as United States
`a2) Patent Application Publication (10) Pub. No.: US 2004/0218406 A1
`(43) Pub. Date: Nov.4, 2004
`
`Janget al.
`
`US 20040218406A1
`
`(54) CONTACTLESS ELECTRICAL ENERGY
`TRANSMISSION SYSTEM HAVING A
`PRIMARY SIDE CURRENT FEEDBACK
`CONTROL AND SOFT-SWITCHED
`SECONDARY SIDE RECTIFIER
`
`(76)
`
`Inventors: Yungtaek Jang, Cary, NC (US); Milan
`M. Jovanovic, Cary, NC (US)
`
`Correspondence Address:
`VENABLE, BAETJER, HOWARD AND
`CIVILETTI, LLP
`P.O. BOX 34385
`
`WASHINGTON,DC 20043-9998 (US)
`
`(21) Appl. No.:
`
`10/426,721
`
`(22)
`
`Filed:
`
`May1, 2003
`
`Publication Classification
`
`(SV)
`
`Inte CI? ecccccccceeesccscssssnneseeeceeceennnnneess H02M 5/45
`
`(52) US. Che
`
`sssssssssssssessstsntsesnesnststsntensvesnee 363/37
`
`(57)
`
`ABSTRACT
`
`Acontactless electrical energy transmission system includes
`a transformer having a primary winding that is coupled toa
`power source through a primary resonant circuit and a
`secondary winding that
`is coupled to a load through a
`secondary resonant circuit. The primary and secondary
`resonant circuits are inductively coupled to each other. A
`primary control circuit detects current changes through the
`primary resonant circuit to control the switching frequency
`of a controllable switching device for maintaining a sub-
`stantially constant energy transfer between the primary
`winding and secondary winding in responseto at least one
`of a power source voltage change and a load change. As a
`result, excessive circulating energy of the CEET system is
`minimized providing a tight regulation of the output voltage
`over the entire load and input voltage ranges without any
`feedback connection between the primary side and the
`secondary side.
`
`Controlled ZVS
`
`Full-Bridge.Rectifier
`
`Controlled
`ZVS
`Rectifier
`
`+
`Vo
`-
`
`:
`
`}
`Variable Frequency
`ResonantInverter
`INDUCTIVE
`t
`: COUPLING
`
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`Frequency
`Resonant
`
`Inverter
`
`
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`
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`
`
`
`
`
`Primary-Current
`Feedback
`
`Frequency
`Control
`
`
`
`
`
`PWM
`Secondary-
`Current
`
`Output Voltage
`
`Zero-Cross
`Feedback
`
`
`Detection
`Control
`
`
`
`
`
`Synchronized
`Ramp Signal
`Generator
`
`
`1
`
`ANKER 1010
`ANKER1010
`
`

`

`Patent Application Publication Nov. 4, 2004 Sheet 1 of 8
`
`US 2004/0218406 Al
`
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`
`Controlled
`Rectifier
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`Frequency
`Contral
`
`(PRIOR ART)
`Fig. 1
`
`2
`
`

`

`Patent Application Publication Nov. 4, 2004 Sheet 2 of 8
`
`US 2004/0218406 Al
`
`!
`
`Variable Frequency
`ResonantInverter
`
`:
`
`Controlled
`Rectifier
`
`;
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`Vs
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`ve
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`ifferentia
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`Voltage
`Divider
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`VoO(SENSE)
`
`Error Amplifier
`with Compensator
`
`VREF
`
`Variable-Gain
`Input Voltage
`Sensing
`
`Voltage
`Controlled
`Oscillator (VCO)
`
`(PRIOR ART)
`Fig. 2
`
`3
`
`

