U.S. patent application number 09/881429 was filed with the patent office on 2003-01-02 for rotary transformer with synchronized operation.
Invention is credited to Michaels, Paul A., Rea, Irvin B..
Application Number | 20030001707 09/881429 |
Document ID | / |
Family ID | 25378466 |
Filed Date | 2003-01-02 |
United States Patent
Application |
20030001707 |
Kind Code |
A1 |
Michaels, Paul A. ; et
al. |
January 2, 2003 |
ROTARY TRANSFORMER WITH SYNCHRONIZED OPERATION
Abstract
A rotary transformer includes a resonant circuit and a coil
drive circuit. The resonant circuit includes a resonating capacitor
connected to a power MOS transistor, coupled across the primary
coil of the transformer. The coil drive circuit includes a diode
connected to a power MOS transistor coupled across the primary coil
of the transformer. A microprocessor detects changes in the voltage
across the primary coil. The resonant circuit is connected and
disconnected from the transformer during a power transfer mode and
a data transfer mode, respectively. During the power transfer mode,
stored energy in the leakage inductance of the primary coil is used
for power coupling, via the resonant circuit, instead of being
dissipated as heat. The resonant circuit is disconnected from the
rotary transformer during the data transfer mode to maximize
bandwidth for two-way data transfer between the primary and
secondary sides of the transformer. The transformer uses a
synchronous mode of operation in which the power MOS transistor of
the coil drive circuit is turned on when the voltage across the
primary coil changes from a positive to a negative value during the
power transfer mode. The synchronous mode of operation virtually
eliminates a current spike through the diode of the coil drive
circuit and provides the microprocessor an appropriate amount of
time to recognize the voltage changes across the primary coil.
Inventors: |
Michaels, Paul A.; (Livonia,
MI) ; Rea, Irvin B.; (Royal Oak, MI) |
Correspondence
Address: |
RADER, FISHMAN & GRAUER PLLC
39533 WOODWARD AVENUE
SUITE 140
BLOOMFIELD HILLS
MI
48304-0610
US
|
Family ID: |
25378466 |
Appl. No.: |
09/881429 |
Filed: |
June 14, 2001 |
Current U.S.
Class: |
336/182 |
Current CPC
Class: |
H01F 38/18 20130101 |
Class at
Publication: |
336/182 |
International
Class: |
H01F 021/04 |
Claims
What is claimed is:
1. A rotary transformer, comprising: a primary coil; a secondary
coil; a resonant circuit coupled to the primary coil; a coil drive
circuit coupled to the primary coil; and a microprocessor connected
to the primary coil for detecting a voltage across the primary
coil, wherein a control voltage is input to the coil drive circuit
when the microprocessor detects a transition of the voltage across
the primary coil from a positive voltage to a negative voltage.
2. The transformer according to claim 1, wherein the coil drive
circuit includes a diode connected to the primary coil and a first
drive transistor connected to the diode.
3. The transformer according to claim 2, wherein the resonant
circuit includes a resonating capacitor connected to the primary
coil and a second drive transistor connected to the resonating
capacitor.
4. The transformer according to claim 3, wherein a control voltage
input to the second drive transistor turns the second drive
transistor on to connect the capacitor to the primary coil during a
power transfer mode.
5. The transformer according to claim 4, wherein the transition of
the voltage across the primary coil from the positive voltage to
the negative voltage occurs during the power transfer mode.
6. The transformer according to claim 5, wherein the transition
occurs at approximately half cycle of the power transfer mode.
7. The transformer according to claim 3, wherein a control voltage
input to the second drive transistor turns the second drive
transistor off to disconnect the capacitor from the primary coil
during a data transfer mode.
8. The transformer according to claim 1, further comprising a
full-wave rectifier coupled to the secondary coil.
9. A rotary transformer, comprising: a primary coil; a secondary
coil; a resonant circuit coupled to the primary coil, the resonant
circuit including a capacitor connected to the primary coil and a
first drive transistor connected to the capacitor; and a coil drive
circuit coupled to the primary coil, the coil drive circuit
including a diode connected to the primary coil and a second drive
transistor connected to the diode, wherein a control voltage input
to the second drive transistor turns the second drive transistor on
when a voltage across the primary coil transitions from a positive
voltage to a negative voltage.
