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Patent for Sale:

Power Factor Correction    

Digital Control to Operate Frequency Domain Rather than the Time Domain


PFC saves money by saving electrical power. As the demand for consumer electronics continues to increase, the emphasis on the cost of power consumption will continue to get attention. Power Factor Correction (PFC) technology saves money by saving electrical power, therefore the value of PFC technology will continue to increase.

PFC technology can be used all around the world, and its requirements and specifications usually vary from country to country. PFC typically minimizes inefficient and costly reactive loads on the power grid. PFC also minimizes any transients/harmonics that can feed back into the home/building electrical grid and disrupt the power source to other appliances.

This patent portfolio can help established suppliers of Power Factor Correction (PFC) products improve their technology and/or it can provide newcomer companies needed patents/IP to be able to gain a significant part of the growing PFC market. As the trend continues to switch from analog to digital control, the patents from this portfolio are of particular value because they are optimized for digital control architectures and implementations.

The Problem Solved by the Technology

The natural voltage waveform from electric power generators is a sinusoid. A sine wave is, by definition, a single frequency. The electrical system works well with sinusoidal voltage and current. Transformers, transmission lines and generators all work best with a single frequency. If a purely resistive load is placed on a generator the current follows the voltage. There are any number of ways to mess up this nice linear relationship. Reactive components such as motors can place a load on the generator which shifts the phase of the current with respect to the voltage. Non-linear loads can do damage to the sinusoidal current. For example a circuit which does not turn on below a certain voltage and then turns on at higher voltage will produce spikes in the current. These spikes are repetitive, happening every cycle. It can be shown that these spikes contain harmonics of the fundamental frequency.

These spikes and harmonics do not work well in the power distribution system. There are several bad effects: The current spikes have to be handled by the electrical system so the size of components may be affected by the higher peak currents. The generators can feel increased stress as a result of the spikes. Finally, these spikes can be the cause of greater power dissipation in the power distribution system. One of the greatest non-linear loads which can produce harmonics on the power lines is the power supply for electronics. These power supplies are in computers, server farms, TVs, radios, stereo systems, etc. These loads are consuming ever greater amounts of power compared to the traditional resistive loads like incandescent lamps, toasters, stoves, etc. The result is that power grid operators and governments have set limitations on the departure of sinusoidal character of the current wave form. The measure of the departure from a resistive load is called Power Factor. A power factor of 1 indicates a resistive load. Anything less than 1 is a departure.

Power Factor Correction (PFC) is a technique for making the power supplies in electronics look more like a resistive load where the current input is proportional to the input voltage. Voltage conversion is accomplished with switching regulator, or chopper, technology. Chopper technology uses inductors to store energy in the form of a magnetic field. The inductor is surrounded by switches which steer the energy flow into and out of the inductor. One configuration of the switched boosts the input voltage to an output voltage. Another configuration reduces, or bucks, the input voltage to generate a lesser voltage. Then there is a combination of the two to produce a buck-boost configuration which will generate a fixed output voltage with a changing input voltage.

To make an AC to DC converter, a bridge rectifier changes the input voltage from a sine wave to a rectified sine wave. This input voltage is then used to drive a chopper, usually in the boost mode. The principle task of the chopper is to produce a constant DC output voltage from a variable, rectified AC, sine wave. A secondary task is to do power factor correction such that the current into the chopper circuit is proportional to the input voltage.

PFC is not a new technology. It has been at least 30 years since the first circuits were designed. The majority of these circuits are built around a fundamental technique: they force the currents into a sinusoidal shape by an active closed loop. Typically these circuits utilize a chopper operating at a much higher frequency than the AC mains. In the operation of these chopper circuits, during each cycle of the chopper, the current is compared to a sinusoidal signal and is forced to align. This requires a fast current measurement at low signal levels in a noisy environment. Often these types of circuits work well over a limited range of inputs, outputs and current and have difficulty on the edges, which might be significant. This requirement generally means that the chopper frequency remains low.

How the Technology Solves the Problem

In the present day many of the high current or high voltage AC-DC power supplies are becoming digitally controlled. The patented invention, doing PFC in an AC-DC chopper, is particularly adapted to digital control. In fact this invention might be more difficult to do in non-digital techniques. There are no high speed loops involved. There is still a chopper section running at higher frequency but there is no cycle-by-cycle current control of that chopper.

If the current wave form is analyzed the fundamental frequency is the frequency of the power mains. In a perfectly resistive load, the only frequency component is that fundamental frequency. An AC-DC power supply can distort the current wave form from the sinusoidal form. This distortion can be analyzed by the fourier transform to show that higher frequencies have been introduced into the wave form.