`

`Patent Application Publication Nov. 4, 2004 Sheet 3 of 8
`
`US 2004/0218406 Al
`
`Variable Frequency
`ResonantInverter
`
`i
`
`Controlled ZVS
`Full-Bridge,Rectifier
`
`:
`;
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`+
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`Output Voltage
`Feedback
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`Current
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`
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`Control i|Oetection Control
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`
`
`Synchronized
`Ramp Signal
`
`Generator
`
`
`
`prccsrewmecsences
`
`4
`
`

`

`Patent Application Publication Nov. 4, 2004 Sheet 4 of 8
`
`US 2004/0218406 Al
`
`Variable Frequency
`: Resonant Inverter
`}
`:
`:
`
`:
`:
`:
`
`Controlled
`ZVS Full-Bridge
`Rectifier
`
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`:
`
`
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`
`Primary-Current
`Feedback
`
`Frequency
`Control
`
`PWM
`
`
`Secondary-
`
`Current
`Output Voltage
`
`Zero-Cross
`Feedback
`
`
`Detection
`Control
`
`
`
`
`
`Synchronized
`
`Ramp Signal
`Generator
`
`
`Fig. 4
`
`5
`
`

`

`Patent Application Publication Nov. 4, 2004 Sheet 5 of 8
`
`US 2004/0218406 Al
`
`
`
`Fig. 5
`
`6
`
`

`

`Patent Application Publication Nov. 4, 2004 Sheet 6 of 8
`
`US 2004/0218406 Al
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`
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`

`Patent Application Publication Nov. 4, 2004 Sheet 7 of 8
`
`US 2004/0218406 Al
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`

`Patent Application Publication Nov. 4, 2004 Sheet 8 of 8
`
`US 2004/0218406 Al
`
`Variable Frequency
`ResonantInverter
`
`:
`
`Vs
`
`:
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`
`
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`Full-Bridge Rectifier
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`RampSignal
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`Generator 2
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`
`9
`
`

