WO1994014228A1 - Method for notch modulating a rectifier for three phase operation - Google Patents

Method for notch modulating a rectifier for three phase operation Download PDF

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Publication number
WO1994014228A1
WO1994014228A1 PCT/US1993/011994 US9311994W WO9414228A1 WO 1994014228 A1 WO1994014228 A1 WO 1994014228A1 US 9311994 W US9311994 W US 9311994W WO 9414228 A1 WO9414228 A1 WO 9414228A1
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WO
WIPO (PCT)
Prior art keywords
step
notch portion
input current
determining
transistors
Prior art date
Application number
PCT/US1993/011994
Other languages
French (fr)
Inventor
George W. Oughton
Original Assignee
Exide Electronics Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to US98672592A priority Critical
Priority to US986,725 priority
Application filed by Exide Electronics Corporation filed Critical Exide Electronics Corporation
Publication of WO1994014228A1 publication Critical patent/WO1994014228A1/en

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Classifications

    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02MAPPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00Details of apparatus for conversion
    • H02M1/42Circuits or arrangements for compensating for or adjusting power factor in converters or inverters
    • H02M1/4208Arrangements for improving power factor of AC input
    • H02M1/4233Arrangements for improving power factor of AC input using a bridge converter consisting of active switches
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02MAPPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/02Conversion of ac power input into dc power output without possibility of reversal
    • H02M7/04Conversion of ac power input into dc power output without possibility of reversal by static converters
    • H02M7/12Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/21Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/217Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M7/219Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only in a bridge configuration
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02BCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B70/00Technologies for an efficient end-user side electric power management and consumption
    • Y02B70/10Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion
    • Y02B70/12Power factor correction technologies for power supplies
    • Y02B70/126Active technologies
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02PCLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P80/00Climate change mitigation technologies for sector-wide applications
    • Y02P80/10Efficient use of energy
    • Y02P80/11Efficient use of energy of electric energy
    • Y02P80/112Power supplies with power electronics for efficient use of energy, e.g. power factor correction [PFC] or resonant converters

Abstract

Methods for converting alternating current into direct current, using a pulse width modulated (PWM) converter (10) connected to a source (12) of alternating current are disclosed. The methods include determining the occurrence of the notch portion of the input current and generating control signals, in response to determining the notch portion for switching the plurality of bi-directional switches (20, 22, 26, 28, 32, 34) contained in the PWM converter so that the input current is modified only during the notch portion. It is preferred to modify the input current to a generally sinusoidal shape so that the total harmonic distortion of said input current is between five and ten percent. The step of determining the occurrence of the notch portion can include sensing the input voltage, preferably determining when said input voltage reaches a desired level, or sensing the input current to determine when it reaches a desired level. It is especially preferred to only switch the bi-directional elements (20, 22, 26, 28, 32, 34) during the notch portion.

Description

METHOD FOR NOTCH MODULATING A RECTIFIER FOR THREE PHASE OPERATION

Field of the Invention

This invention relates to the field of full wave rectifiers used in power supplies for converting alternating current into direct current and, more particularly, to methods for modulating such rectifiers in order to reduce harmonic distortion in the input current.

The invention will be described in association with an uninterruptable power supply (UPS) system as the invention was developed for such use, however, it is to be understood that the invention is not limited to such use. The invention may be used in other power supply systems which utilize rectifier techniques. The invention will also be described in terms of a polyphase power system, in particular a three phase power system, however, it is to again be understood that the invention is not limited to such use. Background of the Invention Rectifiers are utilized in UPS systems for the conversion of alternating current (AC) provided from a power source into direct current (DC) for application to a load. Presently, there are three known techniques for such power conversion, namely, the diode bridge technique, the silicon controlled rectifier (SCR) technique and the pulse width modulation (PWM) converter technique. Although the invention involves an improvement to the PWM technique, an understanding of the other techniques will highlight the novelty and advantages achieved by the invention. The diode bridge technique involves a number of diodes arranged in a bridge configuration connected to an AC source. Depending on the number of phases provided by the AC source, the number of diodes utilized will vary. An example of a diode bridge for particular use with a three phase AC source is shown in Fig. 1.

In Fig. 1, each phase of line to neutral from a three phase AC source is connected to a point between two diodes and the diodes are connected to either a plus (+) buss or a minus (-) buss. It will be understood that power conversion from AC to DC occurs because only the largest line to neutral voltage of the three will be applied to the output. Each line voltage is sufficiently phase separated so that a relatively constant (DC) voltage is applied to the output .

