Abstract:
A switching power supply system is disclosed for powering electrical equipment while minimizing disturbance to an AC power line source. The system includes first and second AC switches which are operated at alternate intervals with respect to each other to permit current to flow between the AC power line source and the load over intervals of the AC voltage cycle. An energy storage element is included in an output filter and stores energy during intervals of the AC voltage cycle and releases the stored energy during the alternate intervals of the AC voltage cycle. By the disclosed switching power supply, the voltage applied to the load and the current flow between the source and the load are sinusoidal, have minimal energy in frequencies other than the fundamental AC power line frequency, have minimal harmonic distortion, result in a power factor close to unity, and are steady and non fluctuating.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a switching power supply for controlling the amount of power supplied to a load from an alternating current (AC) power source and more specifically to a switching power supply for reducing disturbance to the AC power distribution system. 
     BACKGROUND OF THE INVENTION 
     Recently, more stringent standards have been proposed which seek to limit the level at which power-consuming devices are permitted to introduce noise, power frequency harmonics, or other disturbance onto the AC power line as a result of their operation. The IEC555-2 and IEC555-3 standards were initiated by the IEC, amended, approved, renumbered as EN60555-2 and EN60555-3, and implemented by CENELEC for use by members of the European Union. The EN60555-2 and EN60555-3 standards have more recently been updated and renumbered as EN61000-3-2 and EN61000-3-3. All of these standards will be collectively called the “IEC555 Standard”. The IEC555 Standard regulates the effects of the power draw of the load upon the current and voltage characteristics of the AC power line. Herein, the term disturbance is used to refer to any of the above-identified effects, as well as one or more of the following: increased average or root mean squared (rms) line current, reduced power factor, and distortions of the AC line voltage, including flattening of the peak voltage levels and/or periodic changes that would cause a visual flicker of the lighting. These specific types of disturbance to the AC power line voltage and current are known to be present when existing electrical equipment such as computers, audio-visual reproduction and recording equipment, lamp dimmers, motor drives, electronic ballast lights, and photocopying equipment, among others, are powered from an AC power line source. 
     Prior to the invention disclosed herein, no system known to the inventors was capable of supplying variable power from an AC power line source to a resistive AC load that could also meet the requirements of the IEC555 Standard for minimizing disturbance to the AC power line voltage and current. A system in use prior to the proposal of the IEC555 Standard is known as thyristor AC phase control. An example of such system is described in U.S. Pat. No. 5,373,224 to Rabier (“the Rabier Patent”). A further example is shown in FIG. 6A of an extended range full wave phase control circuit. In these types of systems, which may commonly be used in photocopiers, lamp dimmers, heater controls and cooking appliances, a triac placed between the AC power line and the load is “fired”, i.e. switched on, at some delay relative to the start of each half cycle of the AC power line voltage, such that power is supplied to the load during only a predetermined portion of each half cycle of the AC line voltage. In that way, the triac controls delivery of power to the load in accordance with the relative proportion of each AC power cycle in which the triac is switched on. 
     However, while prior triac-controlled switching power supplies are capable of meeting the voltage fluctuating requirements of the IEC555-3 (now EN61000-3-3) standard, they are incapable of meeting the harmonic requirements of IEC555-2 (now EN61000-3-2) standard because the voltage and current waveforms supplied to the load are not sinusoidal at substantially a single frequency. 
     An alternate method of controlling the triac-controlled power supply is to switch it on for several cycles and then off for several cycles, resulting in a very slow modulation frequency. An example of such a system is shown in FIG. 6B employing a solid state relay with zero crossing turn on. The circuit of FIG. 6B is similar to that of FIG. 6A, however, it can be controlled by a computer. Accordingly, the triac can be on for extended periods of time and is slowly modulated on and off to control the average power. Although this method, which is currently in use for many heater type applications in the electronics industry such as photocopiers meets the harmonic distortion requirements, it does not meet the fluctuating voltage restrictions and causes visual flicker to the lighting. Accordingly, it fails the standard IEC555-3 (now EN6100-3-3). 
