Abstract:
A method and apparatus is provided to, among other things, supply power to a load under various load conditions. Output voltage transient responses of the system, such as may be caused by transients changes in the load conditions, may be controlled through current transformation on the output in order to correct or impede over-voltage conditions of the transient response.

Description:
TECHNICAL FIELD 
     The present application relates to regulated power supply systems and methods for controlling transient responses in such systems. 
     BACKGROUND 
     Voltage transients caused by load changes or unstable load conditions can be difficult to correct quickly enough to prevent over-voltage conditions on the power supply output. 
     For example, unstable load conditions causing oscillations in supply voltage tend to occur when a negative impendence load is supplied in power by a conventional regulated power supply system. This is because negative impendence characteristics, in contrast with conventional resistive loads and inductive loads, generate current variations which are 180 degrees out of phase with supply voltage variations. Hence, for a negative impedance load supplied with constant power, a slight increase in output voltage tends to decrease the current absorbed by the load, which in turn tends to cause the load voltage to rise even further leading to an unstable condition which may damage the power supply system and its loads. 
     There is thus a need for a regulated power supply system which exhibits an improved response to transient load changes or unstable load conditions. 
     SUMMARY 
     In accordance with one aspect, there is provided a power supply system for controlling an output fluctuation, the system comprising: a current controlled current source, the source having an output circuit and a control circuit, the control circuit including a DC current source connected thereto for generating a control current, the circuits being inductively coupled such that current in the control circuit is proportional to current in the output circuit, the output circuit connected to a load; and a current transformer having a primary coil connected in series with the output circuit and a secondary connected in series with the control circuit. 
     In accordance with another aspect, there is provided a power supply apparatus for controlling an output fluctuation to a load, the system comprising: a permanent magnet generator/alternator assembly having at least one primary winding and at least one control winding, the primary winding connected to an output circuit including a load, the control winding connected to a control circuit including a DC control current source, the assembly having means for inductively coupling the primary and control windings such that current is in the primary is proportional to current in the control; and a current transformer having a primary coil connected in series with the output circuit and a secondary connected in series with the control circuit. 
     In accordance with aspect, there is provided a method for controlling an transient in a load circuit of a power supply, the method comprising: providing a current controlled current source having the output circuit inductively coupled to a control circuit such that current in the control circuit is proportionally to current in the output circuit; providing a DC control current to the control circuit and operating the current controlled current source to provide a current to a load via output terminals of an output circuit; inductively coupling an output terminal of the output circuit to the control circuit, such that a sudden decrease in current at the output terminal effects a proportional decrease in control current, thereby permitting the control circuit to control a transient load response in the output circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details will be apparent from the following detailed description, taken in combination with the appended figures, in which: 
         FIG. 1  is a schematic illustration of an example power supply system; 
         FIG. 2  is a flow chart for an example method of controlling a transient response of a power supply to a load; 
         FIG. 3  is a schematic illustration of one possible embodiment of the power supply system of  FIG. 1 ; and 
         FIG. 4  is a schematic partial cross-sectional view of an alternator/motor. 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , the power supply system  10  has two output terminals A and B connected to a load  11 . The power supply system  10  has a current controlled current source  12 , a filtering device  14 , a current transformer  16  and control circuitry  18 . 
     A current transformer  16 , having a primary  20  and a secondary  22 , is connected in series with one of the power supply output conductors and directly in series with the load circuit  11 . In particular, the primary  20  of the current transformer  16  is connected in series with the load  11  (i.e. between the output terminal B and the filtering device  14 ). DC output current supplied from the current controlled current source  12  flows to (in this example) the load via the current transformer primary  20 . Thus, output current of the current controlled current source  12  provided to the external load  11  also flows through the primary  20  of the current transformer  16 . The secondary  22  of the current transformer  16  is connected in series with the control circuitry  18 , such that any transient current requested from the source  12  by current in the control circuitry  18 , also flows in the secondary  22  of the current transformer  16  as well as in the control circuitry  18 . 