Fig. 2 depicts the input current iA associated with line voltage VA LN resulting from connection to diodes Dl and D2 of Fig. 1. It will be understood that the depicted input current includes a positive portion, a notch portion and a negative portion. ' Although the diode bridge technique yields a high power factor, approximately 0.95, it will be appreciated from Fig. 2 that a significant amount of harmonic distortion occurs in iA, i.e. on the order of 30% total harmonic distortion (THD) . However, the trend has been to utilize converters having a high power factor and low harmonic distortion, i.e., on the order of between 5 and 10% THD. Moreover, the output voltage and current are unregulated in the diode bridge technique.

In the SCR technique, the diodes in Fig. 1 are generally replaced with SCRs. An SCR is a high current device which permits current to pass upon receipt of an appropriate control signal . In the SCR technique phase control is utilized to generate the desired control signals. U.S. Patent No. 3,849,719 - Geiersbach et al . discloses such an SCR based power rectifier utilizing phase control.

Although the use of the SCR technique results in a regulated output, the low power factor associated with such devices and the presence of harmonic distortion tends to limit their application.

In the PWM technique, a PWM inverter is in effect utilized "backwards." PWM inverters are commonly found in applications where it is desired to convert DC to AC. Examples of such inverters are shown in U.S. Patent Nos . 4,357,655 - Beck, 4,533,836 - Carpenter et al . , 4,730,242 - Divan, 4,882,120 - Roe et al . and 4,975,822 - Lipman. It will be understood that a PWM inverter typically includes a bridge-like arrangement of transistors or bi-directional switches which are selectively switched OFF and ON by pulses of varying width, thereby converting DC applied to the input into a sinusoidal signal being produced at an output .

In the PWM technique for rectification (conversion of AC to DC) , a PWM inverter is operated "backwards."

Pulses of varying width are applied to the transistor bases in a known manner resulting in a DC output . Use of the PWM technique can result in a relatively high power factor and low harmonic distortion. These advantages are recognized in U.S. Patent No. 4,729, 082 - Sato which discloses use of the PWM technique in relation to thyristors . However, the PWM technique introduces other factors such as the continuous operation of the switching elements, i.e., through all occurring levels of current, i.e., the positive, notch and negative levels, and the associated transistor losses and switching losses. Designing a PWM technique converter capable of handling high currents and associated heat dissipation has to date yielded high cost devices.

Consequently, a need still exists for a PWM converter which exhibits a high power factor, low THD and does not result in excessive conduction and switching losses in the switching devices. Summary of the Invention

The previously described problems are resolved and other advantages are achieved in novel methods for converting alternating current into direct current, using a pulse width modulated (PWM) converter connected to a source of alternating current. The methods include determining the occurrence of the notch portion of the input current and generating control signals, in response to determining the notch portion, for switching the plurality of bi-directional switches contained in the PWM converter so that the input current is modified only during the notch portion. It is preferred to modify the input current to a generally sinusoidal shape so that the total harmonic distortion of said input current is between five and ten percent . The step of determining the occurrence of the notch portion can include sensing the input voltage, preferably determining when said input voltage reaches a desired level, or sensing the input current to determine when it reaches a desired level. It is especially preferred to only switch the bi- directional elements during the notch portion.

In one embodiment, the bi-directional switches each include a transistor. In such a situation, the step of generating the control signals so that the input current is modified involves switching the transistors so that the input current is modified only in the notch portion. It is within the scope of the invention to either complimentarily switch the transistors or to only switch one transistor at a time . Description of the Drawings The foregoing and other objects and advantages of the invention will become more readily apparent from the following description of the preferred embodiment of the invention when taken in conjunction with the accompanying drawings which are a part hereof and wherein: Fig. 1 is a schematic diagram showing a prior art diode bridge rectifier;

Fig. 2 is a graph depicting the input current iA occurring in the prior art rectifier of Fig. 1;

Fig. 3 is a schematic diagram of a rectifier constructed in accordance with the present invention;

Fig. 4 is a graph depicting the input current iA occurring in the rectifier of Fig. 3 when operated in accordance with the present invention and the current which would occur if the transistors depicted in Fig. 3 were continuously OFF;

