U.S. patent application number 09/968079 was filed with the patent office on 2002-03-07 for generalized method of paralleling voltage amplifiers.
Invention is credited to Stanley, Gerald R..
Application Number | 20020027790 09/968079 |
Document ID | / |
Family ID | 26797352 |
Filed Date | 2002-03-07 |
United States Patent
Application |
20020027790 |
Kind Code |
A1 |
Stanley, Gerald R. |
March 7, 2002 |
Generalized method of paralleling voltage amplifiers
Abstract
A generalized method for balancing paralleled power converters
is disclosed wherein (N) power converters, generally voltage
amplifiers, are paralleled and have current sensors positioned so
as to form a differencing equation for the circulating current, and
use that difference current as feedback to the paralleled power
converters to force the circulating current to zero. The current
sensors are current transforming transducers, where (N-1)
transducers are included and where the feedback from the (N-1)
transducers is distributed to summing amplifiers, which according
to their gain distribution, balances the power converters. The
system also includes passive magnetic devices to facilitate current
sharing, where the devices are generally inductors which are
designed to store no magnetic energy when under balanced
excitation.
Inventors: |
Stanley, Gerald R.;
(Osceola, IN) |
Correspondence
Address: |
NICHOLAS C. NAHAS
49 THORNLEY DRIVE
CHATHAM
NJ
07928-1360
US
|
Family ID: |
26797352 |
Appl. No.: |
09/968079 |
Filed: |
October 1, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09968079 |
Oct 1, 2001 |
|
|
|
09329759 |
Jun 10, 1999 |
|
|
|
6297975 |
|
|
|
|
60100602 |
Sep 16, 1998 |
|
|
|
Current U.S.
Class: |
363/65 |
Current CPC
Class: |
H02M 7/4807
20130101 |
Class at
Publication: |
363/65 |
International
Class: |
H02M 001/00 |
Claims
1. A system of two or more (N) parallel joined power converters
having outputs connected in parallel, comprising current sensors to
directly measure the circulating currents and by use of negative
feedback, regulate the circulating current to zero at a main
output.
2. The system of claim 1, wherein said negative feedback is
produced by the circulating current being distributed between the
paralleled power converters in such a manner as to sum to zero a
combined correction signal injected into the main output
signal.
3. The system of claim 1, wherein said current sensors are DC
responding current transducers based on magnetic flux sensing, each
current transducer having a primary.
4. The system of claim 3, wherein each of said power converters
being paralleled is incident to the primary of the current
transducers.
5. The system of claim 4, wherein each current transducer primary
has a number of primary turns, a total of the primary turns on each
current transducer [is] being zero when reversed polarity windings
are counted as negative turns.
6. The system of claim 1, including (N-1) current sensors.
7. The system of claim 6, wherein said current sensors are DC
responding current transducers based on magnetic flux sensing.
8. The system of claim 1, wherein said negative feedback is passed
through summing amplifiers which distribute the feedback to said
parallel joined power converters.
Description
[0001] This is a continuation of U.S. patent application Ser. No.
09/329,759, filed Jun. 10, 1999, which claims priority based upon
U.S. Provisional Patent Application Ser. No. 60/100,602 filed on
Sep. 16, 1998.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention is directed to a method for adding two or more
power amplifiers in parallel, and balancing the current between the
parallel joint amplifiers.
[0004] 2. Prior Art
[0005] Paralleling of amplifiers (fast four quadrant DC to AC power
converters) has been done.
[0006] for some time, but presently, the amplifiers are changing
from linear to switch-mode technology. Also the environment in
which they operate is continuing to demand larger amounts of power.
When paralleled amplifiers do not share current, costly
inefficiencies arise.
[0007] At first paralleling of amplifiers was done by using simple
passive ballasting. Linear amplifiers had wide bandwidth and fairly
small phase errors which led to substantial conformity of gain and
phase characteristics. High frequency circulating currents were
reduced by using a highly coupled center tapped inductor whose
center tap joined to the loads and whose ends attached to an
amplifier output. If the amplifiers are delivering equal currents,
such an inductor will store no net energy and thus no signal
voltage will be lost to inductance. It is important not to loose
signal voltage as the costs of generating large amounts of power
are also large.
[0008] When the demands on the ballast resistors grew to more than
250 Watts of dissipation, negative current feedback was used to
synthesize an effective amplifier output resistance (lossless).
This constituted a second and improved generation of paralleling
design.
[0009] With the advent of high efficiency switch-mode amplifiers
additional issues have arisen. Output currents are typically larger
and the gain and phase characteristics are now much looser in
tolerance, potentially making current sharing more difficult.
