U.S. patent application number 13/079612 was filed with the patent office on 2011-09-29 for synchronous rectifier and controller for inductive coupling.
Invention is credited to Jonathan Robert Knight.
Application Number | 20110234157 13/079612 |
Document ID | / |
Family ID | 40252949 |
Filed Date | 2011-09-29 |
United States Patent
Application |
20110234157 |
Kind Code |
A1 |
Knight; Jonathan Robert |
September 29, 2011 |
SYNCHRONOUS RECTIFIER AND CONTROLLER FOR INDUCTIVE COUPLING
Abstract
A synchronous rectifier is arranged to rectify inductively
coupled power using PETs (field effect transistors) to minimize the
voltage drop of the rectifier, which minimizes power loss. Power
loss is an important consideration in applications where fairly
significant power is coupled to a device (such as a battery charger
or other energy storage device) for a fairly short time (such as
less than one hour) at a fairly low voltage (such as around 2.5 to
4.5 volts). Body diodes of the PETs can be used to supply power for
bootstrapping and control logic for controlling the PFTs.
Inventors: |
Knight; Jonathan Robert;
(Tokyo, JP) |
Family ID: |
40252949 |
Appl. No.: |
13/079612 |
Filed: |
April 4, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11777710 |
Jul 13, 2007 |
7920396 |
|
|
13079612 |
|
|
|
|
Current U.S.
Class: |
320/108 ;
307/104 |
Current CPC
Class: |
H02M 3/33592 20130101;
Y02B 70/10 20130101 |
Class at
Publication: |
320/108 ;
307/104 |
International
Class: |
H02J 7/02 20060101
H02J007/02; H01F 38/14 20060101 H01F038/14 |
Claims
1. A synchronous switch circuit, comprising: an inductor for
producing an alternating current voltage in response to a received
changing electromagnetic field; a synchronous rectifier comprising
a first and second high side switch and a first and second low side
switch that rectify the alternating current voltage to produce a
direct current voltage; an energy storage device for storing energy
from the direct current voltage; and control circuitry for
controlling the first and second high side switch and a first and
second low side switch in response to a state of the alternating
current voltage signal.
2.-20. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present disclosure generally relates to alternating
current rectification. More particularly, the present disclosure
relates to synchronous rectification of inductively coupled
power.
BACKGROUND
[0002] Power control in battery-powered mobile devices is of
increased concern in recent times. Most battery-powered mobile
devices use a voltage regulator that can both limit and regulate
power that is being supplied to the various electronic circuits in
the device. Examples of battery-powered mobile devices include
personal data assistants (PDAs), cellular telephones, portable
messaging devices for email and text messages, digital cameras,
personal music player devices, and others.
[0003] Various circuit arrangements have also been made for
charging the battery and otherwise powering the portable device.
For example, a coil in the battery-powered mobile device can be
used to inductively generate a sinusoidal signal that is then
rectified by a full-wave rectifier to generate a direct current
(DC) voltage. The DC voltage is then filtered and regulated, for
example, by a low dropout (LDO) voltage regulator to produce a
regulated 5 volt source for powering the electronic circuits in the
device.
[0004] Conventional full-wave bridge rectifiers utilize junction
diodes. Each junction diode has a forward bias voltage drop of
approximately 0.7V. Thus, for a full-wave bridge rectifier, the
resulting voltage drop relative to the input voltage can be as high
as 1.4 volts. A voltage drop of 450 millivolts can also be present
across the LDO voltage regulator. Both of these voltage drops can
generate an amount of heat that is often undesired.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] Non-limiting and non-exhaustive embodiments are described
with reference to the following drawings.
[0006] FIG. 1 is a schematic diagram illustrating a synchronous
rectifier and controller for inductive coupling of power.
[0007] FIG. 2 is a schematic diagram illustrating another
synchronous rectifier and controller for inductive coupling of
power.
[0008] FIG. 3 is a schematic diagram illustrating the use of NFET
transistors for low-side rectification and PFET transistors for
high-side rectification.
[0009] FIG. 4 is a schematic diagram illustrating the use of NFET
transistors being used for both low-side- and
high-side-switching.
