Patent Publication Number: US-2007109827-A1

Title: Ac to dc converter circuit

Description:
The present application is based on and claims the benefits of U.S. provisional patent application Ser. No. 60/528,572, filed Dec. 10, 2003, titled “AC to DC power converter with high efficiency conversion,” and U.S. provisional patent application Ser. No. 60/532,207, filed Dec. 22, 2003, titled “Lithium ion battery charger,” and U.S. provisional patent application Ser. No. 60/585,447, filed Jul. 2, 2004, titled “Converter circuit,” the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      As illustrated in PRIOR ART  FIG. 1A , a DC power supply  100  is energized from an AC power source  102 . The power source  102  comprises a regulated AC power line with a nominal line voltage and nominal line frequency. The power supply  100  comprises a transformer  104  with a magnetic core  106  and an excitation winding or primary  108  connected across the power line. The primary  108  conducts a primary current  110  supplied by the AC power source  102 . The primary current  110  induces magnetization of the core  106  and provides power to a load on a secondary winding  116 .  
      As illustrated in PRIOR ART  FIG. 1B , the transformer  104  ( FIG. 1A, 1B ) is typically an E-I laminated transformer for 50/60 Hz applications. The transformer  104  has a magnetic core  106  that provides a closed loop, low reluctance, effective magnetic path  210  of length L transverse to an effective magnetic core cross-section  212  with a cross-sectional area AM. The magnetic path  210  surrounds a window  214  with an effective cross sectional area AW. The primary winding  108 , as well as the secondary winding  116  pass through the window  214 .  
      In sizing the transformer core  106  for a specified power line frequency (such as 50/60 Hz or 400 Hz) and a specified magnetic core material, the mechanical dimensions AM, AW, L of the transformer core tend to decrease as the power level specification for the power supply decreases. This reduction in mechanical dimensions of the transformer core allows for the possibility of extreme miniaturization of the power supply, provided that other aspects of the power supply can be miniaturized. As the mechanical dimensions of the transformer decrease, the number of turns required in the primary increases for a specified AC power line voltage.  
      Once the approximate number of turns is determined, then wire diameters are chosen for the primary and secondary windings so that the selected number of primary and secondary turns will substantially fill the window area AW. The window area AW sets a limit on a cross sectional area of windings that can be wound on the transformer  104 .  
      In extending the transformer design process described above to miniaturized power supplies with power levels below about 1 watt, however, additional design problems are encountered due to the extremely small window area AW. A large number of primary winding turns are needed (at line voltage) to prevent saturation of the transformer core  106 . In order to fit this large number of primary turns through the window  214  (along with secondary turns), extremely small diameter magnet wire is needed for the primary winding  108 . However, the extremely small diameter magnet wire is fragile and breaks easily during manufacturing of the transformer  104 . In an effort to overcome this problem, a separate power resistor  112  ( FIG. 1A ) is placed in series with the primary winding  108  ( FIG. 1A ) and sized to reduce the primary voltage of the transformer, which allows a smaller number of larger diameter turns to be used for the primary winding. The use of resistor  112  avoids the use of extremely small diameter magnet wire. The power resistor  112 , however, is physically large for a selected line voltages in the range of 90-280 VAC, and dissipates a large amount of power that overheats other power supply components (such as bridge and regulator circuits  120 ) in the close confines of a miniature DC power supply design package  114 . Either the benefits of low power consumption, the benefits of freedom from overheating or the benefits of miniaturization are lost when a series power resistor  112  is used.  
      A method and circuit are needed that provide low power consumption, freedom from overheating, and miniaturization to take advantage of the small transformer size in a low power DC power supply.  
     SUMMARY OF THE INVENTION  
      Disclosed is an AC to DC converter circuit that includes AC input contacts couplable to an AC line voltage, and DC output contacts couplable to a DC load. The converter circuit also includes a transformer having primary and secondary windings, a rectifier bridge coupled to the secondary winding, a DC filter capacitor coupled to the rectifier bridge, and a voltage regulator coupled the DC filter capacitor and to the DC output contacts.  
