U.S. patent number RE47,276 [Application Number 15/183,726] was granted by the patent office on 2019-03-05 for rf isolation for power circuitry.
This patent grant is currently assigned to Lam Research Corporation. The grantee listed for this patent is Lam Research Corporation. Invention is credited to Neil Benjamin.
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United States Patent |
RE47,276 |
Benjamin |
March 5, 2019 |
RF isolation for power circuitry
Abstract
System and method for providing isolated power to a component
that is also subject a set of RF signals that includes at least a
first RF signal having a first RF frequency is provided. There is
included providing a DC voltage signal and modulating the DC
voltage signal into an isolated power signal using an isolation
transformer. The isolated power signal has an intermediate
frequency that is higher than 60 Hz and lower than the first RF
frequency. There is included supplying the DC voltage signal to the
primary winding and obtaining the isolated power signal from the
secondary winding; and delivering the isolated power to the
component using the isolated power signal.
Inventors: |
Benjamin; Neil (East Palo Alto,
CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Lam Research Corporation |
Fremont |
CA |
US |
|
|
Assignee: |
Lam Research Corporation
(Fremont, CA)
|
Family
ID: |
43879178 |
Appl.
No.: |
15/183,726 |
Filed: |
June 15, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
12603326 |
Oct 21, 2009 |
8755204 |
Jun 17, 2014 |
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02M
3/335 (20130101); H02M 7/44 (20130101) |
Current International
Class: |
H02M
5/45 (20060101); H02M 7/44 (20060101); H02M
3/335 (20060101) |
Field of
Search: |
;363/34,37 ;307/2,3
;336/84R,84C,84M ;219/121.43,121.58 |
References Cited
[Referenced By]
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Other References
"International Search Report", PCT Application No.
PCT/US2010/053263, Mailing Date: Jun. 24, 2011. cited by applicant
.
"Written Opinion", PCT Application No. PCT/US2010/053263, Mailing
Date: Jun. 24, 2011. cited by applicant .
"Internationai Preliminary Report on Patentability", PCT
Application No. PCT/US2010/053263, Mailing Date: May 3, 2012. cited
by applicant.
|
Primary Examiner: Nguyen; Minh
Attorney, Agent or Firm: Martine Penilla Group, LLP
Claims
What is claimed is:
1. A method for providing isolated power to a .[.component.].
.Iadd.heater of an RF chuck .Iaddend.of a plasma processing
chamber.[.-that.]..Iadd., the RF chuck of the plasma processing
chamber .Iaddend.is also subject to a set of RF signals, said set
of RF signals including at least a first RF signal having a first
RF frequency, comprising: providing a DC voltage signal; modulating
said DC voltage signal into an isolated power signal using an
isolation transformer, said isolated power signal having an
intermediate frequency that is higher than 60 Hz and lower than
said first RF frequency, wherein said set of RF signals includes a
plurality of RF signals having different RF frequencies, said first
RF frequency representing the lowest RF frequency among said
different RF frequencies, said isolation transformer having a
primary winding that is not electrically connected to a secondary
winding, .[.including:.]. supplying said DC voltage signal to said
primary winding, and .[.obtaining.]. .Iadd.transferring
.Iaddend.said isolated power signal from said secondary winding
.Iadd.to said heater of said RF chuck.Iaddend., .[.thereby.].
.Iadd.said isolation transformer is configured to
.Iaddend.substantially .[.blocking.]. .Iadd.block capacitive
coupling of .Iaddend.said first RF frequency .Iadd.provided to said
RF chuck.Iaddend.; .[.and delivering said isolated power to said
component using said isolated power signal;.]. wherein .Iadd.said
isolation transformer transfers said isolated power signal from
said primary winding to .Iaddend.said secondary winding .Iadd.via
mutual inductance, and wherein said secondary winding .Iaddend.is
wound with a larger diameter.Iadd., .Iaddend.around a core of said
isolation transformer resulting in an air gap between said
secondary winding .[.and.]. .Iadd.that is over .Iaddend.said core
for reducing the capacitive coupling between said secondary winding
and said core.Iadd., .Iaddend.than a diameter with which said
primary winding is wound around said core.
