U.S. patent application number 11/287146 was filed with the patent office on 2006-08-24 for active lc filtering damping circuit with galvanic isolation.
Invention is credited to James P. Detweiler, Craig R. Weggel, Donald A. Yost.
Application Number | 20060186963 11/287146 |
Document ID | / |
Family ID | 36912048 |
Filed Date | 2006-08-24 |
United States Patent
Application |
20060186963 |
Kind Code |
A1 |
Weggel; Craig R. ; et
al. |
August 24, 2006 |
Active LC filtering damping circuit with galvanic isolation
Abstract
Active damping of voltage across a LC output filter includes
providing a feedback signal from a feedback circuit in relation to
operation substantially at a resonant frequency of the LC
circuit.
Inventors: |
Weggel; Craig R.; (Willow
Grove, PA) ; Yost; Donald A.; (Lansdale, PA) ;
Detweiler; James P.; (Lansdale, PA) |
Correspondence
Address: |
WESTMAN CHAMPLIN & KELLY, P.A.
SUITE 1400
900 SECOND AVENUE SOUTH
MINNEAPOLIS
MN
55402-3319
US
|
Family ID: |
36912048 |
Appl. No.: |
11/287146 |
Filed: |
November 23, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60630888 |
Nov 24, 2004 |
|
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Current U.S.
Class: |
330/291 |
Current CPC
Class: |
H03F 1/38 20130101; H03F
2200/541 20130101; H03F 1/34 20130101; H03F 3/21 20130101 |
Class at
Publication: |
330/291 |
International
Class: |
H03F 1/38 20060101
H03F001/38 |
Claims
1. A method of actively damping voltage across an electrical load
coupled to or having a resonant LC output filter, the voltage being
provided to the load by power amplifier electronics, the method
comprising: providing a feedback circuit operably coupled to the LC
output filter, the feedback circuit having a transformer-coupled
voltage feedback element; sensing a condition of the LC output
filter; and providing a feedback signal from the feedback circuit
in relation to operation substantially at a resonant frequency of
the LC circuit so as to actively damp the voltage across the
load.
2. The method of claim 1 wherein sensing power comprises sensing
current, and the voltage feedback element comprises a current
transformer.
3. The method of claim 2 wherein providing the feedback signal
comprises operation of the feedback circuit at the resonant
frequency of the LC output filter.
4. The method of claim 3 wherein the feedback circuit includes a
sense capacitor operably connected to the LC output filter and a
burden resistor operably connected to the current transformer, and
wherein providing the feedback signal when a frequency of AC
voltage across the sense capacitor substantially reaches the
resonant frequency of the LC output filter.
5. The method of claim 3 and further comprising combining the
feedback signal with a command signal to provide a system error
signal to the power electronics.
6. The method of claim 1 wherein providing the feedback signal
comprises operation of the feedback circuit at the resonant
frequency.
7. The method of claim 1 and further comprising combining the
feedback signal with a command signal to provide a system error
signal to the power electronics.
8. An apparatus for providing electrical power through terminals to
a load wherein a LC filter circuit is connectable to the terminals,
the apparatus comprising: power electronics configured to provide
power to a load based in part on a command signal; a feedback
circuit operably couplable to the LC output filter, the feedback
circuit having a transformer-coupled voltage feedback element; and
a circuit configured to combining the command signal with the
feedback signal.
9. The apparatus of claim 8 wherein the voltage feedback element
comprises a current transformer.
10. The apparatus of claim 9 wherein the feedback circuit is
configured to operate and provide the feedback signal at the
resonant frequency of the LC output filter.
11. The apparatus of claim 9 wherein the feedback circuit includes
a sense capacitor operably connected to the LC output filter and a
burden resistor operably connected to the current transformer, and
wherein the feedback circuit is configured to provide the feedback
signal when a frequency of AC voltage across the sense capacitor
substantially reaches the resonant frequency of the LC output
filter.
12. The apparatus of claim 8 wherein the feedback circuit is
configured to operate and provide the feedback signal at the
resonant frequency of the LC output filter.
13. The apparatus of claim 12 wherein the feedback circuit is
configured such that the feedback signal is proportional to current
in the LC circuit at the resonant frequency.
14. An apparatus for providing electrical power, the apparatus
comprising: power electronics configured to provide power to a load
based in part on a command signal; a LC output filter operably
connected to the power electronics; a feedback circuit operably
connected to the LC output filter, the feedback circuit having a
transformer-coupled voltage feedback element; and a circuit
configured to combining the command signal with the feedback
signal.