     The operation of power supply system  10  may be better understood with reference to a specific implementation of the system, such as is presented in  FIG. 3  and will now be discussed. 
     Referring to  FIG. 3 , in one example the current controlled current source  12  may include a permanent magnet generator/alternator  12  of the general type described in U.S. Pat. No. 7,262,539, the full contents and teachings of which patent are incorporated herein by reference. Further in this example, the generator/alternator  12  may be filtered by a filtering device  14  and may be modulated or regulated to provide a regulated DC output voltage, as is described in United States Published Patent Application US20080067982A1, the full contents and teachings of which published application are incorporated herein by reference. It will be understood, in light of the teachings herein and in the incorporated references, that controlling the control current delivered to the generator/alternator  12  allows the generator/alternator to behave as a current controlled current source. 
     The generator/alternator  12  in this example has multiple alternator phase coils  52  which are inductively coupled to a control coil (or coils)  44  as described in U.S. Pat. No. 7,262,539, so that current in the control coil(s)  44  proportionally affects the output power of by the generator/alternator  12 . A transfer ratio may be provided between the control coil(s)  44  and the phase coils  52 , such as a transfer ratio of 5:1 in this example. The control current flowing in the control coil  44  may optionally be externally controlled by a variable DC current source  46 , as described in US20080067982A1, to vary the current flowing in the secondary coil inversely to a variation in current occurring in the primary coil. A voltage feedback  54  of the type described in US20080067982A1 be provided relative to a reference signal  5 . Filtering device  14  may be provided by a rectifier circuit  48 , which may include a capacitor  50 . Any suitable filtering device  14  may be used. The skilled reader will appreciate that, although useful the purpose of the present description,  FIG. 3  is highly schematic and does not necessarily show all system components or show all components in their correct number or exact physical placement. 
     Referring to  FIG. 4 , alternator/motor  12  has a rotor  112  with permanent magnets  114  which is mounted for rotation relative to a stator  120 . Stator  120  has at least one power winding  52  and preferably at least one control winding  44 , and this embodiment stator  120  has a 3-phase design with three electromagnetically-independent power windings  52  (the phases are denoted by the circled numerals  1 ,  2 ,  3 , respectively) and, correspondingly, three independent control windings  44 . The power and control windings are separated in this embodiment by a winding air gap  126  and disposed in radial slots  128  between a plurality of adjacent teeth  130 . (For ease of illustration in  FIG. 4 , the adjacent elements of control winding  44  are shown unconnected. For ease of description, the adjacent slots  128  are indicated as C, D, E, F etc.) Power winding  52  and control winding  44  are electrically isolated from one another. A back iron  132 , or control flux bus as it is described in this application, extends between slots  128 . A rotor air gap  134  separates rotor  112  and stator  120  in a typical fashion. A core or “bridge” portion or “power flux bus”  136  portion of stator also extends between adjacent pairs of teeth  130  between adjacent portions of power winding  52 . 
     Referring again to  FIG. 4 , in use, in a alternator mode rotor  112  is moved relative to stator  120 , and the interaction of magnets  114  and power windings  52  creates a primary magnetic flux within PM machine  12  along a primary magnetic flux path or magnetic circuit  160 . The primary flux induces a voltage in the power winding, which when an electrical load is connected results in an induced current, and the induced current causes a secondary magnetic flux to circulate an adjacent secondary magnetic flux path or magnetic circuit  162 . The primary and secondary circuits are thus magnetically coupled when a current flows in the power winding. The secondary magnetic circuit  162  is for the most pail isolated from the rotor and primary magnetic circuit  160 . (It is to be understood that this description applies only to phase “1” of the described embodiment, and that similar interactions, etc. occur in respect of the other phases). The skilled reader will appreciate in light of this disclosure that it may be desirable in many situations to include a regulation apparatus to maintain a minimum current in the power winding during no-load conditions. 
