Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
1999-06-18
2001-08-28
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S132000
Reexamination Certificate
active
06282111
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to power inverters and DC to DC converters and, more specifically, to both improved power inverters and DC to DC converters operating in current mode utilizing a novel slope modulation regulation scheme.
BACKGROUND OF THE INVENTION
Inverters change one type of electrical current into another. There exist two types of electrical current, direct current (DC) and alternating current (AC). The electricity commonly available in mobile situations via batteries or generated via alternative means, e.g., wind generators or solar panels, is DC. DC can be easily stored using well known means such as batteries or capacitors. To be used with common appliances and other wall-powered devices, DC must be converted to AC.
Direct current is current which flows in the same direction at all points in time. If one were to measure the voltage of a DC circuit at different instants in time, the measurement would remain constant. As mentioned, the advantage of DC is that it is easy to store.
Alternating current is current which periodically reverses its direction of movement over various periods of time. If one were to measure the voltage of an AC circuit at different instants in time, the voltage would fluctuate, being in a cycle of continuous reversal. In the U.S. this cycling occurs 60 times per second, i.e., 60 Hz. The advantages of AC are that it is very easy to step voltages up or down (through transformers) and thus easier to distribute over long distances with smaller wire than would be possible with DC. This is because as electricity is carried, energy is thermally dissipated due to the resistance of the wire. However, the relative loss decreases as the voltage increases.
As mentioned previously, DC must be converted into AC to power appliances and other wall powered devices. This is the role inverters play. Many methods of DC to AC conversion are well known in the art. However, they all present serious shortcomings that the present invention addresses in a novel fashion.
One method known in the art for DC to AC conversion and DC to DC conversion is voltage controlled pulse width modulation. High frequency switched DC to AC inverters generally use a voltage controlled pulse width modulation scheme such as the system
100
exemplified in
FIG. 1
(
FIG. 1
) wherein DC current enters at terminals
108
and AC current leaves at terminals
109
. This system has a full bridge configuration of switching transistors and commutating diodes
106
. Said bridge could, for example, comprise transistors of type Bipolar, IGBT (insulated gate bipolar transistor), MOSFET (metal-oxide semiconductor field effect transistor), or gate controlled SCR (silicon controlled rectifier). Said bridge is then connected to an LC (inductor and capacitor) output filter
107
. The semiconductors are enabled by conventional drive circuitry
105
. The circuit operates by pulse width modulating a constant frequency drive to the switching transistors in such a way that the average output from them, when smoothed by the LC filter
107
, is the required low frequency sine wave.
A sawtooth generator
102
provides a constant frequency constant amplitude sawtooth ramp signal derived from a conventional relaxation oscillator operating at the required high switching frequency. A low voltage reference sine wave is generated by
101
by conventional means and has a peak to peak amplitude slightly less than that of the sawtooth ramp. In the case of a DC to DC converter the sine wave reference is replaced by a DC voltage reference.
The sine wave or DC voltage reference and sawtooth reference are then compared by a conventional analog comparator
104
which acts here as a pulse width modulator to generate a pulse width modulated logic level signal which if passed through a low pass filter will accurately reproduce the sine wave or DC reference. The modulated signal is then buffered and isolated by the transistor drive circuits
105
for connection to the bridge power switching transistors and commutating diodes
106
. An LC filter
107
removes high frequency components to leave a low frequency sinusoidal or DC voltage output.
However, line and load regulation are quite poor with this type of circuit. One method to improve the regulation is shown with the addition of an output meter
103
which produces a DC error signal to control the sine wave reference output voltage. Such control is by its very nature slow and reacts poorly to switched and non linear loads. Other output correction schemes have an error amplifier connected in the same way as for a DC to DC converter but in this case the phase shift caused by the LC output filter
107
is considerable, even at the low output frequency, and it is hard or impossible to achieve the high loop gain that is necessary for good performance when the inverter or DC to DC converter drives non-linear or pulsed loads.
Another method well known in the art for DC to AC or DC to DC conversion is current mode with pulse width modulation.
FIG. 2
(
FIG. 2
) shows a modification of the voltage controlled pulse width modulated system
200
to allow current mode control wherein DC current enters at terminals
108
and AC or DC current leaves at terminals
109
. A current sense point
201
is inserted between the switching power transistors
106
and the LC output filter
107
to provide a reference voltage proportional to the instantaneous current. In this system the inverter or DC to DC converter output voltage at terminals
109
is compared to a reference sine wave by an error amplifier
202
. The intention is to make the current flowing through the power switches
106
proportional to this error voltage and as a consequence the power stage becomes a high impedance current source; the output inductor impedance is absorbed into the high impedance source and thus the maximum phase shift through said filter
107
is now only 90 degrees compared to 180 degrees for a voltage control system.
The error voltage from the error amplifier, or voltage comparator
202
as it is often referred to in a current controlled system, is compared with the current reference signal in the current comparator
203
to produce a current error signal. This signal is now compared with a high frequency sawtooth reference by comparator/pulse width modulator
104
and the high frequency digital output is connected to the transistor drive circuits
105
as in the above disclosed voltage controlled pulse width modulated inverter
100
.
The resulting system provides true current mode control but unfortunately inherits the enormous disadvantage of an inherent form of instability known as “subharmonic oscillation” that is prevalent in current mode systems for which the duty cycle is either more or less than 50% depending on the configuration. As an inverter requires pulse widths between 0% and 100% of the duty cycle the problem is unavoidable with this type of control.
In practice the effects of subharmonic oscillation do not become significant until the output filter inductor is made small and the high frequency components of the inductor current exceed 5% of the maximum current. This restriction makes the system unsuitable for very small, lightweight inverters.
Another method known in the art for DC to AC inverters is hysteretic current control. High performance high frequency switching inverters and DC to DC converters require gain around the control loop at frequencies many multiples of the baseband sine wave. This is particularly true in the case of inverters driving non-linear loads such as diode rectifiers with capacitor filters for which a high loop gain at frequencies greater than ten or twenty times the baseband frequency is essential if the waveform distortion is to be minimized. Hysteretic current control achieves such performance without becoming prone to subharmonic oscillations. Unfortunately it does not work well at switching frequencies above 50 kHz where circuit delays and power component switching times become so long that circuit currents change significantl
Avionic Instruments INC
Patel Rajnikant B.
Ward & Olivo
LandOfFree
Inverter control using current mode slope 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 Inverter control using current mode slope modulation, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Inverter control using current mode slope modulation will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2474617