Method and apparatus for voltage regulation in multi-output switched mode power supplies

• US 6,239,509 B1
• Filed: 03/30/2000
• Issued: 05/29/2001
• Est. Priority Date: 07/16/1999
• Status: Expired due to Term

• Current Assignee: Semtech Corporation
• Original Assignee: Semtech Corporation
• Inventors: Rader, William Edward III, Fogg, John
• Primary Examiner(s): Riley, Shawn
• Application Number:
  US09/539,275
• Time in Patent Office: 425 Days
• Field of Search: 323/282, 323/284, 323/224, 323/274, 363/16, 363/131, 307/11, 307/33, 307/34
• US Class Current: 307/11
• CPC Class Codes: H02M 1/009 having two or more independently controlled outputs for DC-DC converter with intermediate AC H02M3/33561
• CPC Class Codes: H02M 3/1563 without using an external clock H02M3/158 takes precedence

Abstract

A step-down switched-mode power supply circuit includes a transformer having at least one primary winding and at least one secondary winding, a current sensing device for sensing a current through a primary winding of the transformer, a first switch and a second switch, a first comparator for determining if the current through the current sensing device exceeds a threshold, a voltage regulator coupled to the secondary winding to produce a regulated voltage, a second comparator for determining if the regulated voltage has drooped below an acceptable level, a counter coupled to the second comparator for generating a signal having a fixed number of switch cycles, and control circuitry for generating signals controlling the first switch and the second switch and responsive to the first comparator to enter a power saving mode disabling the signals, and to the second comparator to temporarily exit the power saving mode for a fixed number of cycles when the regulated voltage has drooped below an acceptable level.

Claims

1. A step-down switched-mode power supply circuit having a transformer including at least one primary winding and at least one secondary winding, a current sensing device for sensing a current through the primary winding of the transformer, a first switch, a second switch, a voltage regulator coupled to the secondary winding of the transformer to produce a regulated voltage on an output line, and control circuitry for generating a pair of complimentary signals controlling the first switch and the second switch, the circuit comprising:

a first comparator for determining if the current through the current sensing device exceeds a threshold;

a second comparator coupled to the output line of the voltage regulator for determining if the regulated voltage has drooped below an acceptable level; and

a counter coupled to the second comparator for asserting a power save disable signal for a fixed number of switch cycles when the second comparator determines that the regulated voltage has drooped below the acceptable level,

wherein the control circuitry is responsive to the first comparator to enter a power saving mode disabling the complimentary signals and is responsive to the power save disable signal to temporarily exit the power saving mode while the power save disable signal is asserted.

2. A step-down switched-mode power supply circuit having a first voltage input node for receiving a first voltage level, a second voltage input node for receiving a second voltage level, a first switch and a second switch, the first switch connecting the first voltage node to a phase node when the first switch is active and the second switch connecting the second voltage node to the phase node when the second switch is active, a pulse width modulated synchronous switching control circuit generating a first switching signal and a second switching signal, the first switching signal and the second switching signal being complementary, the first switching signal controlling the first switch and the second switching signal controlling the second switch, a transformer having at least one primary winding and at least one secondary winding, the primary winding coupled to the phase node and to a current sensing device for sensing a current through a primary winding of the transformer, and a voltage regulator coupled to the secondary winding to produce a regulated voltage on an output line, the circuit comprising:

a first comparator for determining if the current through the current sensing device exceeds a threshold;

a second comparator coupled to the output line of the voltage regulator for determining if the regulated voltage has drooped below an acceptable level; and

a counter coupled to the second comparator for asserting a power save disable signal for a fixed number of switch cycles when the second comparator determines that the regulated voltage has drooped below the acceptable level,

wherein the control circuit responsive to the first comparator to enter a power saving mode disabling the first switching signal and the second switching signal and is responsive to the power save disable signal to temporarily exit the power saving mode while the power save disable signal is asserted.

3. A step-down switched-mode power supply circuit having a first voltage input node for receiving a first voltage level, a second voltage input node for receiving a second voltage level, a pulse width modulated switching control circuit, a transformer having at least one primary winding and at least one secondary winding with the primary winding coupled to a phase node and to a current sensing device for sensing a current through the primary winding of the transformer, and a voltage regulator coupled to the secondary winding of the transformer to produce a regulated voltage on an output line, the circuit comprising:

a first switch for connecting the first voltage node to the phase node when the first switch is active wherein the first switch is controlled by a switching signal from the pulse width modulated switching control circuit;

a second switch for connecting the second voltage node to the phase node when the second switch is active;

a first comparator for determining if the current through the current sensing device exceeds a threshold;

a second comparator coupled to the output line of the voltage regulator for determining if the regulated voltage has drooped below an acceptable level; and

a counter coupled to the second comparator for asserting a power save disable signal for a fixed number of switch cycles when the second comparator determines that the regulated voltage has drooped below the acceptable level,

wherein the control circuit is responsive to the first comparator to enter a power saving mode disabling the switching signal and is responsive to the power save disable signal to temporarily exit the power saving mode while the power save disable signal is asserted.

4. The circuit as defined in claim 3, wherein the second switch is a diode.

5. The circuit as defined in claim 3, wherein the second switch is a transistor connected to form a diode.

6. A step-down switched-mode power supply circuit having a first switch and a second switch alternately coupling a primary winding of a transformer to a first input voltage and a second input voltage, the transformer also having a secondary winding, a current sense element connected to measure a current through the primary winding, and a voltage regulator powered by the secondary winding for providing a regulated voltage, the circuit comprising:

means for causing the first switch and the second switch to stop switching and to disengage both the first input voltage and the second input voltage from the primary winding of the transformer in response to detecting a current below a threshold with the current sense element; and

means for comparing the regulated voltage with a reference voltage level to determine if the regulated voltage has drooped below the reference voltage and, if so, causing the first switch and the second switch to cycle through a fixed number of switching cycles.

7. A step-down switched-mode power supply circuit having a first switch and a second switch switching by alternately coupling a primary winding of a transformer to a first input voltage node and a second input voltage node, the transformer also having a secondary winding, a current sense element connected to measure a current through the primary winding, and a voltage regulator powered by the secondary winding for providing a regulated voltage, the circuit comprising:

means for detecting with the current sense element a low current demand condition of the circuit;

means for causing the first switch and the second switch to stop switching and to disengage both the first input voltage and the second input voltage from the primary winding of the transformer in response to the means for detecting;

means for comparing the regulated voltage with a reference voltage level to determine if the regulated voltage has drooped below the reference voltage and, if so, asserting a signal to temporarily restart switching; and

means for restarting temporarily, responsive to the asserted signal, the switching of the first switch and the second switch for a fixed number of switching cycles.

8. The circuit as defined in claim 7, wherein the means for restarting further comprises:

means for causing a counter to count for a fixed number of cycles;

means for strobing the counter with a switching signal tracking at least one switching signal applied to at least one of the first switch and the second switch; and

means for asserting a power save disable signal while the counter is counting.

9. The circuit as defined in claim 8, wherein the means for causing a counter to count includes means for loading a value into the counter and allowing it to count down to zero.

10. The circuit as defined in claim 8, wherein the means for causing a counter to count includes means for zeroing the counter and allowing it to count to a predetermined value other than zero.