Fig. 5 is a more detailed graph of the input current iA depicted in Fig. 4;

Fig. 6 is a graph depicting that portion of the input current iA resulting from the operation of the bi¬ directional switches depicted in Fig. 3 in accordance with the present invention; and Fig. 7 is an enlarged graph depicting the input current iA previously shown in Fig. 4 together with a reference sinusoidal wave. Description of the Preferred Embodiment

A new and novel pulse width modulated converter constructed and operated in accordance with the present invention is shown in Fig. 3 and generally designated 10. Converter 10 converts alternating current to direct current and provides such direct current to a LOAD. Although the invention is best applied in relation to polyphase alternating current and is being so described, the scope of the invention is not so limited.

Three phase AC voltages VA LN, VBιLN and VC LN are supplied by a polyphase source 12 to the input of converter 10. Each input voltage has an associated input current 1A, iB and ic. Converter 10 includes a positive buss 14 and a negative buss 16. VA LN is applied to busses 14 and 16 through inductor 18 and bi-directional switches 20 and 22. Switches 20 and 22 are connected in series with the output of inductor 18 being provided therebetween. Similarly, VB LN is applied to busses 14 and 16 through inductor 24 and bi¬ directional switches 26 and 28. Switches 26 and 28 are connected in series with the output of inductor 24 being provided therebetween. Likewise, VC LN is applied to busses 14 and 16 through inductor 30 and bi-directional switches 32 and 34. Switches 32 and 34 are connected in series with the output of inductor 30 being provided therebetween. Bi-directional switches 20, 22, 26, 28, 32 and 34 are shown to each include a transistor and a diode connected in parallel. Respectively the transistor/diode pairs include transistors 36, 40, 44, 48, 52, 56 and diodes 38, 42, 46, 50, 54, 58. The base of each transistor is driven by controller 60. As will be described below, controller 60 generates control signals sufficient to turn the transistors ON and OFF.

It will be appreciated from discussion above that should transistors 36, 40, 44, 48, 52, 56 remain OFF, converter 10 would operate significantly similar to the rectifier depicted in Fig. 1. To that end the input current flowing through each of inductors 18, 24 and 30 would include notch portions, i.e., portions where the input current equaled zero. By utilizing controller 60 in accordance with the present invention, the transistors are switched using PWM type pulses to "fill in" the notch portions of the input current, so that the input current associated with each line voltage is substantially sinusoidal in shape, thereby significantly reducing the total harmonic distortion.

To this end, controller 60 generally determines the occurrence of the notch portions and generates the control signals in response to such determination, so that the input currents associated with the input voltages are modified only during the notch portions. As will be seen, it is possible for controller 60 to reduce total harmonic distortion to between five and ten percent, namely to approximately seven percent, for each input current. It is noted that a specific arrangement of electrical components making up controller 60 does not constitute a part of the present invention. Consequently, no specific arrangement is described. It is well within the level of skill in this art, for someone knowing how controller 60 should operate to develop an appropriate arrangement of electrical components to achieve such operation. In order to more fully appreciate the object or goal to be achieved by operation of controller 60, consider the waveforms shown in Fig. 4. The waveform VA LN is depicted for purposes of illustration. It will be appreciated that the units of measurement for VA LN, i.e., volts, are different than the units of measurement for the input current iA, i.e., amps. The units shown in Fig. 4 are amps, e.g. input current iA (old) is shown to peak at 18 amps (18 Ap) . VA LN is shown to be a sinusoidal wave. It will be noted that an input current exists for each input voltage. The input current iA (old) is depicted as if transistors 36 and 40 were continuously OFF.

The object for augmenting the input current iA associated with VA#LN is to operate transistors 36 and 40 in a manner which introduces or adds notch current iA notch to the input current iA (old) making the input iA (new) current more sinusoidal shaped. Since the input voltage VA LN and the input current iA are in phase, shown in Fig. 4, VA LN can be used to determine where the notch portions of iA will occur, thereby indicating where transistors 36 and 40 are to be switched. The augmentation of current is also performed for iB and ic.

A more complete understanding of the operation of controller 60 can be had by reference to Fig. 5. Again the input voltage VA LN and the input current iA (old) are shown, however, voltages VB LN and VC LN are also shown together with input notch currents of iB and ic in order that the operation of all the transistors will be appreciated. VA LN has been divided into its degree components, wherein every thirty degree interval is designated. Every sixty degrees, beginning with -30°, current is being added for a different phase, i.e., either phase iA, iB or ic. The dominant transistor for each thirty degree portion is designated.

