Drive circuit for synchronous rectifier and method of operating the same

A drive circuit for driving a rectifier switch, a method of driving the rectifier switch and a power converter employing the drive circuit or the method. 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, and (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.

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.sup.-9 coulombs.times.8 volts.times.500,000
 Hz.times.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 purposes of the present invention. Those skilled
 in the art will also realize that such equivalent constructions do not
 depart from the spirit and scope of the invention in its broadest form.

DETAILED DESCRIPTION
 Referring initially to FIG. 1, illustrated is a schematic diagram of an
 embodiment of a power converter 100 constructed according to the
 principles of the present invention. The power converter 100 has an input
 couplable to a source of electrical power 105 having an input voltage Vin
 and an output that provides an output voltage Vout to a load 190. The
 power converter 100 includes a power switch 120 coupled to the input. In
 the illustrated embodiment, the power switch 120 is a metal oxide
 semiconductor field-effect transistor (MOSFET). Of course, other
 controllable switches, such as bipolar junction transistors (BJTs) and
 gallium arsenide field-effect transistors (GaAsFETs) are well within the
 broad scope of the present invention.
 The power converter 100 further includes an output filter 170, having a
 filter inductor LF and a filter capacitor CF, coupled to the output. The
 power converter 100 further includes a synchronous rectifier coupled to a
 node A between the power switch 120 and the filter inductor LF. In the
 illustrated embodiment, the synchronous rectifier includes a rectifier
 switch Q1. While the illustrated rectifier switch Q1 is an n-channel metal
 oxide semiconductor field-effect transistor (MOSFET), other controllable
 switches, such as bipolar junction transistors (BJTs) and gallium arsenide
 field-effect transistors (GaAsFETs), are well within the broad scope of
 the present invention. The rectifier switch Q1 has an intrinsic input
 capacitance therein, explicitly represented in FIG. 1 by an intrinsic
 capacitor C1 coupled to a control terminal of the rectifier switch Q1. Of
 course, the synchronous rectifier may include any number of rectifier
 switches as may be required by a particular application.
 The power converter 100 further includes a drive circuit 140 that generates
 and delivers a drive signal to drive the control terminal of the rectifier
 switch Q1. The power converter 100 still further includes a control
 circuit 180 coupled to the power switch 120. The control circuit 180
 monitors the output voltage Vout and adjusts the duty cycle of the power
 switch 120 to regulate the output voltage Vout despite variations in the
 input voltage Vin or the load 190. Of course, the control circuit 180 may
 monitor other control points within the power converter 100 as desired.
 The drive circuit 140 includes a bias energy source that, in the
 illustrated embodiment, is a battery BAT1. Of course, other types of
 energy sources, such as a typical internal bias supply or an external bias
 supply, are well within the broad scope of the present invention. The bias
 energy source BAT1 includes an optional bias capacitor C2 coupled
 thereacross. The bias capacitor C2 may be employed to store a portion of
 the energy supplied by the bias energy source BAT1. The bias capacitor C2
 is preferably sufficiently large such that it is capable of acquiring a
 nominally constant voltage throughout the resonant operational cycles of
 the drive circuit 140 and the rectifier switch Q1.
 The drive circuit 140 further includes a first blocking diode D2 coupled to
 a first terminal B of the bias energy source BAT1 and configured to block
 reverse current flow thereto. The drive circuit 140 further includes an
 inductor L1 coupled to the rectifier switch Q1. The drive circuit 140
 still further includes a switching circuit 150 that, in the illustrated
 embodiment, is interposed between the first blocking diode D2 and the
 inductor L1.
 The switching circuit 150 includes series-coupled first and second switches
 Q2, Q3. The first switch Q2 is configured to resonantly transfer energy
 from the bias energy source BAT1 to the control terminal of the rectifier
 switch Q1 via the inductor L1 to turn the rectifier switch Q1 ON. The
 second switch Q3 is configured to resonantly discharge the energy through
 the control terminal of the rectifier switch Q1 to turn the rectifier
 switch Q1 OFF. While the first and second switches Q2, Q3 are illustrated
 as n-channel metal oxide semiconductor field-effect transistors (MOSFETs),
 other controllable switches, such as bipolar junction transistors (BJTs)
 and gallium arsenide field-effect transistors (GaAsFETs), are well within
 the broad scope of the present invention.
