Patent Publication Number: US-4729081-A

Title: Power-factor-corrected AC/DC converter

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to AC-to-DC converters, particularly of a type providing relatively constant-magnitude DC voltage while at the same time drawing power from the AC source with a relatively high power factor. 
     2. Elements of Prior Art 
     To obtain a substantially constant-magnitude DC voltage from an ordinary 120 Volt/60 Hz AC source, it is only necessary to rectify and filter. However, the power factor associated with such a simple AC-to-DC conversion means is very low. 
     To improve the power factor by which power is drawn by the AC/DC conversion means, various schemes and arrangements are available. However, these usually involve the use of filter means comprising relatively large and heavy inductor means. 
     SUMMARY OF THE INVENTION 
     Objects of the Invention 
     An object of the present invention is that of providing an improved AC-to-DC converter means. 
     Another object is that of providing a means by which to obtain a substantially constant-magnitude DC voltage from an AC voltage source while at the same time drawing power from the AC voltage source with a relatively high power factor. 
     Yet another object is that of providing a cost-effective means of providing a relatively ripple-free DC voltage from an AC voltage source while drawing power from the AC voltage source with a relatively high power factor. 
     These are well as other objects, features and advantages of the present invention will become apparent from the following description and claims. 
     Brief Description 
     In its preferred embodiment, the AC/DC converter of the present invention comprises a half-bridge electronic self-oscillating inverter powered from the non-filtered full-wave-rectified 120 Volt/60 Hz power line voltage, and its resulting amplitude-modulated 30 kHz output voltage is applied to a series-resonant L-C circuit. 
     The 30 kHz voltage developing across the tank capacitor of this series-resonant L-C circuit is rectified and applied as DC to an energy-storing capacitor, from which the AC/DC converter&#39;s DC output voltage is supplied. 
     The self-oscillating half-bridge inverter is of such nature that is has to be triggered into self-oscillation. At the same time, the inverter is so arranged that it ceases to oscillate each time the magnitude of its DC supply voltage falls below a predetermined level. 
     Thus, with its DC supply voltage being a series of sinusoidally-shaped DC voltage pulses (which is what constitutes unfiltered full-wave-rectified 120 Volt/60 Hz voltage), it becomes necessary--if inverter oscillation is desired and since inverter oscillation necessarily ceases at the end of each DC voltage pulse--to trigger the inverter at the beginning of each of these DC voltage pulses. 
     Trigger pulses are controllably provided to trigger the inverter into self-oscillation at the beginning of each DC voltage pulse. However, as soon as the magnitude of the DC voltage across the energy-storing capacitor exceeds a first level, the trigger pulses cease to be provided. And, as soon as the magnitude of the DC voltage on the energy-storing capacitor falls below a second level, the trigger pulses are again provided. 
     Whenever the inverter is in operation, the current pulled from the power line is essentially of constant magnitude and therefore providing for a power factor of about 90%. Of course, when the inverter is not oscillating, no power is being pulled from the power line. 
     Thus, at maximum load, power to subject AC/DC converter is pulled from the power line continuously and at a high power factor. At below maximum load, the inverter cycles on and off in such manner as to keep the energy-storing capacitor fully charged; and power is then pulled from the power line at high power factor in intermittent spurts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the preferred embodiment of the AC/DC converter of the present invention. 
     FIG. 2 illustrates various voltage and current waveforms associated with the preferred embodiment of the AC/DC converter. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Details of Construction 
     In FIG. 1, a source S provides a 120 Volt/60 Hz voltage to a bridge rectifier BR, the DC output of which is applied between a B+ bus and a B- bus, with the B+ bus being of positive polarity. 
     A first high-frequency bypass capacitor BPCa is connected between the B+bus and a junction JC; and a second high-frequency bypass capacitor BPCb is connected between junction JC and the B- bus. 
     A first switching transistor Qa is connected with its collector to the B+ bus and with its emitter to a junction JQ; and a second switching transistor Qb is connected with its collector to junction JQ and with its emitter to the B- bus. 