      The converter circuit includes an AC reactance coupled in a series circuit with the primary winding and the AC input contacts. The AC reactance limits AC excitation voltage at the primary winding to less than the AC line voltage.  
      In a preferred embodiment, the AC reactance comprises a capacitor with a capacitive impedance that is greater than the impedance on the primary winding of the transformer. The arrangement provides a desired high efficiency in a low power converter circuit.  
      Other features and benefits that characterize embodiments of the present invention will be apparent upon reading the following detailed description and review of the associated drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1A  illustrates a PRIOR ART power supply circuit.  
       FIG. 1B  illustrates a PRIOR ART transformer.  
       FIG. 2  illustrates a first embodiment of a converter circuit.  
       FIG. 3  illustrates impedances for examples of three AC excitation circuits for primary windings.  
       FIG. 4  illustrates a second embodiment of a converter circuit.  
       FIG. 5  illustrates a third embodiment of a converter circuit.  
       FIG. 6  illustrates a fourth embodiment of a converter circuit.  
       FIG. 7  illustrates a fifth embodiment of a converter circuit.  
       FIG. 8  illustrates a sixth embodiment of a converter circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      In the embodiments described below in  FIGS. 2-8 , an AC to DC converter circuit is energized by an AC line voltage at AC input contacts. The converter provides a DC power supply voltage at DC output contacts to a DC load. The converter includes a transformer, a rectifier bridge, a DC filter capacitor and a voltage regulator. An AC reactance (such as a capacitor or inductor) is coupled in a series circuit with the primary winding and the AC input contacts. The AC reactance limits (lowers) AC excitation voltage at the primary winding to less than the AC line voltage. With the lowered excitation voltage, a smaller number of primary winding turns with a larger wire size can be used and still fit into the same transformer core window size that as a winding with more turns and finer, more fragile, wire size that would connect directly to the full AC line voltage. As a result of the addition of the series AC capacitor, the use of smaller, fragile wire sizes is avoided and reliability of the power supply in increased. Also, low power consumption, freedom from overheating and miniaturization are achieved.  
       FIG. 2  illustrates a miniaturized converter circuit  300  in a housing  302 . The converter circuit  300  includes AC contacts  304 ,  306  for connection to a regulated source of AC voltage  301 , for example nominal 115 VAC or 230 VAC power mains. In an application where the housing  302  comprises a plug assembly, the AC contacts  304 ,  306  can comprise pins or blades extending through the housing  302  that are adapted for plugging into a standard electrical outlet. In applications where the housing  302  encloses a larger apparatus such as a television set, battery charger, or the like, the contacts  304 ,  306  can comprises circuit board connectors such as pins, sockets, wire leads or the like that connect indirectly to a regulated source of AC power. The miniaturized converter circuit  300  can be integrated with other circuits on a circuit board, in which case the AC contacts  304 ,  306  typically comprise circuit board leads or pads. The contacts  304 ,  306  can also comprise a power cord.  
      An input or excitation current  308  flows mainly through a series circuit that comprises an optional fuse (X 1 )  310 , a capacitor (C 1 )  312 , and a primary winding  314  of a power transformer (U 1 )  316 . Substantially all of the excitation current  308  flows through the primary winding  314  and the capacitor  312 , however, an optional bleed resistor (R 1 )  318  can be provided to discharge any residual charge on capacitor  312  in a fraction of a second when the contacts  304 ,  306  are disconnected from the source of AC power. In an instance where the contacts  304 ,  306  are pins or blades that can be unplugged and exposed, the use of the bleed resistor  318  reduces the possibility of an electrical shock. When used, the bleed resistor  318  typically has a resistance of 10 megohms or more and uses a negligible amount of current and power in comparison with that provided to the primary winding  314 . The bleed resistor  318  can be connected in a series loop with the primary winding  314  and the capacitor  312  as illustrated. Alternatively, the bleed resistor  318  can be connected in a series loop with only the capacitor  312 . In an instance where the contacts are connected to other circuits inside housing  302  that provide a suitable resistive discharge path, the bleed resistor can be omitted.  
      As described in more detail below in connection with  FIG. 3 , the capacitor  312  has an impedance ZC that is selected in consideration of the power line frequency and an impedance ZP on the transformer primary winding  314  in order to provide low power consumption and high efficiency, and to enable miniaturization of a transformer  316 .  