 The drive circuit 140 further includes a second blocking diode D3 coupled
 between the switching circuit 150 and a second terminal C of the bias
 energy source BAT1. The second blocking diode D3 may thus protect the
 switching circuit 150 from reverse current flow.
 The power converter 100 operates as follows. During a first interval, when
 the power switch 120 is ON (conducting), the source 105 provides energy to
 the load 190 as well as to the filter inductor LF. Then, during a second
 interval when the power switch 120 is OFF (non-conducting), the inductor
 current flows through the rectifier switch Q1, transferring some of its
 stored energy to the load 190.
 The rectifier switch Q1 may have a substantial intrinsic capacitance
 (represented by the intrinsic capacitor C1). The amount of energy that is
 stored in the intrinsic capacitor C1 each switching cycle (as the
 rectifier switch Q1 is turned ON and OFF) is related to the conduction
 losses experienced by the rectifier switch Q1. To increase the efficiency
 of the power converter 100, a substantial portion of the energy stored in
 the intrinsic capacitor C1 should be recovered each switching cycle.
 Further, the turn ON and turn OFF of the rectifier switch Q1 should be
 synchronized with the operation of the power switch 120.
 The drive circuit 140 turns ON the first switch Q2 of the switching circuit
 150 to form a first conductive path for energy to be resonantly
 transferred from the bias energy source BAT1, through the first blocking
 diode D2, the first switch Q2 and the inductor L1, to the control terminal
 of the rectifier switch Q1. The energy charges the intrinsic capacitor C1
 (of the rectifier switch Q1) causing the voltage at the control terminal
 of the rectifier switch Q1 to increase thus turning ON the rectifier
 switch Q1. The first blocking diode D2 blocks the reverse flow of current
 to the bias energy source BAT1, resulting in a first half-cycle of
 resonant current flow to charge the intrinsic capacitor C1. Additionally,
 the second switch Q3 is OFF, also blocking current flow. Once the
 intrinsic capacitor C1 has been resonantly charged, the first switch Q2
 can be turned OFF (after the first half-cycle of resonant current flow).
 When the rectifier switch Q1 is to be turned OFF, the second switch Q3 is
 turned ON to form a second conductive path for energy to be resonantly
 discharged through the control terminal of the rectifier switch Q1. The
 turn ON of the second switch Q3 allows a second half-cycle of resonant
 current flow to discharge the intrinsic capacitor C1. The second blocking
 diode D3 blocks reverse current flow to the switching circuit 150 and thus
 terminates the second half-cycle of resonant current flow. Once the
 intrinsic capacitor C1 has been resonantly discharged, the second switch
 Q3 can be turned OFF (after the second half-cycle of resonant current
 flow).
 By employing a resonance between the inductor L1 and the intrinsic
 capacitor C1, the transfer of energy to and from the rectifier switch Q1
 may be done in a substantially lossless manner.
 Turning now to FIG. 2, illustrated is a schematic diagram of another
 embodiment of a power converter 200 constructed according to the
 principles of the present invention. The power converter 200 has an input
 couplable to a source of electrical power 205 having an input voltage Vin
 and an output that provides an output voltage Vout to a load 290. The
 power converter 200 includes an inverter 210 coupled to the input. The
 power converter 200 further includes a transformer T1 having a primary
 winding S1 coupled to the inverter 210 and a secondary winding S2. The
 power converter 200 further includes a synchronous rectifier 230 having a
 rectifying diode D1 and a rectifier switch Q1. The synchronous rectifier
 230 is coupled to the secondary winding S2 and rectifies a periodic
 waveform supplied by the secondary winding S2.