What the patented inventions do is measure the harmonics introduced by the chopper circuit and apply a correction signal to the chopper at the frequency of the AC mains, which is 120 Hz, for the rectified AC input. It does this in a closed loop fashion which forces the magnitude of the harmonics to zero.

The result is an almost perfect power factor of “1”. This is not a fast reacting circuit, but nothing in this off-line AC to DC conversion can happen extremely fast by the nature of the line frequency. Perfect power factor can be achieved in a few AC cycles of a stable load. If the power controller is a small, special purpose, microprocessor, the patented invention can be added to the voltage control general routine without adding a large computational overhead. This is the result of the method of overtone measurement. The harmonic content is extracted by a Fast Fourier Transform which is a very fast and compact method for extracting harmonic content. This harmonic content is then subtracted from the chopper timing which is used to regulate the output voltage.

This invention can advance the abilities of digital power to move into this area of PFC. It can provide a power factor of “1” over the full range of operation. This can, in turn, save a few per cent of power. In a large server farm this can amount to a large power savings which accrues directly to the bottom line.

It can also allow the chopper to run at a much higher frequency. This, in turn, can have a big effect on the size of the passive components. The inductor is an expensive element with a magnetic core. The higher frequency can reduce the size of the magnetic core substantially. Also, it is a potential game changer. It allows for new architectures in the AC to DC conversion with PFC. For example the typical chopper produces a 400 volt output.

This technology lends itself to going directly to 12 volts and thus skipping several stages of voltage reduction and the additional losses and added cost. Normally the AC-DC converter runs in the boost mode because it is easier to do power factor correction in the boost mode. To generate a voltage lower than the peak AC input voltage a buck-boost configuration must be used. When the AC voltage is lower than the output voltage, the input must be boosted to a higher voltage. When the AC voltage is higher than the input voltage the voltage must be bucked to this lower voltage. One of the patents in this portfolio deals with that buck-boost configuration, to do power factor correction. This is not feasible with the present technology. The new invention can sense the overtones in the buck-boost circuit and remove them in a similar manner.

A great deal of the current PFC correction technology uses the chopper in the critical conduction mode (CCM). The CCM works like this, the chopper turns on and charges the inductor. The control loop, which regulates the output voltage, generates a current signal such that when the charge current in the inductor reaches this signal, the chopper commutes and the energy in the inductor passes into the load. As the current in the inductor falls, a sensor detects when it passes through zero. When it passes through zero the circuit switches to charge mode and repeats the cycle. This circuit is “self oscillating,” that is the parameters of the circuit cause it to oscillate. Self-oscillation has the drawback that the frequency of oscillation ranges over a wide range, plus the fact that two separate high speed current sense comparisons have to be made every cycle. The advantage of the new technology is that no high speed current comparisons need to be made. The current sense takes place over a whole cycle of 8.33 milliseconds and adjustments to the duty cycle are made as a result of that measurement. The chopper frequency is constant and only the fraction of the charge time is varied, the so called duty factor. Because of the closed loop nature of the new technology, the power factor is made close to unity no matter what happens to the input, output, and components as a function of time and temperature.

The seller would like to be granted a license back.

Patent Summary

U.S. Patent Classes & Classifications Covered in this listing:

Class 363: Electric Power Conversion Systems

Conversion systems wherein a single electrical source circuit is coupled to a single electrical load circuit.

Subclass 39: With means to introduce or eliminate frequency components
Subclass 45: Including means for reducing ripples from the output
Subclass 46: With ripple responsive, automatic control

Class 324: Electricity: Measuring And Testing

This is the residual home for all subject matter, not elsewhere classified, relating to the measuring, testing (or sensing) of electric properties, (e.g., determining ground resistivity, determining frequency of an alternating current, determining kilowatt hour demand), or the measuring, testing or sensing of nonelectric properties by electric means (e.g., determining moisture, a nonelectric property, by measuring conductance with a resistance bridge; determining speed, a nonelectric property by use of an electric tachometer).

Subclass 623: Harmonic

Class 327: Miscellaneous Active Electrical Nonlinear Devices, Circuits, And Systems

This is the residual class for electrical devices, circuits or systems having an output not directly proportional to its input and comprising at least one component which can provide gain or can route electrical current and which device, circuit or system does not form a complete system such as is classified specifically elsewhere or a subcombination of utility only in such elsewhere classified system.

Subclass 330: With rectifier or nonlinear impedance