      The transformer  316  includes a secondary winding  320  that is preferably electrically insulated from the primary winding  314 . The secondary winding  320  connects to a rectifier bridge  322 . The secondary winding  320  provides AC excitation to the rectifier bridge  322 , and the rectifier bridge  322  rectifies the excitation and provides rectified (DC) excitation at rectifier output conductors  324 ,  326 . The rectifier bridge  322  can comprise a full wave bridge of rectifier diodes (D 1 , D 2 , D 3 , D 4 ) and provide a full wave rectified output at output conductors  324 ,  326  as illustrated. The rectifier  322  can alternatively comprise only two rectifier diodes in an instance where the secondary winding  320  is center-tapped and provide a full wave rectified output at output conductors  324 ,  326 . The rectifier bridge  322  can alternatively comprise a single rectifier diode and provide a half wave rectified output at output conductors  324 ,  326 .  
      A DC filter capacitor (C 2 )  328  is connected to output conductors  324 ,  326  to reduce AC ripple in the rectified output. A regulator  330  is also connected to the output conductors  324 ,  326  to regulate a DC output voltage at DC output contacts  332 ,  334 . It will be understood by those skilled in the art that the DC load connected to the DC output contacts  332 ,  334  can include a DC filter capacitor, a regulator, or both, making it unnecessary to include DC filter capacitor  328  or regulator  330  in the housing  302  itself. The regulator serves to maintain the output voltage constant with changes in the load current and the variations of the AC input voltage, as for example when the input is 90-280 VAC. The regulator  330  can be a series regulator, a shunt regulator or other known type of regulator. In the example illustrated, an exemplary shunt regulator is shown that comprises a voltage divider (R 2 , R 3 ) providing a reference voltage  336  to a shunt regulator integrated circuit  338 . The adjustable regulator integrated circuit  338  is preferably a type TL431 adjustable precision shunt regulator from ON Semiconductor of Denver, Colo. Some advantageous features of the converter circuit  300  are described below in connection with  FIG. 3 . In  FIG. 2 , and throughout the application, various regulators  330  are identified by a stippled background to better distinguish the regulators  330  from other components.  
       FIG. 3  graphically illustrates AC input impedances ZIN 1  (example  1 ), ZIN 2  (example  2 ), ZIN 3  (example  3 ) that are presented as a load to an AC power source. Example  1  is the circuit in  FIG. 1A  with resistor  112  at zero ohms, in other words, short circuited. Example  2  is the circuit in  FIG. 1A  with resistor  112  at a non-zero resistance so that a significant portion of the AC line voltage is dropped across resistor  112 . Example  3  is the circuit of  FIG. 2  which includes an AC capacitor  312  in series with a primary winding  314 .  
       FIG. 3  provides a transform plane representation of complex impedances. A horizontal axis  352  represents a series resistive, heating, or real component of impedance. A vertical axis  354  represents a series reactive, lossless, or imaginary component of impedance. An origin  356  represents zero AC input impedance. The converter circuit examples  1 ,  2 ,  3  each have approximately the same number of primary winding ampere-turns, each delivers approximately the same amount of power to a DC load, but each draws a different amounts of power from the AC line, and each has a different amount of internal heating.  
      In Example  1  ( FIG. 1A ), the primary winding  108  connects directly to the AC power source  102 , there is no added series impedance (i.e., resistor  112  is zero ohms), and the primary winding  108  has a large number of turns N that carry a primary current I through a primary wire with a wire cross sectional area A. The primary wire is extremely small diameter and subject to breakage, making the transformer difficult to manufacture.  
      In Example  2  ( FIG. 1A ), the primary winding is connected to the AC power source  102  through a resistor  112  that has a resistance R that is larger than a primary winding impedance ZP=ZP 2 . In Example  2 , the AC voltage applied to the primary winding  108  is reduced, and the primary winding  108  has a reduced number of turns (N×0.707, for example) that carry an increased current (I×1.414) for example. In Example  2 , the primary wire has a larger cross sectional area (A×2, for example). The power resistor  112  dissipates a large amount of power, leading to low efficiency and overheating the power supply in Example  2 .  