 The power converter 200 further includes a drive circuit 240 that drives a
 control terminal of the rectifier switch Q1. In the illustrated
 embodiment, the drive circuit 240 generates and delivers a drive signal to
 the control terminal of the rectifier switch Q1. The power converter 200
 further includes an output filter 270, having a filter inductor LF and a
 filter capacitor CF, that filters the rectified waveform to provide the
 output voltage Vout at the output of the power converter 200. The power
 converter 200 still further includes a control circuit 280, coupled to the
 inverter 210, that monitors the output voltage Vout and adjusts the
 switching of the inverter 210 to regulate the output voltage Vout despite
 variations in the input voltage Vin or the load 290. Of course, the
 control circuit 280 may monitor other control points within the power
 converter 200 as desired.
 In the illustrated embodiment, the inverter 210 includes a power switch 215
 coupled to the input of the power converter 200. The control circuit 280
 periodically switches the power switch 215 to apply the input voltage Vin
 across the primary winding S1. The inverter 210 further includes a
 series-coupled auxiliary switch 220 and capacitor 225, coupled across the
 power switch 215, that clamps a voltage across the windings of the
 transformer T1 when the power switch 215 is OFF (non-conducting). While
 the embodiment illustrated and described contains an inverter 210 with an
 active clamp forward switching topology, those skilled in. the art will
 realize that the principles of the present invention may be employed with
 a wide variety of switching topologies, including those not employing an
 active clamp.
 The rectifier switch Q1 of the synchronous rectifier 230 is coupled to the
 secondary winding S2. In the illustrated embodiment, the rectifier switch
 Q1 is an n-channel metal oxide semiconductor field-effect transistor
 (MOSFET) controllably switched by the drive circuit 240 to rectify the
 periodic waveform supplied by the secondary winding S2. Of course, other
 controllable switches, such as bipolar junction transistors (BJTs) and
 gallium arsenide field-effect transistors (GaAsFETs), are well within the
 broad scope of the present invention. The rectifier switch Q1 has an
 intrinsic capacitance therein, explicitly represented in FIG. 2 by an
 intrinsic capacitor C1 coupled to the control terminal of the rectifier
 switch Q1.
 The drive circuit 240 includes a bias energy source that, in the
 illustrated embodiment, is a battery BAT1. Of course, other types of
 energy sources, such as a typical internal bias supply or an external bias
 supply, are well within the broad scope of the present invention. The bias
 energy source BAT1 includes an optional bias capacitor C2 coupled
 thereacross. The bias capacitor C2 may be employed to store a portion of
 the energy supplied by the bias energy source BAT1 The bias capacitor C2
 is preferably sufficiently large such that it is capable of acquiring a
 nominally constant voltage throughout the resonant operational cycles of
 the drive circuit 240 and the rectifier switch Q1.
 The drive circuit 240 further includes a first blocking diode D2 coupled to
 a first terminal A of the bias energy source BAT1 and configured to block
 reverse current flow thereto. The drive circuit 240 further includes an
 inductor L1 coupled to the rectifier switch Q1. The drive circuit 240
 still further includes a switching circuit 250 that, in the illustrated
 embodiment, is interposed between the first blocking diode D2 and the
 inductor L1.
 The switching circuit 250 includes series-coupled first and second switches
 Q2, Q3. The first switch Q2 is configured to resonantly transfer energy
 from the bias energy source BAT1 to the control terminal of the rectifier
 switch Q1 via the inductor L1 to turn the rectifier switch Q1 ON. The
 second switch Q3 is configured to resonantly discharge the energy through
 the control terminal of the rectifier switch Q1 to turn the rectifier
 switch Q1 OFF. While the first and second switches Q2, Q3 are illustrated
 as n-channel metal oxide semiconductor field-effect transistors (MOSFETs),
 other controllable switches, such as bipolar junction transistors (BJTs)
 and gallium arsenide field-effect transistors (GaAsFETs), are well within
 the broad scope of the present invention.
 The drive circuit 240 further includes second blocking diode D3 coupled
 between the switching circuit 250 and a second terminal B of the bias
 energy source BAT1. The second blocking diode D3 may thus protect the
 switching circuit 250 from reverse current flow.