2. The method of claim 1 further comprising: obtaining an AC line
signal having a frequency in the range of 50 Hz to 60 Hz; and
rectifying said AC line signal to obtain said DC voltage
signal.
3. The method of claim 1 further comprising providing an
electrostatic shield between said secondary winding and said
core.
4. The method of claim 1 further comprising providing an
electrostatic shield between said secondary winding and said
primary winding.
5. The method of claim 1 wherein said secondary winding is wound
over said primary winding.
6. The method of claim 5 further comprising providing an
electrostatic shield between said secondary winding and said
primary winding.
7. The method of claim 1 wherein said intermediate frequency is
between about 500 Hz and about 2 MHz.
8. The method of claim 1 wherein said core is formed of a core
material having a mu value between about 10 and about 5000.
9. The method of claim 1 further comprising providing a low-side
control circuit for controlling at least one of an amplitude of
said DC voltage signal and a turn ratio of said isolation
transformer.
10. The method of claim 1 further comprising providing a modulation
circuit for modulating said isolated power signal.
11. The method of claim 10 further comprising providing a high-side
control circuit for controlling said modulation circuit.
12. A system for providing isolated power to a .[.component that.].
.Iadd.heater element of a chuck of a plasma processing chamber, the
chuck .Iaddend.is also subject to a set of RF signals, said set of
RF signals including at least a first RF signal having a first RF
frequency, comprising: first and second terminals for providing a
DC voltage signal; and a modulating circuit to modulate said DC
voltage signal into an isolated power signal using an isolation
transformer, said isolated power signal having an intermediate
frequency that is higher than 60 Hz and lower than said first RF
frequency, wherein said set of RF signals includes a plurality of
RF signals having different RF frequencies, said first RF frequency
representing .[.the.]. .Iadd.a .Iaddend.lowest RF frequency among
said different RF frequencies, said isolation transformer having a
primary winding that is not electrically connected to a secondary
winding, wherein said DC voltage signal is supplied to said primary
winding and said isolated power signal is obtained from said
secondary winding .[.thereby.]. .Iadd.and provided to said heater
element, the primary winding is physically spaced apart from
secondary winding to facilitate .Iaddend.substantially blocking
.Iadd.capacitive coupling of .Iaddend.said first RF frequency
.Iadd.provided to said chuck via the heater element.Iaddend.,
wherein said isolated power is delivered .Iadd.via mutual
inductance through said isolation transformer .Iaddend.to said
.[.component.]. .Iadd.heater element .Iaddend.using said isolated
power signal, wherein said secondary winding is wound with a larger
diameter.Iadd., .Iaddend.around a core of said isolation
transformer resulting in an air gap between said secondary winding
.[.and.]. .Iadd.that is over .Iaddend.said core for reducing the
capacitive coupling between said secondary winding and said
core.Iadd., .Iaddend.than a diameter with which said primary
winding is wound around said core.
13. The system of claim 12 further comprising an electrostatic
shield between said secondary winding and said primary winding.
14. The system of claim 12 further providing an electrostatic
shield between said secondary winding and said core.
15. The system of claim 12 wherein said secondary winding is wound
over said primary winding.
16. The system of claim 15 further providing an electrostatic
shield between said secondary winding and said primary winding.