      In Example  3  ( FIG. 2 ), the primary winding is connected to the AC power source  102  through a capacitor  312  that has a capacitance C. In Example  3 , the AC voltage applied to the primary winding  314  is reduced, and the primary winding  314  has a reduced number of turns (N×0.707, for example) that carry an increased current (I×1.414) for example. In Example  3 , the primary wire has a larger cross sectional area (A×2, for example). The capacitor  312  dissipates negligible power and provides a reduced voltage to the primary winding  314 , allowing a larger diameter wire to be used that is relatively free of breakage during transformer manufacture. The capacitor  312 , which has a negligible power loss, does not overheat the power supply in Example  3 .  
      Impedances of various circuit components in the power supply circuits of  FIGS. 1A, 2  are illustrated as vectors in the  FIG. 3  transform plane. A vector ZC represents an impedance of the capacitor  312  in  FIG. 3 . A vector R represents an impedance (resistance) of the resistor  112  in  FIG. 1A . A vector ZP 1  represents an input impedance on the transformer primary winding  108  of N turns in  FIG. 1A  when resistor  112  is zero ohms. A vector ZP 2  represents an input impedance of (N×0.707) turns on the transformer primary winding  108  in  FIG. 1A  when the resistor  112  has a resistance (impedance) R&gt;|ZP 2 |. The vector ZP 2  also represents an input impedance of (N×0.707) turns on the transformer primary winding  314  in  FIG. 2  that is used in series with capacitor  312  that has a capacitive impedance |ZC|&gt;|ZP 2 |.  
      It will be recognized by those skilled in the art that the impedance encountered at a primary winding such as impedance ZP 1  has a first impedance portion  370  that is due to the primary winding per se (magnetizing impedance), and also a second impedance portion  372  that is due to secondary load as it is reflected at the primary impedance. As illustrated in  FIG. 3 , the magnetizing impedance  370  and the reflected load impedance  372  add up vectorially to impedance ZP 1 .  
      AC input impedances ZIN 1 , ZIN 2 , ZIN 3  of the comparable power supply Examples  1 ,  2 ,  3  are represented as dots on the transform plane. The input impedances are the vector sums of the series components. The AC input impedances can be represented as vectors (not shown) extending from the origin  356  to the dots. ZIN 1  represents the input impedance ZIN illustrated in  FIG. 1A  with RSERIES=0 and ZP=ZP 1 . ZIN 2  represents the input impedance ZIN illustrated in  FIG. 1A  with ZP=ZP 2 , R&gt;ZP 2  and ZP 2 &lt;ZP 1 . ZIN 3  represents the input impedance ZIN illustrated in  FIG. 2  with ZP=ZP 2 , ZC&gt;ZP 2  and ZP 2 &lt;ZP 1 . As can be seen by inspection of  FIG. 3 , Example  1  has a resistive power consumption  358 . In Example  2 , the number of winding turns is reduced by use of a series resistor, but the resistive power consumption is increased greatly to power loss  360 . In Example  3 , the number of winding turns is reduced by use of a capacitor, and the power consumption is reduced to a reduced power consumption level  362 . The power supply circuit in Example  2  is preferred for low power levels below about 50 milliwatts where the lower efficiency (compared to  FIGS. 4-7 ) does not cause excessive heating of the converter circuit.  
       FIG. 3  illustrates that use of an AC capacitor in series with a transformer primary allows an adequate number of ampere-turns for excitation of a low power miniature transformer with increased primary wire size, low primary voltage and low power consumption in a miniature housing that is free of overheating.  
       FIG. 4  illustrates a miniaturized converter circuit  400  that is similar to the miniature converter circuit  300  illustrated in  FIG. 2 . The converter circuit  400  can be used at higher power levels and provides higher efficiency than the converter circuit illustrated in  FIG. 2 . Reference numbers used in  FIG. 4  that are the same as reference numbers used in  FIG. 2  indicate the same or functionally similar features.  
       FIGS. 4-7  illustrate converter circuits that can be used at higher power levels and that provide higher efficiency in comparison to the converter circuits illustrated in  FIGS. 2, 8 .  