[0010] One of the preferred uses of the subject paralleled
amplifiers is in the medical industry, for use with magnetic
resonance imaging (MRI), where the load on the system is the
gradient coil of the MRI device. This environment is relatively
hostile for gradient signal processing, because the MRI device has
large amounts of peak RF power (<=20 KW) supplied to coils which
are immediately inside the gradient coils. With such intimate
coupling, it is necessary to place low-pass filters in the feed
lines to the gradient coils to contain the RF currents. These
filters tend to aggravate an already bad situation for establishing
wide bandwidth negative current feedback. Large phase response lags
within the amplifiers and distributed capacitances in the gradient
coils already have limited the amounts of feedback that can be used
to control the system. Any controls added to effect current sharing
dare not corrupt the output signal as there is insufficient
feedback to correct any significant injected non-linear errors.
Therefore some of the methods practiced by the DC to DC converter
industry for current sharing are not applicable here.
[0011] What is desired is a lossless means of sensing circulating
(unbalance) currents caused by mismatched parallel power converters
and introducing output corrections in such a manner as to not
influence the net output available to the load. This implies that
the entire method is lossless and also has no net output inductance
added to the load circuit.
SUMMARY OF THE INVENTION
[0012] The objects of the invention have been accomplished by
providing a system of two or more (n) parallel joined power
converters which use current sensors to directly measure the
circulating currents and by use of negative feedback regulates the
circulating current to zero. Preferably this system uses passive
magnetic devices to facilitate current sharing, which devices are
each designed to store no magnetic energy when under balanced
excitation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic view of an N of 2 paralleled balanced
output amplifier, showing specific circuitry for the novel features
and characteristics; and
[0014] FIG. 2 is a schematic view of a further embodiment of the
invention, including an N of 3 paralleling method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0015] With respect first to FIG. 1, an N of 2 paralleled balanced
amplifier circuit is shown generally as reference numeral 2 which
is generally used for the purpose of providing balanced output to a
load 4, such as a gradient coil 4. With reference still to FIG. 1,
the circuit 2 would further include an assembly of state of the art
circuitry shown generally at 6, which would encompass input
amplifiers and filters, current control feedback and output
monitors, and the like. The circuit 2 would also include first and
second power modules 8 and 10 which comprise individual amplifiers
8a, 8b and 10a and 10b respectively. The circuit 2 further
comprises current sensing transducers 12 and 14 and passive devices
16 and 20. A main current transducer 22 is provided medially
positioned between the load and the passive device 20 providing a
feedback loop to the state of the art circuitry 6.
[0016] With respect still to FIG. 1, an input signal is provided to
the power modules 8 and 10 via buses 24 and 26, while the input to
individual amplifiers 8a and 8b is via buses 28a and 28b
respectively; and to amplifiers 10a and 10b via buses 30a and 30b
respectively. Meanwhile, the outputs of amplifiers 8a and 8b are
interconnected to current sensing transducers 12 and 14 via buses
32a and 32b respectively, while the outputs of amplifiers 10a and
10b are directed through current sensing transducers 12 and 14 via
buses 34a and 34b. The current sensing transducer 12 is
interconnected to the passive device 16, while the passive device
16 is interconnected to the load 4 by way of a bus 38. Likewise the
current sensing transducer 14 is joined to the passive device 20,
which in turn is interconnected to the main current transducer 22
by way of bus 42, and then directly to the load by way of bus
44.
[0017] With reference still to FIG. 1, pre-amps 46 and 48 are
interconnected to the transducers 12, 14 and then to further amp
circuits 50 and 52, by way of buses 54 and 56. The output at 55 of
amp 50 is then diverted to summing amp circuits 60, 62 via buses
55a, 55b, respectively. Meanwhile, the output at 57 from amp 52 is
diverted to summing amp circuits 64, 66 via buses 57a, 57b. The
loop is closed when the output is again joined to the main power
amps 8a, 10a by buses 28a, 30a; and when the output of the amps 64,
66 is again joined to the power amps 8b, 10b by buses 28b, 30b.
[0018] With reference now to FIG. 1, the operation of the invention
will be described relative to its diagrammatic sketch, and in
relation to the preferred embodiment of the invention. It should be
understood that, one of the preferred modes of operation for the
invention is for use in the amplification within magnetic resonance
imaging (MRI) devices, but the invention is not so limited to such
a use. It should also be understood that while this specific
application, that is for use with an MRI, requires a full bridge
configuration, that the invention in its broadest sense is not so
limited, but rather could be used in some applications in a half
bridge configuration, for example, for use in driving a poly-phase
motor, etc. FIG. 1 shows by way of the dashed line, the symmetry
line for the half bridge configuration. It should also be
appreciated that there are two balancing signals involved because
there are two half bridge pairs coming together, that is, two half
bridge pairs that are going to be combined, that is amplifiers 8a
and 10a.
[0019] When these two signals are not balanced, there will not be
perfect gain coming to the load and a circulating current will be
formed which flows around the loop through the passive device 16.
For this reason, sensor 12 is precisely placed within the circuit,
is actually sensing the difference current in buses 32a and 34a. It
is therefore represented with a positive mark and a negative mark
because of the passing through in opposite directions; so that
twice the difference is actually sensed by its core. The core then
reports that as a dc coupled signal which is then amplified by the
error amplifier 46, 50 which integrates, and which error signal is
sent back to become part of the input signal to the amplifier. It
should be appreciated that the identical course of action is true
on the opposite half-bridge, that is through transducer 14, sensing
the difference current through buses 32b and 34b.