 The power converter 200 operates as follows. In steady-state operation, the
 power switch 215 is ON (conducting) for a primary duty cycle D to apply
 the DC input voltage Vin across the primary winding S1. The power switch
 215 is then OFF (non-conducting) for a complementary duty cycle 1-D to
 allow the auxiliary switch 220 to reset the transformer T1.
 The rectifier switch Q1 may have a substantial intrinsic capacitance
 (represented by the intrinsic capacitor C1). The amount of energy that is
 stored in the intrinsic capacitor C1 each switching cycle (as the
 rectifier switch Q1 is turned ON and OFF) is related to the conduction
 losses experienced by the rectifier switch Q1. To increase the efficiency
 of the power converter 200, a substantial portion of the energy stored in
 the intrinsic capacitor C1 should be recovered each switching cycle.
 Further, the turn ON and turn OFF of the rectifier switch Q1 should be
 synchronized with the operation of the power switch 215 and the auxiliary
 switch 220.
 The drive circuit 240. turns ON the first switch Q2 of the switching
 circuit 250 to form a first conductive path for energy to be resonantly
 transferred from the bias energy source BAT1, through the first blocking
 diode D2, the first switch Q2 and the inductor L1, to the control terminal
 of the rectifier switch Q1. The energy charges the intrinsic capacitor C1
 (of the rectifier switch Q1) causing the voltage at the control terminal
 of the rectifier switch Q1 to increase thus turning ON the rectifier
 switch Q1. The first blocking diode D2 blocks the reverse flow of current
 to the bias energy source BAT1, resulting in a first half-cycle of
 resonant current flow to charge the intrinsic capacitor C1. Additionally,
 the second switch Q3 is OFF, also blocking current flow. Once the
 intrinsic capacitor C1 has been resonantly charged, the first switch Q2
 can be turned OFF (after the first half-cycle of resonant current flow).
 When the rectifier switch Q1 is to be turned OFF, the second switch Q3 is
 turned ON to form a second conductive path for energy to be resonantly
 discharged through the control terminal of the rectifier switch Q1. The
 turn ON of the second switch Q3 allows a second half-cycle of resonant
 current flow to discharge the intrinsic capacitor C1. The second blocking
 diode D3 blocks reverse current flow to the switching circuit 250 and thus
 terminates the second half-cycle of resonant current flow. Once the
 intrinsic capacitor C1 has been resonantly discharged, the second switch
 Q3 can be turned OFF (after the second half-cycle of resonant current
 flow).
 By employing a resonance between the inductor L1 and the intrinsic
 capacitor C1, the transfer of energy to and from the rectifier switch Q1
 may be done in a substantially lossless manner.
 Turning now to FIGS. 3A-3D, illustrated are schematic diagrams of various
 embodiments of drive circuits 300, 325, 350, 375 constructed according to
 the principles of the present invention. The drive circuit 300 illustrated
 in FIG. 3A is configured to drive a rectifier switch Q1 (having an
 intrinsic capacitance represented by intrinsic capacitor C1). The drive
 circuit 300 is couplable to a bias energy source (not shown) and receives
 energy therefrom. The drive circuit 300 includes a first blocking diode D1
 coupled to the bias energy source and configured to block reverse current
 flow thereto. The drive circuit 300 further includes a switching circuit
 310, having series-coupled first and second switches Q2, Q3, coupled to
 the first blocking diode D1. The switching circuit 310 is analogous to the
 switching circuit 250 illustrated and described with respect to FIG. 2 and
 therefore will not hereinafter be described in detail.
 The drive circuit 300 further includes a second blocking diode D2 coupled
 to the switching circuit 310 and configured to block reverse current flow
 thereto. The drive circuit 300 further includes an inductor L1 coupled to
 a node A between the first and second switches Q2, Q3. The drive circuit
 300 further includes a blocking capacitor C2 coupled between the switching
 circuit 310 and a control terminal the rectifier switch Q1. The blocking
 capacitor C2 is employable to provide DC isolation between the switching
 circuit 310 and the control terminal of the rectifier switch Q1. The
 blocking capacitor may acquire a DC voltage during the operation of the
 drive circuit 300.