`

`US 2004/0218406 Al
`
`Nov.4, 2004
`
`CONTACTLESS ELECTRICAL ENERGY
`TRANSMISSION SYSTEM HAVING A PRIMARY
`SIDE CURRENT FEEDBACK CONTROL AND
`SOFT-SWITCHED SECONDARYSIDE RECTIFIER
`
`FIELD OF THE INVENTION
`
`[0001] Generally, the present invention relates to the field
`of contactless electrical energy transmission (CEET) sys-
`tems, more particularly,
`to CEET systems that provide
`highly regulated powerto a load.
`
`BACKGROUND OF THE INVENTION
`
`[0002] Contactless electrical energy transmissions are
`knownfor the convenience by which they deliver powerto
`a load. Generally, CEET systems transfer power via an
`air-gap inductive coupling without there being any direct
`electric connection between a primary side and a secondary
`side. As such, in some applications, CEET systems offer
`distinct advantages over energy transmission systems that
`use wires and connectors. For example, CEET systems are
`preferred in hazardous applications such as mining and
`underwater environments due to the elimination of the
`
`sparking andthe risk of electrical shocks. Other exemplary
`applications
`that use CEET systems include charging
`devices that safely and reliably transfer power to consumer
`electronic devices and medical devices.
`
`[0003] Atypical CEET system consists of a transmitter in
`the primary side, a transformer, and a receiver in the
`secondary side. Such CEET system employs a primary
`inverter at the transmitter and a secondary rectifier at the
`receiver. The inverter and rectifier are coupled to each other
`via the primary and secondary windingsofthe transformer.
`Since the primary winding and the secondary winding are
`inductively coupled through the air-gap, electric power is
`transferred from the primary side to the secondary side as
`magnetic energy obviating the need for any physical elec-
`trical interconnections.
`
`[0004] However, power transmission via the inductive
`coupling of the CEET transformer has certain drawbacks in
`terms of low efficiency and unregulated delivery of power to
`the load. This is because the leakage inductance of the CEET
`transformer with air-separated primary and secondary wind-
`ings is muchlarger than the leakage inductance of a con-
`ventional transformerthat uses well interleaved primary and
`secondary windings. The CEET primary and secondary
`windings can store high amounts of leakage inductance
`energy that can cause high parasitic ringing and losses.
`Moreover, in CEET systems, it is very difficult to regulate
`power transmission mainly because there is no physical
`connection between the primary side and the secondaryside
`that would provide feedback information for regulating the
`power transmission.
`
`[0005] FIG. 1 shows one CEET system that achieves high
`efficiency by recovering the energy stored in the leakage
`inductance of the transformer. This system, which is more
`fully described in U.S. Pat. No. 6,301,128 B1, issued to
`Delta Electronics, Inc., the assignee of the present invention,
`incorporates the leakage inductance of each one of the
`primary and secondarysides in its power stage. The primary
`side includes a variable-frequency resonant inverter and the
`secondary side includes a controlled rectifier. An input-
`voltage feed forward control block controls the output
`
`inverter in
`frequency of the variable-frequency resonant
`response to source voltage variations, while a pulse width
`modulated (PWM) output voltage feedback control block
`controls the controlled rectifier output in response to load
`variations. Underthis arrangement, the PWM output voltage
`feedback control block and the input-voltage feed forward
`control block act as independent controls for regulating the
`output voltage without any feedback connection between the
`primary and secondarysides. FIG. 2 shows a more detailed
`schematic block diagram of the power stage and the con-
`trollers shown in FIG.1.
`
`In conventional CEET systems, lack of any feed-
`[0006]
`back information from the secondary side to the primary
`side prevents adjusting energy transfer from the primary side
`in response to load variations that occur on the secondary
`side. Thus, the maximum transferable power through the
`inductive coupling of the primary and secondary sides can
`vary under a range of light-load to high-load conditions.
`Such variations can create extra circulating energy and
`conduction losses. Moreover, for pulse width modulated
`control of energy transfer on the secondary side, the ratio of
`the duty cycle variations can be very large at high-load and
`light-load conditions. As a result, guaranteeing reliable
`operation over the entire load range requires complex cir-
`cuitry for implementing a suitable feedback control.
`
`[0007] Finally, switch S, of the controlledrectifier in FIG.
`2 turns on with hand switching, i.c., when the MOSFET
`switch turns on whenthe voltage across the switch is equal
`to the output voltage. The hard switching is not desirable,
`becauseit increases conductive noise and energy loss in the
`CEETsystem.
`
`[0008] Therefore, there exists a need for a simple CEET
`solution that provides a highly regulated power transfer
`between the primary and secondary sides and avoids harm-
`ful hard switching conditions.
`
`SUMMARYOF THE INVENTION
`
`[0009] Briefly, according to the present invention, a con-
`tactless electrical energy transmission system couples a
`powersource to a load. The system includes a transformer
`having a primary winding that is coupled to the power
`source through a primary resonantcircuit of an inverter and
`a secondary winding that is coupled to the load through a
`secondary resonant circuit of a rectifier. The primary and
`secondary resonant circuits are inductively coupled to each
`other. A primary control circuit is responsive to a current
`change through the primary resonant circuit to control the
`switching frequency of a controllable switching device for
`maintaining a substantially constant energy transfer between
`the primary winding and secondary winding in response to
`either one or both of a power source voltage change and a
`load change.
`
`[0010] According to another aspect, a secondary control
`circuit generates one or more pulse width modulated control
`signals for controlling the amountof energy delivered to the
`load under varying load conditions. The pulse width modu-
`lated signals are generated in response to a voltage variation
`across the load and a zero current crossing through the
`secondary resonantcircuit.
`
`[0011] According to yet another aspect of the present
`invention, a secondary controllable switching circuit
`is
`10
`10
`
`