17. A method for providing isolated power to a .[.component.].
.Iadd.load of a chuck .Iaddend.of a plasma processing chamber
.[.that.]..Iadd., the chuck .Iaddend.is also subject to a plurality
of RF signals, said plurality of RF signals including at least a
first RF signal having a first RF frequency, said method
comprising: providing an AC line signal having a frequency in the
range of 50 Hz to 60 Hz; rectifying said AC line signal to obtain a
quasi-DC voltage signal; modulating said quasi-DC voltage signal
into an isolated power signal using an isolation transformer, said
isolated power signal having an intermediate frequency that is
higher than 60 Hz and lower than said first RF frequency, said
first RF frequency representing .[.the.]. .Iadd.a .Iaddend.lowest
RF frequency among all RF frequencies of said plurality of RF
signals, said isolation transformer having a primary winding that
is not electrically connected to a secondary winding,
.[.including:.]. supplying said quasi-DC voltage signal to said
primary winding, and .[.obtaining.]. .Iadd.transferring
.Iaddend.said isolated power signal from said secondary winding.[.,
thereby.]. .Iadd.to said load of said chuck, said isolation
transformer is configured to enable .Iaddend.substantially blocking
.Iadd.capacitive coupling of .Iaddend.said first RF frequency
.Iadd.provided to said chuck.Iaddend.; wherein .Iadd.said isoaltion
transformer transfers said isolated power signal from said primary
winding to .Iaddend.said secondary winding .Iadd.via mutual
inductance; wherein said secondary winding .Iaddend.is wound with a
larger diameter.Iadd., .Iaddend.around a core of said isolation
transformer resulting in an air gap between said secondary winding
.[.and.]. .Iadd.that is over .Iaddend.said core for reducing the
capacitive coupling between said secondary winding and said
core.Iadd., .Iaddend.than a diameter with which said primary
winding is wound around said core.[.; and delivering said isolated
power to said component of said plasma processing chamber using
said isolated power signal.]..
Description
BACKGROUND OF THE INVENTION
Plasma has long been employed to process substrates (e.g., wafers)
into semiconductor products, such as integrated circuits. In many
modern plasma processing systems, a substrate may be placed onto an
RF chuck for plasma processing inside a plasma processing chamber.
The RF chuck may be biased with an RF signal, using RF voltages in
the range from tens to thousands of volts and RF frequencies in the
range from tens of KHz to hundreds of MHz. Since the RF chuck also
acts as a workpiece holder, proper control of the RF chuck
temperature is an important consideration to ensure repeatable
process results.
Generally speaking, the RF chuck's temperature is maintained by one
or more electric heaters, which may be integrated or coupled with
the RF chuck. Electrical power to the electric heater is typically
obtained from line AC voltage via an appropriate control circuit to
maintain the RF chuck at a desired temperature range. By way of
example, the electric heater may be powered by DC, line frequency
(e.g., 50/60 Hz AC) or KHz range AC power.
In this configuration, the DC/low frequency power needs to be
coupled to the RF chuck assembly, which is also simultaneously
subject to substantial levels of RF power either by stray coupling
or by direct connection. To prevent an undesirable apparent RF
short to ground, loss of RF power and high levels of signal
interference, even damage via the electric heater power supply
and/or control circuitry, RF isolation is required.
To facilitate discussion, FIG. 1 shows relevant portions of an
example system that employs AC line (e.g., 50/60 Hz) voltages or DC
voltages to power a heater or other load circuits at the RF hot or
"high side". Referring to FIG. 1, AC line voltages or DC voltages
are supplied via leads 102 and 104 to RF filter circuit 106. RF
filter circuit 106 is shown to be a single-channel (includes 2
wires for 1 complete circuit, to power 1 heater zone),
dual-frequency filter and may include L-C circuits of a known
design to present a high impedance to RF frequencies of interest
(e.g., 2 MHz and 13.5 MHz) such that a relative RF short to ground
via leads 102 and 104 and any attached circuitry, e.g. heater
control/powering circuitry is effectively prevented. For
illustration purposes, these RF frequencies are coupled to heater
114 via lead 116 as shown. In the example of FIG. 1, 114 is the
load including the heater and high side control circuitry. On the
other hand, 116 represents a leakage path, such as stray
capacitance that would allow RF from the plasma or applied to the
chuck to .[.how.]. .Iadd.flow .Iaddend.back via the heater load,
for example, RF filter 106 may have different designs and multiple
stages to handle a wide range of discrete RF frequencies. The
operation of RF filter 106 in its various implementations is
basically known technology and will not be elaborated here.
Filter outputs 110 and 112 provides power to a load, e.g. heater,
114. A control circuit (not shown) may be coupled to leads 102 and
104 to turn on/off the input AC line voltages or DC voltages to
control the temperature of an RF chuck, for example. The control
may be performed in a proportional or in a binary on/off manner.
Temperature sensing of the RF chuck may be employed as a feedback
signal to the control circuit, for example.
