Abstract:
One embodiment of an impedance stabilizer for use with a switching voltage regulator supplied by a source of an electrical voltage has an impedance and a switch controllable to permit current from a source to flow through the impedance. Control circuitry to operate the switch cyclically with a controlled duty cycle is responsive to variations in the voltage of the source having a frequency lower than a cycle rate of the switch to increase the duty cycle of the switch as the voltage of the source increases.

Description:
BACKGROUND 
       [0001]    Power supplies for electronic devices commonly use a two-stage system, in which incoming utility power, typically 100 to 250 V AC, 50-60 Hz, is first transformed and rectified to produce about 12 V DC power (which may be smoothed to a greater or lesser extent) and is then regulated to produce a sufficiently smooth and stable DC supply for the load device. The regulator may be a switching converter such as a DC-DC “buck converter,” in which the power intake is regulated by rapidly switching on and off an intake transistor or other switch. 
         [0002]    These converters are typically regulated to produce a constant output voltage into a resistive load, and thus effectively a constant output power, at least in the short term. If the supply voltage increases, the duty cycle of the switch is reduced, reducing the average current flow into the converter. As measured at frequencies low compared with the switching speed of the converter, this produces the effect of a negative real marginal input impedance. In general, impedance may be purely resistive, purely reactive, or complex with both resistive and reactive parts. If the combined negative input impedance of all the switching converters in a given power supply circuit outweighs the positive real impedances in the part of the circuit from the first-stage transformer/rectifier to the switching converters, the circuit can become unstable. Since the only positive real impedance may be the output impedance of the first-stage transformer/rectifier, that is a very real possibility. It has previously been proposed to stabilize such circuits by including a resistive and/or capacitive load impedance alongside the switching converter to provide additional positive real impedance. However, a predominantly resistive stabilizing load wastes power. A predominantly capacitive stabilizing load requires a substantial capacitance if the stabilizing impedance is to be effective. At low frequencies, typically below a few kHz, the physical size of the capacitor becomes a significant problem for the circuit designer. 
         [0003]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The accompanying drawings, which are included to provide further understanding and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention, without being limiting. 
           [0005]    In the drawings: 
           [0006]      FIG. 1  is a circuit diagram of a first embodiment of an impedance correction circuit. 
           [0007]      FIG. 2  is a circuit diagram of part of an embodiment of a power supply circuit including a second embodiment of an impedance correction circuit. 
           [0008]      FIG. 3  is a circuit diagram of a third embodiment of an impedance correction circuit. 
           [0009]      FIG. 4  is a circuit diagram of a fourth embodiment of an impedance correction circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Reference will now be made in detail to various embodiments illustrated in the accompanying drawings. 
         [0011]    Referring to  FIG. 1  of the accompanying drawings, a first embodiment of an impedance correction circuit, indicated generally by the reference numeral  10 , comprises a switch  12  and an impedance  14  that in operation are connected between power supply rails  16  and  18 . The impedance  14  may be resistive, reactive, or a complex impedance with both resistive and reactive components. The switch  12  is controlled by control circuitry  20 . In use, the control circuitry  20  is responsive to variations in the voltage V R  between the power supply rails  16  and  18  to control the switch  12  so that, as the voltage V R  increases, the duty cycle (the proportion of time for which the switch is on), and thus the average current flowing through the impedance  14 , increase. This produces the effect of a positive impedance drawing current between the power supply rails  16  and  18 . 
         [0012]    Referring now to  FIG. 2 , one embodiment of a power supply circuit, indicated generally by the reference numeral  50 , comprises a pair of power rails  52  and  54 , to which are connected a DC power source  56 , a load device  58 , and a second form of impedance stabilizer  60 . 
         [0013]    The DC power source  56  may be, by way of example, an AC to DC converter such as a rectifier or transformer rectifier that produces an imperfectly smoothed DC voltage V R  between the power supply rails  52 ,  54 . Such DC power sources are well known and, in the interests of conciseness, power source  56  is not described in more detail here. 
         [0014]    In the embodiment shown in  FIG. 2 , load device  58  is a “buck converter” that draws current from the power supply rails  52 ,  54  and supplies a lower but more constant output voltage to a load, represented by resistor  62 . The buck converter  58  shown in  FIG. 2  comprises a switch  64 , controlled by control circuitry in the form of a comparator  66  and an amplifier  68 , that alternately charges an inductor  70  and permits the inductor to discharge. 
         [0015]    In use, in steady-state operation of the resistive load  62 , the comparator  66  compares the output voltage from the inductor  70  with a reference voltage, so as to maintain the voltage, current, and power supplied to the resistive load  62  constant within a desired tolerance. As a result, the comparator  66  controls the duty cycle of switch  64  so as to draw similarly constant power from the DC power supply  56  through the power supply rails  52 ,  54 . Therefore, if the DC voltage V R  increases, the duty cycle of switch  64  and the average current drawn by the buck converter  58  decrease, and vice versa, producing a negative apparent marginal resistance. If the negative marginal resistance of the buck converter  58  is greater than the positive output resistance of the DC power supply  56 , the power supply circuit  50  could become unstable and could oscillate or resonate undesirably. If several load devices  58  with negative marginal impedance are connected to a single power supply  56 , this undesirable unstable condition may be even more likely to occur. 
         [0016]    In the circuit  50  shown in  FIG. 2 , the impedance stabilizer  60  provides a positive marginal impedance that compensates for the negative marginal impedance of the load device  58 . 
         [0017]    The impedance stabilizer  60  comprises a switch  80 , which in the embodiment shown in  FIG. 2  is a field effect transistor, in series with a resistor  82  between the power supply rails  52 ,  54 . A comparator  84  compares the voltage across the resistor  82  (which represents the current I R  drawn by the impedance stabilizer  60 ) with the supply voltage V R . The output of comparator  84  may drive the switch  80  through a suitable amplifier  90 . 
         [0018]    Both inputs to the comparator  84  are fed through band pass filters  86 ,  88  so that the impedance stabilizer  60  is responsive only to voltage fluctuations in a desired range of frequencies, typically below a few kilohertz, at which conventional stabilization by a capacitive load would require an inconveniently large capacitor. The circuit components are selected so that the switching rate of switch  80  under control of comparator  84 , and the switching rate of switch  64  in the load device  58 , are fast compared with the upper cutoff of band pass filters  86 ,  88 . Thus, comparator  84  does not respond to the rapid fluctuations caused by the switches, and perceives the current I R  averaged over the duty cycle of the switch  80 . 
         [0019]    The band pass filters may have an upper cut-off at least an order of magnitude, preferably, at least three orders of magnitude, lower than a switching rate of the switch  80  in normal operation. In an example, the impedance stabilizer may be directed primarily to stabilizing the 100-120 Hz fluctuations of full-wave rectified AC utility power, and the switching rate of the switch  80  may be over 1.2 kHz, preferably, over 120 kHz. In a simple feedback circuit, the switching cycle time, as well as the duty cycle within the switching cycle, of the switch  80  may vary, but a maximum cycle rate is typically determined by the response times of the components in the feedback loop. 
         [0020]    The comparator  84  is configured to control the duty cycle of the switch  80  so that the current I R  is related to the voltage V R  by the relationship: 
         [0000]    
       