From -30° through 0° to +30 ° transistors 36 and 42 are being turned ON and OFF in a pulse width modulated fashion in order to draw current through inductor 18. Such switching action generates current or augments the current iA filling in the shaded section. The length of ON and OFF times will draw more or less current thereby providing more or less augmentation.

Transistor 40 is turned ON just long enough to get the current up to a desired level, i.e. to a level which will generate a near sinusoidal shape and then as the current begins to rise too high transistor 40 is turned OFF and transistor 36 is turned ON. When transistor 40 is turned OFF the current will decrease. As the current gets too low, transistor 40 is turned ON again and transistor 36 is turned OFF. The ON/OFF cycle of transistors 36 and 40 is repeated during the notch portion from -30° to +30° augmenting iA (old) in a manner that shapes iA (old) as a sinusoid, i.e. fills in the shaded regions, to form iA (new) . It is noted that during the period from -30° to 0° transistor 36 will predominate and from 0° to +30° transistor 40 will predominate. It will be appreciated that in order to determine whether the input current iA is at a desired level, sensing the level of this current by controller 60 will be necessary. Controller 60 will sense all currents iA, iB and ic. Inductor 18 serves to integrate the effect of the switching so the shape of iA is smooth.

From 30° to 60° iB is being augmented by predominantly switching transistor 48 among transistors 44 and 48 in a manner as described for transistors 36 and 40. From 60° to 90° iB is being augmented by predominantly switching transistor 44 among transistors 44 and 48 in a manner as described for transistors 36 and 40. From 90° to 120° ic is being augmented by predominantly switching transistor 52 among transistors 52 and 56 in a manner as described for transistors 36 and 40. From 120° to 150° ic is being augmented by predominantly switching transistor 56 among transistors 52 and 56 in a manner as described for transistors 36 and 40. From 150° to 180° iA is being augmented by predominantly switching transistor 40 among transistors 36 and 40 in a manner previously described. From 180° to 210° iA is being augmented by predominantly switching transistor 36 among transistors 36 and 40 in a manner previously described. From 210° to 240° iB is being augmented by predominantly switching transistor 44 among transistors 44 and 48 in a manner as described for transistors 36 and 40. From 240° to 270° iB is being augmented by predominantly switching transistor 48 among transistors 44 and 48 in a manner as described for transistors 36 and 40. From 270° to 300° ic is being augmented by predominantly switching transistor 56 among transistors 52 and 56 in a manner as described for transistors 36 and 40. From 300° to 330° ic is being augmented by predominantly switching transistor 52 among transistors 52 and 56 in a manner as described for transistors 36 and 40. In order to determine the particular notch portion occurring at a given instant in time, controller 60 can sense the input voltages. If the input voltage is sensed, shown in Fig. 3, determining the existence of notch portions can be made based on the relationship: sin (ωt) = 0.5 VPEAK where: ω = the frequency of said input voltage; and V PEAK = the peak voltage of said input voltage. By using either the peak positive and negative values for voltage and current, the occurrence of all notch portions can be determined.

It is noted that filling or augmenting the input current in the notch portions involves relatively small levels of current, i.e., the augmentation currents are much lower than the main current will become. As shown in Fig. 5, each filled-in triangle has a maximum current level of only .5 peak current . The main input current pulse passes through diodes 38 and 42 and not the transistors without having to turn 36 or 40 ON or OFF at all. In other words the transistors are only operated during notch portions and with current levels less than half the peak current.

Graphically the current which is to be augmented or added to any each of the input currents is shown in Fig. 6. When such current is added, the input current will be preferably shaped as shown in Fig. 7. A sinusoid has been superimposed in Fig. 7 in order to demonstrate the now low harmonic distortion present in the input current. It has been determined that the harmonic distortion is approximately seven percent .

One primary advantage of the present invention is that the transistors are only operating 1/6 of the time if only one transistor is switched at any one time or 1/3 of the time if transistors are switched complimentarily.