When RF filters are employed to provide RF isolation in a high
power, high RF frequency application, several drawbacks are
encountered. The high RF frequency (e.g., in the MHz range)
necessitates the use of large air core inductors in some designs,
rendering the filtering circuit bulky. Furthermore, the goal is to
maintain a sufficiently high input impedance (when viewed from the
RF hot direction) across all the frequencies of interest. RF
isolation design is complicated, however, by the .[.filet.].
.Iadd.fact .Iaddend.that some plasma processing systems employ RF
frequency tuning during processing. RF frequency tuning employs a
range of frequencies during operation, thus making the RF isolation
filter design significantly more challenging and complex due to the
need to handle variable RF frequencies (and hence a wide range of
RF impedances) and the desire to maintain system-to-system RF
impedance and attenuation consistency. Even if an RF isolation
filter design can handle a wide range of operating frequencies
(fundamentals as well as their harmonics) and can be carefully
matched to provide acceptable system-to-system uniformity with
respect to RF isolation, care must still be taken to avoid high
voltage discharge or arcing/breakdown and excessive heat generation
or power dissipation may still be problematic. The design task is
seriously complicated by the magnitude of the RF signal, which may
be up to the range of thousands .[.oh.]. .Iadd.of .Iaddend.volts
and up to the range of .[.thou.]. .Iadd.thousands .Iaddend.of
watts.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
The present invention is illustrated by way of example, and not by
way of limitation, in the figures of the accompanying drawings and
in which like reference numerals refer to similar elements and in
which:
FIG. 1 shows relevant portions of an example prior art system that
employs AC line (e.g. 50/60 Hz) voltages or DC voltages to power a
heater or other load circuits.
.Iadd.FIG. 2A shows a chamber, a chuck and a heater..Iaddend.
FIG. 2 shows, in accordance with an embodiment of the present
invention, relevant portions of an isolation transformer
implementation to provide high DC or intermediate frequency AC
power to a load that is also coupled to one or more high frequency
RF signals.
FIGS. 3A-3C show, in accordance with embodiments of the invention,
some example control schemes.
DETAILED DESCRIPTION OF EMBODIMENTS
The present invention will now be described in detail with
reference to a few embodiments thereof as illustrated in the
accompanying drawings. In the following description, numerous
specific details are set forth in order to provide a thorough
understanding of the present invention. It will be apparent,
however, to one skilled in the art, that the present invention may
be practiced without .[.sonic.]. .Iadd.some .Iaddend.or all of
these specific details. In other instances, well known process
steps and/or structures have not been described in detail in order
to not unnecessarily obscure the present invention.
Embodiments of the invention employ an innovative approach to RF
isolation in a high power, high frequency environment in one or
more embodiments of the invention, an AC source power signal is
rectified into a DC power signal then modulated into an
intermediate frequency power signal to be supplied to the primary
winding of an isolation transformer. As the term is employed
herein, the DC power signal may be a true DC power signal (i.e.,
having no frequency or ripple component similar to that supplied by
a battery) or a rectified DC power signal (which may have some
ripple components typical of DC signals rectified from AC signals).
Being an isolation transformer, there is no direct electrical
connection between the primary and the secondary windings of the
isolation transformer. Instead, an isolated power signal having the
intermediate frequency is generated across the secondary winding of
the isolation transformer via mutual inductance.
The use of an intermediate frequency, which is intentionally chosen
to be higher than AC line frequencies (e.g., 50 Hz or 60 Hz) but
typically lower than the RF frequency to be blocked, and preferably
lower than the lowest of the RF frequencies to be blocked if
multiple high frequencies RF signals are involved, renders it
possible to reduce the size of the isolation transformer while
innovative techniques are employed to reduce the
primary-to-secondary capacitive coupling, as well as to reduce the
secondary-to-core capacitive coupling.
In one or more embodiments of the invention, the intermediate
frequency of the power signal supplied to the primary
winding.[.,.]. of the isolation transformer is in the range of
about 500 Hz to about 2 MHz, more preferably in the range of about
5 KHz to about 200 KHz, and in a preferred embodiment in the range
of about 10 KHz to about 50 KHz. The selection of the appropriate
intermediate frequency is critical since a lower operating
frequency tends to result in an undesirably large isolation
transformer and a higher frequency tends to enable a reduction in
the size of the magnetic components (e.g., the isolation
transformer) while the drive circuit and the magnetic core material
tend to become less efficient at higher operating frequencies.