`

`US 2004/0218406 Al
`
`Nov.4, 2004
`
`responsive to one or more pulse width modulated control
`signals. The secondary controllable switching circuit has
`one or more switchesthat are activated at substantially zero
`voltage to avoid hard switching conditions.
`
`[0012] According to some of the more detailed features of
`the present invention, the secondary control circuit detects a
`zero current crossing through the secondary resonant circuit
`to generate synchronized ramp signals for controlling the
`pulse width modulated control signals. In an exemplary
`embodiment, the synchronized ramp signals are 180° out of
`phase from each other.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`{0013] FIG. 1 shows a block diagram of a known CEET
`system,
`
`[0014] FIG. 2 shows a more detailed block diagram of the
`CEET system of FIG.1;
`
`[0015] FIG. 3 shows a block diagram of a CEET system
`according to the present invention;
`
`[0016] FIG. 4 shows a more detailed block diagram of the
`CEET system of FIG.3;
`
`{0017] FIG. 5 showsan equivalentcircuit diagram of the
`CEETsystem of the present invention;
`
`[0018] FIG. 6(2)-() show various topological stages for
`the equivalent circuit of FIG. 5;
`
`[0019] FIG. 7(a)-(g) show someof the waveformsfor the
`equivalent circuit of FIG. 5; and
`
`[0020] FIG. 8 shows a more detailed block diagram of the
`CEETsystem of FIG.3.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`[0021] FIG. 3 shows an exemplary block diagram of the
`CEETsystem in accordance with the present invention. The
`system of FIG. 3 includes a variable frequency resonant
`inverter at a primary side and a controlled rectifier at a
`secondary side that includes a load. The primary side and
`secondary side are inductively coupled through the primary
`and secondary windings of a transformer. As shown, the
`inverter couples a power source having a power voltage Vg
`to the primary winding through a primary resonant circuit
`comprising inductive and capacitive elements in the primary
`side. As describedlater in detail, a primary-current feed back
`frequency control block controls a primary switching fre-
`quency for regulating the power transfer between the pri-
`mary and secondary sides. On the secondary side,
`the
`rectifier, which is a controlled zero-voltage switching (ZVS)
`rectifier, couples the secondary winding to a load through a
`secondary resonant circuit comprising inductive and capaci-
`tive elements in the secondary side. The primary resonant
`circuit and the secondary resonant circuit are inductively
`coupled each other through the primary and secondary
`windings of the transformer.
`
`In accordance with one aspect of the present inven-
`[0022]
`tion, current through the primary winding is controlled in
`response to a sensed current change that is caused by a
`powervoltage V. or a load change. As such either one of a
`power voltage change or load change or both regulate the
`power transfer between the primary and secondary sides.
`
`More specifically, a primary controllable switching device
`has a switching frequency that controls the current flow
`through the primary winding. This aspect of the present
`invention senses primary resonant current changes for con-
`trolling the switching frequency of the primary controllable