     Primary magnetic circuit  60  includes rotor  112 , rotor air gap  134 , power flux bus  136  and the portion of stator teeth  130  between rotor  112  and power flux bus  136 . Primary magnetic circuit encircles a portion of power winding  52  and, in use as an alternator causes a current flow in power winding  52 . Secondary magnetic circuit  162  includes power flux bus  136 , control bus  132  and the portion of stator teeth  130  between control bus  132  and power flux bus  136 . In this embodiment, secondary magnetic circuit encircles the portions of the power winding  52  and control winding  44  in slot  128   b . Power flux bus  136  divides slot  128  into two slot portions or openings  128   a  and  128   b , with one opening  128   a  for the power winding only, and another opening  128   b  for the power and control windings. The primary magnetic circuit encircles an opening  128   a  while the secondary magnetic circuit encircles an opening  128   b . Opening  128   a  is preferably radially closer to the rotor than opening  128   b . Power flux bus  136  is preferably common to both the primary and secondary magnetic circuit paths and thus the primary and secondary magnetic circuits are magnetically coupled, as mentioned. 
     A tertiary magnetic circuit  164  preferably circulates around control bus  132 , as partially indicated in  FIG. 4  (i.e. only a portion of the tertiary circuit is shown, as in this embodiment the tertiary circuit circulates the entire stator). The control flux bus  132  is preferably common to both the secondary and tertiary magnetic circuit paths and thus the secondary and tertiary magnetic circuits are also magnetically coupled. As mentioned, at least a portion of control flux bus  132  is saturable. 
     In use, as is described in more detail US20080067982A1, the current delivered by such a generator/alternator  12  is proportional to the control current provided to the control coil(s)  44  of the alternator by the source  46 . The generator/alternator  12 , its associated control circuit  18 , and the filtering device  14  thus form together an apparatus useful for generating regulated output voltage. The system  10  may thus be used to provide regulated power. 
     Referring still to  FIG. 3 , transient control may be provided by connection of system  10  to a current transformer  16 , as will now be described. A primary coil  40  of the transformer  16  is connected in series with the DC output terminal B of the power supply system  10 , while a secondary coil  42  of the transformer is connected in series with the control coil  44  and allows for a current to flow in a direction reverse to a direction of a current flowing in the primary coil  40 , thereby having the effect of cancelling DC fluxes occurring in the core of the current transformer  16 . A diode  56  is provided across the transformer secondary in the control circuit of this example to prevent the voltage across the secondary from reversing polarity. 
     The transformer primary-to-secondary ratio may be matched to the current controlled current source transfer ratio. For example, the generator/alternator  12  of  FIG. 3  may have a transfer ratio of 5:1, meaning that the output current of the generator/alternator  12  is 5 times the control current input. While the current controlled current source may have any suitable current transfer ratio, matching the current transformer  16  primary-to-secondary ratio to the current transfer ratio of the current controlled current source may assist with ensuring that the current transformer  16  core remains unsaturated, since ampere turns in the primary are equal and opposite to the ampere turns in the secondary, thus resulting in cancellation of the flux in the core of the transformer. Consequently, the current transformer  16  may also be provided with a primary-to-secondary ratio of 5:1. 
     Referring still to  FIG. 3 , in use, it will be understood that changes in currents flowing respectively in the primary  40  and the secondary  42  of the current transformer  16  are related, such that if there should be an unrequested change in the current in the load circuit  11 , for example caused by a sudden open circuiting of the load (a breaker circuit opening, for example), the current flowing in the secondary  42  will be influenced by the primary current such that the current flowing in the secondary  42  will be reduced at virtually the same instant. This will cause, in this example, the control current provided by the circuit  18  to the current controlled source  12  to be suddenly reduced, as well. As noted above, since output current is proportional to control current in current controlled current source  12 , reducing the control current will also reduce the output current from the source  12 , virtually in synchronism with the sudden loss of load. Without this current transformer  16  arrangement, the output voltage of the current source  12  would otherwise suddenly increase in response to an open circuit on the load, since the output load resistance has suddenly greatly increased. the skilled reader will appreciate that, if a voltage feedback  54  (as is further described in US20080067982A1) is provided, the output voltage of the source  12  would eventually (i.e. after some transient time) return to the desired/set output voltage through the control action of the voltage feedback, however the current transformer of the present arrangement provides a faster response time. 