In one or more embodiments of the invention, the isolation
transformer is specifically designed with reduced dimensions and to
operate efficiently at the aforementioned intermediate frequency
range while presenting a high impedance to high frequency signals
at all RF frequencies of interest. As mentioned, in one or more
embodiments, the secondary winding is physically spaced apart from
the primary winding to reduce capacitive coupling. In a preferred
embodiment, the secondary winding is spaced as far as possible from
the primary winding to minimize this capacitive coupling. In one or
more embodiments, the secondary winding is wound with a large
diameter, resulting in an air gap between the secondary winding and
the magnetic core, thereby reducing the capacitive coupling between
the secondary winding and the core. In one or more .[.embodiment.].
.Iadd.embodiments.Iaddend., the secondary winding is wound over the
primary winding but with a larger diameter, thereby reducing the
capacitive coupling between the primary winding and the secondary
winding, as well as reducing the capacitive coupling between the
secondary winding and the core. If desired, one or more shields may
be interposed between the secondary winding and the primary
winding, between the secondary winding and the core, and/or between
the primary winding and the core to control the capacitive
coupling, such shields being slotted in a fashion known as a
Faraday shield in .[.art.]. .Iadd.order .Iaddend.to minimize the
induction of eddy currents in the shield.
The features and advantages of embodiments of the invention may be
better understood with reference to the figures and discussions
that follow. FIG. 2 shows, in accordance with an embodiment of the
present invention, relevant portions of an isolation transformer
implementation to provide high DC or AC line power to a load that
is also coupled to one or more high frequency RF signals. In the
example of FIG. 2, the load is a heater .Iadd.264 .Iaddend.for an
.[.RF coupled.]. .Iadd.RF-coupled .Iaddend.chuck .Iadd.262
.Iaddend.in a plasma processing chamber .Iadd.260, as shown in FIG.
2A, .Iaddend.although other loads may also benefit from embodiments
of the invention.
With respect to FIG. 2, a source power signal in the form of AC
line voltages and frequencies (e.g., 50 Hz or 60 Hz) is supplied
via leads 202 and 204 to a rectifier/filter circuit 206. Rectifier
circuit 206, which may be implemented by a bridge rectifier and/or
may employ triac, SSR, or thyristor controls, converts the AC line
input power signal to a quasi-DC power signal which may be
subsequently filtered into smooth.[...]. DC if desired. In the
example of FIG. 2, the AC source power signal on leads 202/204 may
be a single phase signal or a 3-phase signal as desired, and
rectifier circuit 206 is correspondingly a single-phase or
three-phase rectifier. If a DC power signal is available as input
power, then no rectification may be necessary. It should be noticed
that the high current drawn from the AC line into the input filter
may require the power factor correction circuitry.
The DC power signal output by rectifier circuit 206 is then
supplied to a drive circuit 208, which converts the DC power signal
received on leads 210 and 212 to an intermediate signal having an
intermediate frequency. The intermediate frequency is in the range
of about 10 KHz to about 1 MHz, more preferably in the range of 10
KHz to a few hundred KHz, and in an embodiment in the range of
about 10 KHz to about 200 KHz. As such, the intermediate frequency
is intentionally higher than the AC line frequency of 50-60 Hz but
preferably lower than the RF frequency to be blocked (which tends
to be in the multiple MHz range). Being higher than the AC line
frequency, the intermediate frequency renders it possible to use a
smaller isolation transformer 220. In one or more embodiments,
drive circuit 208 is a switch-mode power supply, which pulse-width
modulates the received.[...]. DC power signal to the desired
intermediate frequency. In one or more embodiments, the duty cycle
after pulse-width modulation may vary from slightly above zero to
about 50%. If desired, an appropriate drive circuit 208 may
modulate the received DC power signal to an AC sine signal having
an intermediate frequency. Reducing the harmonic content in this
fashion can prevent interference and noise issues and simplifies
any filtering requirements. Alternate power modulation schemes
including zero crossing and on/off control may also be implemented
either solo, or in combination.