15. The apparatus of claim 14 wherein the voltage feedback element
comprises a current transformer.
16. The apparatus of claim 15 wherein the feedback circuit is
configured to operate and provide the feedback signal at the
resonant frequency of the LC output filter.
17. The apparatus of claim 15 wherein the feedback circuit includes
a sense capacitor operably connected to the LC output filter and a
burden resistor operably connected to the current transformer, and
wherein the feedback circuit is configured to provide the feedback
signal when a frequency of AC voltage across the sense capacitor
substantially reaches the resonant frequency of the LC output
filter.
18. The apparatus of claim 14 wherein the feedback circuit is
configured to operate and provide the feedback signal at the
resonant frequency of the LC output filter.
19. The apparatus of claim 18 wherein the feedback circuit is
configured such that the feedback signal is proportional to current
in the LC circuit at the resonant frequency.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is based on and claims the benefit
of U.S. provisional patent application Ser. No. 60/630,888, filed
Nov. 24, 2004, the content of which is hereby incorporated by
reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] The discussion below is merely provided for general
background information and is not intended to be used as an aid in
determining the scope of the claimed subject matter.
[0003] It is common to provide filtering of power electronic
amplifiers in order to remove high frequency elements therein. One
common approach is to use a LC (inductor-capacitor) in the output
stage across which the output power for the load is obtained.
Although such a filter is effective in removing high frequency
components, a problem arises if the resonant frequency of the LC
filter is in the operational range of the power amplifier. In such
cases, an undesirable large voltage can develop across the load at
the resonant frequency. Thus, a method of damping the natural
response of the LC filter to prevent unwanted and excessive load
voltage is necessary.
[0004] Various approaches of damping have been used. In a first
form, damping is provided by using a dissipative approach,
generally in the form of a resistor or a combination of a resistor
and a capacitor. However, this approach results in unnecessary and
potentially high levels of power dissipation. In an alternative
approach, active damping is used. However, active damping requires
the use of a control loop and therefore, a method of sensing the
output voltage.
[0005] Two known methods of sensing output voltage have been used.
The first method requires the use of high impedance resistors and a
differential operational amplifier. However, this method does not
provide galvanic isolation, and therefore can result in limited or
even prohibited use in circuits requiring a high level of
electrical isolation. A second known method requires the use of
relatively expensive and electrically complex Hall Effect, or a
similar type close-loop current sensor. In this method, the current
sensor is configured to measure the current flowing through a
resistor disposed across the terminals of a voltage signal to be
measured. The current measurement is proportional to the voltage of
interest. Drawbacks of this second approach include high cost and
complexity.
SUMMARY OF THE INVENTION
[0006] This Summary and the Abstract are provided to introduce some
concepts in a simplified form that are further described below in
the Detailed Description. This Summary and the Abstract are not
intended to identify key features or essential features of the
claimed subject matter, nor is it intended to be used as an aid in
determining the scope of the claimed subject matter. In addition,
the description herein provided and the claimed subject matter
should not be interpreted as being directed to addressing any of
the short-comings discussed in the Background.
[0007] Generally, an aspect of the present invention includes a
method of actively damping a resonant LC filter using a control
loop combined with a transformer-coupled voltage feedback element.
The output voltage is sensed using only a small number of
non-complex and low-cost components, while offering full galvanic
isolation. A voltage control loop is compensated to receive a
voltage signal with a desired AC characteristic.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic/block diagram having an active LC
damping circuit with galvanic isolation.
[0009] FIG. 2 is a schematic circuit diagram having an active LC
damping circuit with galvanic isolation.
[0010] FIG. 3 illustrates plots of various signals.
[0011] FIG. 4 illustrates plots of various signals.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] FIG. 1 illustrates an active LC damping circuit 10 with
galvanic isolation. The schematic/block diagram is provided in FIG.
1 because as appreciated by those skilled in the art, components
therein can be implemented in hardware (digital and/or analog) and
or software modules as is well known in the art.
[0013] FIG. 2 is a second representation of the active LC damping
circuit 10. The schematic diagram of FIG. 2 is typically used to
model circuit 10 using analytical tools such as SPICE modeling
techniques. Numbers have been used in FIGS. 1 and 2 to identify
similar components. FIG. 2 includes additional electrical
components generally used to model parasitic elements of actual
components.