10. The transformer according to claim 9, wherein a control voltage
input to the first drive transistor turns the first drive
transistor on to connect the capacitor to the primary coil during a
power transfer mode.
11. The transformer according to claim 10, wherein a control
voltage input to the first drive transistor turns the drive
transistor off to disconnect the capacitor from the primary coil
during a data transfer mode.
12. The transformer according to claim 10, wherein the transition
of the voltage across the primary coil from the positive voltage to
the negative voltage occurs during the power transfer mode.
13. The transformer according to claim 12, wherein the transition
occurs at approximately half cycle of the power transfer mode.
14. The transformer according to claim 9, further comprising a
full-wave rectifier coupled to the secondary coil.
15. A method of operating a rotary transformer having a primary
coil, a second coil, a resonant circuit coupled to the primary
coil, a coil drive circuit coupled to the primary coil, the method
comprising the steps of: supplying a control voltage to the
resonant circuit; detecting a transition of a voltage across the
primary coil when the control voltage is supplied to the resonant
circuit; and supplying a control voltage to the coil drive circuit
when the transition of the voltage across the primary coil is
detected in the detecting step.
16. The method according to claim 15, wherein the transition of the
voltage across the primary coil comprises a transition from a
positive voltage to a negative voltage.
17. The method according to claim 15, wherein the resonant circuit
comprises a resonating capacitor connected to the primary coil and
a second drive transistor connected to the resonating
capacitor.
18. The method according to claim 15, wherein the coil drive
circuit comprises a diode connected to the primary coil and a
second drive transistor connected to the diode.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention is directed to rotary transformers,
and more particularly to a rotary transformer with a synchronized
mode of operation that transfers both power and data between two
structures.
[0003] 2. Description of the Related Art
[0004] Rotary transformers are often used for transmitting both
data and power between two structures that rotate relative to one
another, such as between a vehicle tire and its corresponding wheel
axle in a tire pressure sensor system. In another example, a rotary
transformer can be used to couple data and power from a steering
column to a steering wheel, as disclosed in co-assigned U.S. Pat.
No. 6,121,692, the entire contents of which are herein incorporated
by reference.
[0005] As is known in the art, loosely coupled power transformers
do not conduct power efficiently between the primary and secondary
of the transformer. Instead, a part of the input current into the
primary coil stores energy in the leakage inductance of the coil.
Prior art structures often include a Zener diode across the primary
to absorb the energy of the voltage spike that occurs in the
transformer when the current to the primary coil is turned off.
More particularly, the Zener diode will conduct current before the
drive transistor in the primary side breaks down. However, under
this approach, the stored energy is dissipated as heat, thereby
wasting the energy built up in the primary coil's leakage
inductance and lowering the power coupling efficiency of the
transformer.
[0006] To overcome this problem, conventional rotary transformer
designs tend to focus on methods of increasing the coupling
efficiency by constructing a magnetically efficient structure for
power transmission, such as by using more expensive,
high-efficiency core materials, and then adding a complex load
impedance mechanism for providing limited two-way communication
through the transformer. This results in an overly complicated
structure requiring close mechanical tolerances, which increases
the manufacturing cost of the system. Further, the bandwidth for
these structures tends to be relatively narrow, which limits the
amount of data or the speed at which data can be transmitted
between the primary and secondary sides of the transformer.
[0007] To increase the bandwidth in the rotary transformer, a
loosely coupled rotary transformer that includes a resonant
circuit, such as a resonating capacitor connected to a power MOS
transistor, may be coupled across the primary coil of the
transformer, as described in co-pending, co-assigned U.S. patent
application Ser. No. 09/395,817 filed on Sep. 14, 1999, the entire
contents of which are herein incorporated by reference. In the
loosely coupled rotary transformer, the resonant circuit is
connected and disconnected from the transformer during a power
transfer mode and a data transfer mode, respectively. During the
power transfer mode, stored energy in the leakage inductance of the
primary coil is used for the power coupling, via the resonant
circuit, instead of being dissipated as heat. The resonant circuit
is disconnected from the rotary transformer during the data
transfer mode to maximize bandwidth for two-way data transfer
between the primary and secondary sides of the transformer.
Including the resonant circuit in the loosely coupled transformer
optimizes data and power transfer without requiring the use of
high-cost, high-efficiency magnetic structures in the core of the
transformer.
