Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2003-04-30
2004-12-21
Sherry, Michael (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
Reexamination Certificate
active
06833693
ABSTRACT:
BACKGROUND OF THE INVENTION
Most modem electronic devices manufactured today contain at least one electrical signal line which is an unwanted source of electrical “noise”, thereby adversely affecting other electronic circuits, both within and external to the electronic device. Generally speaking, this noise exists in the form of electromagnetic interference (EMI) of nearby electrical signals by the offending electrical signal. This EMI may be conducted from the offending electrical signal line to others by way of an electrically conductive path. Alternately, the interference may be radiated from the offending electrical signal line to nearby circuits without the benefit of a directly conductive connection. Oftentimes, the result of such radiated or conducted noise is erroneous or improper operation of the circuit being affected by the EMI, due primarily to unexpected voltage changes in the affected circuit. As a result, protecting electrical circuits from EMI that is generated by other signal lines has long been an important facet of the electronic circuit and device design process.
One example of a source of such noise is a switching power supply or converter, which typically is an electrical circuit designed to convert a power source from one form into another that is usable by another electrical circuit. For example, a direct-current/direct-current (DC/DC) converter transforms an input DC power source, such as a 12 volt (V) DC power source, into an output DC power source with a higher or lower voltage compared to the input source. Other switching power converters, such as AC/DC converters, DC/AC converters, and the like, can exhibit similar noise properties.
One simple example of a DC/DC converter is the buck converter
1
shown in FIG. A. A switch S, which is typically a transistor, is employed to energize an inductor L intermittently via an input DC voltage V
IN
SO that an output voltage V
OUT
remains substantially consistent. The inductor L thus is used as an energy-storage component, with the overwhelming majority of that energy then being delivered to a load Z
out
. The diode D is employed to provide a closed circuit for energy dissipation of the inductor when the switch S is open. The values for the inductor L, a capacitor C, and a resistor R are chosen to restrict certain characteristics of the converter
1
to levels that are acceptable to the load driven. These characteristics include, for example, overshoot and peak-to-peak ripple of the output voltage V
OUT
.
The opening and closing of the switch S is determined by a switching control circuit
2
. The switching control circuit
2
is often comprised in part of an output voltage monitor circuit
3
, which monitors the output voltage V
OUT
of converter
1
. The output voltage monitor circuit
3
may consist of, for example, a voltage divider formed by a first and second resistors R
1
and R
2
. The output of the voltage divider is then presented to an input of a first voltage comparator COMP
1
, which compares that voltage against a DC reference voltage V
REF
, thus generating an output voltage monitor signal V
ovm
. A feedback impedance Z
f
may also be used to control the output of the first comparator COMP
1
.
Aside from the output voltage monitor circuit
3
, the switching control circuit
2
also includes a second comparator COMP
2
, which compares the output voltage monitor signal V
ovm
with an oscillating signal V
osc
. Often the oscillating signal V
osc
is a periodic ramp voltage, although other types of oscillating signals, such as square waves and sinusoidal waves, may also be employed. The output of the second comparator COMP
2
thus serves as the switch control signal V
control
, operating in pulse-width-modulation (PWM) mode, for opening and closing the switch S based on the demands of the load Z
out
.
While switching power supplies are well-known for their high efficiency, the typically high current switching levels of the energy storage component, such as the inductor L of the buck converter
1
of FIG. A, normally generate conducted and radiated EMI into surrounding electronic circuits. The power spectral density of this EMI typically takes the form of noise spikes at the fundamental frequency and harmonic frequencies of the PWM control signal used to open and close the switching element of the switching power supply.
Several methods of protecting circuits from EMI generated by switching power supplies have been employed previously. Many such methods involve protecting the sensitive circuits of the electronic device involved from the noise effects of the power converter. For example, the electronic circuit designer often attempts to structure the physical layout of the electronic circuits on a printed circuit board (PCB) so that the generated EMI of the converter will have an attenuated effect on other surrounding circuits. Such efforts include physically routing any offending signals remotely from other sensitive signal lines and circuits, utilizing additional ground planes within the PCB to electrically shield and separate the power converter from surrounding circuits, and the like. Unfortunately, such efforts normally require exorbitant amounts of a PCB designer's time and effort, and are also error-prone, requiring multiple circuit design revisions in order to reduce sufficiently the effects of the noise on the device.
Other similar solutions involve more substantive circuit additions to shield radiated and conducted noise from circuits that are sensitive to that noise. These additions include the use of large and complex filters on the PCB, chokes, additional metal shielding, shielded cables, and so on.
In contrast to the solutions above, more recent approaches to the problem involve changing the nature of the offending power supply to make that signal less of a noise source to surrounding circuitry. For example, one proposed solution has been to “dither” the oscillating signal V
osc
by adding a small noise signal to the oscillating signal itself. Dithering of the oscillating signal results in displacing the frequency spectrum of the offending noise a small amount, but does not lower the power level of the frequency spectrum. This solution has been utilized in devices in which other circuits within the device are sensitive to noise at particular frequencies, because the small displacement in the frequency spectrum of the oscillating signal may aid in reducing the effects of the noise on that circuit. However, many electronic devices are susceptible to noise across a wide range of frequencies, making this solution inapplicable in such cases. For example, dithering of the oscillating signal is particularly ineffective for electronic devices such as electronic test and measurement instruments, which often are employed to investigate electronic signals over a very wide band of the frequency spectrum.
Other prior art solutions, such as those indicated in “Current control technique for improving EMC in power converters,” ELECTRONIC LETTERS, Vol. 37, No. 5, pp. 274-275 (Mar. 1, 2001) by Giral et al., and “Improvement of power supply EMC by chaos,” ELECTRONIC LETTERS, Vol. 32, No 12, p. 1045 (Jun. 6, 1996) by Deane et al., focus on the use of chaotic control of DC/DC power converters to reduce the electromagnetic interference normally generated by such circuits. Such solutions succeed in reducing the peaks of the frequency spectrum due to the control signal associated with such converters by spreading out the power of the spectrum at the fundamental and harmonic frequencies. However, such solutions typically do not ensure failsafe operation of the converter being driven by the offending control signal due to its chaotic nature. Adding chaotic control as described by the prior art does not guarantee that the switch will not remain in the closed position, thus potentially causing permanent damage to the inductor of the converter by way of sustained electrical current. By the same token, the circuit described may not prevent excessive periods of time during which the inductor is not being char
Agilent Technologie,s Inc.
Laxton Gary L.
Sherry Michael
Way Kyle J.
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