[0014] Referring back to FIG. 1 power amplifier electronics 12
provide output power to an LC filter indicated at 14, that in turn,
provides power to the desired load 16. Power amplifier electronics
12 includes modules (hardware and/or software) for receiving a
command input signal and controlling power control elements to
provide output power. Power amplifier electronics 12 are well known
and can take many forms, the design of which is not important for
purposes of providing this description. In an aspect of the present
invention, a voltage control loop or feedback circuit 18 provides
feedback for active damping. The voltage control loop 18 includes
an error amplifier 20 (represented herein with summer 22 and gain
stage 24), a voltage sensor 30, and a compensation network 32. In
particular, the voltage sensor 30 comprises a current transformer
36, which provides galvanic isolation. As illustrated, the current
transformer 36 is operably coupled to sense current flowing through
a sense capacitor 38 where the output terminals, or secondary
terminals, of the current transformer 36 are coupled to a burden
resistor 40. A voltage signal across the burden resistor 40 is in
proportion to the sensed current flowing through capacitor 38.
[0015] At this point, it should be noted that capacitor 38 need not
be a separate capacitor from the LC filter 14, but rather, can
advantageously be one of the capacitors used therein. In this
manner, no additional cost is incurred in order to provide a
separate sense capacitor.
[0016] A particular advantageous feature of the present invention,
in one embodiment, is that the voltage feedback of the control loop
18 becomes noticeably active at least, or only, when the frequency
range of the AC voltage across the sense capacitor 38 corresponds
to the resonant frequency of the LC circuit 14, which is
substantially higher than the corner frequency determined by the
current transformer 36 and the burden resistor 40. The corner
frequency is thus selectable. When the voltage feedback becomes
"active" (i.e. no longer negligible and accurate or proportional
with respect to the current flowing in the LC filter 14), the
voltage feedback signal leads the output voltage across load 16 by
approximately 90.degree.. FIG. 3 illustrates the feedback signal at
41, the output voltage at 42 and the current in the sense capacitor
38 at 44. The feedback control loop 18 operates over a wide range,
but its active influence on the output voltage 42 occurs in a
narrow range of frequencies resulting from, and centered about, the
resonant frequency of the LC output filter 14.
[0017] The feedback or voltage signal across burden resistor 40, is
scaled by feedback compensation circuit 32 and is summed with a
desired command signal provided at 50 by the error amplifier 20 in
order to provide a system error signal 52. In a preferred
embodiment, the gain of the error amplifier 20 is configured so as
to provide unity gain in the command path. In this manner, the
signal by the control loop 18 is negligible at low frequencies.
With the voltage feedback provided as above, attenuation or damping
of the power amplifier electronic 12 output voltage signal is
achieved specifically at the point of resonance of the LC output
filter 14. This is illustrated in FIG. 4 where the amplifier output
voltage is indicated at 60 and the voltage across the load is
indicated at 62.
[0018] It should be noted that inductor L2 70 and capacitor C6 72
are optional if the power stage output is referenced to ground. In
other words, in another embodiment of the present invention, a
single LC circuit would suffice. In the embodiment illustrated in
FIG. 2, both of the terminals from the power electronics include
unwanted high frequency electrical activity so in that embodiment,
filtering is provided for each of the output terminals.
[0019] It should also be noted that scaling of the feedback voltage
may not be necessary in some applications, for example, a simple
voltage divider may be used, if desired, in combination with the
burden resistor 40 to provide the desired feedback voltage.
Furthermore, a low pass filter can be added to the feedback signal
to compensate, or further attenuate, the feedback signal at high
frequencies since the gain of the feedback signal increases with
frequency due to the reduction in impedance of capacitor 38 with
frequency.
[0020] A particular advantageous feature of the present invention
is that the forward gain provided by the error amplifier 20 can be
unity. In this manner, at low frequencies, the feedback signal has
a very low amplitude and thus a negligible effect on the voltage
command signal. This is due at least in part to the nature of the
current transformer 36 used with the sense capacitor 38. Stated
another way, the compensation network gain is adjusted such that at
the desired frequency (i.e., the resonant frequency of the LC
filter), or the other components of the voltage feedback signal,
are adjusted so that the voltage feedback signal is strong enough
to provide compensation. However, at low frequencies, it is as if
the voltage feedback signal does not exist and the voltage command
signal is passed with unity gain through the error amplifier 20.
Thus, damping is provided for the output voltage at the resonant
frequency, while allowing the power electronics to have an operable
range across and including the resonant frequency.
[0021] Although the subject matter has been described in language
specific to structural features and/or methodological acts, it is
to be understood that the subject matter defined in the appended
claims is not limited to the specific features or acts described
above as has been held by the courts. Rather, the specific features
and acts described above are disclosed as example forms of
implementing the claims.
* * * * *