 The drive circuit 300 further includes a clamping circuit 315 coupled to
 the rectifier switch Q1. In the illustrated embodiment, the clamping
 circuit 315 includes a diode D3 configured to clamp a lower voltage
 excursion of a drive signal supplied to the control terminal of the
 rectifier switch Q1 by the drive circuit 300. By limiting a negative
 excursion of the drive signal, the clamping circuit 315 may reduce an
 amount of charge transferred to the rectifier switch Q1 thereby increasing
 an overall efficiency of the synchronous rectifier employing the rectifier
 switch Q1. Alternatively, the diode D3 may be configured to clamp an upper
 voltage excursion of the drive signal.
 The diode D3 is further configured to allow the blocking capacitor C2 to
 acquire an appropriate charge such that the blocking capacitor C2 may
 provide a small negative voltage to the control terminal of the rectifier
 switch Q1 when the rectifier switch Q1 is turned OFF.
 The clamping circuit 315 further includes a bleeder resistor R1 coupled to
 the control terminal of the rectifier switch Q1. The bleeder resistor is
 employed to provide a leakage path for voltages that may be present at the
 control terminal of the rectifier switch Q1.
 Turning now to FIG. 3B, illustrated is another embodiment of a drive
 circuit 325 constructed according to the principles of the present
 invention. The drive circuit 325 is analogous to the drive circuit 300
 illustrated and described with respect to FIG. 3A with variations as
 hereinafter described.
 The drive circuit 325 includes a first blocking diode D1 couplable to a
 bias energy source (not shown) and configured to block reverse current
 flow thereto. The drive circuit 325 further includes a switching circuit
 330, having series-coupled first and second switches Q2, Q3, coupled to
 the first blocking diode D1. The drive circuit 325 further includes a
 second blocking diode D2 coupled to the switching circuit 330 and
 configured to block reverse current flow thereto. The drive circuit 325
 further includes an inductor L1 coupled to a node A between the first and
 second switches Q2, Q3. The drive circuit 325 further includes a blocking
 capacitor C2 coupled between the switching circuit 330 and a control
 terminal the rectifier switch Q1.
 The drive circuit 325 further includes a clamping circuit 335 coupled to
 the rectifier switch Q1. In the illustrated embodiment, the clamping
 circuit 335 includes a diode D3 configured to clamp a lower voltage
 excursion of a drive signal supplied to the control terminal of the
 rectifier switch Q1 by the drive circuit 325. By limiting a negative
 excursion of the drive signal, the clamping circuit 335 may reduce an
 amount of charge transferred to the rectifier switch Q1 thereby increasing
 an overall efficiency of the synchronous rectifier employing the rectifier
 switch Q1. Alternatively, the diode D3 may be configured to clamp an upper
 voltage excursion of the drive signal.
 In the illustrated embodiment, the diode D3 is coupled to a voltage source
 (represented by battery BAT1). By adjusting a voltage of the voltage
 source BAT1, a turn-OFF voltage of the rectifier switch Q1 may be
 correspondingly adjusted. The clamping circuit 335 further includes a
 bleeder resistor R1 coupled to the control terminal of the rectifier
 switch Q1. The bleeder resistor R1 is employed to provide a leakage path
 for voltages that may be present at the control terminal of the rectifier
 switch Q1.
 Turning now to FIG. 3C, illustrated is another embodiment of a drive
 circuit 350 constructed according to the principles of the present
 invention. The drive circuit 350 is analogous to the drive circuit 325
 illustrated and described with respect to FIG. 3B with variations as
 hereinafter described.
 The drive circuit 350 includes a clamping circuit 365 coupled to the
 rectifier switch Q1. The clamping circuit 365 is substantially similar to
 the clamping circuit 335 illustrated and described with respect to FIG.
 3B. The clamping circuit 365 includes a diode D3 coupled to a voltage
 source (represented by battery BAT1). The polarity of the diode D3 is
 reversed from the configuration illustrated and described with respect to
 FIG. 3B to allow the diode D3 to clamp an upper voltage excursion of a
 drive signal supplied to the control terminal of the rectifier switch Q1
 by the drive circuit 350. The upper voltage excursion is substantially
 equal to a voltage of the voltage source BAT plus about a diode voltage
 drop.