[0010] FIG. 5 is a schematic diagram illustrating the use of NFET
transistors implemented using a monolithic process, but where the
isolation of the NFET transistors from a P-epitaxial substrate is
not desired.
[0011] FIG. 6 is a schematic diagram illustrating gate driving
circuitry and control circuitry powered by a synchronous
rectifier.
DETAILED DESCRIPTION
[0012] Various embodiments will be described in detail with
reference to the drawings, where like reference numerals represent
like parts and assemblies throughout the several views. Reference
to various embodiments does not limit the scope of the invention,
which is limited only by the scope of the claims attached hereto.
Additionally, any examples set forth in this specification are not
intended to be limiting and merely set forth some of the many
possible embodiments for the claimed invention.
[0013] Throughout the specification and claims, the following terms
take at least the meanings explicitly associated herein, unless the
context clearly dictates otherwise. The meanings identified below
are not intended to limit the terms, but merely provide
illustrative examples for use of the terms. The meaning of "a,"
"an," and "the" may include reference to both the singular and the
plural. The meaning of "in" may include "in" and "on." The term
"connected" may mean a direct electrical, electro-magnetic,
mechanical, logical, or other connection between the items
connected, without any electrical, mechanical, logical or other
intermediary therebetween. The term "coupled" can mean a direct
connection between items, an indirect connection through one or
more intermediaries, or communication between items in a manner
that may not constitute a connection. The term "circuit" can mean a
single component or a plurality of components, active and/or
passive, discrete or integrated, that are coupled together to
provide a desired function. The term "signal" can mean at least one
current, voltage, charge, data, or other such identifiable
quantity.
[0014] Briefly stated, the present disclosure generally relates to
synchronous rectification of inductively coupled power. Inductively
coupled power can be rectified using FETs (field effect
transistors) to minimize the voltage drop of the rectifier, which
minimizes power loss. Power loss is an important consideration in
applications where fairly significant power is coupled to a device
(such as a battery charger or other energy storage device) for a
fairly short time (such as less than one hour) at a fairly low
voltage (such as around 2.5 to 4.5 volts). Body diodes of the FETs
can be used to supply power for bootstrapping and control logic for
controlling the FETs.
[0015] FIG. 1 is a schematic diagram illustrating a synchronous
rectifier and controller for inductive coupling of power. Circuit
100 receives power from receive coil L110. Receive coil L110 can be
a receive coil from a telephone. The output voltage of the receive
coil is sinusoidal (because of the inductive coupling), so
rectification, peak detection and filtering is typically performed
by circuit 100 (in a similar fashion to rectification and filtering
in a conventional AC supply).
[0016] In operation, zero-crossing detection comparator U102 is
used to detect a zero crossing on power signals "plus" and "minus."
In response to a detection of a zero-crossing, signals are
generated to turn on an appropriate low-side switch. For example,
when a positive-going zero crossing is detected, comparator U102
activates plus phase low-side switch M102. When a negative zero
crossing is detected, comparator U102 activates minus phase
low-side switch M103.
[0017] High-side switches M101 and M104 are enabled in response to
the output of the zero-crossing detection comparator as well as the
as phase rectifier comparators. For example, plus phase high-side
switch M104 conducts when the output of the zero-crossing detection
comparator indicates a positive-going zero crossing and the plus
phase rectifier comparator U101 indicates that signal "plus" is
greater than "Vout." Minus phase high-side switch M101 conducts
when the output of the zero-crossing detection comparator indicates
a negative-going zero crossing is detected and the minus phase
rectifier comparator U103 indicates that signal "minus" is greater
than "Vout." Thus a comparator is used to compare the output
capacitor C101 with each phase voltage and will be high whenever
the phase voltage is higher.
[0018] Power for low-voltage control circuitry can be derived by
using a passive bridge rectifier driving a series regulator. In
another example, the synchronously-rectified phase voltage in FIG.
1 can be used to supply power to another series regulator. The
output of these two regulators can be tied in parallel to provide
power for the control circuits. As described below with reference
to FIGS. 3-6, body diodes that are inherent in the rectifier FETs
can also be used as the passive bridge rectifier. Thus, a single
LDO supplied from the output of the synchronous rectifier can be
used to supply power to the control circuitry when body diodes of
the rectifier FETs are used.