     In the case where the control circuit  18  has an intrinsic inductance, such as where the circuit includes one or more control coils, the time to reduce the current in the control circuit may be dependant on the voltage which is available within the control circuit. As current in the control circuit changed, the inductively-generated back EMF (i.e. V=L*dI/dT, where V is voltage, L is inductance, I is current and T is time) relative to the available voltage across the control circuit tends to limit how quickly the control current can be changed. However, in the case where, say, a 5:1 transfer ratio is present between control and output in the current controlled source, the output voltage available on the secondary of the current transformer is 5 times greater than the voltage change at the current transformer primary and, as such, provides a control action which is 5 times faster than may otherwise be obtained from the voltage control portion of the control circuit  18 . 
     Referring again to  FIG. 1 , therefore when a change (also referred to as an output fluctuation or a transient) in the output current at the output terminals A and B occurs, a control current flowing in the control circuit  18  instantaneously changes direction in a suitable direction to change the output power to correct the output power generated by the generator/alternator  12 . The direction of the control current reduces the output power supplied through inductive coupling effects of the control circuit within the generator/alternator  12 . The current on the control circuit, is influenced in a direction that adjusts the output current according to the load demand for transient conditions. In this example, the net control current will reduce/increase in response to a load transient (depending on the transient to be controlled). Therefore, a sudden drop in load current (e.g. due to an open circuit on the load) will also cause a drop in control current, which will effect a drop in generated current from the source. This reduction in generated current, in turn, reduces the output voltage and DC output current through the primary conductive device  20 , thus mitigating positive output voltage transients due to sudden load reductions. 
     The described approach may thus provide a direct feedback mechanism useful, in one example, in case of sudden, unrequested transients in a condition of the load  11 . The feedback mechanism allows the reduction of voltage transients caused by sudden changes in a load condition or an unstable load condition. 
       FIG. 2  illustrates one example method of controlling a transient response of a power supply system, as will now be described. 
     In step  30  a current controlled output current is generated. 
     In step  32 , the output voltage is optionally monitored and controlled by comparing the output voltage of the source to a reference voltage, and the control current is adjusted to maintain the output voltage at a predetermined rate/level. 
     In step  34 , a current transformer is provided with the primary in series with the output current terminals of the current controlled current source and the secondary in series with a control current circuit controlling the current controlled current source. 
     In step  36 , the current transformer polarity is configured such that load-induced changes in system output current automatically provide proportional changes to the control current in the control current circuit, to thereby effect corrections to output current requested from the current controlled current source in response to load transients. 
     It will be understood that constant power loads often exhibit negative impedance instability characteristics. In the present arrangement, as current absorbed by the constant power load decreases, the transformer  16  reacts to the change in the supplied output current at the terminals A and B such that the output current is reduced in a controlled manner. The controlled reduction in the output current to the load, in turn, reduces the output voltage at the load. This tends to reduce the amount of phase shift between the current and the voltage at the load which is usually seen when the load exhibits negative impedance characteristics. The instabilities may therefore be alleviated through operation of the transformer  16 . 
     It will also be understood that other variants of the power supply system  10  are possible in accordance with given practical applications. For example, the current controlled current source  12  may be any suitable current controlled current source. The embodiments described above therefore are intended to be exemplary only, and are susceptible to modification without departing from the present application. The application is intended to be limited solely by the scope of the appended claims.