      In  FIG. 4 , a secondary winding  320  is center-tapped, and a bridge rectifier  322  includes two rectifier diodes D 1  and D 4 . In contrast with  FIG. 4 , in  FIG. 2 a  secondary winding  320  is not center-tapped and the bridge rectifier  322  requires four rectifier diodes D 1 , D 2 , D 3 , D 4 . A person of ordinary skill in the art would recognize that either rectifier arrangement can be used in a converter circuit, dependent on factors such as the availability of a center tap on the transformer and the desired DC output voltage.  
      In  FIG. 4 , the transformer  316  includes an auxiliary secondary winding  402  that is galvanically isolated from the center-tapped secondary winding  320 . The secondary winding  402  provides energization for a regulator circuit  330  in  FIG. 4 . In contrast with  FIG. 4 , the converter circuit  300  in  FIG. 2  does not include an auxiliary secondary winding.  
      In  FIG. 4 , a regulator circuit  330  regulates power supply voltage at DC output contacts  332 ,  334  by varying current through a shunt regulator that is connected in parallel with transformer primary  314 . In contrast with  FIG. 4 , the converter circuit  300  in  FIG. 2  regulates power supply voltage at DC output contacts  332 ,  334  by varying current through a shunt regulator  338  that is connected in parallel with the DC output contacts  332 ,  334 .  
      In the regulator  330  in  FIG. 4 , current through a regulator integrated circuit  338  varies as a function of DC output voltage, and the current passes through an input of optocoupler  404 . The optocoupler  404  provides galvanic isolation between circuits coupled to the DC output and circuits coupled to the AC input. An output of the optocoupler  404  couples along line  406  to an input of a type 555 timer  408 . A bridge rectifier  410  (connected to isolated secondary winding  402 ) and a filter capacitor  412  provide a galvanically isolator supply voltage for energizing the timer  408  and the output of the optocoupler  404 . An output of the timer  408  on line  414  couples to the gates (inputs) of field effect transistors  416 ,  418 . The timer  408  actuates the field effect transistors  416 ,  418  with voltage pulses to bypass current away from the primary winding  314 . As explained above in connection with Example  3  in  FIG. 3 , the impedance of the primary winding  314  is low in comparison to the impedance of the capacitor  312 . The AC voltage at the primary winding  314  is thus considerably less than the line voltage, and relatively low cost, low voltage field effect transistors  416 ,  418  and commutating diodes  417 ,  419  can be used. With the regulation arrangement shown in  FIG. 4 , so-called “universal line voltage” performance can be easily accomplished. In other words, the converter circuit  400  can be connected, without adjustment, to any line voltage in the worldwide AC power voltage range of about 90-250 VAC and operate with high efficiency over that entire range. With the shunt regulation arrangement in  FIG. 4 , AC current is shunted off before it can reach the transformer, and heating is reduced.  
       FIG. 5  illustrates a miniaturized converter circuit  500  that is similar to the miniature converter circuits illustrated in  FIGS. 2, 4 . Reference numbers used in  FIG. 5  that are the same as reference numbers used in  FIGS. 2, 4  indicate the same or functionally similar features.  
      In  FIG. 5 , a metal oxide varistor (MOV)  502  is connected to a transformer primary  314 . A capacitor  506  is also connected to a transformer primary  314 . As explained in more detail below, the components  502 ,  506  reduce power line transients across the transformer primary  314 .  
      In  FIG. 5 , a regulator circuit  330  regulates power supply voltage at DC output contacts  332 ,  334  by varying current through an optically isolated triac  508  that is connected in parallel with transformer primary  314 . The optically isolated triac  508  functions as a shunt regulator across the transformer primary  314 . In contrast with  FIG. 2 , the circuit  200  in  FIG. 2  regulates power supply voltage at DC output contacts  332 ,  334  by varying current through a shunt regulator  338  that is connected in parallel with the DC output contacts  332 ,  334 .  FIG. 5  provides higher efficiency than  FIG. 2 .  FIG. 5  is preferred for higher power applications, whereas  FIG. 2  is preferred for lower power applications.  