     A tank capacitor C is connected between junction JC and a junction JX; a tank inductor L is connected between junction JX and a junction JY; and primary windings PWa and PWb of positive feedback saturable current transformers CTa and CTb, respectively, are connected in series between junction JY and junction JQ. 
     Secondary winding SWa of transformer CTa is connected between the base and emitter of transistor Qa; and secondary winding SWb of transformer CTb is connected between the base and the emitter of transistor Qb. 
     A first energy-storing capacitor ESCa is connected between junction JC and a positive DC bus DC+; and a second energy-storing capacitor ESCb is connected between junction JC and a negative DC bus DC-. 
     A first charging capacitor CCa is connected in series with a first charging inductor CIa to form a first series-combination, and this first series-combination is connected between junction JX and the anode of a first high frequency rectifier HFRa. The cathode of rectifier HFRa is connected with the DC+ bus. 
     A second charging capacitor CCb is connected in series with a second charging inductor CIb to form a second series-combination, and this second series-combination is connected between junction JX and the cathode of a second high frequency rectifier HFRb. The anode of rectifier HFRb is connected with the DC- bus. 
     A trigger resistor TR is connected between the B+bus and a junction JT; and a trigger capacitor TC is connected between junction JT and the B- bus. A trigger Diac TD is connected between junction JT and the base of transistor Qb. 
     The contacter terminals of a magnetic reed switch RS are connected across trigger capacitor TC, which is to say: between junction JT and the B- bus. Around the reed switch is placed a magnetizing winding MW, the terminals of which are connected in series with an adjustable resistor AR to form a series-combination, and this series-combination is connected between junction JC and the DC- bus. 
     A DC load means DCLM is connected across DC output terminals DCOT+ and DCOT-; which DC output terminals are connected with the DC+ bus and the DC- bus, respectively. 
     DETAILS OF OPERATION 
     The detailed operation of the circuit of FIG. 1 may best be understood with reference to the various waveforms of FIG. 2, wherein: 
     FIG. 2a shows the waveform of the 120 Volt/60 Hz AC voltage provided from source S; 
     FIG. 2b shows the waveform of the full-wave-rectified 120 Volt/60 Hz AC voltage; 
     FIG. 2c shows the trigger pulses provided to the base of transistor Qb; 
     FIG. 2d shows the waveform of the high-frequency voltage provided between junctions JY and JC when the inverter is oscillating; 
     FIG. 2e shows the waveform of the current drawn from the 120 Volt/60 Hz AC voltage source S; and 
     FIG. 2f shows the waveform of the charging current provided to one of the energy-storing capacitors ESCa/ESCb. 
     The circuit arrangement of FIG. 1 comprises a half-bridge inverter; which half-bridge inverter consists principally of the following components: bypass capacitors BPCa/BPCb, transistors Qa/Qb, and positive feedback current transformers CTa/CTb. The operation of such a self-oscillating half-bridge inverter is well known and is described in various ways in U.S. Pat. Nos. Re. 31,758, 4,506,318 and 4,581,562 to Nilssen. 
     The output of this half-bridge inverter is provided between junctions JC and JY and is illustrated in FIG. 2d as being an amplitude-modulated high-frequency voltage. Connected with the inverter&#39;s output, between junctions JC and JY, is a series-tuned L-C circuit consisting of tank inductor L and tank capacitor C. This series-tuned L-C circuit is series-resonant at or near the fundamental frequency of the inverter&#39;s amplitude-modulated high-frequency output voltage; which fundamental frequency is on the order of 30 kHz. 
     In the preferred embodiment, as long as indeed provided, the trigger pulses occur at a point approximately 30 degrees after the beginning of each sinusoidally-shaped DC supply voltage pulse (see FIGS. 2b/2c). Moreover, the inverter is arranged to cease oscillation whenever the instantaneous magnitude of its DC supply voltage falls below the level associated with a point just a little less than 30 degrees before the end of each sinusoidally-shaped DC supply voltage pulse. The resulting inverter output voltage will then be as illustrated in FIG. 2d; which results in a current draw from the AC voltage source (S) as illustrated in FIG. 2e and in a charging current provided to energy-storing capacitors ESC1/ESC2 as illustrated in FIG. 2f. 