     FIG. 1 contains a set of waveforms plotted versus time for 1) the voltage Vs and current Is on an AC power line with minimal disturbance present; 2) the voltage VL and current IL as supplied to a load through a prior triac-controlled supply; and 3) an example of the AC power line voltage Vs 1  and current Is 1  under disturbed conditions, i.e. as voltage and current being supplied to the load by the triac-controlled power supply. As shown in FIG. 1, irregular, non-sinusoidal voltage and current are supplied to the load as waveforms VL and IL The irregular current draw, in turn, disturbs the AC power line characteristics, resulting in the disturbed voltage and current waveforms Vs 1  and Is 1 . 
     Like the prior thyristor AC phase control system, the present invention is designed to supply power to AC loads used in photocopiers, lamp dimmers, heater controls, cooking appliances, and many other types of equipment which draw sinusoidal AC power. The present invention operates to minimize the level of disturbance to the AC power line while providing variable AC power to a load and meeting the requirements of the IEC555 standard. 
     Accordingly, it is an object of the present invention to provide a switching power supply which delivers a controlled amount of power to an AC load from an AC power line while minimizing disturbance to the voltage and current which are carried by the AC power line. 
     A further object of the invention is to provide a switching power supply which draws power at timed cycle intervals from a source while delivering a continuous sinusoidal voltage and current to the load. 
     Another object of the invention is to provide a switching power supply which draws power at timed cycle intervals from a source while maintaining substantially sinusoidal voltage and current waveforms at the source. 
     Another object of the invention is to provide a switching power supply which provides power to an AC load at a modulation frequency substantially higher than the AC source frequency so that there will be no fluctuating distortion on the source that would create a visible flicker. 
     Still another object of the invention is to provide a more reliable switching power supply which contains few components. 
     A still further object of the invention is to provide a low-cost switching power supply for use with a full spectrum of consumer and business equipment. 
     SUMMARY OF THE INVENTION 
     These and other objects are provided by the switching power supply of the present invention. In a first preferred embodiment of the present invention, the switching power supply includes an AC switch which responds to a control input to permit current to flow bi-directionally between the power source and the load for a duration proportional to the power level needed. The switching frequency is several orders of magnitude higher than that of the AC line so as not to produce unwanted line distortion and to allow a sinusoidal voltage and current to be realized. The switching power supply further includes an energy storage element which stores energy during the period that the AC switch is closed and releases the stored energy to supply current bi-directionally to the load during the period that the AC switch is off and the load is blocked. The energy storage element preferably includes an inductor, but may include a capacitor instead of or in addition to the inductor. The storage element additionally provides filtering of the high switching frequency to reduce the switching frequency ripple and noise content at the load. The switching power supply further includes an electromagnetic interference (EMI) reducing filter as an input filter to prevent current flow between the source and the load from introducing noise frequencies and harmonics of the fundamental AC power line frequency onto the voltage and current waveforms of the AC power line. 
     Preferably, the switching power supply includes a second AC switch coupled to the energy storage element which is operated at alternate intervals with respect to the first AC switch. The first switch is turned on while the second switch is turned off to permit bi-directional current flow between the source and the load. The second switch is turned on when the first switch is turned off to permit bi-directional current to flow between the energy storage element and the load. 
     Preferably, the switching power supply includes modulation control circuits which could be implemented by, but not limited to, pulse width modulation (PWM), frequency modulation (FM), phase modulation (PM), or any suitable energy modulation technique. These control circuits will generate periodic pulse trains to control the operation of the first and second AC switches. In a preferred embodiment, a first train of pulses is generated to control the operation of the first AC switch and a second train of pulses is generated to control the operation of the second AC switch. The first and second pulse trains are preferably opposite in phase with respect to each other to activate each AC switch at alternate intervals. Preferably, the control circuits are controlled automatically in accordance with the power requirements of the load, e.g. via feedback control signals delivered from load system circuitry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an example of voltage and current waveforms at the input and output of a prior art switching power supply. 
     FIG. 2 shows a block and schematic diagram of the switching power supply of the present invention. 
     FIG. 3 shows an example of drive pulse trains used to operate the switching power supply shown in FIG.  2 . 
     FIG. 4 shows an example of voltage and current waveforms supplied to a load from the output of the switching power supply shown in FIG.  2 . 