`switching device so that the transferred power through the
`transformeris automatically maintained constant relative to
`powervoltage V, and load changes. Also,as described later
`in detail, in accordance with another aspect of the present
`invention, a secondary current zero-cross detection block is
`used with a synchronized ramp signal generator to control a
`pulse width modulated (PWM)feedback control block that
`provides tightly regulated control over a wide range of load
`conditions.
`
`[0023] FIG. 4 shows a more detailed block diagram ofthe
`CEETsystem of FIG.3 with a series resonant inverterin the
`primary side. The primary side is comprised of a pair of
`primary switches S,, and S,, which are shown with their
`antiparallel diodes. These switches form a primary con-
`trolled switching circuit. The inverter also includes a reso-
`nant capacitor C,, which is part of the primary resonant
`circuit. The secondary side is comprised of resonant capaci-
`tor Cg, diodes D, na po» and filter capacitor C. Secondary
`switches S, and S., which are also shown with their anti-
`parallel diodes, form a secondary controlled switching cir-
`cuit.
`
`[0024] FIG. 5 shows an equivalent circuit to the CEET
`system of the invention with leakage Lp, Ls, and magnetiz-
`ing L,,
`inductances of the transformer. To simplify the
`analysis,
`it
`is assumed that the input- and output-ripple
`voltages are negligible so that the voltages across the input
`and outputfilter capacitors can be represented by constant-
`voltage sources V, and Vo, respectively. As such, inductive
`and capacitive elements shown on the primary and second-
`ary sides create respective primary and secondary resonant
`circuits that are inductively coupled to each other.
`
`(0025] To further facilitate the explanation of the opera-
`tion, FIGS. 6(a)-(J) show topological stages of the circuit in
`FIG. 5 during a switching cycle, whereas FIGS. 7(a)(-(q)
`show the power-stage key waveforms for operation. To
`further simplify the analysis,
`the following analysis of
`operation assumesthat all semiconductor componentsin the
`circuit are ideal. i.e., that they exhibit zero resistance when
`in the on state and infinite resistance in the off state.
`Moreover, the magnetizing current i,, in FIG. 5 is in phase
`with resonant current i,,. Nevertheless, these assumptions
`do not have any significant effect on the explanation of the
`principle of operation of the proposed circuit.
`
`[0026] Before secondary switch S,, is turned on at t=T,,
`negative primary side resonant current i;p=iytlp=iyytiy_s/N
`flows through leakage inductance Lp, resonant capacitor Cp,
`and low-side switch S,, whereas, negative secondary-side
`resonant current 1, flows through leakage inductance Lg,
`resonant capacitor C,, output diode D,, and the antiparallel
`diode of secondary switch S,, as shown in FIG.6(/). Atthe
`same time, output diode D, and secondary switch S,are off
`blocking output voltage V,, whereas, high-side switch S,, is
`off blocking input voltage V,. As a result, secondary switch
`S, turns on with ZVS at t=Tp, as shown in FIG. 6(a).
`
`[0027] After secondary switch S, is turned on, the direc-
`tion of the resonant current is not changed until low-side
`switch S, is turned off at t=T,. After low-side switch S, is
`11
`11
`
`