      In  FIG. 5 , an error amplifier sensor (in the shunt regulator integrated circuit  338 ) is used to generate a current I proportional to the error from a reference target. This current is used to drive a light emitting diode (LED)  510  that is used to optically trigger the triac  508  after a threshold in the triac output voltage is reached. The triac  508  is connected across the primary winding  314  of the transformer  316 . When the triac  508  fires (turns ON), presenting a low impedance element across the primary, current is diverted from the transformer primary  314  and the load to the triac  508 , and flows back to the utility return contact  306 . This low impedance (ON) state of the triac is maintained until the current from the capacitor  312  changes polarity. When there is excess voltage available from the utility as controlled by the capacitor  312 , the voltage regulator  330  causes an AC current that is slightly different from a sinusoid because of the step change in voltage across the capacitor  312 . However since this change is voltage is small compared to the total input utility voltage, the resulting deviation of the input current from a sinusoid is minimal. The extra power that would have been delivered to the transformer and load is shunted and absorbed by the reactive component, capacitor  312 . Since the reactive component (capacitor  312 ) is not dissipative, the resulting efficiency of the power supply is higher. There is, however, the small overhead loss in the form of the conduction loss of the triac.  
      The power capability of converter circuit  500  is limited by the current capability of the optically driven triac combination, which is preferably an integrated circuit MOC3042. For higher power applications, MOC3042 maybe used as a gate drive for a higher power rating triac across the primary transformer winding.  
      For loads where the control for the power supply is such that the triac fires close to 180 degrees in the duty cycle, the current from the reactive component during the portion of the cycle is monotonically decreasing. On the other hand, the magnetizing current for the transformer is increasing and close to its maximum and thus in this portion of the power cycle, the load current could possibly be starved. It is observed that under this condition the control of the voltage regulator is irregular and could lead to higher ripple output voltage. Capacitor  506  serves to avoid these problems by storing reserve charge such that there will be current to support the magnetizing current demand and prevent the irregular behavior of the regulator  330 .  
      The current from the utility line is almost a constant current source and a perfect sinusoid because most of the impedance to the power supply is due to the reactance of capacitor  312 . Thus the power supply causes minimal harmonic distortion on the utility input currents. Shunting of the current from the transformer to the triac however generates a spike of current in the reactive component if it is a capacitor. This spike of current is due to the change in the voltage across the reactive capacitive element due to the triac turning ON. The waveform of this current would be dependent on the characteristic of the resulting voltage waveform across the input capacitor. If the triac switches instantaneously, the current would be a large spike, delta function. This produces a large EMI conducted noise with very wide spectrum. If the triac switches with a ramp characteristic of duration tau, the current spike would be a rectangular pulse of duration tau. The resulting conducted emission current due to this pulse has a asymptotic current noise versus frequency profile that would be constant from 120 Hz to 1/(pi*tau) where it would decrease at 20 db/dec in the logarithmic scale. The magnitude of this conducted emission can therefore be reduced if tau were increased such that the 20 db/dec rolloff occurs way before the significant lower frequency of interest for EMC conducted emission which is 150 khz. An inductor  504  in series with the triac serves this purpose. The inductor  504  could also be in series with the capacitor  506  before it is connected to the transformer primary  314 .  
      In both  FIGS. 4-5 , line current is shunted through a switching device (MOSFET or TRIAC) to provide shunt regulation. The switching devices in  FIG. 4-5  are coupled to an AC power line and are not isolated from AC power lines by the transformer  316 . The switching devices in  FIGS. 4-5  are thus subject to damage from transients.  
      In actual utility lines, transients in the form of induced lightning voltage strikes or noise spikes caused by local loads such as motors tuning ON/OFF from household appliances such as washing machines, refrigerators, dishwashers gets coupled directly through the capacitor  312  into the transformer and switching devices and could be large and the cumulative effect of such transients could cause the switching device in  FIGS. 4-5  to fail. The transformer is relatively immune to saturation due to the transient because of the high frequency of the disturbance. Failure would be due to the breakdown of the isolation barrier between the primary and secondary. To avoid this, the embodiments in  FIGS. 4-5  includes transient arresting device such as an MOV  502  (metal oxide varistor) or transient voltage suppressor diodes  417 ,  419  to limit the voltage excursion across the transformer. Because MOVs suffer from long term degradation due to cumulative high energy strikes where the degree of degradation is a function of the cumulative energy of all the strikes, it is advantageous to select the MOV  502  to have a higher energy capacity or use transient voltage suppressors diodes.  