[0020] It should be appreciated from FIG. 1, that two current
sharing methods are introduced, where the first has been described
in relation to the current sensing balance transducers, 12, 14; and
which are for use at low frequencies. At high frequencies, the
coupled magnetic device 16, 20 are used to provide a module to
module inductance, without adding to the output inductance of the
pair. With reference now to FIG. 2, the sizing of the current
sensors such as 12, 14; and the passive devices 16, 20 will now be
described in greater detail.
[0021] Each one of these amplifiers 160, 162, 163, 164, 166 and 167
is shown now with three input ports, where each has a main input
port, each shown as the center port, 160a, 162a, and so on. On
these ports, that is these main ports, the coefficients of gain are
approximately equal, and the value of the coefficient is
immaterial. Each amplifier, 160-167, has two remaining ports, a
b-port and a c-port, which receive balancing signals, and are
asymmetric as it relates to their gain coefficients. The gain
coefficients of the two remaining ports are characterized by two
gain coefficients, a and b, with the gain coefficients being
distributed according to the following table:
1 Gain Coefficients Coefficient a Coefficient b Amplifier Ports
160b, 162b, 162c, 160c, 163b, 164c, and 163c, 164b, 166b, 166c 167b
and 167c
[0022] The relationship between Gain Coefficients a and b, is the
following:
[0023] a=-b/2 when b=k, and where k is an arbitrary constant.
[0024] The relationship between a and b is important for the
balancing, that is upon the error correcting signals coming back
into the summing amplifiers 160-167.
[0025] With reference still to FIG. 2, the current sensing
transducers 112a and 112b, 114a, and 114b, and their internal
wiring will now be described in detail. It should first be noted
that the current transformers 112a, 112b, 114a, 114b form a low
pass structure, whereas the passive members 116, 120 form a high
pass structure.
[0026] Now with respect to the low pass structure, the number of
current sensing transducers required is related to the number of
amplifiers (N) in the system being paralleled, such that the number
of sensors required equals (N-1). With respect now to the wiring,
where "t" is the number of turns in the sensor, the sensors 112a
and 114a will have 2t windings in the primary, and it windings in
the remaining windings, with the latter windings poled the same
way. With respect to sensors 112b and 114b, the windings are
opposite to those of sensors 112a and 114a, as shown in FIG. 2. The
resulting signals represent a pair of difference equations;
differencing the outputs of amplifiers 108, 110 and 111.
[0027] Now with respect to the passive system, the system is
comprised of inductors 116 and 120, which could be a small toroidal
core which are shared by all the windings. In the case of the
passive system, the geometry is not important, but just as in the
active system, that is sensors 112 and 114, the numbers of windings
and polings is. As shown in FIG. 2, the number of windings is shown
for each passive device as either M or 2M, where M is the number of
turns taken on some common shared magnetic circuit. As mentioned
above, the geometry is not the issue, but how the windings are
poled and what the relative number of flux lines that are generated
that is important. As the currents are matched flowing through
these the three separate sections of the passive device, there will
be no field in the core because the current will be in balance.
[0028] In summary, the low frequency loop comprised of the current
sensors 112, 114 monitors imbalance to make sure low frequencies
don't persist on the cores. But the low pass loop has limited
bandwidth, and is not capable of tracking rapid errors allowing for
rapid errors or short term errors that exist between the voltages
found at the output of these amplifiers. The passive device can as
it is a high pass structure. Further advantageously, there is no
net inductance created, nor is there any excess volume of core
material having any net flux stored in the core. This also keeps
its core small and it keeps its cost low.
[0029] As mentioned above, the same winding rules apply to (N-1)
magnetic cores as are applied to the current sensing transducers
which produce the desired result. In this situation, the turns
multiplier for all the current carrying windings may be an integer
greater than 1. Each of the cores will have one winding driven with
reverse poling that has (n-1) times the turns as do any of the
others. Each core's windings are seriesed with those of the next
core's until each of the (n-1) amplifiers has one and only one core
that represents it with a counter-poled winding. A master amplifier
will have no such core and will have passed through identical
minimal windings in all (N-1) cores. Care should be used to keep
the net resistance similar in all of the wiring including the
so-called passive master. Note that the amplifier which is declared
to be the master in the passive system is not required to be the
pseudo-master in the active balance system.
[0030] Advantageously, the passive balance impedance can more
practically be created with simply inductors when (N) is large. The
impact on the net inductance output source impedance is diluted by
(N) regardless. For a small (N) such as 2 or 3, the added output
inductance is more of a concern. In this case, lower mu core
materials will be used in a simple inductor design to minimize the
saturation effects. The case of large (N) also dilutes the need for
this type of active balancing system as the noise of (N)'s simple
active ballast feedback systems is reduced by the square root of
(N). Noise is thereby seen to be less improved by large (N) than is
the output impedance for the simple inductor case.
* * * * *