`

`US 2004/0218406 Al
`
`Nov.4, 2004
`
`[0030] As can be seen, the voltage stress of switches S,,
`turned off at t=T,, resonant current i,, flowing through
`and S, is always limited to input voltage V, while the
`switch S, is diverted from the switch to its output capaci-
`voltage stress of S,,S,, D,, and D, are alwayslimited to the
`tance Cogs,, as Shown in FIG.6(6). Asaresult, the voltage
`output voltage Vo.
`across switch SL starts increasing, whereas the voltage
`across high-side switch S,, starts decreasing,as illustrated in
`FIGS. 7(c) and 7(d), since the sum of the voltage across
`switches S, and S,, is equal to input voltage V,. When the
`voltage across high-side switch S,, reaches zero at t=T3,1e.,
`whenoutput capacitance Cogs; of high-side switch S,, fully
`discharged, the antiparallel diode of high-side switch S,,
`begins to conduct, as shown in FIG.6(c). At the same time,
`low-side switch S, is off blocking input voltage V,. Because
`after t=T, input voltage V, is connected to the resonant
`circuit, the resonant currentstarts increasing. This topologi-
`cal stage ends at
`t=T, when i,p reaches zero and the
`antiparallel diode of high-side switch S,, stops conducting.
`As can be seen from FIG. 7(e), to achieve ZVS of S,,, it is
`necessary to turn on SH while its antiparallel diode is
`conducting.
`
`[0031] FIG. 8 shows an exemplary implementation of the
`CEET system of the present invention. The primary side
`includes a primary control block that uses current feed back
`for frequency control. The primary control block comprises
`an error amplifier with compensator that receives a sensed
`primary current Ipagensry and a reference current signal
`Ign Because of the primary and secondary resonant circuits
`are inductively coupled to each other, the sensed primary
`current Ipx¢sENsr) Varies relative to the power voltage V,
`changes as well as load changes. Based on the inputted
`sensed primary current Ipa¢sensry and reference current
`signal I,.,, the error amplifier circuit generates an error
`signal V.., which is applied to a voltage controlled oscillator
`(VCO). The VCO output sets the primary switching fre-
`quency fs used to control the primary controlled switching
`circuit, which includes primary switches S,, and S,. A driver
`controls the switching states of the primary switches S,, and
`S, by turning them on and off in accordance with the
`primary switching frequency fg.
`
`In FIG. 7(a), high-side switch S,, is turned on at
`[0028]
`t=T, with ZVS. As a result, after t=T, resonant current ip
`continues to flow through closed switch S,,, as shown in
`FIG. 6(e). Because of the assumption that currents iy, and
`i,are in phase with current1, », when the direction of current
`i,p is reversed at t=T’,, the direction of i,, and i,, is also
`reversed,as illustrated in FIGS. 7(e)-7(g). Consequently, at
`t=T,, current i,, which was flowing through output diode D,
`and the antiparallel diode of switch S,, is diverted to the
`antiparallel diode of switch S, and switch S,, as shown in
`FIG. 6(e). This topological stage ends at t=T,, when sec-
`ondary switch S, is turned off.
`
`[0029] After secondary switch S, is turned off at t=T.,
`primary side resonant current i,» flows through leakage
`inductance L,, resonant capacitor C,, and high-side switch
`S,, whereas, secondary-side resonant current i,, flows
`through leakage inductance Lg, resonant capacitor Cg, out-
`put diode D,, and the antiparallel diode of secondary switch
`S., as shown in FIG.6(f). As a result, secondary switch S,
`can be turned on with ZVSat t=T,, as shown in FIG. 6(g).
`This topological stage ends at t=T,, when high-side switch
`S,, is tumed off. After high-side switch S,, is turned off at
`t=T,, resonant current i,, flowing through switch S,, is
`diverted from the switch to its output capacitance Cogsy, aS
`shown in FIG. 6(/). As a result, output capacitance Cogs
`is being charged, whereas output capacitance Cogg;_ is being
`discharged. When output capacitance Cogs, is fully dis-
`charged at t=T,, the antiparallel diode of low-side switch S,,
`begins to conduct, as shown in FIG. 6(i). At the sametime,
`high-side switch S,, is off blocking input voltage V,. This
`topological stage ends at t=T,, when 1,, reaches zero and the
`antiparallel diode of low-side switch S, stops conducting. To
`achieve ZVSof S,, it is necessary to turn on S, while its
`antiparallel diode is conducting. In FIG. 7, low-side switch
`S, is turned on at t=-Tg with ZVS. As a result, after t=T,,
`resonant current1,p continues to flow through closed switch
`S,, as shown in FIG. 60). As shown in FIGS. 6(k) and 7,
`after t=T,,, the direction of currents 1,p, iy, and is are
`reversed so that current i,» flows through S,, whereas,
`current1,< flows through switch S, andthe antiparallel diode
`of switch S,, as shown in FIG.6(k). The circuit stays in this
`topological stage until the next switching cycleis initiated at
`t=T>.
`
