INTEGRATED CIRCUIT WITH A PREHEAT CONTROL FOR A BALLAST
The present invention provides a ballast with preheat function for fluorescent or compact fluorescent lamps. The lamp is connected in series with an inductor and a capacitor to form a resonant circuit. A first switch and a second switch controlled by control circuit are coupled to the resonant circuit for switching the resonant circuit. A RC circuit is composed of a first resistor and a second resistor connected in series to form a voltage divider, and a capacitor is connected in parallel with second resistor. Switching frequency is voltage dependent.
1 . A ballast circuit, comprising:
a resonant circuit, comprising a lamp, an inductor and a capacitor connected in series;
a half-bridge inverter, coupled to the resonant circuit and comprising a first switch and a second switch connected in series;
a RC circuit, comprising a capacitor, a first resistor and a second resistor, for providing a voltage signal for frequency control; and
an integrated circuit, coupled to the half-bridge inverter and the RC circuit for generating a first switching signal and a second switching signal to drive the half-bridge inverter in response to the voltage signal received by the RC circuit, and providing a sequence control, a frequency control and protections to the ballast circuit,
wherein the first switch and the second switch of the half-bridge inverter are complementarily switched on and off.
2 . The ballast circuit as claimed in claim 1 , wherein the first switch of the half bridge inverter is controlled by a first switching signal.
3 . The ballast circuit as claimed in claim 1 , wherein the second switch of the half-bridge inverter is controlled by a second switching signal.
4 . (canceled)
5 . The ballast circuit as claimed in claim 1 , wherein the first resistor and second resistor are connected in series and a capacitor is connected to the second resistor in parallel.
6 . The ballast circuit as claimed in claim 1 , wherein the first resistor and the second resistor connected in series also serves as a voltage divider.
7 . The ballast circuit as claimed in claim 6 , wherein the voltage signal depends on a ratio of the voltage divider.
8 . The ballast circuit as claimed in claim 1 , wherein the RC circuit is employed for gradually increasing the voltage signal in a transient state and stabilizing it to a steady state.
9 . The ballast circuit as claimed in claim 1 , further comprising:
a first threshold voltage corresponding to a first time period for a first switching frequency; and
a steady voltage corresponding to a second time period for sweeping the first switching frequency to a second switching frequency between the first time period and the second time period.
10 . The ballast circuit as claimed in claim 9 , wherein the first threshold voltage is lower than the steady voltage.