 Turning now to FIG. 3D, illustrated is another embodiment of a drive
 circuit 375 constructed according to the principles of the present
 invention. The drive circuit 375 is analogous to the drive circuit 350
 illustrated and described with respect to FIG. 3C with variations as
 hereinafter described.
 The drive circuit 375 includes a clamping circuit 380 coupled to the
 rectifier switch Q1. The clamping circuit 380 is substantially similar to
 the clamping circuit 335 illustrated and described with respect to FIG.
 3C. The clamping circuit 380 includes a Zener diode ZD1 series-coupled in
 opposition to a diode D3. The Zener diode ZD1 is configured to clamp an
 upper voltage excursion of a drive signal supplied to the control terminal
 of the rectifier switch Q1 by the drive circuit 375. The diode D3 is
 configured to prevent the Zener diode ZD1 from conducting in a forward
 direction. Alternatively, the Zener diode ZD1 may be configured to clamp a
 lower voltage excursion of the drive signal.
 Turning now to FIG. 4, illustrated is a schematic diagram of another
 embodiment of a power converter 400 constructed according l; to the
 principles of the present invention. The power converter 400 has an input
 couplable to a source of electrical power 405 having an input voltage Vin
 and an output that provides an output voltage Vout to a load 490. The
 power converter 400 includes an inverter 410 coupled to the input. The
 power converter 400 further includes a transformer T1 having a primary
 winding S1 coupled to the inverter 410 and a secondary winding S2. The
 power converter 400 further includes a synchronous rectifier 430, having
 first and second rectifier switches Q1, Q2. The synchronous rectifier 430
 is coupled to the secondary winding S2 and rectifies a periodic waveform
 supplied by the secondary winding S2.
 The power converter 400 further includes a drive circuit 440 that generates
 and delivers first and second drive signals to drive the first and second
 control terminals of the first and second rectifier switches Q1, Q2. The
 power converter 400 further includes an output filter 470, having a filter
 inductor LF and a filter capacitor CF, that filters the rectified waveform
 to provide the output voltage Vout at the output of the power converter
 400. The power converter 400 still further includes a control circuit 480,
 coupled to the inverter 410, that monitors the output voltage Vout and
 adjusts the switching of inverter 410 to regulate the output voltage Vout
 despite variations in the input voltage Vin or the load 490. of course,
 the control circuit 480 may monitor other control points within the power
 converter 400 as desired.
 In the illustrated embodiment, the inverter 410 includes a power switch 415
 coupled to the input of the power converter 400. The control circuit 480
 periodically switches the power switch 415 to apply the input voltage Vin
 across the primary winding S1. The inverter 410 further includes a
 series-coupled auxiliary switch 420 and capacitor 425, coupled across the
 power switch 415, that clamps a voltage across the windings of the
 transformer T1 when the power switch 415 is OFF (non-conducting).
 The first and second rectifier switches Q1, Q2 of the synchronous rectifier
 430 are coupled to the secondary winding S2. In the illustrated
 embodiment, both the first and second rectifier switches Q1, Q2 are
 n-channel metal oxide semiconductor field-effect transistors (MOSFETs)
 controllably switched by the drive circuit 440 to rectify the periodic
 waveform supplied by the secondary winding S2. Of course, other
 controllable switches, such as bipolar junction transistors (BJTS) and
 gallium arsenide field-effect transistors (GaAsFETs), are well within the
 broad scope of the present invention. The first and second rectifier
 switches Q1, Q2 have intrinsic capacitances therein, explicitly
 represented by first and second intrinsic capacitors C1, C2 respectively
 coupled to the first and second control terminals of the first and second
 rectifier switches Q1, Q2.
 The drive circuit 440 is couplable to a bias energy source that, in the
 illustrated embodiment, is a battery BAT1. Of course, other types of
 energy sources, such as a typical internal bias supply or an external
 energy source, are well within the broad scope of the present invention.
