reference voltage and the output of the third sense winding to provide an indication if the third sense winding output is below the reference. This indicates if there is a short in the output of the lamp ballast. The secondary is wound on multiple sections of a bobbin with one section being a low voltage section. The sense winding is wound under the secondary in the bobbin secondary low voltage section. The ballast additionally has a track and hold circuit to prevent an indication of a short in the lamp voltage when the lamp voltage is dimmed with a dimming circuit. The ballast also has a preheat logic signal to prevent a short circuit indication during the preheat mode at startup.
Koninklijke Philips Electronics N.V.
H H
H05B H05B
inventional inventional
Roderick T. Chin
Hinman Chang