[0008] The loosely coupled rotary transformer utilizes a fixed
frequency drive circuit, and a resonant drive mode that is very
power efficient compared to other known rotary transformer drive
methods. The transformer resonant frequency and drive frequency is
matched for the nominal supply voltage and secondary load. However,
a problem may arise when the supply voltage is not properly
regulated, or the secondary load is subject to large changes from a
nominal level. In both cases, the power coupling efficiency may
decrease from a nominal level.
[0009] The inventors of the present invention have recognized this
problem and have modified the operation of the rotary transformer
drive circuit to maintain high power efficiency for changes in
either supply voltage or secondary load. This is especially
important for vehicle operation where the supply voltage for proper
operation may vary between a voltage of approximately 9.0 and
approximately 16.0 volts, which is almost a 2:1 ratio.
SUMMARY OF THE INVENTION
[0010] The invention comprises a rotary transformer with a
synchronous mode of operation to facilitate the transfer of power
and two-way communications between two structures, such as a column
and steering wheel of a vehicle. During normal operation, the
rotary transformer repetitively alternates between a power transfer
mode and a data transfer mode by multiplexing time across the
rotary transformer. A microprocessor supplies a pulse train that
periodically applies full power from a power supply, such as a
vehicle battery to the transformer's primary coil, or "column
coil." In the referenced prior patent, during the power mode when
the pulses supplied to the primary coil are "on", the
microprocessor disconnects a resonating or tuning capacitor C1 from
the primary coil. When the pulses are "off", the resonating
capacitor is reconnected, at which time energy stored in the
resonating capacitor is supplied across the rotary transformer. By
connecting the resonating capacitor C1 to the primary coil only
when the pulses are turned "off", the power required to drive the
rotary transformer is minimized and the energy recovered from the
primary coil is maximized. For synchronized operation, the subject
of this application, the tuning capacitor is connected during the
entire power mode. It is disconnected only during the data transfer
mode. The microprocessor can also adjust the width of the pulses
supplied to the primary coil to maintain a constant power level at
the wheel circuit using means well known in the art, such as a
voltage regulator.
[0011] After a preset length of time allotted for the power
transfer mode, the microprocessor causes the primary circuit to
change to the data transfer mode. During this mode, the primary
circuit transmits a preset number of data bits to the secondary
side across the rotary transformer, and then the secondary side
transmits a preset number of bits to the primary circuit across the
rotary transformer. Then, the circuit returns to the power transfer
mode and repeats the sequence.
[0012] One aspect of the invention is that during the power
transfer mode, the drive transistor of the coil drive circuit is
switched "on" when the voltage across the primary coil changes from
positive to negative at approximately one half of a cycle to
provide a synchronous mode of operation. This synchronized mode of
operation virtually eliminates a current spike through the diode of
the coil drive circuit that exists using conventional modes of
operation, which turn the drive transistor "on" at the end of the
power transfer mode, such as in fixed frequency and variable
frequency modes of operation. By preventing the current spike
through the diode of the coil drive circuit, the stress to the
driving transistor and electromagnetic interference to the
operating environment of the rotary transformer are minimized. In
addition, the synchronous mode operation of the invention provides
the microprocessor a sufficient amount of time to recognize the
change of the resonant waveform of the primary circuit during the
power transfer mode so that it can change the output ports, unlike
conventional modes of operation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] In the drawings:
[0014] FIG. 1 shows a rotary transformer drive circuit used for
fixed frequency, variable frequency or synchronized modes of
operation.
[0015] FIG. 2 shows the associated waveforms for the rotary
transformer drive circuit of FIG. 1 in the fixed frequency
operation mode.
[0016] FIG. 3 shows the waveforms for the rotary transformer drive
circuit of FIG. 1 when the supply voltage is larger than the
minimum level and the secondary load is less than its maximum
level.
[0017] FIG. 4 shows the waveforms of a rotary transformer drive
circuit having synchronous operation according to an embodiment of
the invention.
[0018] FIG. 5 shows the waveforms for the rotary transformer drive
circuit of the invention in FIG. 4 for a nominal supply voltage of
approximately 14V and a nominal secondary load.
