Switching power source

• US 7,394,670 B2
• Filed: 03/22/2005
• Issued: 07/01/2008
• Est. Priority Date: 03/23/2004
• Status: Active Grant

• Current Assignee: Sanken Electric Company Limited
• Original Assignee: Sanken Electric Company Limited
• Inventors: Koike, Kengo
• Primary Examiner(s): Han; Jessica
• Assistant Examiner(s): PHAM, EMILY P
• Application Number:
  US11/087,278
• Publication Number:
  US 20050213355A1
• Time in Patent Office: 1,197 Days
• Field of Search: 363/21.16, 363/19, 363/21.13, 363/21.18, 363/97
• US Class Current: 363/21.16
• CPC Class Codes: H02M 1/10 Arrangements incorporating converting means for enabling loads to be operated at will from different kinds of power supplies, e.g. from ac or dc
• CPC Class Codes: H02M 3/33523 with galvanic isolation between input and output of both the power stage and the feedback loop

Abstract

A control circuit 8 of a switching power source comprises a voltage detector 31 for detecting a voltage V_DC of a DC power supply 1 to produce a detection voltage V_DT; a comparator 41 for producing an output signal V_CP when detection voltage V_DT from voltage detector 31 exceeds a reference voltage V_R2; a bottom voltage detector 51 for detecting a bottom point of voltage V_DS across a MOS-FET 3 after energy has been discharged from transformer 2; and a switching controller 61 for selectively turning MOS-FET 3 on depending on existence or absence of the output signal V_CP from comparator 41. When the input voltage V_DC from DC power supply 1 so rises that detection voltage V_DT of voltage detector 31 is above the reference voltage V_R2, switching controller 61 serves to late turn MOS-FET 3 on at the time bottom voltage detector 51 detects the second or later bottom point of the voltage V_DS across MOS-FET 3, extending the off period of MOS-FET 3 to reduce switching frequency of MOS-FET 3.

Claims

1. A switching power source comprising a DC power supply;

a transformer having primary and secondary windings;

a switching element connected to the DC power supply through the primary winding of the transformer;

a rectifying smoother connected to the secondary winding of the transformer to produce a DC output voltage; and

a control circuit for producing drive signals to the switching element to turn the switching element on and off in order to keep the DC output voltage substantially constant,

said control circuit comprising a voltage detector for detecting a voltage of the DC power supply to produce a detection voltage;

a comparator for producing an output signal when the detection voltage from the voltage detector exceeds a reference voltage;

a bottom voltage detector for detecting a bottom point in the voltage across the switching element after energy has been discharged from the transformer; and

a switching controller for selectively turning the switching element on depending on existence or absence of the output signal from the comparator.

2. The switching power source of claim 1, wherein the switching controller serves to turn the switching element on at the time of detection of a first bottom point in the voltage across the switching element by the bottom voltage detector when the comparator does not produce the output signal, and

the switching controller serves to turn the switching element on at the time of detection of plural bottom points of the voltage by the bottom voltage detector when the comparator produces the output signal.

3. The switching power source of claim 1, further comprising an input rectifying circuit for converting AC voltage input from AC power source into DC voltage.

4. The switching power source of claim 1, wherein the switching controller serves to turn the switching element on at the time of detection of a first bottom point in the voltage across the switching element by the bottom voltage detector when the comparator produces the output signal, and

the switching controller serves to turn the switching element on at the time of detection of plural bottom points of the voltage by the bottom voltage detector when the comparator does not produce the output signal.

5. The switching power source of claim 1, wherein said comparator compares plural detection voltages from the input voltage detector with the reference voltage to produce plural output signals;

said switching controller serves to turn the switching element on at the time the bottom voltage detector detects plural bottom points in the voltage across the switching element in view of plural output signals from the comparator.

6. The switching power source of claim 1, wherein said comparator has hysteretic characteristics which comprises first and second different reference voltages to switch the output signal upon elevation and drop of the power voltage.

7. A switching power source comprising a DC power supply;

a transformer having primary, secondary and drive windings;

a switching element connected to the DC power supply through the primary winding of the transformer;

a rectifying smoother connected to the secondary winding of the transformer to produce a DC output voltage; and

a control circuit for producing drive signals to the switching element to turn the switching element on and off in order to keep the DC output voltage substantially constant,

said control circuit comprising a voltage detector for detecting a voltage produced on the drive winding during the on period of the switching element to produce a detection voltage;

a comparator for producing an output signal when the detection voltage from the voltage detector exceeds a reference voltage;

a bottom voltage detector for directly or indirectly detecting a bottom point in the voltage across the switching element after energy has been discharged from the transformer; and

a switching controller for selectively turning the switching element on depending on existence or absence of the output signal from the comparator.

8. The switching power source of claim 7, wherein said comparator has hysteretic characteristics which comprises first and second different reference voltages to switch the output signal upon elevation and drop of the power voltage.

9. The switching power source of claim 7, wherein said comparator compares plural detection voltages from the input voltage detector with the reference voltage to produce plural output signals;

said switching controller serves to turn the switching element on at the time the bottom voltage detector detects plural bottom points in the voltage across the switching element in view of plural output signals from the comparator.

10. The switching power source of claim 7, further comprising an input rectifying circuit for converting AC voltage input from AC power source into DC voltage.

11. A switching power source comprising an input rectifying circuit connected to AC power source for converting AC voltage from the AC power source into DC voltage;

a transformer having primary and secondary windings;

a switching element connected to the input rectifying circuit through the primary winding of the transformer;

a rectifying smoother connected to the secondary winding of the transformer to produce a DC output voltage; and

a control circuit for producing drive signals to the switching element to turn the switching element on and off in order to keep the DC output voltage substantially constant,

said control circuit comprising a voltage detector for detecting a peak value, average or effective value in AC voltage from the AC power source to produce a detection voltage;

a comparator for producing an output signal when the detection voltage from the voltage detector exceeds a reference voltage;

a bottom voltage detector for detecting a bottom point in the voltage across the switching element after energy has been discharged from the transformer; and

a switching controller for selectively turning the switching element on depending on existence or absence of the output signal from the comparator.

12. The switching power source of claim 11, wherein said comparator has hysteretic characteristics which comprises first and second different reference voltages to switch the output signal upon elevation and drop of the power voltage.

13. The switching power source of claim 11, wherein the switching controller serves to turn the switching element on at the time of detection of a first bottom point in the voltage across the switching element by the bottom voltage detector when the comparator does not produce the output signal, and

the switching controller serves to turn the switching element on at the time of detection of plural bottom points of the voltage by the bottom voltage detector when the comparator produces the output signal.

14. The switching power source of claim 11, wherein said comparator compares plural detection voltages from the input voltage detector with the reference voltage to produce plural output signals;

said switching controller serves to turn the switching element on at the time the bottom voltage detector detects plural bottom points in the voltage across the switching element in view of plural output signals from the comparator.