     More particularly, being excited by the intermittent amplitude-modulated 30 kHz squarewave voltage of FIG. 2d, and being series-resonant at or near the 30 kHz fundamental frequency of this squarewave voltage, the voltage developed across the tank capacitor (C) will (by way of so-called Q-multiplication) increase in magnitude until it gets limited by loading; which means that it will increase to the point of providing substantial charging current to energy-storing capacitors ESCa and ESCb. 
     In turn, as long as the inverter operates to produce the output voltage indicated by FIG. 2d, the magnitude of the DC voltage developing across energy-storing capacitors ESCa and ESCb will increase (in a step-wise manner) until one or the other of the following events occur: 
     (i) the current drain caused by DC load means DCLM equals the average charging current being provided from the inverter&#39;s output by way of the series-resonant L-C circuit; 
     (ii) the magnitude of the voltage across energy-storing capacitor ESCb gets to be so high as to cause enough current to pass through magnetizing winding MW to cause reed switch RS to close, thereby causing the inverter to stop operation. 
     Thus, as long as the AC/DC converter of FIG. 1 is loaded at or beyond a certain level (by DC load means DCLM), the magnitude of the DC output voltage (which exists between DC output terminals DCOT+ and DCOT-) is either at or below a certain predetermined magnitude (which is determined by the setting of adjustable resistor AR), and the inverter then operates in the intermittently continuous manner shown in FIG. 2d. 
     On the other hand, if the AC/DC converter is loaded below said certain level, the magnitude of the DC output voltage will gradually increase until it exceeds the predetermined level. At that point the reed switch closes and the inverter ceases operation, thereby ceasing altogether to provide output. 
     Thereafter, the magnitude of the DC output voltage will gradually decrease until the amount of current flowing through magnetizing winding MW gets to be so low as to cause reed switch RS to open, thereby to cause the inverter to start operating again, thereby to cause the magnitude of the DC output voltage to start increasing again in a gradual step-wise manner. 
     In other words, when loaded to or beyond a certain point, the inverter in the AC/DC converter will continuously operate in the (120 Hz) interrupted manner indicated by FIG. 2d; whereas, when loaded below that certain point, the inverter will interruptedly operate in the interrupted manner indicated by FIG. 2d (i.e., operating in a doubly interrupted manner). 
     While the rate of interruption of the inverter&#39;s output voltage is a constant 120 Hz, the rate at which this constant 120 Hz interruption is interrupted is dependent upon the degree of loading applied to DC output terminals DCOT1/DCOT2 as well as the degree of hysteresis built into the magnetic reed switch: the less hysteresis, the higher the rate of interruption; the less loading, the lower the rate of interruption. 
     ADDITIONAL COMMENTS 
     (a) The basic nature of a series-excited parallel-loaded high-Q resonant L-C circuit, when excited by a series-connected voltage source, is that of essentially constituting a current source to its parallel-connected load. Moreover, the magnitude of the current provided to the parallel-connected load is substantially proportional to the magnitude of the voltage provided by the series-connected voltage source. 
     Moreover, when such an L-C circuit is parallel-loaded with a substantially constant-voltage load (such as in instant case), the loading provided by the L-C circuit to the series-connected voltage source is substantially a constant-current. That is: a parallel-connected constant-magnitude-voltage load converts into a constant-magnitude-current load as seen from the viewpoint of a series-connected source. 
     (b) One result of the above-described basic nature of a high-Q series-excited parallel-loaded resonant L-C circuit is that the magnitude of the charging current provided to the energy-stored capacitors ESC1/ESC2 (see FIG. 2f) is roughly proportional to the magnitude of the inverter output voltage (see FIG. 2d), which in turn in proportional to the magnitude of the inverter&#39;s DC supply voltage (see FIG. 2b). 
     (c) Another result is that the magnitude of the current drawn by the inverter from its DC voltage supply will be about proportional to the magnitude of the DC voltage present across the energy-storing capacitors. 