 The drive circuit 440 further includes a first blocking diode D1 coupled
 to a first terminal A of the bias energy source BAT1 and configured to
 block reverse current flow thereto. The drive circuit 440 further includes
 an inductor L1 and a blocking capacitor C3 coupled to the rectifier switch
 Q1. The blocking capacitor C3 is employable to provide DC isolation
 between a switching circuit 450 (interposed between the first blocking
 diode D1 and the inductor L1) and the first control terminal (of the first
 rectifier switch Q1).
 In the illustrated embodiment, the switching circuit 4t50 includes
 series-coupled first and second switches Q3, Q4. The first switch Q3 is
 configured to resonantly transfer energy from the bias energy source BAT1
 and the second control terminal (of the second rectifier switch Q2) to the
 first control terminal (of the first rectifier switch Q1), via the
 inductor L1, to turn the second rectifier switch Q2 OFF and to turn the
 first rectifier switch Q1 ON. The second switch Q4 is configured to
 resonantly transfer the energy through the first control terminal (of the
 first rectifier switch Q1) to the second control terminal (of the second
 rectifier switch Q2) to turn the first rectifier switch Q1 OFF and to turn
 the second rectifier switch Q2 ON. While the first and second switches Q2,
 Q3 are illustrated as n-channel metal oxide semiconductor field-effect
 transistors (MOSFETs), other controllable switches, such as bipolar
 junction transistors (BJTs) and gallium arsenide field-effect transistors
 (GaAsFETs), are well within the broad scope of the present invention.
 The drive circuit 440 further includes second blocking diode D2 coupled
 between the switching circuit 450 and a second terminal B of the bias
 energy source BAT1. The second blocking diode D2 may thus protect the
 switching circuit 450 from reverse current flow.
 The drive circuit 440 further includes first and second clamping circuits
 460, 465, respectively coupled to the first and second rectifier switches
 Q1, Q2. In the illustrated embodiment, the first clamping circuit 460
 includes a diode D3 configured to clamp a lower voltage excursion of a
 first drive signal supplied to the first control terminal (of the first
 rectifier switch Q1) by the drive circuit 440. The second clamping circuit
 465 includes a diode D4 configured to clamp a lower voltage excursion of a
 second drive signal supplied to the second control terminal (of the second
 rectifier switch Q2) by the drive circuit 440. By limiting a negative
 excursion of the first and second drive signals, the first and second
 clamping circuits 460, 465 may reduce an amount of charge transferred to
 the respective first and second rectifier switches Q1, Q2 thereby
 increasing an overall efficiency of the synchronous rectifier 430.
 Alternatively, the first and second clamping circuit 460, 465 may be
 configured to clamp upper voltage excursions of the first and second drive
 signals, respectively.
 The first clamping circuit 460 further includes a bleeder resistor R1
 coupled to the first control terminal (of the first rectifier switch Q1).
 The second clamping circuit 465 includes a bleeder resistor R2 coupled to
 the second control terminal (of the second rectifier switch Q2). The
 bleeder resistors R1, R2 are employed to respectively provide first and
 second leakage paths for voltages that may be present at the first and
 second control terminals of the first and second rectifier switches Q1,
 Q2.
 The power converter 400 operates as follows. In steady-state operation, the
 power switch 415 is ON (conducting) for a primary duty cycle D to apply
 the DC input voltage Vin across the primary winding S1. The power switch
 415 is then OFF (non-conducting) for a complementary duty cycle 1-D to
 allow the auxiliary switch 420 to reset the transformer T1.
 The first and second rectifier switches Q1, Q2 may have substantial
 intrinsic capacitances (represented by the first and second intrinsic
 capacitors C1, C2). The amount of energy that is stored in the first and
 second intrinsic capacitors C1, C2 each switching cycle (as the first and
 second rectifier switches Q1, Q2 are turned ON and OFF) is related to the
 conduction losses experienced by the synchronous rectifier 430. To
 increase the efficiency of the power converter 400, a substantial portion
 of the energy stored in the first and second intrinsic capacitors C1, C2
 should be recovered each switching cycle. Further, the turn ON and turn
 OFF of the first and second rectifier switches Q1, Q2 should be
 synchronized with the operation of the power switch 415 and the auxiliary
 switch 420.