[0019] Drive voltage for the NMOS synchronous rectifier FETs can be
derived from a bootstrap supply such as provided from a capacitor,
which can be supplied externally to an integrated circuit that
implements circuit 100. Depending on processing technology of the
integrated circuit, PFETs can be used, which do not necessarily
need the bootstrap supply.
[0020] The above circuit is normally used to replace the
conventional passive diode bridge rectifier, which is responsible
for approximately 1.4V of drop during conduction. The voltage drop
across the FET switches is determined by their Rds-on, but is
typically be reduced to 200 mV (or less) with a 500 mA load current
and for typical switch sizes.
[0021] The output voltage normally depends on the degree of
coupling between the charging coil (charging station) and the
receive coil (inside the phone or other portable device) and the
load current. A considerable amount of open-circuit voltage is
usually dropped across the leakage inductance (shown as L101 at 10
.mu.H in FIG. 1) of the transformer which can be formed using
charge and receive coils (such as receive coil 110).
[0022] The leakage inductance of the "temporary" transformer (which
usually comprises the coil in the charging station and the coil in
the equipment with the battery to be charged) is often relatively
large. The relatively large leakage inductance of the transformer
causes the output voltage of the rectifier to go up in response to
a reduction of the load current. For this reason, using a buck
switching regulator instead of a linear regulator/LDO can save very
significant amounts of power and avoid generation of heat when the
load is less than a maximum expected value.
[0023] The combination of using the synchronous rectifier to
minimize voltage drop at varying load currents, and by using a buck
switching regulator (as described below) to minimize power loss at
less than maximum load currents yields more efficient power
transference than do conventional power transfer systems.
Minimizing power dissipation is typically even more efficient at
lighter load currents.
[0024] A buck switching regulator can be supplied by the rectifier
circuit to regulate the output voltage down to an appropriate value
for a battery charging system. In another example, a buck converter
can be configured to charge the battery directly by designing it to
operate in constant current or constant voltage mode, in response
to a charging state of the battery.
[0025] Additionally, power storage devices other than batteries can
be charged by circuit 100. For example, capacitors and fuel cells
can be charged efficiently. Power loss can be significantly lowered
in applications where fairly significant power is coupled to a
device for a fairly short time (such as less than one hour) at a
fairly low voltage (such as around 2.5 to 4.5 volts.
[0026] FIG. 2 is a schematic diagram illustrating another
synchronous rectifier and controller for inductive coupling of
power. In this example, rectification and regulation are combined
into one function and performed with the rectifier switches
M201-M204. Circuit 200 receives power from receive coil 210, which
drives power signals "plus" and "minus."
[0027] In operation, zero-crossing detection comparator U2 is used
to detect a zero crossing on the power signals. In response to a
detection of a zero-crossing on the power signals, signals are
generated to turn on an appropriate low-side switch. For example,
when a positive-going zero crossing is detected, comparator U202
activates plus phase low-side switch M202. When a negative zero
crossing is detected, comparator U202 activates minus phase
low-side switch M203.
[0028] High-side switches are enabled in response to the output of
the zero-crossing detection comparator as well as the as phase
rectifier comparators. For example, plus phase high-side switch
M204 conducts when the output of the zero-crossing detection
comparator indicates a positive-going zero crossing is detected and
the plus phase rectifier comparator U201 indicates that signal
"plus" is greater than "Vout." Minus phase high-side switch M201
conducts when the output of the zero-crossing detection comparator
indicates a negative-going zero crossing is detected and the minus
phase rectifier comparator U203 indicates that signal "minus" is
greater than "Vout." Thus a comparator is used to compare the
output capacitor C201 with each phase voltage and will be high
whenever the phase voltage is higher.
[0029] Comparator U204 compares the filtered output voltage (Vout)
to a reference voltage (for example, 5V as shown in FIG. 2). Thus
an additional condition is used in order to turn on the high-side
switches. The phase voltage is normally kept above the output
voltage and the output voltage is normally kept below the reference
voltage. Latches such as A204 and A205 can be used to prevent
potential "chattering" (such as frequent on/off switching of the
rectifier phase switches) and is not necessary in every
implementation.