     (d) Yet another result is that the magnitude of the current drawn by the series-resonant L-C circuit when powering a constant-voltage parallel-connected load, is substantially constant. 
     Thus, since--for a given setting of the adjustable resistor (AR)--the magnitude of the voltage on the energy-storing capacitors is substantially constant, the magnitude of the current provided by the inverter into the series-tuned L-C circuit is approximately constant; which, in turn, means that the magnitude of the current drawn by the inverter from its DC supply voltage will be approximately constant--as indicated in FIG. 2e. 
     (e) By virtue of their basic nature, magnetic reed switches have hysteresis. Thus, the magnitude of the current through the magnetizing winding (MW) required for causing the reed switch (RS) to close is higher than the magnitude of the current required to cause it to open. 
     Within a wide range, the amount of hysteresis can be designed to be just about any degree required. In the preferred embodiment, the hysteresis is about 20%; which implies that the magnitude of the DC output voltage will be regulated to within about plus/minus 10%. 
     (f) By changing the setting of the adjustable resistor (AR), the magnitude of the DC output voltage can likewise be set. 
     In this connection, it is important to note that the magnitude of the DC output voltage can be set to virtually any level: higher or lower than the peak magnitude of the DC supply voltage, higher or lower than the peak magnitude of the inverter&#39;s output voltage, etc. 
     (g) In the preferred embodiment, the inverter is so arranged as to oscillate approximately only in the intervals between 30 and 120 degrees, as well as between 210 and 330 degrees, of the 120 Volt/60 Hz supply voltage. As a result, current is drawn from the source only during those intervals; the implication of which is to minimize the third harmonic content of the current drawn from the source, which feature is important in situations where power is provided by a single phase of a three-phase power distribution system--a situation that is frequently significant in connection with powering fluorescent lighting systems. 
     (h) The current drawn from the power line by the circuit of FIG. 1 is illustrated by FIG. 2e in an idealized form, which would only occur if using perfect components, including an infinitely high Q of the L-C tuned circuit. 
     With such perfect components--as long as the conduction angle of the current approximately covers the indicated two thirds of the total period of the power line voltage (i.e., the middle 120 degrees out of each half-cycle)--the power factor of the power drawn from the power line would be about 85%. 
     However, in reality, the current waveshape will not have quite as flat a top as is shown in FIG. 2e. Rather, the waveshape will exhibit a slightly curved top--with a raised center. As a result, the power factor of the power actually drawn from the power line will be closer to about 90%. 
     (i) In FIG. 1, the principal purpose of elements CCa/CCb is that of providing DC isolation between junction JX and energy-storing capacitors ESCa/ESCb, and the principal purpose of elements CIa/CIb is that of improving rectification efficiency by way of mitigating the effect of the reverse recovery time of rectifiers HFRa/HFRb. 
     (j) In overall operation, the circuit of FIG. 1 functions in such manner as to convert the unfiltered full-wave-rectified 120 Volt/60 Hz voltage (which is provided to the half-bridge inverter as a pulsed DC voltage source having near-negligible internal impedance) (see FIG. 2b) to a 30 kHz squarewave AC voltage provided at the inverter&#39;s output terminals JC and JY (see FIG. 2d). The internal source impedance associated with the inverter&#39;s output is also of near-negligible magnitude; which is to say that the inverter&#39;s DC supply voltage as well as the inverter&#39;s squarewave output voltage both constitute nearly perfect voltage sources. 
     On the other hand, by virtue of the action of the series-resonant L-C circuit, the AC output provided between output terminals JC and JX constitutes a near-perfect current source; which, of course, is equivalent to saying that the DC charging current provided to the two energy-storing capacitors (ESCa/ESCb) is provided from a near-perfect current source. 
     (k) It is believed that the present invention and its several attendant advantages and features will be understood from the preceeding description. However, without departing from the spirit of the invention, changes may be made in its form and in the construction and interrelationships of its component parts, the form herein presented merely representing the presently preferred embodiment.