[0019] FIG. 6 shows the waveforms for the rotary transformer drive
circuit of the invention in FIG. 4 for a low supply voltage of 8V
and a nominal secondary load.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] FIG. 1 shows a rotary transformer 100 using a fixed
frequency to transfer power and data between two structures, such
as a column circuit and a wheel circuit of a steering wheel of a
vehicle. The transformer 100 includes a primary side 101 having a
primary coil 102 and a secondary side 103 having a secondary coil
104. The voltage potential across the primary coil 102 is referred
to as Vp, and the current passing through the primary coil 102 is
referred to as Ip. The voltage potential across secondary coil 104
is referred to as Vs. Resistors R1 and R3 are placed across the
primary coil 102 and secondary coil 104, respectively, to control
any ringing produced by the transformer 100 due to the loose
coupling. Typically, the resistance values of resistors R1 and R3
are reduced until the primary and secondary resonant circuits
formed by the transformer's 100 leakage inductance and stray
capacitance are critically damped. As a result, when the capacitor
C1 is disconnected during the power transfer mode, the
transformer's 100 bandwidth is very large, allowing the invention
to transmit digitally controlled pulse trains as well as various
limited bandwidth sine wave coding schemes, such as frequency-shift
keying (FSK) or other comparable schemes. Thus, the large bandwidth
produced by the structure in FIG. 1 allows large amounts of
virtually any data type to be transmitted between the primary and
secondary sides, which is advantageous in various automotive
applications.
[0021] A resonating capacitor C1 and a drive transistor Q2 are
placed across the primary coil 102 to form a resonant circuit 107.
As a result, the stored energy in the leakage inductance of the
primary coil 102 is coupled to the resonating capacitor C1 when the
drive transistor Q2 is turned off. In doing so, the primary side of
the transformer 100 continues to couple energy to the secondary
side after the drive transistor Q2 is turned off, increasing the
power coupling efficiency and decreasing the overall amount of heat
generated by the transformer 100.
[0022] The rotary transformer 100 also includes a diode D1
connected to the collector of the transistor Q1, which is
illustrated as an n-channel MOS driver to form a coil drive circuit
105. The diode D1 has a negligible effect on the data transfer and
permits the primary voltage Vp to go below ground, as illustrated
in FIG. 2, thus extending the period of active power coupling
between the primary and secondary sides of the transformer 100. The
increase in the power coupling time generally increases the overall
power efficiency enough to more than compensate for the additional
loss due to the forward voltage drop across diode D1. If transistor
Q1 is a bipolar NPN transistor rather than an n-channel MOS driver
as described above, diode D1 may not be needed, provided that the
collector swing of the bipolar NPN transistor is less than its
base-emitter breakdown voltage.
[0023] FIG. 1, a series connected capacitor C2 and a resistor R2 is
connected between the driven side of the coil 102 and the common
ground. This permits the AC voltage at the drive side of coil 102,
Point C, to be coupled to a microcomputer 106, which controls the
signals applied to control inputs A and B. In a preferred
embodiment, the pulse train is supplied at a frequency of
approximately 25 kHz. The pulse duration of the pulse train
determines the amount of power that is transferred from the primary
side to the secondary side of transformer 100. As shown in FIG. 1,
a full wave rectifier 108 may be connected to the transformer 100
to extract the power being coupled to the secondary side during the
power transfer mode.
[0024] A preferred set of waveforms for the power transfer mode of
the non-synchronized operating mode of the prior art is shown in
FIG. 2. In operation, a positive control voltage V.sub.A applied to
the gate of drive transistor Q1 of the coil drive circuit 105 turns
Q1 on. Simultaneously an inverted control voltage V.sub.B is
applied to the gate of transistor Q2 of the resonant circuit 107 to
turn Q2 off. As a result, the primary coil 102 has a voltage
potential very close to ground potential, and the voltage Vp across
the primary coil 102 is approximately equal to V.sub.BATT. After a
delay of Td, the drive transistor Q1 of the coil drive circuit 105
is turned off and the drive transistor Q2 of the resonant circuit
107 is turned on. The stored energy in the leakage inductance of
the primary coil 102 generates a damped sine wave for voltage Vp
across the primary coil 102. Provided that the load is at its
maximum level and the battery voltage V.sub.BATT is at its minimum
level, and the resonating capacitor C1 is sized for these levels,
the sine wave will have just completed one cycle at time Tp when
the cycle will be repeated.
