Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2000-04-04
2001-06-05
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S090000
Reexamination Certificate
active
06243278
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to power conversion and, more specifically, to a drive 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, generally 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. Of course, depending on the type of rectifier switch, an opposite drive polarity may be employed. Although a capacitive charge within the rectifier switch may only be 30 to 50 nanocoulombs per device (rectifier switch), in situations where as many as a dozen or more devices may be used, a high drive current may be required for a brief period of time to change conduction modes.
The power required by the process of charging the control terminal(s) of the rectifier switch(s) (gate terminal, in the case of a FET) may be represented as the drive bias voltage multiplied by the total gate charge multiplied by the switching frequency and divided by the efficiency of the bias energy source employed. The power required may, for example, be equivalent to: 50*10
−9
coulombs×8 volts×500,000 Hz×12 devices/0.8 bias efficiency=3 watts. In addition, typical drive currents may be 10 amperes or greater, lasting for tens of nanoseconds. The need to provide substantial power to the rectifier switch(s) to change conduction modes thus reduces some of the advantages of the synchronous rectifier.
Accordingly, what is needed in the art is a drive circuit for driving the rectifier switch of a synchronous rectifier that overcomes the deficiencies of the prior art.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides, for use with a synchronous rectifier having at least one rectifier switch, a drive circuit for driving the rectifier switch. In one embodiment, the drive circuit includes (1) a blocking diode couplable to a bias energy source and configured to block reverse current flow thereto, (2) an inductor coupled to the rectifier switch, (3) a switching circuit, coupled to the blocking diode and the inductor, configured to: (3a) resonantly transfer energy from the bias energy source to a control terminal of the rectifier switch via the inductor to turn the rectifier switch ON, and (3b) resonantly discharge the energy through the control terminal to turn the rectifier switch OFF.
The present invention introduces, in one aspect, a drive circuit that employs resonance to transfer energy to and from a rectifier switch in a substantially lossless manner. The resonance is a result of the interaction between, among other things, the inductor of the drive circuit and a gate capacitance of the rectifier switch.
In one embodiment of the present invention, the switching circuit includes series-coupled first and second switches. The first switch is configured to resonantly transfer the energy from the bias energy source to the control terminal, via the inductor, to turn the rectifier switch ON. The second switch is configured to resonantly discharge the energy through the control terminal to turn the rectifier switch OFF. The rectifier switch may thus be turned ON or OFF by activating an appropriate one of the first and second switches of the switching circuit.
In a more specific embodiment, wherein the blocking diode is a first blocking diode coupled to a first terminal of the bias energy source, the drive circuit further includes a second blocking diode coupled between the switching circuit and a second terminal of the bias energy source. The second blocking diode may thus protect the switching circuit from reverse current flow.
In one embodiment of the present invention, the bias energy source includes a bias capacitor coupled there across and configured to store at least a portion of the energy. The bias capacitor is, in one embodiment, sufficiently large such that it is capable of acquiring a nominally constant voltage throughout the resonant operational cycles of the drive circuit.
In one embodiment of the present invention, the drive circuit includes a blocking capacitor coupled between the switching circuit and the control terminal of the rectifier switch. The blocking capacitor provides DC isolation between the switching circuit and the control terminal of the rectifier switch. The blocking capacitor may acquire a DC voltage during the operation of the drive circuit.
In one embodiment of the present invention, the drive circuit includes a clamping circuit coupled to the rectifier switch. The clamping circuit is configured to clamp a lower or upper voltage excursion of a drive signal supplied to the control terminal of the rectifier switch. In a related embodiment the clamping circuit includes a diode. In another related embodiment, the clamping circuit further includes a Zener diode series-coupled in opposition to the diode and is configured to clamp an upper or lower voltage excursion of the drive signal supplied to the control terminal of the rectifier switch. In another related embodiment, the drive circuit includes a voltage source coupled to the clamping circuit. The voltage source is employable to set the clamping voltage of the clamping circuit to a predetermined level. In another embodiment, the clamping circuit further includes a bleeder resistor coupled to the control terminal of the rectifier switch. The bleeder resistor provides a leakage path for voltages that may be present at the control terminal of the rectifier switch.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same pur
Han Jessica
Tyco Electronics Logistics A.G.
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