 The drive circuit 440 turns ON the first switch Q3 of the switching circuit
 250 to form a first conductive path for energy to be resonantly
 transferred through the second control terminal (of the second rectifier
 switch Q2) and the bias energy source BAT1, through the first blocking
 diode D1, the first switch Q3, the inductor L1 and the blocking capacitor
 C3, to the first control terminal (of the first rectifier switch Q1). As
 the second input capacitance C2 discharges through the second control
 terminal, the second rectifier switch Q2 turns OFF. The energy charges the
 first intrinsic capacitor C1 (associated with the first rectifier switch
 Q1) causing the voltage at the first control terminal (of the first
 rectifier switch Q1) to increase, thereby turning ON the first rectifier
 switch Q1. The first blocking diode D1 blocks the reverse flow of current
 to the bias energy source BAT1, resulting in a first half-cycle of
 resonant current flow to charge the first intrinsic capacitor C1.
 Additionally, the second switch Q4 is OFF, also blocking current flow.
 Once the first intrinsic capacitor C1 has been resonantly charged, the
 first switch Q3 can be turned OFF (after the first half-cycle of resonant
 current flow).
 When the first rectifier switch Q1 is to be turned OFF and the second
 rectifier switch Q2 is to be turned ON, the second switch Q4 is turned ON
 to form a second conductive path for energy to be resonantly transferred
 through the first control terminal (of the first rectifier switch Q1),
 through the blocking capacitor C3, the inductor L1, the second switch Q4
 and the second blocking diode D2, to the second control terminal (of the
 second rectifier switch Q2). As the first input capacitance C1 discharges
 through the first control terminal, the first rectifier switch Q1 turns
 OFF. The energy charges the second intrinsic capacitor C2 (of the second
 rectifier switch Q2) causing the voltage at the second control terminal
 (of the second rectifier switch Q2) to increase, thereby turning ON the
 second rectifier switch Q2. The turn ON of the second switch Q4 allows a
 second half-cycle of resonant current flow to discharge the first
 intrinsic capacitor C1. The second blocking diode D2 blocks reverse
 current flow to the switching circuit 450 and thus terminates the second
 half-cycle of resonant current flow. Once the first intrinsic capacitor C1
 has been resonantly discharged, the second switch Q4 can be turned OFF
 (after the second half-cycle of resonant current flow).
 By employing a resonance between the inductor L1 and the first and second
 intrinsic capacitors C1, C2, the transfer of energy to and from the first
 and second rectifier switches Q1, Q2 may be done in a substantially
 lossless manner.
 Those skilled in the art should understand that the previously described
 embodiments of the power converter and drive circuit are submitted for
 illustrative purposes only and other embodiments are well within the broad
 scope of the present invention. Additionally, exemplary embodiments of the
 present invention have been illustrated with reference to specific
 electronic components. Those skilled in the art are aware, however, that
 components may be substituted (not necessarily with components of the same
 type) to create desired conditions or accomplish desired results. For
 instance, multiple components may be substituted for a single component
 and vice-versa.
 The principles of the present invention may be applied to a wide variety of
 power circuit topologies, including circuit topologies not employing an
 active clamp. Additionally, the drive circuit of the present invention may
 be used with various half bridge, full bridge, flyback, and boost
 converter topologies employing discrete or integrated magnetics. For a
 better understanding of a variety of power converter topologies employing
 discrete and integrated magnetic techniques, see, Modern DC-to-DC
 Switchmode Power Converter Circuits, by Rudolph P. Severns and Gordon
 Bloom, Van Nostrand Reinhold Company, New York, N.Y. (1985), which is
 incorporated herein by reference in its entirety.
 Although the present invention has been described in detail, those skilled
 in the art should understand that they can make various changes,
 substitutions and alterations herein without departing from the spirit and
 scope of the invention in its broadest form.