     FIG. 5 shows an example of voltage and current waveforms of the AC power line source at the input of the switching power supply shown in FIG. 2 while power is being supplied to a load. 
     FIG. 6A shows a prior circuit having thyristor AC phase control. 
     FIG. 6B shows a prior circuit having a slowly modulated triac control. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a set of waveforms plotted versus time for the following: 1) the voltage Vs and current Is on an AC power line source with minimal disturbance present; 2) the voltage VL and current IL as supplied to a load through a prior art triac controlled switching power supply; and 3) an example of the voltage Vs 1  and current Is 1  on an AC power line under disturbed conditions, e.g. when power is supplied to a load by the prior art triac controlled switching power supply having voltage and current characteristics as shown by the VL and IL waveforms. 
     As shown in FIG. 1, the voltage Vs and current Is on the AC power line with no disturbance present appear as sine curves having a single fundamental frequency at the power line frequency, e.g. a fixed frequency at 50 Hz, 60 Hz, or 400 Hz, etc. Ideally, the AC power line voltage Vs and current Is have no energy in frequencies other than the single fundamental frequency, such that there will be no energy in higher harmonics of the fundamental frequency, and no energy in other frequencies due to various sources of noise. If an AC power line is coupled to a load through a power modulation circuit that has the voltage VL and current IL characteristic shown in FIG. 1, disturbance is introduced onto the AC power line such that the voltage and current which remain on the AC power line more closely resemble the disturbed waveforms shown as Vs 1  and Is 1  in FIG.  1 . 
     FIG. 2 is a block and schematic diagram showing the switching power supply of the present invention. As shown in FIG. 2, the switching power supply includes a first AC switch consisting of Q 1 , D 1 , D 2 , D 3 , &amp; D 4 . This switch is used to conduct power from a first (variable potential) AC input line  12  to a first AC output line  14  through an output filter  16  during either a positive or negative portion of the half cycle of the AC power line voltage. A second AC switch which consists of Q 2 , D 5 , D 6 , D 7 , &amp; DS is used to permit the flow of current to be continuous between an energy storage element such as inductor L 1  in output filter  16  and the load through output line  14  when switch element Q 1  is turned off. First and second AC switch elements Q 1  and Q 2  can be of any one of many types of relatively fast switching voltage controlled or current controlled switches, such as bipolar junction transistors and field effect transistors, IGBTs, vacuum tubes, relays, etc., so long as their switching frequencies are at least one and preferably several orders of magnitude higher than the fundamental AC power line frequency. The first and second AC switches are coupled to the input lines of the source, i.e. a first (variable potential) AC input line  12  and a second (variable potential or common) AC input line  18 , via an input filter  17  for reducing electromagnetic interference (EMI). 
     Output filter  16  serves to continue the flow of current between the source and the load during portions of the positive and negative half cycles of the AC power line voltage when switch element Q 1  is turned off. For that purpose, output filter  16  includes an energy storage element shown representatively as inductor L 1 , and may also include a capacitor C 1  which also serves as an energy storage element and/or for the purpose of eliminating rapid fluctuations in the voltage or current flow between the load and the energy storage element or the source. Capacitor C 1  may also be used to correct for a phase lag between the voltage and the current supplied to the load. 
     Diodes D 1 -D 8  permit current to flow only in a single direction in switch elements Q 1  and Q 2 . Diodes D 1  and D 4  serve to permit the flow of current from AC input line  12  through connection point A to connection point B when switch element Q 1  is turned on during a positive half cycle of the AC voltage, and diodes D 2  and D 3  serve to permit the flow of current from connection point B through connection point A to AC input line  12  when switch element Q 1  is turned on during a negative half cycle of the AC voltage. When switch element Q 1  is turned off and switch element Q 2  is turned on during a positive half cycle of the AC voltage, diodes D 6  and D 7  permit the flow of current between inductor L 1  of output filter  16  to the AC output line  14  to the load to the AC (common) input line  18  through connection points C to B. Likewise diodes D 5  and D 8  permit the flow of current between inductor L 1  of output filter  16  through connection points B to C to the AC (common) input line  18  to the load to the AC output line  14  during a negative half cycle. 