      
       k 
       1 
       V 
       R 
       −k 
       2 
       I 
       R 
       =k 
       3  
      
     
         [0000]    where k 1 , k 2 , and k 3  are suitable constants. 
         [0021]    The apparent marginal impedance of the impedance stabilizer  60  is 
         [0000]        dV   R   /dI   R   =k   2   /k   1 , which is constant. 
         [0000]    The marginal impedance is selected to be sufficient that the overall marginal impedance of the power supply circuit  50  in regions of operation is positive. 
         [0022]    Because only the marginal impedance, and not the average impedance, is of concern, k 3  may be selected to reduce the actual current draw. For example, I R  may be set to zero at the highest value of V R  that is expected to occur in the normal range of use of the circuit  50 . 
         [0023]    The impedance stabilizer  60  may be less efficient than those described below, because the primary impedance is the resistor  82  which is dissipative, but is very simple. 
         [0024]    Referring now to  FIG. 3 , a third form of impedance stabilizer, indicated generally by the reference numeral  100 , comprises two switches  102 ,  104  connected in series between power supply rails  106 ,  108 . The switches  102 ,  104  are controlled by a pulse width modulation (PWM) circuit  110  that turns the switches on alternately. The PWM circuit  110  is controlled by comparator  112 , which compares the supply voltage V R  with the voltage at an output node between the two switches  102 ,  104 . The output node is provided with an LCR circuit similar to that in the buck converter  58  shown in  FIG. 2 . As may be seen by comparing buck converter  58  with impedance stabilizer  100 , the second switch  104  may be replaced with a reverse-biased diode. 
         [0025]    The impedance stabilizer  100  can be manufactured very economically, because most of its circuitry is a standard buck converter. 
         [0026]    Referring now to  FIG. 4 , a fourth form of impedance stabilizer, indicated generally by the reference numeral  120 , comprises a switch  122  controlled by a PWM circuit  124  that is controlled by a comparator  126 , which compares the supply voltage V R  with the voltage at an output of the switch  122 . The switch  122  is connected in series with a resistor  128  and an inductor  130  between power supply rails  132 ,  134 . A reverse-biased diode  136  is connected in parallel with inductor  130 . 
         [0027]    In this configuration, the resistor  128  serves primarily as a current sense input to the comparator  126  for the current through the switch  122 , and may have a low resistance to reduce resistive heating and dissipation of power. The primary impedance is the inductor  130 . When the switch  122  is turned on, the inductor  130  stores energy, and when the switch  122  is turned off, the inductor discharges stored energy back into the power supply rail  132 . Thus, if properly configured the impedance stabilizer  120  can be almost lossless, because the power that it draws is mostly regenerated. 
         [0028]    Various modifications and variations can be made to the illustrated embodiments without departing from the spirit or scope of the invention. 
         [0029]    For example, although several embodiments of impedance stabilizer have been described, the skilled reader will understand how features from different embodiments may be combined to produce alternative embodiments. The impedance stabilizer  10 ,  100 , or  120  may be used instead of the impedance stabilizer  60  in the power supply circuit  50  shown in  FIG. 2 , and any of the impedance stabilizers may be used with other forms of power supply circuit. 
         [0030]    An impedance stabilizer may be combined in a single module with a load circuit having negative marginal input impedance, and the impedance stabilizer may then be configured so that the module as a whole has non-negative marginal impedance, or has a negative marginal impedance sufficiently low that it will reliably be stabilized by the positive output impedance of any likely power supply  56 . 
         [0031]    Where a power supply circuit comprises more than one load circuit, individual load circuits may be provided with associated impedance stabilizers, or one impedance stabilizer may be provided for a plurality of load circuits, or another arrangement may be used. 
         [0032]    Thus, it is intended that the description cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.