The intermediate signal output by drive circuit 208 is then
supplied to the primary winding 222 of isolation transformer 220.
Primary winding 222 is shown wound around one segment of a core
224. Core 224 may be formed of manganese zinc or nickel zinc or
another suitable high magnetic permeability material (e.g., mu in
the 2000 range). Alternatively, powdered iron (mu of 10 to 40),
core materials commonly known as MPP, Sendust (mu of 50 to 300),
NiZn and MnZn ferrites (mu of 100 to 5000), etc., may be employed.
In general, materials having a higher mu may be employed for lower
frequency operation and vice versa. For power application between
20 KHz and 2 MHz, materials with a mu value between about 200 to
about 3000 may be suitable, in an embodiment in an embodiment,
materials having a .[.nm.]. .Iadd.mu .Iaddend.range value between
about 10 to about 5000 may be employed. In an embodiment, materials
having a mu range value between about 100 to about 2000 may be
employed, in an embodiment, materials having a mu range value
between about 200 to about 1000 may be employed.
In one or more embodiments, an air gap 230 (the location shown in
FIG. 2 is only an example) may be provided in core 224 to prevent,
saturation and to linearize the magnetic characteristics as well as
potentially reduce the temperature dependency of isolation
transformer 220. If air gap 230 is present, primary winding 222 is
preferably wound to the sides of the air gap 230 instead of over
air gap 230 to reduce dissipation in the winding.
Secondary winding 236, which is not directly coupled to primary
winding 222 by conduction, is also wound around core 224. In one or
more embodiments, to reduce capacitive coupling between primary
winding 222 and secondary winding 236, secondary winding 236 is
positioned apart from primary winding 222 to reduce the
primary-to-secondary capacitive coupling and to achieve a high
degree of isolation, particularly for the higher frequency RF
signals. For example, secondary winding 236 may be positioned
opposite primary winding 222 around core 224 as shown. Although
this separation of the windings may result in considerable leakage
inductance, appropriate designs can readily accommodate this
issue.
Generally speaking, it is preferable that the RF coupling be mostly
by stray capacitance providing the core material is a dielectric.
Ferrite materials are metal oxides with high resistance and
dielectric constants typical of other ceramics, say 10-100, may be
employed to achieve capacitance between the primary and secondary
sides of a few pica farads. However certain .[.termites.].
.Iadd.ferrites .Iaddend.such as the common MnZn materials may have
dielectric constants orders of magnitude higher such that secondary
insulation such as airgaps on Teflon liners may be required to
control stray capacitance. For example, in one or more embodiments,
the capacitive coupling is preferably limited to the single-digit
picofarad range (such as 1 pF to about 20 pF).
In one or more embodiments, the choice of the core material and
design of the core involves tradeoffs, for while epsilon both real
and imaginary typically decreases somewhat with increasing
frequency for ferrites, thus lowering the stray capacitance, the
loss tangent still suffers a maximum at some particular frequency
and it is desirable that the power transmission be operated well
below this frequency in order to avoid excessive core loss. The
stray capacitance tends to be somewhat independent of frequency,
but lowers as frequency is increased such that isolation improves
at higher frequencies so the particulars of the RF in use tend to
not matter as long as the high frequency RF signal is applied in a
common mode (no net .[.Mix.]. .Iadd.Flux .Iaddend.in the core).
Thus this approach potentially offers a universal solution across
all frequency ranges and produces far less broadband loading than
the filters currently in use. While it is in general true that the
RF blocking is universal and broadband, care should be taken in the
design of the coils and transformer to prevent the accidental
introduction of resonant antenna circuits such that the impedance
would be unacceptably lowered at particular frequencies. It should
further be arranged that these frequencies if any do not coincide
to the RF frequencies applied.