[0030] As in circuit 100, circuit 200 can be used to provide the
battery charge function by forcing the regulator to charge the
battery directly. Constant current mode charging can be implemented
by sensing load current and turning off the high-side switches,
rather than sensing the output voltage and actuating the high-side
switches to control the charging.
[0031] FIG. 3 is a schematic diagram illustrating the use of NFET
transistors for low-side rectification and PFET transistors for
high-side rectification. Circuit 300 comprises receive coil L302,
NFET transistors M310 and M320, PFET transistors M330 and M340, and
capacitor C304.
[0032] Circuit 300 can be fabricated using a single P-epitaxial
CMOS monolithic process or by using discrete FET transistors. In
both examples, the body diodes of the FETs can be used to function
as passive rectifiers such that a source of power is provided for
the gate drive and control circuitry. Likewise, the body diodes of
the FETs can be used to prevent voltage spikes that could possibly
damage the FETs when they are switched off. If an N-epitaxial
process is used, then an isolation option (such as isolated wells)
for the PFETs is normally used.
[0033] FIG. 4 is a schematic diagram illustrating the use of NFETs
being used for both low-side- and high-side-switching (which
reduces gate drive current requirements and improves efficiency at
light load currents). Circuit 400 comprises receive coil L402, NFET
transistors M410, M420, M430, and M440, and capacitor C404.
[0034] Circuit 400 can be fabricated using a monolithic process to
save die area (by eliminating isolation options, for example).
Circuit 300 can be fabricated using the monolithic process or by
using discrete power FETs. As illustrated above with respect to
circuit 300, the body diodes of the FETs of circuit 400 form a
passive bridge that provide a source of power for the gate drive
and control circuitry. Circuit 400 can be implemented using
bootstrap drive circuitry for the high-side FETs. The high-side
FETS M430 and M440 are normally isolated from the substrate
(depending on the process used).
[0035] FIG. 5 is a schematic diagram illustrating the use of NFETs
implemented using a monolithic process, but where the isolation of
the NFETs from the P-epitaxial substrate is not desired. Circuit
500 comprises receive coil L502, NFET transistors M510, M520, M530,
and M540, and capacitor C504.
[0036] In circuit 500, the backgates of the high-side NFETs are the
substrate itself. The passive diodes of the high-side NFETs are
coupled in parallel with the Source/Drain junction of the high-side
FETs in order to provide a source of power for the gate drive and
control circuitry and to also prevent voltage spikes that could
possibly damage the FETs when they are switched off.
[0037] FIG. 6 is a schematic diagram illustrating gate driving
circuitry and control circuitry powered by a synchronous rectifier.
Circuit 600 comprises receive coil L602, NFET transistors M610,
M620, M630, and M640, LDO X610, comparators X620 and X630, control
circuitry X640 and capacitor C604.
[0038] Circuit 600 comprises LDO X610 that is coupled between the
output of the rectifier (which comprises the four body diodes
illustrated by FETs M610, M620, M630, and M640) and the control
circuitry X640. Because the output of the synchronous rectifier
typically rises with decreasing load current, the LDO is used to
maintain a supply of sufficiently low voltage to protect the
low-voltage control circuitry. The full swing of the controlled LDO
output voltage can be applied to the low-side NFET gates in many,
if not all, implementations of the synchronous rectifier.
[0039] Level shifting circuitry can be used between the output of
the control circuitry and the gates of the high-side rectifier FETs
M630 and M640. Level shifting circuitry is often used because the
gate drive swing must usually be larger than the output voltage of
the LDO. For example, the gate drive swing can be up to the
rectifier output voltage in the case of PFET high-side FETs and up
to the bootstrap voltage in the case of NFET high-side FETs.
[0040] Although the invention has been described herein by way of
exemplary embodiments, variations in the structures and methods
described herein may be made without departing from the spirit and
scope of the invention. For example, the positioning and/or sizing
of the various components may be varied. Individual components and
arrangements of components may be substituted as known to the art.
Since many embodiments of the invention can be made without
departing from the spirit and scope of the invention, the invention
is not limited except as by the appended claims.
* * * * *