Low-noise switching power supply having variable reluctance transformer

• US 4,994,952 A
• Filed: 09/20/1989
• Issued: 02/19/1991
• Est. Priority Date: 02/10/1988
• Status: Expired due to Fees

• Current Assignee: Electronics Research Group Incorporated
• Original Assignee: Electronics Research Group Incorporated
• Inventors: Silva, LeRoy F., Silva, Donald M., Fehsenfeld, Fred M. Sr.
• Primary Examiner(s): BEHA JR, WILLIAM
• Application Number:
  US07/410,142
• Time in Patent Office: 517 Days
• Field of Search: 363/17, 363/25, 363/26, 363/90, 363/95, 363/97, 363/98, 323/247, 323/249, 323/250, 323/254, 323/331, 323/335, 336/73
• US Class Current: 363/56.01
• CPC Class Codes: H01F 2029/143 with control winding for generating magnetic bias
• CPC Class Codes: H01F 2038/026 non-linear inductive arrangements for converters, e.g. with additional windings
• CPC Class Codes: H01F 29/14 with variable magnetic bias amplitude modulation by means of variable impedance element H03C1/08 ; magnetic amplifiers H03F; circuits for automatic telephonic communication H04M3/00
• CPC Class Codes: H02M 3/3372 of the parallel type
• CPC Class Codes: Y10S 174/17 in an electric power conversion, regulation, or protection system

Abstract

There is provided a low-noise switching power supply for producing a regulated output voltage from an unregulated supply. The power supply includes a main transformer having first, second and third legs. A primary winding associated with a first leg produces an alternating magnetic flux in the core. A load winding associated with a second leg produces the output voltage from the power supply. A control winding is associated with the third leg. The regulated output voltage is compared to a reference voltage and used to operate a control circuit which alternately short circuits the control winding so as to vary the reluctance pass in the transformer and maintain the output voltage at a substantially constant predetermined value. A feedback circuit including a coupling transformer connects the primary coil to the switching circuit to improve regulation and provide a "soft start" capability.

A second embodiment of the present invention utilizes a constant frequency source to drive the main switching transistors of the unregulated power supply instead of the self oscillating scheme.

Claims

1. A low noise switching power supply for producing a regulated output voltage from an unregulated supply to a load, comprising:

(a) a transformer including;

i. a transformer core including first, second and third core legs;

ii. a set of transformer windings associated with said transformer including;

a. a control winding associated with said first core leg;

b. a primary winding associated with said second core leg; and

c. a load winding associated with said third core leg for producing said output voltage; and

(b) control means for controlling said output voltage comprising;

i. switching means responsive to said unregulated supply for providing an oscillating voltage of substantially constant frequency to said primary winding;

ii. means for comparing the actual output voltage produced at said load winding with a reference representative of the desired output voltage value to produce an error signal representative of the difference between said actual output voltage and said reference; and

iii. means responsive to said error signal for short circuiting said control winding as required to maintain said output voltage at said desired value.

2. A constant frequency switching power supply for producing a regulated output voltage from an unregulated supply to a load, comprising:

(a) a transformer including;

i. a transformer core including first, second and third core legs; and

ii. a set of transformer windings associated with said transformer including;

a. a control winding associated with said first core leg;

b. a primary winding associated with said second core leg; and

c. a load winding associated with said third core leg; and

(b) control means for controlling said output voltage comprising;

i. a constant frequency source applied to a pair of switching transistors in said unregulated supply for providing an oscillating voltage of substantially constant frequency to said primary winding independent of the load winding;

ii. means for comparing the actual output voltage produced at said load winding with a reference representative of the desired output voltage value to produce an error signal representative of the difference between said actual output voltage and said reference; and

iii. means responsive to said error signal for short circuiting said control winding as required to maintain said output voltage at said desired value.

3. The constant frequency switching power supply of claim 2, wherein said short circuiting means short circuits said control winding when the switching transistors of said unregulated supply are not conducting current, the flux in the transformer being changed while the unregulated supply is not providing said oscillating voltage.

4. The constant frequency power supply of claim 3, wherein said short circuiting means discontinues short circuiting of said control winding when one of the switching transistors of said unregulated supply is conducting current.

5. The constant frequency switching power supply of claim 4, including a dead band between the conduction and nonconduction a current in the switching transistors of said unregulated supply.

6. The constant frequency switching power supply of claim 5, said short circuiting means initiating short circuiting of said control winding during the dead band and removing said short circuiting when one of the switching transistors resumes conducting current.

7. The constant frequency switching power supply of claim 2, said constant frequency source further comprising:

(a) a clock having a specified frequency; and

(b) means for driving the switching transistors of said unregulated supply, said driving means being controlled by said clock.

8. The constant frequency switching power supply of claim 7, including a means for enabling said driving means to oscillate only when the control means are powered up and operating properly.

9. The constant frequency switching power supply of claim 8, including an over voltage protection means which interrupts the enabling means and causes the driving means to cease operating when the output voltage of the supply exceeds its rated value.

10. The constant frequency switching power supply of claim 2, including a means for determining current at the load winding and signaling the control means to reduce the output voltage of the supply when the rated value of the load current is exceeded.

11. The constant frequency switching power supply of claim 2, said regulated output voltage being substantially five volts or twelve volts.