Changing switching frequency of a primary side power converter to compensate for inductance variation

• US 20080259650A1
• Filed: 07/26/2007
• Published: 10/23/2008
• Est. Priority Date: 04/23/2007
• Status: Active Grant

• Current Assignee: Active-Semi, Inc. (Qorvo, Inc.)
• Original Assignee: Active-Semi International, Inc. (Qorvo, Inc.)
• Inventors: Grant, Matthew, Huynh, Steven, Kunst, David
• Granted Patent: US 7,667,987 B2
• Time in Patent Office: Days
• US Class Current: 363/21.12
• CPC Class Codes: H02M 1/0009 Devices or circuits for detecting current in a converter
• CPC Class Codes: H02M 3/33507 with automatic control of the output voltage or current, e.g. flyback converters H02M3/33561, H02M3/33569 take precedence
• CPC Class Codes: H02M 3/33523 with galvanic isolation between input and output of both the power stage and the feedback loop

Abstract

A comparing circuit and a control loop are used to maintain the peak level of current flowing through an inductor of a flyback converter. An inductor switch control signal controls a switch through which the inductor current flows. The inductor current increases at a ramp-up rate during a ramp time and stops increasing at the end of the ramp time. The comparing circuit generates a timing signal that indicates a target time at which the inductor current would reach a predetermined current limit if the inductor current continued to increase at the ramp-up rate. The control loop then receives the timing signal and compares the target time to the end of the ramp time. The pulse width of the inductor switch control signal is increased when the target time occurs after the end of the ramp time. Adjusting the frequency and pulse width controls the peak of the inductor current.

Claims

1. A method comprising:

(a) receiving a feedback signal indicative of a first time and of an input voltage of a power converter, wherein an inductor current flowing through a primary winding of the power converter stops increasing at the first time, and wherein the primary winding exhibits an inductance whose magnitude deviates from a stated nominal magnitude;

(b) generating an inductor switch control signal that has a switching frequency; and

(c) adjusting the switching frequency of the inductor switch control signal to compensate for the deviating magnitude of the inductance and to maintain a constant output current of the power converter.

2. The method of claim 1, wherein the power converter is a flyback converter.

3. The method of claim 1, wherein the inductance of the primary winding is larger than the stated nominal magnitude, and wherein the adjusting in (c) compensates for the larger inductance by decreasing the switching frequency.

4. The method of claim 3, wherein the first time occurs later when the inductance of the primary winding is larger.

5. The method of claim 1, wherein the adjusting in (c) is performed by an integrated circuit that is packaged in an integrated circuit package having no more than four terminals.

6. The method of claim 1, wherein the feedback signal is generated using a voltage across an auxiliary winding, and wherein the auxiliary winding is magnetically coupled to the primary winding.

7. The method of claim 1, wherein the inductor switch control signal has a pulse width, further comprising:

(d) receiving a switch signal indicative of a ramp-up rate at which the inductor current increases;

(e) generating a timing signal that indicates a target time at which the inductor current would reach a predetermined current limit if the inductor current continued to increase at the ramp-up rate; and

(f) adjusting the pulse width of the inductor switch control signal such that the first time occurs nearer to the target time.

8. The method of claim 7, wherein the timing signal is generated in (e) by comparing charge that accumulates on timing capacitors of different sizes.

9. The method of claim 7, wherein the timing signal is generated in (e) by comparing charge that accumulates on same-sized timing capacitors that are charged by current sources of different sizes.

10. The method of claim 7, wherein the timing signal is generated in (e) using a current comparator.

11. A device comprising:

a primary winding that exhibits an inductance whose magnitude deviates from a stated nominal magnitude, wherein an inductor current flowing through the primary winding stops increasing at a first time;

a switch that is controlled by an inductor switch control signal, wherein the inductor switch control signal has a switching frequency; and

an oscillator that generates a switching frequency signal, wherein the switching frequency signal governs the switching frequency of the inductor switch control signal, and wherein the oscillator generates the switching frequency signal to compensate for the deviating magnitude of the inductance and to maintain a constant output current of the power converter.

12. The device of claim 11, wherein the device is a flyback converter.

13. The device of claim 11, wherein the inductance of the primary winding is larger than the stated nominal magnitude, and wherein the oscillator compensates for the larger inductance by decreasing the switching frequency.

14. The device of claim 11, wherein the device controls the inductor current such that the inductor current reaches the predetermined current limit at the first time.

15. The device of claim 11, wherein the switch and the oscillator are part of an integrated circuit that is packaged in an integrated circuit package having no more than four terminals.

16. A power converter that outputs an output current, comprising:

a secondary side comprising a secondary winding; and

a primary side comprising a primary winding, an auxiliary winding, an oscillator and a switch, wherein an inductor current flows through the primary winding when the switch is on, wherein the switch is controlled by an inductor switch control signal, wherein the oscillator generates a switching frequency signal using feedback from magnetic coupling between the auxiliary winding and the secondary winding, wherein the switching frequency signal has a first frequency, wherein the inductor switch control signal has a second frequency that depends on the first frequency, wherein no feedback from the secondary side is used to control the second frequency, and wherein the oscillator controls the first frequency such that the output current remains constant.

17. The power converter of claim 16, wherein the power converter is a flyback converter.

18. The power converter of claim 16, wherein the primary winding exhibits an inductance whose magnitude deviates from a stated nominal magnitude, and wherein the oscillator controls the first frequency to compensate for the deviating magnitude of the inductance.

19. The power converter of claim 16, wherein the primary side further comprises an adaptive current limiter, wherein the inductor switch control signal has a pulse width, and wherein the adaptive current limiter adjusts the pulse width of the inductor switch control signal such that the output current remains constant.

20. A system comprising:

an inductor switch that is turned on by an inductor switch control signal;

an inductor whose actual inductance deviates from a stated inductance, wherein an inductor current flows through the inductor and ramps up to a peak current while the inductor switch is turned on, and wherein the inductor switch control signal has a switching frequency; and

means for maintaining the peak current at a constant magnitude while adjusting the switching frequency to compensate for the actual inductance deviating from the stated inductance.

21. The system of claim 20, wherein the means includes a control loop.

22. The system of claim 20, wherein the means includes an oscillator that is part of an integrated circuit.

23. The system of claim 20, wherein the means is part of an integrated circuit that is packaged in an integrated circuit package having no more than four terminals.

24. The system of claim 20, wherein the inductor current ramps up at a ramp-up rate to the peak current, and wherein the means generates a timing signal that indicates a target time at which the inductor current would reach a predetermined current limit if the inductor current continued to increase at the ramp-up rate.