Moreover, when the transistors are active, they only involve levels of current up to a maximum of 1/2 either the positive or negative peak. Consequently, the average current through the cross hatch triangles shown in Fig. 5, is only 1/4 the maximum. Since the transistors are on for only 1/3 of time, and the average current that you are processing during that time is only 1/4 of the main current, the switched volt-amps (VA) through the transistors is only 1/12 the total VA of the converter. Therefore, use of the invention yields smooth input currents with only 1/12 of the cost and efficiency penalty that would normally be incurred by operating the transistors full time as is the case in the above described PWM technique.

While the invention has been described in detail herein in accord with certain embodiments thereof, many modifications and changes therein may be effected by those skilled in the art. As for example, sensing the output voltage or output current in order to control the switching of the transistors. Accordingly, it is intended by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.

Claims

ClaimsWhat is claimed is:
1. A method for converting alternating current into direct current, using a pulse width modulated converter connected to a source of alternating current, wherein said converter comprises a plurality of bi-directional switches, wherein the input current of said converter exhibits a notch portion, said switches being responsive to a control signal, said method comprising the steps of : determining the occurrence of said notch portion; and generating said control signals, in response to determining the occurrence of said notch portion, so that said input current is modified only during said notch portion.
2. The method of claim 1, wherein said step of generating said control signals so that said input current is modified comprises the step of switching said bi¬ directional switches so that said input current is modified to a generally sinusoidal shape.
3. The method of claim 2, wherein said step of switching said bi-directional switches comprises the step of modifying said input current so that the total harmonic distortion of said input current is between five and ten percent.
4. The method of claim 1, wherein said source applies an input voltage to said converter and wherein said step of determining the occurrence of said notch portion comprises the steps of sensing said input voltage and determining the occurrence of said notch portion in response to sensing said input voltage.
5. The method of claim 4, wherein said step of determining the occurrence of said notch portion in response to sensing said input voltage comprises the step of determining when said input voltage reaches a desired level.
6. The method of claim 5, wherein the step of determining when said input voltage reaches a desired level comprises the step of determining the existence of the relationship: sin (ωt) = 0.5 VPEAK where: ω = the frequency of said input voltage; and VPEAK = the peak voltage of said input voltage.
7. The method of claim 1, wherein said step of determining the occurrence of said notch portion comprises the steps of sensing said input current and determining the occurrence of said notch portion in response to sensing said input current .
8. The method of claim 7, wherein said step of determining the occurrence of said notch portion in response to sensing said input current comprises the step of determining when said input current reaches a desired level .
9. The method of claim 8, wherein the step of determining when said input current reaches a desired level comprises the step of determining when said input current reaches 0.866 of the peak input current value.
10. The method of claim 1, wherein said step of generating said control signals comprises the step of switching said bi-directional elements only during said notch portion.
11. The method of claim 1, wherein said bi¬ directional switches each comprise a transistor and wherein said step of generating said control signals so that said input current is modified comprises the steps of switching said transistors so that said input current is modified only in said notch portion.
12. The method of claim 11, wherein said converter includes first and second transistors connected in series and said source being connected between said first and second transistors and wherein said step of switching said transistors comprises the step of complimentarily switching said first and second transistors.
13. The method of claim 11, wherein said converter includes first and second transistors connected in series and said source being connected between said first and second transistors and wherein said step of switching said transistors comprises the step of first switching one of said first and second transistors during said a first part of said notch portion and then switching the other of said first and second transistors during the remainder of said notch portion.
PCT/US1993/011994 1992-12-08 1993-12-08 Method for notch modulating a rectifier for three phase operation WO1994014228A1 (en)

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US98672592A true 1992-12-08 1992-12-08
US986,725 1992-12-08

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7508094B2 (en) 2006-03-17 2009-03-24 Eaton Corporation UPS systems having multiple operation modes and methods of operating same

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4729082A (en) * 1985-11-21 1988-03-01 Kabushiki Kaisha Toshiba Control device for power converter
US4864483A (en) * 1986-09-25 1989-09-05 Wisconsin Alumni Research Foundation Static power conversion method and apparatus having essentially zero switching losses and clamped voltage levels

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4729082A (en) * 1985-11-21 1988-03-01 Kabushiki Kaisha Toshiba Control device for power converter
US4864483A (en) * 1986-09-25 1989-09-05 Wisconsin Alumni Research Foundation Static power conversion method and apparatus having essentially zero switching losses and clamped voltage levels

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7508094B2 (en) 2006-03-17 2009-03-24 Eaton Corporation UPS systems having multiple operation modes and methods of operating same

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