In one or more embodiments, secondary winding 236 is wound around
core 224 with a larger diameter 238 when compared to the manner
with which primary winding 222 is wound around core 224. The larger
diameter 238 helps to reduce the secondary-to-core capacitive
coupling. In one or more embodiments (not shown in FIG. 2),
secondary winding 236 is wound over primary winding 222, with a
larger diameter to reduce the secondary-to-primary capacitive
coupling as well as to reduce the secondary-to-core capacitive
coupling. The reduction in these capacitive couplings reduces the
frequency dependency and is an important aspect of some embodiments
of the present invention.
In contrast, primary winding 222 tends to be wound closer to core
224 to help reduce the leakage inductance. To reduce turn-to-turn
or turn-to-core capacitance (which may, in some cases, contribute
to self-resonance), a small gap and/or an insulating layer may be
interposed between primary winding 222 and core 224.
Secondary winding 236 may have a 1:1 ratio with primary winding 222
or may have an n:1 winding relationship with primary winding 222 to
step up or down the voltage. Higher voltage, lower current power
signals tend to be more efficient for transmission purposes and may
be desired in some cases.[., in.]..Iadd.. In .Iaddend.a preferred
embodiment, 208 volt AC may be rectified to over 300 volts DC for
operational use. Via mutual inductance, an isolated power signal
having the intermediate frequency is generated across secondary
winding 236. The isolated power signal having the intermediate
frequency, which is output by isolation transformer 220, may then
be employed to drive the load, or may be converted to an isolated
DC power signal to drive the load. If desired, output filtering may
be performed prior to driving the load with the isolated power
signal.
In one or more embodiments, a shield is provided to further reduce
the capacitive coupling. For example, a shield may be provided
between primary winding 222 and secondary winding 236 to reduce the
primary-to-secondary capacitive coupling. As another example, a
shield may alternatively or additionally be provided between
secondary winding 236 and core 224 to reduce the secondary-to-core
capacitive coupling. As yet another example, a shield may
alternatively or additionally be provided between primary winding
222 and core 224 to reduce the primary-to-core capacitive coupling.
The shield may be grounded, in one or more embodiments, to conduct
any current developed thereon to ground. However, the detailed
design and geometry selected should take care to avoid
significantly increasing the secondary to ground capacitance. In
one or more embodiments, one or more slits are provided in the
shield (e.g., in the toroidal winding direction) to reduce eddy
currents and prevent the shield from acting like a shorted turn.
The presence of the shield has been found to reduce self-resonance
.Iadd.(e.g., .Iaddend.antenna effect) in the primary and secondary
windings, thereby smoothing out the impedance characteristics and
contributing to the frequency-independent characteristic of the
design.
In one or more embodiments, filters may be employed to allow the
high frequency RF signal (i.e., the RF signal to be blocked) to be
presented to isolation transformer 220 as a common mode signal. In
the example of FIG. 2, capacitor 245 is coupled to leads 244 and
246 respectively to accomplish the goal of presenting the high
frequency RF signal to isolation transformer 220 as a common mode
signal. Filters of other designs well known to those skilled in the
art may also be employed. While there may be stray capacitances
(represented by 240 and 242), these stray capacitances may be
dominated by capacitor 245 for the purpose of insuring that the
output signal RF coupling is common mode signal. Capacitor 245
should be appropriately sized (not too large) to avoid resonating
at critical frequencies.
Once the power is transferred across the RF isolation (transformer)
.[.h.]. .Iadd.it .Iaddend.can be used to power a passive circuit
such as a heater directly, either as AC at the switching frequency,
or rectified into deeply modulated DC or filtered back to smoothed
DC. It may also be rectified or controlled at the high side if
desired.
In one or more embodiments, power control is applied at the low
side (e.g., primary winding 222 and circuitry toward the AC line
side of FIG. 2) using for example SSRs as is currently done for
heaters, or by PWM (pulse width modulation) or by ON/Off burst
modulation of the switching circuit. The control scheme may
implement open loop, feed forward, or feed back control.
Feedback can be implemented by monitoring low side power/current
draw, or more accurately using a high side electrical sensor of
power, voltage and or current or some other sensed variable. In the
case of a .[.beater.]. .Iadd.heater.Iaddend., a temperature sensor
may be employed, e.g. a resistance temperature detector,
fluoro-optic probe or thermocouple. In the event that the high side
sensor is electrical, another isolation channel may be provided,
e.g. opto-coupler or optic fiber or low current RF filter.