[0025] As shown in the waveforms of FIG. 2, the resonating
capacitor C1 is disconnected by turning drive transistor Q2 off
whenever drive transistor Q1 is turned on. As a result, drive
transistor Q1 does not have to supply any current I.sub.C1 to
resonating capacitor C1, allowing all of the drive current to go to
the transformer 100. When the drive transistor Q2 is turned off,
the stored energy in the primary leakage inductance resonantly
couples the resonating capacitor C1 to the transformer 100 and then
moves back to the primary leakage inductance for continuous power
coupling with the secondary side. In other words, placing the
resonating capacitor C1, rather than a Zener diode, across the
primary coil 102 allows the energy stored in the primary leakage
inductance of the coil 102 to be used for power coupling rather
than wasted as dissipated heat. Note that power MOS transistors can
conduct in either direction, a function that is necessary for
resonating capacitor C1 to be effective as a resonating capacitor
in the illustrated embodiment. If a bipolar NPN transistor were to
be used instead of the power MOS transistor Q2, a diode would need
to be placed between the collector and emitter terminals of the
bipolar NPN transistor for the circuit to function in the same
manner as a circuit containing the power MOS transistor.
[0026] Resonating capacitor C1 increases the power coupling
efficiency of the inventive transformer 100. However, the
resonating capacitor C1 tends to limit the bandwidth of the data
transfer to an undesirably low level. To avoid this problem, the
invention preferably time-multiplexes the data and the power modes,
continuously switching between the two modes to provide both
efficient power transfer and a wide bandwidth for two-way data
transfer. More particularly, control voltage V.sub.B is input into
drive transistor Q2, turning drive transistor Q2 on and off to
connect and disconnect resonating capacitor C1 and switch the
transformer 100 between operating in the power transfer mode for a
fixed time period, e.g. 5 ms, and in the data mode for a fixed time
period, e.g. 500 .mu.s.
[0027] The transformer 100 preferably cycles continuously between
the two modes. The bit rate and/or the duration of the data
transfer mode can be modified in any known manner to optimize the
amount of data transferred between the primary and secondary sides.
For example, using a 100 kHz data rate (10 .mu.s period) transfers
50 bits of data between the primary side and the secondary side in
500 .mu.s. Experimental studies with a low-cost air core
transformer show that data bit rates over 1 MHz are possible in the
invention. Furthermore, inserting a 500 .mu.s data transfer period
once every 5 ms of power transfer time reduces the power mode duty
factor by only 10%. Depending on the particular application in
which the inventive transformer circuit is used, the length of the
data transfer period can be smaller than 0.1% of the power transfer
period.
[0028] One advantage of the invention is that the transformer 100
provides both an acceptable power transfer and data transfer
without requiring specialized, higher-cost magnetic materials,
allowing the inventive circuit to be manufactured with lower-cost,
easily available air core transformers. More particularly,
including a resonant control circuit 103 across the primary coil
102 in a loosely coupled transformer allows energy stored in the
leakage inductance of the primary coil 102 to be coupled to the
secondary side rather than being wasted as dissipated heat.
Further, the invention can switch between power transfer and data
transfer modes by simply connecting and disconnecting the resonant
control circuit 103, making the circuit of the invention much
simpler than known structures using complex load impedance
mechanisms for generating data transfer capabilities in a
transformer.
[0029] Under normal operating conditions, the supply voltage
V.sub.BATT is larger than the minimum level, and the load is less
than its maximum level. The pulse width will be decreased to
maintain a constant secondary voltage Vs. Under these conditions
the resonant sine wave will be interrupted after more than a
complete cycle has been completed. The resulting waveforms, shown
in FIG. 3, disclose a very high spike in the current through diode
D1 as the driving transistor Q1 is turned on. The spike not only
increases stress to the driving transistor Q1, but also causes
electromagnetic interference that is not desirable in the
automotive environment.
[0030] One solution to these problems is to operate the transformer
100 at a variable frequency, rather than at a fixed frequency
described above. Operating the transformer 100 at a variable
frequency permits precisely one cycle of the data transfer mode
before switching to the power mode of operation. Operating the
transformer 100 at a variable frequency could be accomplished by
monitoring the transformer primary voltage Vp at point C and
turning the drive transistor Q1 on and the resonating capacitor C1
off just as the primary voltage Vp changes from negative to
positive at time Td.
