Wave transmission lines and networks – Coupling networks – Frequency domain filters utilizing only lumped parameters
Reexamination Certificate
1999-08-17
2001-08-28
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Coupling networks
Frequency domain filters utilizing only lumped parameters
C333S177000, C330S20700P, C330S251000
Reexamination Certificate
active
06281767
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an output filter for eliminating the switching residue of a carrier signal from a modulating signal, for example in order to eliminate the switching residue from the signal supplied by a generator, for example a PWM generator, to a load connected to said system.
BACKGROUND OF THE INVENTION
In particular, in the sector of the audio band amplifiers there are currently used, in addition to linear amplifiers, switching devices which can be schematically described as a pair of switches controlled at a high switching frequency which switch the current to the load in an alternating way. The timing of the switching instants follows a modulation criterion according to the signal to be reproduced. The output signal, consisting of the sum of the carrier signal and the modulating signal, is demodulated in a low-pass filter which extracts the low-frequency modulating signal from the high-frequency carrier. A diagram of an amplifier and of a corresponding low-pass filter of this type which are currently known is provided in FIG.
1
. The numbers
1
and
3
indicate two electronic switches controlled by a PWM circuit indicated in a general way by
5
. The chopped signal at the output is sent to a low-pass filter formed by a single L-C network comprising an inductance L and a capacitance C. The L-C network has a cut-off frequency of the order of 25 kHz to eliminate the high-frequency switching signal from the signal sent to the load Z, so that a signal containing only the frequencies in the audio range is supplied to the load (for example a loudspeaker).
With this system, attenuations of 12 dB/octave are obtained. To obtain greater attenuations, it is possible to dispose two or more L-C networks in cascade connection, with a consequent increase of the output impedance of the amplifier.
The system may also be of the balanced type, in other words with a bridge configuration of the type illustrated in
FIG. 2
, where the load Z is disposed between two pairs of controlled switches
1
,
3
and
1
′,
3
′. The control signals of the two pairs of switches
1
,
3
and
1
′,
3
′ are equal but in opposite phase. The symbol L, C and L′, C′ indicate the components of the low-pass output filters.
By comparison with amplifiers of the linear type, switching amplifiers have the advantage of a higher theoretical efficiency, equal to 100%, since there are only two possible conditions of the switches, namely: closed switch, zero voltage, non-zero current, zero power dissipation; and open switch, non-zero voltage, zero current, zero power dissipation.
At the present time, however, this technology has considerable problems, particularly for application to the audio sector, arising from the limits of the output reconstruction filter which, in order to attenuate the switching residue, also has drastic effects in the pass band, with natural resonances, distortion, and an increase in output impedance at high frequencies beyond the acceptable limits. The limits imposed by the output reconstruction filter are principally as follows:
a) increase of the output impedance of the filter as a function of the number of networks and the decrease of the cut-off frequency of each network;
b) high variation of the frequency and phase response characteristics as a function of the load impedance;
c) insufficient attenuation of the switching residue in cases in which the ratio between the pass band and the switching frequency is high. This problem arises whenever a compromise between quality and switching losses is sought;
d) increase of the phase rotation of the output signal in proportion of the number of L-C networks used;
e) increase of the resistive part of the filter with an increase in the number of L-C networks which leads to an increase in the conduction losses in the filter and consequently a loss of efficiency;
f) increase of the volume and total cost of the filter with an increase in the number of L-C networks for a given switching frequency;
g) resonances in the pass band which are dangerous for the operation of the amplifier in no-load conditions.
SUMMARY OF THE INVENTION
The purpose of the present invention is to provide an output filter for a switching system of the type mentioned above which overcomes the disadvantages of conventional filters.
The solution proposed by the invention consists in configuring the output filter on the basis of an L-C network, with attenuation and phase characteristics in the pass band that are acceptable in terms of quality, and adding to this a dipole or cancellation network which provides a very high attenuation at the switching frequency and a minimum effect in the pass band. In practice, the dipole generates a signal equal and opposite to the switching residue at the terminals of the load, in such a way that the sum of the two signals is canceled and no current at the switching frequency flows to the load.
The resonant frequency of the L-C network is lower than the switching frequency, so that it presents an inductive behavior to the generator.
Further advantageous characteristics of the device according to the invention are indicated in the attached claims and will be described in greater detail with reference to some diagrams showing examples of embodiments. The invention will be more clearly understood from the description and the attached drawing, which shows non-restrictive practical examples of the invention.
REFERENCES:
patent: 4628275 (1986-12-01), Skipper et al.
patent: 5113145 (1992-05-01), Ideler et al.
patent: 195 12 754 A1 (1996-10-01), None
patent: 2020505 (1979-11-01), None
patent: 55-082516 (1980-06-01), None
Bettendorf Justin P.
McGlew and Tuttle , P.C.
LandOfFree
Reconstruction filter for eliminating the switching residue... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Reconstruction filter for eliminating the switching residue..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Reconstruction filter for eliminating the switching residue... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2541003