Furthermore, modulation may be performed on the AC or DC isolated
power signal on the high side to enable high-side control.
In the low side control scheme, each high side device (e.g. heater
element) may employ its own power channel. As an alternative it is
possible to isolate the power to the high side using a common power
channel, which can either be an even larger power isolator
(transformer), or by running several lower power channels in
parallel. In this case, the control may be implemented using active
circuitry at the high side. The control signals corresponding to a
set point (e.g. temperature or power) may also be from the low side
and may be isolated (e.g., using opto-coupling).
FIGS. 3A-3C show, in accordance with embodiments of the invention,
some example control schemes. In these circuits, three load options
are shown and the current may be delivered to the load (see right
side of the figure) alternatively (one option at a time or
concurrently (2 or more options simultaneously). Further, power
factor correction circuitry (not shown) may be employed with one or
more of FIGS. 3A-3C. Generally speaking, one or both of low-side
and high-side (e.g. secondary winding 236 and circuitry toward the
high frequency RF side of FIG. 2) control schemes are possible. In
FIG. 3A, no active control is provided with either the
rectifier/filter circuit (e.g., 306), the driver circuit (e.g.,
308) or to the high-side modulation circuit (314).
However, as with all of FIGS. 3A-3C and with other implementations,
it is possible to control the delivery of the AC source power
signal. For example, an appropriately isolated (e.g., using,
opto-isolation) temperature sensing signal may be provided to a
source power signal control circuit to turn on and off the AC
source power signal to control the temperature of a chuck. In the
chuck example, since the thermal mass is fairly high, even simple
on/off control of the AC source power signal (as opposed to
proportional) has the potential of producing good performance. This
is particularly true if the AC source power signal is controlled
using a microprocessor that can rapidly cycle the AC source power
signal on/off.
In FIG. 3B, low side control is implemented. In FIG. 3B, a
microprocessor control unit 320 may be employed to control the
rectifier/filter circuit (e.g., rectifier/filter circuit 306) to
regulate the amplitude of the DC output signal. Alternatively or
additionally, microprocessor control unit 320 may be employed to
change the switching characteristics, the pulse width duration,
and/or the operating frequencies of the switch mode power supply
and/or other characteristics of driver circuit 308. As another
example, processor/controller unit 320 may be employed to change
tap points on the transformer primary, thereby effectively changing
the turn ratio of isolation transformer 310.
In FIG. 3C, high side control is implemented. In this case, a
high-side temperature sensor may send isolated sensor signals
(e.g., opto-isolated or low power filtering) to a
processor/controller unit 330, which then issues commands (which
may also be isolated using, for example, opto-isolation or low
power filtering) to a high-side isolated controller 352 to control
the high-side modulation circuit 354. In the example of FIG. 3C,
house-keeping voltages and currents for high-side modulation
controller may be supplied using an isolated power signal from the
low-side.
As can be appreciated from the foregoing, embodiments of the
invention substantially obviate the frequency dependency of the
prior art filtering approach as well as the difficulties with
matching passive filter components inherent in that approach. The
resonance problem associated with the prior art filtering approach
is also substantially eliminated. By using an intermediate
frequency with the isolation transformer, it is possible to
substantially shrink the physical size of the isolation
transformer. By appropriately designing the isolation transformer
to provide a high degree of RF isolation while presenting a high
impedance to all high RF frequencies of interest, it is possible to
efficiently provide power from an AC source or a DC source to a
component or assembly that is also coupled to a high RF signal.
While this invention has been described in terms of several
preferred embodiments, there are alterations, permutations, and
equivalents, which fall within the scope of this invention.
Although various examples are provided herein, it is intended that
these examples be illustrative and not limiting with respect to the
invention. Also, the title is provided herein for convenience and
should not be used to construe the scope of the invention herein.
If the term "set" is employed herein, such term is intended to have
its commonly understood mathematical meaning to cover zero, one, or
more than one member. It should also be noted that there are runny
alternative ways of implementing the methods and apparatuses of the
present invention.
* * * * *