      Whenever there are reactive elements such as capacitor  312 , inductor  504 , capacitor  506  and primary  314 , the possibility of unwanted resonance also occurs. This resonance is suppressed so that damping coefficient is close to 1 by the resistance of the polychem fuse  310 .  
      In the prior art circuit of  FIG. 1A , efficiency can be degraded when the power supply operates at the universal range for example of 90VAC to 280VAC because the series regulator needs to absorb the added voltage drop due to the increase in input voltage. This degradation is greatly reduced in the converter circuits presently disclosed. Power supplies for printers, fax machine and battery chargers for laptops have improved efficiency with the presently disclosed converter circuits are used.  
      The embodiment shown in  FIG. 5  can also be modified such that the triac MOC3042 is connected across a secondary winding  320 . In this modification, the efficiency would be degraded compared to  FIG. 5  because of the addition of the power loss due to the resistance of the primary and secondary winding during the shunting of the power from the load. This modification, however, has the benefit that the inductor  504  used for EMC control can be eliminated, since it is effectively replaced by the naturally occurring leakage inductance of the primary and secondary windings of the transformer. Because there is natural protection by the transformer on high voltage, high frequency content inputs due to the leakage inductances, the transient protection of MOV or voltage suppressor diodes is not needed in this modification.  
      Similar to the modification described above, Mosfet switches can be used in place of the triac, as described below in connection with  FIG. 6 .  
       FIG. 6  illustrates a miniaturized converter circuit  600  that is similar to the miniature converter circuit  300  illustrated in  FIG. 2 . Reference numbers used in  FIG. 6  that are the same as reference numbers used in  FIG. 2, 4 ,  5  indicate the same or functionally similar features.  
      In  FIG. 6 , a secondary winding  320  is center-tapped, and a bridge rectifier  322  includes two rectifier diodes D 1  and D 4 .  
      In  FIG. 6 , the fuse  310  is also located external to housing  302  as illustrated. In  FIG. 6 , the transformer  316  does not includes an auxiliary secondary winding since shunt regulation is performed across the secondary winding  320  rather than the primary winding, and galvanic isolation is not needed in a regulator circuit  330 . In  FIG. 6 , a regulator circuit  330  regulates power supply voltage at DC output contacts  332 ,  334  by varying current through a shunt regulator that is connected in parallel with transformer secondary  320 .  
      In the regulator  330  in  FIG. 6 , current through a regulator integrated circuit  338  varies as a function of DC output voltage on line  324  and the regulator  338  provides a control voltage to an input of a timer  408 . No optocoupler is provided since all regulator components are on the secondary side and no galvanic isolation is needed. An output of the timer  408  on line  406  couples to the gates (inputs) of field effect transistors  416 ,  418 . The timer  408  actuates the field effect transistors  416 ,  418  with voltage pulses to load the secondary winding  320 . With the regulation arrangement shown in  FIG. 6 , so-called “universal line voltage” performance can be easily accomplished. In other words, the converter circuit  600  can be connected, without adjustment, to any line voltage in the worldwide AC power voltage range of about 90-250 VAC and operate with high efficiency over that entire range.  
       FIG. 7  illustrates a miniaturized converter circuit  700  that is generally similar to the miniature converter circuit  600  illustrated in  FIG. 6  above, however, an optically triggered triac  508  is used for a shunt regulator across a secondary winding  320  instead of the mosfets of  FIG. 6 . Reference numbers used in  FIG. 7  that are the same as reference numbers used in  FIG. 2, 4 ,  5 ,  6  indicate the same or functionally similar features.  
