Variable capacitor with very fine resolution

• US 5,463,285 A
• Filed: 03/14/1994
• Issued: 10/31/1995
• Est. Priority Date: 03/14/1994
• Status: Expired due to Fees

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: El-Hamamsy, Sayed-Amr
• Primary Examiner(s): Pascal, Robert J.
• Assistant Examiner(s): Ratliff, Reginald A.
• Application Number:
  US08/209,290
• Time in Patent Office: 596 Days
• Field of Search: 315/248, 315/209 R, 315/290, 315/240, 315/DIG. 4, 315/307
• US Class Current: 315/248
• CPC Class Codes: H03J 2200/10 Tuning of a resonator by means of digitally controlled capacitor bank
• CPC Class Codes: H05B 41/2806 with semiconductor devices and specially adapted for lamps without electrodes in the vessel, e.g. surface discharge lamps, electrodeless discharge lamps
• CPC Class Codes: Y02B 20/00 Energy efficient lighting technologies, e.g. halogen lamps or gas discharge lamps
• CPC Class Codes: Y10S 315/04 Dimming circuit for fluorescent lamps

Abstract

The resolution ΔC of a binary capacitance ladder having a plurality of N branches is precisely determined by appropriately selecting the value of a fixed capacitor in each branch relative to the parasitic capacitance of a corresponding series-connected switch. The capacitance of each respective branch varies by 2^n-1 ΔC, where n is the number of the branch, adjacent branches being numbered sequentially. The total capacitance is variable between C_Total and C_Total -(2^N -1)ΔC in steps of ΔC, where C_Total is the sum of the capacitances in the ladder with all switches closed, and C_Total -(2^N -1)ΔC is the capacitance of the ladder with all switches open. For any particular branch of the ladder, the switch capacitance may be further controlled by adding a fixed capacitance in parallel with the respective switch. Such a variable capacitance is useful as a tuning capacitor in an electrodeless HID lamp ballast.

Claims

1. A variable capacitor, comprising:

a binary capacitance ladder comprising a plurality N of parallel connected branches and having a predetermined resolution Δ

C, each of said branches comprising a fixed capacitor of capacitance C_n coupled in series with a switch having a parasitic capacitance C_n '"'"' when open, adjacent branches being numbered sequentially, n being the number of a respective branch, said binary capacitance ladder having a maximum total capacitance C_Total given by;

space="preserve" listing-type="equation">C.sub.Total =C.sub.1 +C.sub.2 +. . .+C.sub.n,

each respective branch having a variation in capacitance Δ

C_n due to opening and closing of the respective switch S_n therein given by;

##EQU3## such that said binary capacitance ladder has a total capacitance variation between values of C_Total and C_Total -(2^N -1)Δ

C in steps of Δ

C, said variable capacitor further comprising an additional fixed capacitor coupled in parallel with at least one said switch, such that the capacitance C_n of said switch includes the capacitance of said additional fixed capacitor.

2. The variable capacitor of claim 1, further comprising a fixed-branch capacitor having a capacitance C_f coupled in parallel with said N parallel connected branches, such that said variable capacitor has a nominal capacitance given by:

space="preserve" listing-type="equation">C.sub.nom =C.sub.f +C.sub.Total -kΔ

C.sub.Total, where 0≦

k≦

1.

3. The variable capacitor of claim 2 wherein k=1/2.

6. The ballast of claim 3, further comprising a fixed branch capacitor having a capacitance C_f coupled in parallel with said N parallel connected branches, such that said variable capacitor has a nominal capacitance given by:

space="preserve" listing-type="equation">C.sub.nom =C.sub.f +C.sub.Total -kΔ

C.sub.Total, where 0≦

k≦

1.

7. The ballast of claim 6 wherein k=1/2.

4. A ballast for an electrodeless high intensity discharge lamp of the type having an excitation coil situated about an arc tube for exciting an arc discharge therein upon application of a radio frequency signal to said excitation coil, said ballast comprising:

at least one switching device;

a ballast driver coupled to said switching device for driving said switching device; and

a resonant circuit coupled to said switching device, said resonant circuit comprising said excitation coil and at least one variable capacitor, said variable capacitor comprising a binary capacitance ladder comprising a plurality N of parallel connected branches and having a predetermined resolution Δ

C, each of said branches comprising a fixed capacitor of capacitance C_n coupled in series with a switch having a parasitic capacitance C_n '"'"' when open, adjacent branches being numbered sequentially, n being the number of a respective branch, said binary capacitance ladder having a maximum total capacitance C_Total given by;

space="preserve" listing-type="equation">C.sub.Total =C.sub.1 +C.sub.2 +. . .+C.sub.n,

each respective branch having a variation in capacitance Δ

C_n due to opening and closing of the respective switch S_n therein given by;

##EQU4## such that said binary capacitance ladder has a total capacitance variation between values of C_Total and C_Total -(2^N -1)Δ

C in steps of Δ

C.

5. The ballast of claim 4, further comprising an additional fixed capacitor coupled in parallel with at least one said switch, such that the capacitance C_n of said switch includes the capacitance of said additional fixed capacitor.