Modulators – Pulse or interrupted continuous wave modulator – Pulse width modulator
Reexamination Certificate
2001-11-30
2003-10-21
Pascal, Robert (Department: 2819)
Modulators
Pulse or interrupted continuous wave modulator
Pulse width modulator
C327S172000
Reexamination Certificate
active
06636124
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates in general to pulse width modulation circuits, and in particular to pulse width modulation circuits used in conjunction with circuits for controlling heavier and noise sensitive electrical loads, such as laser driver power circuits, on printed circuit boards and like substrates, in a highly accurate manner.
2. Discussion
Pulse width modulation (PWM) circuits have been in use for over forty years, and have grown in popularity and widespread use significantly in the last 30 years or so. In many applications, the accuracy of the timing of the individual pulse width modulation signals is not a particular concern, especially in some circuit applications like motor drive circuits, that will be discussed below. However, in a variety of precision circuit applications, where having a stable duty cycle of the individual pulses of a PWM pulse train is important, there have been continuing attempts to improve and refine duty cycle stability of such circuits and/or reduce their susceptibility to external or internal interferences. Nonetheless, there are at least noise or interference problems that appears to have been not solved by those PWM circuits that have been seen. Further this particular noise or interference problem is frequently present on PWM circuits implemented on printed circuit boards, particularly if there is ground-loop feedback among the control, comparator and/or power circuits. This particular problem will now be explained with reference to
FIGS. 1 and 2
.
FIG. 1
shows the conventional design of a pulse width modulation (PWM) circuit
20
, which has a comparator
22
provided with two inputs, namely noninverting input
24
and inverting input
26
, and an output
28
. One of the inputs, such input
24
, is connected to a triangle wave source Vtrwv(t), while another input
26
is connected to a control signal, Vcntr(t). As shown in
FIG. 2A
, the triangle waveform may take that the form of a conventional symmetrical rising and falling shape
34
with a generally uniform frequency, or it may take the form a conventional sawtooth waveform (not shown), also with a generally uniform frequency, depending on the design of the comparator circuit and/or the waveform generator voltage. The control signal, Vcntr(t), may vary with time, and in
FIG. 2A
, is represented by a straight line
36
. A resulting PWM signal, Vout(t), is generated at output
28
of comparator
22
. This PWM signal is represented by waveform
38
in FIG.
2
B. The maximum peak-to-peak value of the triangle wave voltage on input
24
is within the input voltage range of comparator
22
, and it is usually at 3 to 5 volts for a comparator powered by a 5 volt power supply.
As shown in
FIG. 2A
, when an interference voltage, Vint, is added to the control signal input, it will falsely trigger the comparator, and the trigger error in time caused by this interference is: Te=Vint/SR, where SR is the slew rate of the triangle wave voltage at another input of the comparator, its unit of measure being V/t, that is volts/time.
FIG. 2
shows that when this interference signal Vint is added on top of the control input signal Vcntr(t), for example, it results in an earlier switching of comparator
22
from its high state to its low state, which means that this process of adding interference produces a trigger error Te, as shown in FIG.
2
B. In other words, the output signal Vout(t) at output
28
is falsely triggered prematurely by the additive interference signal. Vint of a negative-going interference voltage impressed for any reason upon the control signal Vcntr would also result in false triggering in an opposite or time-delayed sense. In both cases, this error in time will be seen as a noise at the output voltage which will be proportional to the time error Te.
The interference signal or signals may arise from a variety of possible sources, and the severity of this noise may vary depending the specific circuits used, the noise source, the lay-out of circuit components, the routing of conductors and/or ground plane configurations, provisions that are made for shielding circuit components, leads and/or wires from electromagnetic induction, external noise, and the like. In a number of applications, for example, motor drive circuits, slight errors in the timing of individual pulses within a train of digital pulses likely will not have any significant effects. Moreover, in many automatic servo systems which use PWM drive signals, the closed loop nature of the control system automatically compensates for repeated or continuous noise that produces a steady state timing error in the length of individual square wave pulse in a PWM drive signal pulse train. However, in other systems, such as high-speed digital communications systems, including laser-based fiber optic systems which use electro-optical interfaces and PWM-based laser driver circuits, noise may lead to transposition errors in the state of the digital information and/or may result in the need to reduce the operational speed of the circuitry to take into account that interference signals will result in the time-shifting of individual pulses of the laser driver circuits in the manner described in connection with the waveforms shown in FIG.
2
.
A number of earlier patents relating to PWM circuits have shown a variety of techniques to improve the accuracy of and/or the immunity of such circuits from noise or other forms of interference or variations in performance. Such patents include the following:
4,059,807 to Hamada
4,134,076 to Suzuki et el.
4,337,438 to Guggenbuhl et al.
4,504,793 to Yokohama
4,514,863 to Tokumo
4,531,096 to Yokoyama
4,949,048 to Tokumo et al.
4,952,884 to Tokumo et al.
5,001,413 to Ohms
5,508,663 to Konno
5,262,733 to Nakajima et al.
6,300,835 to Kijkmans et al.
The foregoing patents do help show that state of the art with respect to the use and understanding of PWM circuits is well-developed, and that a number of steps have been taken over the years to combat noise and/or interference and/or transient condition problems encountered with PWM circuits used in a variety of applications. Accordingly, all of these patent references are hereby incorporated by reference as helping show what is now generally known in the field of electronic PWM circuits and taught in various U.S. patents. However, none of these patents appears to address how to solve or substantially reduce the interference or noise problem of the type described above with regard to
FIGS. 1 and 2
.
Accordingly, there is a continuing need to develop highly accurate and low noise pulse width modulation circuits that are less susceptible to altered timing of the individual pulses within a train of repetitive pulses of a PWM drive system. There is also a need to provide for PWM circuits which produce more accurate and stable timing of individual pulses even in the presence of interference signals impressed upon the input control signal, no matter what the source of the noise or interference.
Accordingly, it is a primary object of the present invention to provide an circuit apparatus and method which helps substantially reduce the adverse timing effects of interference signals imposed upon either the triangular waveform input signal or the input control signal supplied to a PWM controller. A related object of the present invention is to achieve this object without the use of exotic new control circuits, or the use of extra-high precision components.
Still another object is to develop a method and circuits which substantially solve or reduce the adverse noise/timing problem created by interference imposed upon the input leads to the comparator circuits of PWM controllers, particularly PWM controllers implemented with one or more printed circuit board designs involving power switching devices. A related object of the present invention is to advance the state of the art with respect to PWM methods and circuits which are particularly useful in high-speed data transmission applications, including but not limi
Analog Technologies, Inc.
Cantor & Colburn LLP
Chang Joseph
Pascal Robert
LandOfFree
Method and apparatus for accurate pulse width modulation does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for accurate pulse width modulation, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for accurate pulse width modulation will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3122669