      In  FIG. 7 , a capacitor (such as capacitor  506  in  FIG. 5 ) is not needed unless the leakage inductance provided by the transformer is inadequate for EMC control. For higher power applications, where the MOC3042 triac  508  (with optical triggering by light emitting diode  510 ) has inadequate current carrying capability, the triac  508  can be used to drive a gate of a higher power capability triac that serves as the shunt regulator element. The circuit in  FIG. 7  has the advantage of simple circuit topology with only a small power loss penalty.  
       FIG. 8  illustrates an embodiment of a converter circuit  800  which provides two galvanically isolated DC outputs that are each separately regulated. The converter circuit  800  is generally similar to the miniature power supplies illustrated in FIGS.  2 ,  4 - 7 . Reference numbers used in  FIG. 8  that are the same as reference numbers used in  FIG. 2 ,  4 - 7  indicate the same or functionally similar features.  
      In  FIG. 8 , a main (higher power) output on contacts  332 A,  334 A supplies electronic equipment (such as a television set) which can be turned on or off by a remote control. A remote control (lower power) output on contacts  332 ,  334  supplies remote control circuitry (such as an infrared receiver for a remote control in the television) which is continuously energized and serves to turn the higher power equipment on or off. In  FIG. 8 , the transformer has a first secondary winding  320  for energizing lower power circuitry and a second secondary winding  802  for energizing higher power circuitry. The secondary winding  320  connects to a 4 diode bridge  322  and a regulator  330  that are similar to those described above in connection with  FIG. 2 . The secondary winding  802  connects to a 4 diode bridge  322 A and a DC filter capacitor  328 A and a series regulator  804  to provide a second DC output on contacts  332 A,  334 A.  
      The various embodiments of converter circuits illustrated in  FIGS. 2-8  can be used in a wide variety of applications where DC power is converted from an AC power source in a low power range at or below one watt. These applications include both power supplies and battery chargers. Features shown in one embodiment can be appropriately applied to another embodiment.  
      In each of the disclosed embodiments in FIGS.  2 ,  4 - 8 , An AC to DC converter circuit  300 ,  400 ,  500 ,  600 ,  700  or  800  comprises AC input contacts  304 ,  306  coupling to an AC line voltage, and DC output contacts  332 ,  334  coupling to a DC load. Each of the converter circuits includes a transformer  316  with a primary winding  314  and a secondary windings  320 . Each of the converter circuits includes a rectifier bridge  322  coupled to the secondary winding  320 . Each of the converter circuits includes a DC filter capacitor  328  coupled to the rectifier bridge  322 . Each of the converter circuits includes a voltage regulator  330  coupled to the DC filter capacitor  328  and to the DC output contacts  332 ,  334 . In each of the converter circuits, an AC reactance (AC capacitor  312 ) is coupled in a series circuit with the primary winding  314  and the AC input contacts  304 ,  306 . The AC reactance (AC capacitor  312 ) limits AC excitation voltage at the primary winding  314  to less than the AC line voltage at contacts  304 ,  306 . It will be understood by those skilled in the art that the AC capacitor  312  provides a reactance in the primary winding circuit, and that an inductor, which also provides a reactance, can be substituted for the capacitor  312  while achieving the same benefits of low power consumption and reduction in the number of primary winding turns and increase in the wire size of primary winding turns.  
      The following are advantages of the embodiments disclosed over conventional wall plug power supplies and switch mode power supplies:  
      1. Low Cost  
      2. Approximately the same size as wall plug power supplies operating at the same power levels. For lower power applications, the arrangement shown in  FIG. 2  can be used, resulting in smaller size.  
      3. Higher efficiency than equivalent wall plug power supplies operating at the same power levels.  
      4. Transformer approaches theoretical minimum size of transformer for low power applications, with additional window area to accommodate larger size windings.  
      5. Low component count with resultant high reliability.  
      6. Better efficiency than switch mode power supply at low power levels.  
      7. Use of mostly passive components and uses simple and long proven components adding reliability.  
      8. Low frequency switching (120 Hz) resulting in lower EMC noise  
      9. Easy EMC control  
      10. Reactive component is a capacitor, so power factor is leading and almost zero, which is advantageous to utility supplying power to other loads that are typically lagging.  
      11. Input current almost sinusoidal giving low input current harmonic distortion.  
      Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.