Optimal FM for HF operation of high intensity discharge (HID) lamps

Abstract

A low cost method for operating a high intensity discharge (HID) lamp including a ballast circuit. The method includes generating DC in an AC-to-DC converter, capturing any AC ripple of the DC with a buffer capacitor to generate a control signal, generating a high frequency lamp power signal from DC utilizing an HF inverter circuit and modulating the high frequency power signal utilizing the control signal to generate a frequency swept lamp power signal to drive the lamp while avoiding acoustic resonance.

What is claimed is:  
     
         1 . A ballast circuit for stable operation of a high intensity discharge (HID) lamp, comprising: 
 an AC-to-DC converter for converting power from an AC source to DC, the AC-to-DC converter including a buffer capacitor for capturing AC ripple riding on the DC;    a high frequency inverter and ballasting element (HFIBE) coupled to the AC-to-DC converter and to the HID lamp, the HFIBE including means for generating a lamp driving signal from the DC power drive said HID lamp;    a driver circuit coupled to the HFIBE circuit, the driver circuit including a voltage controlled oscillator (VCO) for generating a driving signal for driving the HFIBE; and    a frequency control circuit coupled to the buffer capacitor, the HFIBE and the driver circuit, wherein the buffer capacitor provides a control signal to the driver circuit, and wherein the VCO in the driver circuit responds to the control signal by controlling the HFIBE to output frequency swept square wave signal to power said HID lamp without driving said lamp into an acoustic resonant state.    
     
     
         2 . The ballast circuit set forth in  claim 1 , further comprising a power correction circuit between the AC-to-DC converter and the HFIBE.  
     
     
         3 . The ballast circuit set forth in  claim 1 , wherein the frequency control circuit is an RC circuit.  
     
     
         4 . The ballast circuit as set forth in  claim 1 , wherein the frequency control circuit is constructed as part of an integrated circuit.  
     
     
         5 . The ballast circuit as set forth in  claim 3 , wherein the frequency control circuit senses the voltage on the buffer capacitor and generates a current which varies in frequency with variance of the voltage in the buffer capacitor.  
     
     
         6 . The ballast circuit set forth in  claim 3 , wherein the frequency control circuit includes a buffer capacitor of approximately 22 nanofarads.  
     
     
         7 . The ballast circuit set forth in  claim 1 , wherein said lamp is a metal halide lamp.  
     
     
         8 . The ballast circuit of  claim 1 , wherein the average lamp power signal generated in the HFIBE includes a frequency of around 80 kHz and a power rating of approximately 39 Watts.  
     
     
         9 . The ballast circuit of  claim 8 , wherein the swept frequency signal out from the HFIBE to the lamp runs from about 75 to about 85 kHz.  
     
     
         10 . The ballast circuit of  claim 1 , where the frequency control circuit includes a capacitor and a first resistor connected in parallel, the parallel combination connected serially to a second resistor.  
     
     
         11 . The ballast circuit of  claim 10 , where the capacitor is around 22 nanofarads and the first and second resistors are about 158 kOhms and 169 kOhms, respectively.  
     
     
         12 . The ballast circuit of  claim 1 , wherein the AC-to-DC converter includes a full-wave rectifier.  
     
     
         13 . The ballast control circuit of  claim 1 , wherein the AC-to-DC converter includes an EMI protect circuit.  
     
     
         14 . A high intensity discharge lamp for stable operation including a very low cost (VLC) ballast circuit which captures a sawtooth-like AC ripple signal in a buffer capacitor to produce a control signal for frequency modulating an average lamp power signal used to drive the lamp outside a state of acoustic resonance, the ballast circuit comprising: 
 an AC-to-DC converter for connection to an AC power source, the AC-to-DC converter including said buffer capacitor;    a high frequency inverter and ballasting element coupled to the AC-to-DC converter and buffer capacitor for generating a square wave signal to drive said HID lamp; and    a coupling circuit for coupling a control signal generated by the buffer capacitor into a driver;    wherein a VCO within the driver responds to the control signal to generate a swept FM signal causing the HF inverter and ballasting element to drive the HID lamp such that it is maintained in a stable state.    
     
     
         15 . A low cost method for operating a high intensity discharge (HID) lamp comprising a ballast circuit, the method comprising the steps of: 
 generating DC in an AC/DC converter;    capturing any AC ripple of the DC with a buffer capacitor to generate a control signal;    generating a high frequency lamp power signal from DC utilizing an HF inverter circuit; and    modulating the high frequency power signal utilizing the control signal to generate a frequency swept lamp power signal to drive said lamp while avoiding acoustic resonance.    
     
     
         16 . The method set forth in  claim 15 , wherein the frequency modulated signal has a range from about 75 to about 85 kHz to operate the HID lamp without going into acoustic resonance.  
     
     
         17 . The method set forth in  claim 15 , wherein the control signal has a frequency range of around 10 to 1000 Hz.