`[0032] Because the primary switching frequency f, con-
`trols the current flow through the primary winding,
`the
`disclosed arrangement maintains a constant energy transfer
`between the primary and secondary sides over the entire
`range of power voltage V. and load variations. Conse-
`quently, the CEET system of the invention providesa tight
`regulation of delivered poweroverthe entire load and power
`source voltage ranges without a physical feedback connec-
`tion between the primary side and secondary side. As sated
`above, the primary switching frequency f, is controlled to
`keep the magnitude of the primary current constant, so that
`the maximum transferable power through the inductive
`coupling is automatically kept constant without an excessive
`circulating energy.
`
`the range of the primary switching
`[0033] Preferably,
`frequency f, is set to be higher than the primary resonant
`frequency to provide a Zero Voltage Switching (ZVS)
`arrangement for the primary switches S,, and S,, thereby
`avoiding hard switching conditions. Alternatively, the pri-
`mary switching frequency f, can be set to be lower than the
`primary resonant frequency primary to operate the primary
`switches S,; and S, with a zero current switching (ZCS)
`arrangement.
`
`In accordance with another aspect of the present
`[0034]
`invention, the CEET system provides the output voltage
`feedback controller with a constant PWM gain over the
`entire load range using synchronized ramp signals. The
`diodes D, na po» Which form the secondary rectifier, are
`controlled by a secondary control block. The secondary
`control block uses a ZVS PWM control to maintain a tight
`regulation of the output voltage in the presence of a varying
`load. The secondary control block includes two PWM
`modulators that are responsive to the output voltage varia-
`tions and the synchronized ramp signals for controlling the
`secondary switches S, and S, during various load conditions
`including light load and high load conditions. Under this
`Arrangement, a sensed output voltage Vorgnnsp) 1S com-
`pared with a reference voltage Vapp at the input of an error
`with compensation amplifier. A generated error signal V,,, at
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`the output of the error amplifier is compared with ramp
`signals Vaan, 20d Vaayps- Ramp signals Veayp, and
`Vap» te Synchronized to the zero crossing of the secondary
`resonant current and 180° out of phase each other as shown
`in FIGS. 7(/) and 7(i). By the comparisons between error
`signal V,, and ramp signals Vaaypi1, and Veampo gate
`signals S, and S, are generated as shown in FIGS. 7(j) and
`7(n).
`
`[0035] According to another aspect of the present inven-
`tion, the gate signals are generated such that the secondary
`switches S1 and S2 turn on whentheirantiparallel diodes are
`conducting. As a result, the CEET system of the present
`invention not only provides ZVS for the primary switches
`S,, and S; but also for the secondary switches S, and S,.
`
`[0036] When S, andS,are shorted,i.e., turned on, the load
`is separated from the secondary resonantcircuit, causing less
`damped resonance and thereby increasing the secondary
`resonant current. This is because the secondary resonant
`current does not go throughthe load and is bypassed through
`the S, and S, causing a short circuit with no damping that
`results in the secondary resonant current
`to increase.
`Because of the inductive coupling provided by the primary
`and secondary windings, the increased current is sensed at
`the primary side. Based on the increased sensed current, the
`primary control block Increases the switching frequency to
`maintain constant current through the primary winding.
`
`In case of above resonant frequency operation,
`[0037]
`when the switching frequency is reduced, higher current and
`thus more energyis delivered to the load. Conversely, when
`the switching frequencyis increased, lower current and thus
`less energy is delivered to the load. This can happen when
`S, and S, are opened,i.e., turnedoff. As a result, the load is
`connectedin series to the secondary resonantcircuit increas-
`ing resonance damping, which reduces secondary resonant
`current flow. As a result, sensed resonant current at
`the
`primary side is reduced,
`thereby reducing the primary
`switching frequency to maintain constant current through
`the primary winding. It should be noted that S, and S,
`operate at the same frequency as the primary side switches
`S, and S,;.
`
`In an exemplary implementation, the performance
`[0038]
`of the CEET system of the invention was evaluated on a
`36-W (12 V/3 A), universal-line-range (90-265 V,.) pro-
`totype circuit operating over a switching frequency range
`from 125 kHz to 328 kHz. The experimental circuit was
`implemented with the following components: switches S,,
`and S,—IRF840; secondary switch S, and S,—SI4810DY;
`and output diode D, and D,=MBR2045CT. Inductive cou-
`pling transformer T wasbuilt using a pair of modifiedferrite
