A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims benefit under 35 USC. § 119(e) of U.S. Provisional Patent Application No. 62/896,648, filed Sep. 6, 2019, entitled “Method to Balance the Secondary Winding Current to Improve the Current Control Stability.”

FIELD OF THE INVENTION

The present disclosure relates generally to power converters that provide a constant DC output current to a load, such as, for example, light-emitting diodes. More particularly, the present disclosure relates to improving the current control stability of power converters using inductive balancing.

BACKGROUND

The usage of light-emitting diodes (LEDs) to provide illumination is increasing rapidly as the cost of LEDs decrease and the endurance of the LEDs increases to cause the overall effective cost of operating LED lighting products to be lower than incandescent lamps and fluorescent lamps providing equivalent illumination. Also, LEDs can be dimmed by controlling the current through the LEDs because LEDs are current driven devices. The current through a plurality of LEDs in a lighting device must be controlled tightly in order to control the illumination provided by the LEDs and to avoid current runaway and LED flickering. An important consideration in LED driver design is that the output current ripple must be within a certain tolerance to avoid overdriving current to the LEDs and visible flickering thereof.

Resonant type half-bridge DC-DC power converters are a very popular topology for use as an LED driver. This type of power converter has good stability and very wide range of operating frequencies. An issue with this type of power converter is that when the output current is high, for example, 2 A to a 45V LED load, the output current of the power converter will show an asymmetric high AC high frequency (HF) ripple which can affect feedback control stability and cause lower frequency LED output flickering.

BRIEF SUMMARY

Exemplary power converters, transformers, methods as disclosed herein may improve current control stability (e.g., feedback control loop stability) by adding inductors to help balancing the secondary winding currents and avoid flickering of the LED load.

One embodiment of a resonant power converter as disclosed herein comprises a primary circuit with one or more switching elements controllably driven at an operating frequency to produce an alternating current (AC) output through a primary winding of a transformer, and a secondary circuit with first and second secondary windings of the transformer. The first secondary winding has a first leakage inductance, the second secondary winding having a second leakage inductance, and the secondary circuit is configured to provide power at an output node based on a power transfer between the primary winding and the first and second secondary windings. At least one balance inductor is coupled in series with the first and second secondary windings, and configured to reduce a difference between the first leakage inductance and the second leakage inductance.

In an exemplary aspect of the above-referenced embodiment, the first secondary winding may have a first secondary winding current peak, and the second secondary winding may have a second secondary winding current peak, wherein the at least one balance inductor is configured to reduce a difference between the first AC current peak and the second AC current peak.

In another exemplary aspect of the above-referenced embodiment, an inductance of the at least one balance inductor is less than 10% of a primary inductance of the primary winding divided by a turns ratio between the primary winding and the first and second secondary windings squared.

In another exemplary aspect of the above-referenced embodiment, the first and second secondary windings are coupled together at a common node, and the at least one balance inductor comprises a single balance inductor coupled to the common node.

In another exemplary aspect of the above-referenced embodiment, the first and second secondary windings are coupled together at a common node, the at least one balance inductor includes a first balance inductor and a second balance inductor, the first balance inductor is coupled in series with the first secondary winding opposite the common node, and the second balance inductor is coupled in series with the second secondary winding opposite the common node.

In another exemplary aspect of the above-referenced embodiment, an inductance of the first balance inductor is less than 10% of a primary inductance of the primary winding divided by a first turns ratio between the primary winding and the first secondary windings squared, and an inductance of the second balance inductor is less than 10% of the primary inductance of the primary winding divided by a second turns ratio between the primary winding and the second secondary windings squared.

In another exemplary aspect of the above-referenced embodiment, the secondary circuit includes a first diode and a second diode, with each of the first and second diodes including an anode and a cathode, and the cathodes of the first and second diodes coupled to the output node of the secondary circuit. The first balance inductor is coupled between the first secondary winding and the anode of the first diode, and the second balance inductor is coupled between the second secondary winding and the anode of the second diode.

In another exemplary aspect of the above-referenced embodiment, the power converter is an LED driver and the load comprises one or more light emitting diodes (LEDs).