     Pulse width modulation (PWM) circuits  20  and  22  generate controlled duration pulses, which in turn, control the duration and timing at which first and second source switch elements Q 1  and Q 2  are activated to permit current to flow between the source and the load. PWM control circuits  20  and  22  are controlled by an external control signal  24  which may be, for example, a reference voltage output of a potentiometer or a feedback control signal generated by load system circuitry to automatically control the amount of power delivered to the load. The external control signal  24  operates a control interface  26  to supply the signals required to operate PWM control circuits  20  and  22 . An optical isolator  28  may also be used, as appropriate, to galvanically isolate the control signal  24  from the AC lines  12  and  18 . 
     The construction of the switching power supply of the present invention having been described, attention will now be turned to its operation. FIG. 3 shows a pair of waveforms Q 1 D and Q 2 D which are output from PWM control circuits  20  and  22 , respectively, and which are used to control the operation of first and second AC switch elements Q 1  and Q 2 , respectively. The Q 1 D and Q 2 D waveforms are trains of pulses which vary between a first state L below the threshold for engaging switch elements Q 1  and Q 2  and a second state H at which switch elements Q 1  and Q 2  are engaged. The Q 1 D and Q 2 D waveforms are timed such that the Q 1 D waveform has the state H when the Q 2 D waveform has the state L and vice versa. Accordingly, the first and second AC switch elements Q 1  and Q 2  are operated such that whenever switch element Q 1  is turned on, switch element Q 2  is turned off, and whenever switch element Q 2  is turned on, switch element Q 1  is turned off. 
     Preferably, switch element Q 1  is turned on by waveform Q 1 D multiple times during every AC voltage cycle, and switch element Q 2  is turned on multiple times during the alternate portions of every AC voltage cycle. Hence, the preferred embodiment of the invention operates as follows for each cycle of the AC power line voltage. During a portion of the positive AC half cycle, switch element Q 1  is turned on multiple times by an ‘H’ pulse of waveform Q 1 D to permit current to flow from the source to the load through AC input line  12  and AC output line  14  through diodes Dl, D 4  and switch element Q 1 . During the alternate portions of the same AC half cycle, switch element Q 1  is turned off and switch element Q 2  is turned on to permit current to flow from an energy storage element such as inductor L 1  or capacitor C 1  in output filter  16  to the load through output line  14  to the AC common  18  and back through diodes D 6 , D 7  and switch element Q 2  to the energy storage element. Next during a portion of the negative AC half cycle, switch element Q 1  is again turned on multiple times by an ‘H’ pulse of waveform Q 1 D to permit current to flow from the load to the source through output filter  16 , connection point B, diodes D 2  and D 3  and switch element Q 1 , connection point A and to AC input line  12  through input filter  17 . During the alternate portions of the negative AC half cycle in which switch element Q 1  is turned off and switch element Q 2  is turned on, current is permitted to flow from a storage element such as inductor L 1  or capacitor C 1  in output filter  16  to connection point B, diodes D 5  and D 8  and switch element Q 2  to connection point C to the load and back to connection point  14 . 
     The voltage and current which appear at AC output line  14  and which are applied to the load by operation of the switching power supply of FIG. 2 have waveforms as shown in FIG.  4 . As will be understood from an examination of FIG. 4, the voltage and current waveforms applied to the load by the switching power supply are sinusoidal at the fundamental AC power line frequency and have minimal energy in harmonic frequencies of the fundamental frequency, and minimal noise energy. 
     During operation of the switching power supply shown in FIG. 2 the voltage and current waveforms which appear on the AC input line  12  have waveforms as shown in FIG.  5 . As will be understood from an examination of FIG. 5, the waveforms for the voltage and current of the AC power line source during operation of the switching power supply of FIG. 2 remain sinusoidal at the fundamental AC power line frequency and have minimal energy in harmonic frequencies of the fundamental frequency, and minimal noise energy. Accordingly, an examination of FIG. 5 shows that the switching power supply of the present invention meets its stated objective of controlling the power to an AC load while minimizing disturbances which could be coupled onto the AC power line source. 
     While the invention has been described in detail herein in accordance with certain preferred embodiments thereof, many modifications and changes therein may be effected by those skilled in the art. 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 present invention.