[0031] However, some problems may exist using the variable
frequency approach described above. One problem is that the rate of
change of the waveform for the primary voltage Vp occurs in about
four (4) to six (6) volts per microsecond. As a result, the
microprocessor 106 would have to recognize the change in the
primary voltage Vp from a negative-to-positive voltage and then
change the output ports all within about 0.2 .mu.s, which may be
impractical for a low cost automotive microprocessor.
[0032] The inventors have recognized this problem and have provided
a transformer 100 with a synchronous mode of operation that
virtually eliminates the spike in the current through diode D1 when
the driving transistor Q1 is turned on while providing the
microprocessor an adequate amount of time to change the output
ports.
[0033] Referring now to FIG. 4, the drive transistor Q1 is switched
on when the voltage at Point C changes from negative to positive at
one half of a cycle at time Ts, rather than at the end of each
complete cycle at time Td as in conventional transformers. When the
drive transistor Q1 is turned on at time Ts, the drive current Ip
will not flow because of diode D1 and a negative primary voltage Vp
at Point C, as shown in FIG. 1. After a half cycle when the voltage
Vp at Point C changes from negative to positive, the drive current
I.sub.Q1 will automatically flow because the drive transistor Q1
has already been switched on. It will be appreciated that the time
Ts may occur when the primary voltage Vp is slightly positive
because the primary voltage Vp may not drop completely to zero
during the data transfer mode.
[0034] Using the synchronous mode of operation of the invention,
the amount of time for the microcomputer 106 to change the output
ports is greatly improved as compared to conventional approaches.
For example, at a resonant frequency of approximately 50 kHz, the
microcomputer 106 has 10 microseconds to recognize a state change a
resonant frequency of approximately 50 kHz using the invention,
instead of 0.2 microseconds using conventional approaches. Thus,
the invention provides a factor of 50 reduction in speed
requirement of the microprocessor 106 and the speed requirement of
the microprocessor 106 practical in an automotive environment.
Moreover, because the capacitor C1 is charged to less than one
volt, the resonating capacitor C1 does not have to be switched off
during the power transfer mode of operation. Because the drive
current I.sub.Q1 is switched from off-to-on and from on-to-off at
essentially zero voltage across the resonating capacitor C1, there
are no large current spikes, as shown in FIG. 4. Instead, the
primary coil 102 of transformer 100 can simply switch between the
resonating capacitor C1 and the drive transistor Q1.
[0035] In summary, the invention switches the drive transistor Q2
on to connect the resonating capacitor C1 to the primary coil 102
during the power transfer mode and disconnected during the data
transfer mode. During the power transfer mode, the microcomputer
106 detects the transition of voltage Vp across the primary coil
102 (at point C) from a positive-to-negative and turns the drive
transistor Q1 on. The drive transistor Q1 is switched off when the
coupled energy to the transformer secondary coil 104 is sufficient
to provide sufficient power to the secondary side.
[0036] One aspect of the invention is that at nominal power loads,
the transformer 100 using the synchronous mode of operation of the
invention has the same power efficiency as a transformer using a
fixed frequency mode of operation. Another aspect of the invention
is that when the supply voltage, V.sub.BATT or the secondary load
changes, the invention can adapt and maintain very high power
efficiency.
[0037] FIG. 5 shows the waveforms for the control voltage V.sub.A
and the transformer primary voltage Vp for a battery voltage
V.sub.BATT of approximately 14 volts and a nominal secondary load
using the synchronous mode of operation of the invention. As can be
seen in FIG. 5, the transitions between the power transfer and data
transfer modes are continuous without any abrupt discontinuities at
the mode changes.
[0038] FIG. 6 shows the waveforms for the control voltage V.sub.A
and the transformer primary voltage Vp that result when the battery
voltage, V.sub.BATT, drops from approximately 14 volts to about 8
volts with approximately the same secondary load as in FIG. 5. As
seen in FIG. 6, the period for the power transfer mode is longer to
supply the same average power to the secondary side at a lower
supply voltage, V.sub.BATT. However, the transitions between the
power transfer and data transfer modes are continuous and lack any
abrupt discontinuities at the mode changes, similar to the
waveforms in FIG. 5.
[0039] While the invention has been specifically described in
connection with certain specific embodiments thereof, it is to be
understood that this is by way of illustration and not of
limitation, and the scope of the appended claims should be
construed as broadly as the prior art will permit.
* * * * *