Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2000-07-17
2001-09-11
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S080000, C363S089000
Reexamination Certificate
active
06288920
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to power conversion and, more specifically, to a drive compensation circuit for a synchronous rectifier in a power converter and a power converter employing the same.
BACKGROUND OF THE INVENTION
A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC—DC converters are frequently employed to advantage. DC—DC converters generally include an inverter, a transformer having a primary winding coupled to the inverter and a rectifier coupled to a secondary winding of the transformer. The inverter generally includes a switching device, such as a field-effect transistor (FET), that converts the DC input voltage to an AC voltage. The transformer then transforms the AC voltage to another value and the rectifier generates the desired DC voltage at the output of the DC—DC converter.
Conventionally, the rectifier includes passive rectifying devices, such as Schottky diodes, that conduct the load current only when forward-biased in response to the input waveform to the rectifier. Passive rectifying devices, however, cannot achieve forward voltage drops of less than about 0.35 volts, thereby substantially limiting a conversion efficiency of the DC—DC converter. To achieve an acceptable level of efficiency, DC—DC converters that provide low output voltages (e.g., 1 volt) often require rectifying devices that have forward voltage drops of less than about 0.1 volts. The DC—DC converters, therefore, generally use synchronous rectifiers. A synchronous rectifier replaces the passive rectifying devices of the conventional rectifier with rectifier switches, such as FETs or other controllable switches, that are periodically driven into conduction and non-conduction modes in synchronism with the periodic waveform of the AC voltage. The rectifier switches exhibit resistive-conductive properties and may thereby avoid the higher forward voltage drops inherent in the passive rectifying devices.
One difficulty with using a rectifier switch (e.g., an n-channel silicon FET) is the need to provide a drive signal that alternates between a positive voltage to drive the device into the conduction mode and a zero or negative voltage to drive the device into the non-conduction mode. Although a capacitive charge within the rectifier switch may only be 30 to 50 nanocoulombs, the rectifier switch requires a high drive current for a brief period of time to change conduction modes. Typical drive currents may be 10 amperes or greater, lasting for tens of nanoseconds. The need to provide substantial power to the rectifier switch to change conduction modes thus reduces some of the advantages of the synchronous rectifier.
The '138 patent, the '482 patent and the '541 patent all describe the use of the secondary winding of the transformer to directly drive the synchronous rectifier. The recognition of the availability of suitable drive voltages from the secondary winding over the entire switching cycle of the inverter led to the development of self-synchronized synchronous rectifiers as disclosed in the aforementioned patents.
The '032 patent describes the use of extra windings in the transformer and voltage-limiting switches to improve the control of the drive signal. The extra windings are particularly useful when the output voltage is so low that the secondary winding does not develop sufficient voltage to ensure that the rectifier switch is fully driven into the conduction mode. The voltage-limiting switches are useful when the input or output voltages are variable, resulting in wide voltage variations in the drive signal. The extra windings and voltage-limiting switches thus allow the transformer to provide drive signals of sufficient voltage to efficiently operate the synchronous rectifier.
When the switching frequency of a DC—DC converter is increased to achieve a more compact design, however, the energy required to charge and discharge the internal capacitance of the rectifier switch can result in substantial losses, detracting from, and ultimately limiting, the benefits ofthe low conduction mode resistance of the rectifier switch. As the duty cycle of the inverter changes to accommodate variations in either the load or the input or output voltage, wide variations in the voltage of the drive signal may result. Further, the transformer generates voltages of both positive and negative polarity, charging the control terminal of the rectifier switch to both positive and negative voltages. The variable nature of the drive signal detracts from the efficiency of the synchronous rectifier and presents an obstacle to increasing the switching frequency of the inverter.
Accordingly, what is needed in the art is a drive compensation circuit for driving the rectifier switch that avoids unnecessarily charging the control terminal of the rectifier switch to substantially negative voltages in the non-conduction mode and further avoids charging the control terminal ofthe rectifier switch to unnecessarily high positive voltages during the conduction mode, thereby increasing an efficiency of the synchronous rectifier. Additionally, in synchronous rectifiers employing at least two rectifier switches, a drive compensation circuit that equalizes the voltages applied to the control terminals of the rectifier switches may further increase the efficiency of the power converter.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides, for use with a synchronous rectifier (or a rectifier) coupled to a secondary winding of a transformer and having a rectifier switch, a circuit for, and method of driving the rectifier switch and a power converter employing the circuit or the method. In one aspect of the present invention wherein the synchronous rectifier has at least first and second rectifier switches, the circuit includes: (1) a series-coupled drive winding and capacitor, coupled between a first control terminal of the first rectifier switch and a second control terminal of the second rectifier switch, that generates first and second drive signals and delivers the first and second drive signals to the first and second control terminals, respectively, and (2) first and second clamps, coupled to the first and second control terminals, respectively, that control first and second capacitive charges within the first and second rectifier switches to limit voltage excursions of the first and second drive signals.
In another aspect of the present invention wherein the rectifier has a rectifier switch and a rectifying device, the circuit includes: (1) a series-coupled drive winding and capacitor, coupled between a control terminal of the rectifier switch and a terminal of the rectifying device, that generates a drive signal and delivers the drive signal to the control terminal, and (2) a clamp, coupled to the control terminal, that controls a capacitive charge within the rectifier switch to limit a voltage excursion of the drive signal.
The present invention therefore introduces a circuit (drive compensation circuit) that reduces an amount of charge transferred to a control terminal of one or more rectifier switches and thereby increases an overall efficiency of the synchronous rectifier. The drive compensation circuit avoids unnecessarily charging the control terminal(s) of the rectifier switch(es) to substantially negative voltages in the non-conduction mode and further avoids charging the control terminal(s) of the rectifier switch(es) to unnecessarily high positive voltages during the conduction mode. Further, when the synchronous rectifier contains at least first and second rectifier switches, the charge from one rectifier switch may be transferred to the other rectifier switch during alternating portions of the switching cycle of the inverter. The drive compensation circuit can equalize the voltages at the control terminals of the rectifier switches, ther
Jacobs Mark E.
Rozman Allen F.
Vu Bao Q.
Wong Peter S.
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