Switching power supply

• US 6,064,580 A
• Filed: 03/05/1999
• Issued: 05/16/2000
• Est. Priority Date: 03/09/1998
• Status: Expired due to Term

• Current Assignee: Shindengen Electric Manufacturing Company Limited
• Original Assignee: Shindengen Electric Manufacturing Company Limited
• Inventors: Watanabe, Haruo, Hatakeyama, Haruhiko
• Primary Examiner(s): Tso, Edward H.
• Assistant Examiner(s): Patel, Rajnikant B.
• Application Number:
  US09/262,917
• Time in Patent Office: 438 Days
• Field of Search: 363/17, 363/16, 363/15, 363/132, 363/127, 363/98, 363/89
• US Class Current: 363/17
• CPC Class Codes: H02M 3/33592 having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

A switching power supply has first and second switching elements that are alternately turned on to cause an alternating current to flow through a primary winding of a transformer. The alternating current flowing through the primary winding induces voltages respectively across first and second secondary windings of the transformer. Synchronous rectifying transistors are turned on by the voltages induced across the first and second secondary windings, causing a current to flow alternately through the first and second secondary windings. Either one of the first and second switching elements is turned on while the other is being turned off, except for short off-times in which both the first and second switching elements are prevented from being turned on. As a result, when currents flow through the first and second secondary windings, the synchronous rectifying transistors are turned on. Therefore, no current flows through internal parasitic diodes of the synchronous rectifying transistors, and hence any loss caused thereby is very small.

Claims

1. A power supply circuit comprising:

first and second choke coils;

first and second capacitors;

first and second switching elements each comprising a MOSFET;

first and second synchronous rectifying transistors each comprising a MOSFET;

a transformer having a primary winding and first and second secondary windings which are magnetically coupled to said primary winding;

said first and second switching elements being connected in series with each other at a junction, jointly making up a series-connected circuit, said primary winding and said first choke coil having respective terminals connected to said junction between the first and second switching elements;

said first and second capacitors being connected in series with each other at a junction, jointly making up a series-connected circuit, said primary winding having another terminal connected to said junction between the first and second capacitors;

said series-connected circuit of the first and second switching elements and said series-connected circuit of the first and second capacitors being connected parallel to each other, jointly making up a parallel-connected circuit; and

a power supply, operably connected to a terminal of said parallel-connected circuit and another terminal of said first choke coil, for applying a voltage between said terminal of said parallel-connected circuit and said another terminal of said first choke coil

wherein when said first and second switching elements are alternately tuned on, an alternating current flows through said primary winding to induce alternating-current voltages respectively across said first and second secondary windings for thereby alternately turning on said first and second synchronous rectifying transistors to cause a current to flow alternatively through said first and second secondary windings for supplying a current to said second choke coil.

4. A power supply circuit according to any one of claims 1 through 3, wherein said first and second secondary windings have respective terminals connected to each other at a junction, said second choke coil having a terminal connected to said junction between said first and second secondary windings, and wherein said first and second secondary windings have respective other terminals connected to each other by said first or second synchronous rectifying transistor at a junction, with an output voltage being extracted between the junction between the other terminals of the first and second secondary windings and another terminal of said second choke coil, the arrangement being such that a voltage at the other terminal of the second secondary winding is applied to a gate terminal of one of said first and second synchronous rectifying transistors which is connected to said first secondary winding, and a voltage at the other terminal of the first secondary winding is applied to a gate terminal of one of said first and second synchronous rectifying transistors which is connected to said second secondary winding.

5. A power supply circuit according to claim 4, wherein either one of said first and second switching elements is turned on for an on-time except for a relatively short off-time in which both of said first and second switching elements are prevented from being turned on at the same time, said on-time being variable to keep constant the output voltage extracted from said second choke coil.

6. A power supply circuit according to claim 5, further comprising an output capacitor for smoothing said output voltage.

2. A power supply circuit comprising:

first and second choke coils;

first and second capacitors;

first and second switching elements each comprising a MOSFET;

first and second synchronous rectifying transistors each comprising a MOSFET;

a transformer having a primary winding and first and second secondary windings which are magnetically coupled to said primary winding;

said first and second switching elements being connected in series with each other at a junction, jointly making up a series-connected circuit, said primary winding and said first choke coil having respective terminals connected to said junction between the first and second switching elements;

said second capacitor being connected parallel to said series-connected circuit of the first and second switching elements, jointly making up a parallel-connected circuit;

said parallel-connected circuit having a terminal connected by said first capacitor to another terminal of said primary winding; and

a power supply, operably connected to said parallel-connected circuit, said first capacitor, and another terminal of said first choke coil, for applying a voltage between a junction between said parallel-connected circuit and said first capacitor, and said another terminal of said first choke coil,

wherein when said first and second switching elements are alternately turned on, an alternating current flows through said primary winding to induce alternating-current voltages respectively across said first and second secondary windings for thereby alternately turning on said first and second synchronous rectifying transistors to cause a current to flow alternatively through said first and second secondary windings for supplying a current to said second choke coil.

3. A power supply circuit comprising:

first and second choke coils;

first and second capacitors;

first and second switching elements each comprising a MOSFET;

first and second synchronous rectifying transistors each comprising a MOSFET;

a transformer having a primary winding, first and second secondary windings which are magnetically coupled to said primary winding, and a tertiary winding which is magnetically coupled to said primary winding and said first and second secondary windings;

said primary winding and said first capacitor being connected in series with each other, jointly making up a series-connected circuit, said first switching element being connected parallel to said series-connected circuit of the primary winding and the first capacitor, jointly making up a parallel-connected circuit;

said parallel-connected circuit having a terminal connected to a terminal of said first choke coil; and

a power supply, operably connected to another terminal of said parallel-connected circuit and another terminal of said first choke coil, for applying a voltage between said another terminal of said parallel-connected circuit and said another terminal of said first choke coil;

said tertiary winding and said second capacitor being connected in series with each other, jointly making up a series-connected circuit, said second switching element being connected parallel to said series-connected circuit of the tertiary winding and the second capacitor,

wherein when said first and second switching elements are alternately turned on, an alternating current flows through said primary winding to induce alternating-current voltages respectively across said first and second secondary windings for thereby alternately turning on said first and second synchronous rectifying transistors to cause a current to flow alternately through said first and second secondary windings for supplying a current to said second choke coil.