`cores (EER28-3F3) with the primary winding (80 turns of
`AWG#44/75 strands Litz wire) and the secondary winding
`(18 turns of AWG#42/150 strands Litz wire). The control
`circuit was implemented with controllers UC3863, LM319,
`AD817, and LM393. ATL431 voltage-reference ICs is used
`for an output voltage reference for the locally controlled
`rectifier. An IR2110 driver is used to generate the required
`gate-drive signals for switches S,, and S,. Two TC4420
`drivers are used to generate the required gate-drive signals
`for switches S, and S,. The output voltage of the experi-
`mental circuit is well regulated with a voltage ripple less
`than 2% overthe entire input-voltage range. The measured
`
`efficiencies are approximately 84.4% at full load and mini-
`mum input voltage and approximately 78.5% at full load and
`maximum input voltage.
`
`1. Acontactless electrical energy transmission system for
`coupling a power source to a load, comprising:
`
`a transformer having a primary winding and a secondary
`winding;
`
`an inverter coupling said power source to said primary
`winding through a primary resonantcircuit;
`
`a primary controllable switching device responsive to a
`switching frequency that controls the flow of current
`through said primary winding;
`
`a rectifier coupling said secondary winding to said load
`through a secondary resonantcircuit that is inductively
`coupled to the primary resonant circuit; and
`
`a primary control circuit responsive to a current change
`through said primary resonant circuit to control the
`switching frequency for maintaining a substantially
`constant energy transfer between the primary winding
`and secondary winding in responseto at least one of a
`powersource voltage change and a load change.
`2. The system of claim 1 further including a secondary
`controllable switching device that is responsive to a load
`change for controlling the amount of energy delivered to the
`load.
`
`3. The system of claim 2, wherein the secondary control-
`lable switching device is responsive to at least one pulse
`width modulated control signal for controlling the amount of
`energy delivered to the load.
`4. The system of claim 3, wherein a secondary control
`circuit generates the at least one pulse width modulated
`control signal
`in response to at
`least one of a voltage
`variation across the load and a zero current crossing detec-
`tion through said secondary resonant circuit.
`5. The system of claim 4, wherein the secondary control-
`lable switching device includes at least one switch respon-
`sive to the pulse width modulated control signal, wherein the
`switch is activated at a substantially zero voltage.
`6. The system of claim 5, wherein the secondary control
`circuit detects a zero current crossing through said second-
`ary resonant circuit to generate synchronized ramp signals
`for controlling the at
`least one pulse width modulated
`control signal.
`7. The system of claim 6, wherein the synchronized ramp
`signals are 180° out of phase with respect to each other.
`8. Acontactless electrical energy transmission system for
`coupling a power source to a load, comprising:
`
`a transformer having a primary winding and a secondary
`winding;
`
`an inverter coupling said power source to said primary
`winding through a primary resonantcircuit;
`
`a primary controllable switching device responsive to a
`switching frequency that controls flow of current
`through said primary winding;
`
`a secondary rectifier coupling said secondary winding to
`said load through a secondary resonant circuit that is
`inductively coupled to the primary resonantcircuit; and
`
`least one
`a secondary control circuit that generates at
`pulse width modulated control signal for controlling
`
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`the amount of energy delivered to the load, wherein the
`at least one pulse width modulated signal is generated
`in response to a voltage variation across the load and a
`zero current crossing through said secondary resonant
`circuit.
`9. The system of claim 8 further including a primary
`control circuit responsive to a current change through said
`primary resonantcircuit to control the switching frequency
`of said primary controllable switching device for maintain-
`ing a substantially constant energy transfer between the
`primary winding and secondary winding in responseto at
`least one of a power source voltage change and a load
`change.
`10. The system of claim 8 further including a secondary
`controllable switching circuit that is responsive to the at
`least one pulse width modulated control signal for delivering
`energy to the load.
`11. The system of claim 10, wherein the secondary
`controllable switching device includes at least one switch
`responsive to the pulse width modulated control signal,
`wherein the switch is activated at a substantially zero
`voltage.
`12. The system of claim 8, wherein the secondary control
`ci