TECHNICAL FIELD

This description relates generally to electronic circuits, and more particularly to a power supply system.

BACKGROUND

Power supply circuits can be implemented in a variety of different ways. Examples of power supply circuits include synchronous rectifier power converters, asynchronous rectifier power converters, resonant power converters, and any of a variety of other types of switching power converters. A typical power supply circuit can thus activate one or more switches to convert an input voltage to an output voltage. Typical power supply circuits can implement a transformer for delivering an output voltage on the secondary winding of a transformer from a square-wave input voltage applied by the switches to the primary winding of the transformer. For an ideal transformer (e.g., zero leakage inductance and zero winding resistance), rectifying the secondary voltage results in a DC output voltage that is equal to the input voltage multiplied by the turns ratio of the transformer. However, introducing physical separation between the primary and secondary windings of the transformer due to high isolation requirements can introduce significant leakage inductance in the transformer.

An increase in the output current of the transformer will cause a voltage drop on the impedance of the leakage inductance which will result in a reduction of the output voltage. Therefore, the output voltage will be dependent on the output current. To eliminate the effect of load current variation on the output voltage, typical power supply circuits include a resonant capacitor in series with the primary winding of the transformer to resonate with the leakage inductance. If the switching frequency of the converter is equal to the resonant frequency of the leakage inductance and the resonant capacitor, the impedances of these components will be equal and opposite and will mutually cancel. Therefore, no voltage drop will be associated with the load current, and the output voltage of the converter would be independent of the load current.

However, in practice, significant tolerances are present in both the resonant frequency of the capacitor and the leakage inductance, as well as in the switching frequency of the power converter. Consequently, the resonant frequency and switching frequency may differ significantly and the mismatch will prevent mutual cancellation of impedances, resulting in unacceptable dependence of the output voltage on the output load. If the switching frequency of the converter is lower than the resonant frequency, the interaction between the resonant capacitor and the magnetizing inductance of the transformer will generate a voltage that will considerably increase the output voltage of the converter.

SUMMARY

A circuit includes a transformer having first and second transformer inputs and first and second transformer outputs. The first transformer output can be adapted to be coupled through a capacitor to a first input of an output stage. The second transformer output can be adapted to be coupled to a second input of the output stage. The circuit also includes a switching system having first and second inputs and first and second switching outputs. The first input can be configured to receive a switching signal. The second input can be configured to receive an input voltage. The first and second switching outputs respectively can be coupled to the first and second transformer inputs.

A switching power supply system includes a switching system having a switching input and a switching output. The switching system includes switches configured to close responsive to a switching signal at the switching input. The switching system can be configured to provide a first voltage at the switching output responsive to closing the switches. The system also includes a transformer including a primary winding and a secondary winding. The primary winding can be coupled to the switching output and configured to receive the first voltage. The secondary winding can be configured to provide a second voltage responsive to the first voltage. The system also includes a capacitor coupled to the secondary winding. The system further includes an output stage coupled to the capacitor. The output stage can be configured to generate an output voltage by rectifying the second voltage.

A switching power supply system includes a switching system having first and second inputs and first and second switching outputs. The first input can be configured to receive a switching signal having a switching frequency. The second input can be configured to receive an input voltage. The system also includes a transformer having a primary winding and a secondary winding. The primary winding can have first and second transformer inputs. The secondary winding can have first and second transformer outputs, in which the first transformer input is coupled to the first switching output, and the second transformer input is coupled to the second switching output. The system also includes an output stage having a first and second voltage inputs and a voltage output. The second voltage input can be coupled to the second transformer output. The output stage can be configured to provide an output voltage at the voltage output responsive to the first and second voltage inputs. The system further includes a capacitor coupled between the first transformer output and the first voltage input, in which the capacitor and the secondary winding have a resonant frequency above the switching frequency.

