Switch mode power supply using transformer flux sensing for duty cycle control

• US 6,396,718 B1
• Filed: 12/19/2000
• Issued: 05/28/2002
• Est. Priority Date: 12/19/2000
• Status: Expired due to Fees

• Current Assignee: Semiconductor Components Industries LLC (ON Semiconductor Corporation)
• Original Assignee: Semiconductor Components Industries LLC (ON Semiconductor Corporation)
• Inventors: Hian, Chew Ming, Ng, Hector Chi Man, Suen, Eddie Chiu Keung
• Primary Examiner(s): Vu, Bao Q.
• Application Number:
  US09/739,602
• Time in Patent Office: 525 Days
• Field of Search: 363/21.07,21.08,21.09,21.1,21.15,21.16,21.17,21.18,41,97,131
• US Class Current: 363/21.07
• CPC Class Codes: H02M 3/33546 with automatic control of the output voltage or current H02M3/33561 takes precedence

Abstract

A switch mode power supply (200, FIG. 2; 600, FIG. 6) includes a transformer (208), a transistor (212) that drives the primary winding of the transformer, and a controller (210, 610). The controller senses whether or not the transformer is reset and applies a switching control signal to a control terminal (220) of the transistor, accordingly. The signal is produced at a switching frequency, and the signal has a duty cycle that is limited based on whether or not the transformer is reset.

Claims

1. An apparatus for controlling a power supply, which includes a transformer having a primary winding and a secondary winding, and a switch that drives the primary winding and is controlled with a switching control signal, the apparatus comprising:

a controller that senses whether or not the transformer is reset and produces the switching control signal at a switching frequency, wherein the switching control signal has a duty cycle that is limited to a first limit when the transformer is not reset, and limited to a second limit when the transformer is reset, wherein the controller includes

a pulse width modulator that produces a pulse width modulated signal at the switching frequency;

a duty cycle controller that modulates a maximum duty cycle of the pulse width modulated signal based on whether or not the transformer is reset, resulting in the switching control signal; and

an oscillator, coupled to the duty cycle controller, which produces a sawtooth waveform, wherein the duty cycle controller causes a slope of the sawtooth waveform to vary based on whether or not the transformer is reset, and the oscillator produces a signal to vary the maximum duty cycle of the pulse width modulated signal based on the slope of the sawtooth waveform and a reference voltage provided by the duty cycle controller.

2. The apparatus of claim 1, wherein the switch is a transistor, and the transistor is switched on and off by the switching control signal, which is applied to a control terminal of the transistor.

3. The apparatus of claim 2, wherein the transistor is a field-effect transistor, and the control terminal is a gate terminal.

4. The apparatus of claim 3, further comprising:

a feedback lead, coupled between the controller and a drain terminal of the field-effect transistor, which enables the controller to sense a drain-source voltage of the field-effect transistor and to determine whether or not the transformer is reset.

5. The apparatus of claim 1, wherein the controller senses whether or not the transformer is reset for each switching cycle.

6. The apparatus of claim 1, wherein the controller further comprises:

a clock that provides a clock signal at a higher frequency than the switching frequency, wherein the clock signal is provided to the duty cycle controller, enabling the duty cycle controller to modulate the maximum duty cycle to the first limit or the second limit.

7. The apparatus of claim 1, wherein the oscillator produces an alternating waveform that includes a charging portion and a discharging portion, and the duty cycle controller adjusts the slope of the charging portion by adjusting a charging current to charge a capacitance within the oscillator.

8. The apparatus of claim 7, wherein the controller further comprises:

a comparator that compares the alternating waveform with a first reference voltage, Vhigh, and wherein the duty cycle controller causes the maximum duty cycle of the switching control signal to be varied by adjusting a value of Vhigh.

9. The apparatus of claim 1, further comprising:

a secondary circuit, coupled to the secondary winding, which produces a regulated output voltage.

10. An apparatus for controlling a power supply that includes an alternating current (AC) voltage source that produces an input voltage, a rectifier, coupled to the AC voltage source, which rectifies the input voltage, resulting in a rectified input voltage, a capacitor, coupled to the rectifier, which smoothes the rectified input voltage, a transformer having a primary winding and a secondary winding, wherein the primary winding is coupled to the capacitor, a secondary circuit, coupled to the secondary winding, which produces a regulated output voltage, and a transistor, coupled to the primary winding, which drives the primary winding and which is responsive to a switching control signal, the apparatus comprising:

a controller that senses whether or not the transformer is reset and produces the switching control signal at a switching frequency, wherein the switching control signal has a duty cycle that is limited to a first limit when the transformer is not reset, and limited to a second limit when the transformer is reset, and wherein the controller comprises

an oscillator that produces an alternating waveform at the switching frequency, wherein each cycle of the alternating waveform includes a charging portion and a discharging portion; and

a duty cycle controller that adjusts a slope of the charging portion based on whether or not the transformer is reset, causing a maximum duty cycle of the switching control signal to be varied; and

a feedback lead, electrically coupled between the transistor and the controller, which enables the controller to sense whether or not the transformer is reset.

11. The apparatus of claim 10, wherein the duty cycle controller adjusts the slope of the charging portion by adjusting a charging current used to charge a capacitance within the oscillator.

12. The apparatus of claim 10, wherein the controller further comprises:

a comparator that compares the alternating waveform with a first reference voltage, Vhigh, and wherein the duty cycle controller causes the maximum duty cycle of the switching control signal to be varied by adjusting a value of Vhigh.

13. The apparatus of claim 10, wherein the controller causes a slope of a sawtooth waveform, produced by an oscillator, to vary based on whether or not the transformer is reset, and

the oscillator produces the sawtooth waveform to vary the maximum duty cycle of the pulse width modulated signal based on the slope of the sawtooth waveform and a reference voltage provided by the duty cycle controller.

14. The apparatus of claim 10, wherein the controller senses whether or not the transformer is reset for each switching cycle.

15. A method for supplying a regulated source of power, the method comprising:

sensing whether or not a transformer is reset;

producing a switching control signal at a switching frequency, wherein the switching control signal has a duty cycle that is limited to a first limit when the transformer is not reset, and limited to a second limit when the transformer is reset, wherein producing the switching signal comprises

providing a clock signal at a higher frequency, wherein the switching frequency is a fraction of the higher frequency,

producing the switching control signal, using the clock signal, by adjusting the duty cycle to the first limit or the second limit,

producing an alternating waveform at the switching frequency, wherein each cycle of the alternating waveform includes a charging portion and a discharging portion, and

adjusting a slope of the charging portion based on whether or not the transformer is reset, causing a maximum duty cycle of the switching control signal to be varied; and

providing the switching control signal to a switch, which drives a primary winding of the transformer based on the switching control signal.