Abstract:
A voltage rectifier circuit having a storage element and a switching stage that is switchable to enable the storage element to capture a peak voltage of an alternating power source. The switching stage includes transistors arranged in a back-to-back configuration. In one example, the storage element is a capacitor and the transistors are PNP bipolar junction transistors. The configuration of the circuit enables reduced loading on the power source, as well as reduced sensitivity to temperature.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    Example aspects described herein relate generally to voltage rectifier circuits, and more particularly, to methods, apparatuses and systems that employ a voltage rectifier circuit having a low-drop diode substitute with minimal loading, to capture the peak voltage of an alternating current source. 
         [0003]    2. Description of the Related Art 
         [0004]      FIG. 1  shows a circuit  101  that is used to capture the positive peak voltage of an alternating current (AC) source  102 . The output current of the AC source  102 , which has certain source impedance  106 , is converted to direct current by a diode  103 . The output voltage of the diode  103  is captured by a capacitor  105 , which is slowly dissipated by a high-value resistor  104  when the output voltage of the AC source  102  is low or zero. 
         [0005]    The circuit  101  has some limitations. First, the diode  103  has a significant voltage drop which can be problematic when the AC signal is in the low-volts range. Because of this, voltage Vout is approximately equal to the peak voltage of the AC source  102  minus the diode voltage drop. For a PN silicon diode, the voltage drop is about 0.7 volts. Assuming that the AC source  102  has a peak voltage of about 3 volts, this voltage drop can lead to a deviation from the peak voltage of about 23%. Second, this voltage drop varies with temperature. Even the voltage drop for a Schottky diode, which is generally smaller than 0.7 volts, is temperature dependent. Third, the source impedance  106  preferably must be kept quite low such that when diode  103  conducts during the peak voltage and the peak current is high, the voltage drop across the source impedance  106  also degrades voltage Vout. However, keeping the source impedance low often is difficult or expensive to do. 
         [0006]    Resistor  104  generally has a high resistance value to slowly “bleed down” the capacitor  105  when the AC source  102  voltage is low or zero. As a result, during the peak input voltage, a current surge must supply all the charge drained away by resistor  104  during the non-peak time. 
       SUMMARY 
       [0007]    The above and other limitations are overcome by a voltage rectifier circuit constructed and operated according to example aspects herein. In one example, the circuit can capture the peak voltage of an alternating current (AC) source, maintains a low voltage drop from the AC source and provides minimal loading on the AC source. 
         [0008]    In one example embodiment herein, the circuit comprises a storage element (e.g., a capacitor), and a switching stage that is switchable to enable the storage element to capture a peak voltage of an alternating power source. The switching stage includes transistors arranged in a back-to-back configuration. In one example embodiment, the transistors are bipolar junction PNP transistors, emitters of the transistors are connected together, and a base of a first one of the transistors is connected with the alternating power source. Also in one example, a base and a collector of a second of the transistors are connected together. 
         [0009]    In one example embodiment, the circuit further comprises a resistor connected in parallel with the storage element, and further comprises a voltage source connected with the emitters. Also, a base and a collector of one of the transistors are connected with the storage element and the resistor. 
         [0010]    Preferably, an output current of one the transistors is set to be a peak current of the alternating power source. 
         [0011]    In still a further example embodiment herein, the transistors are identical, and include a plurality of transistors. 
         [0012]    In another example embodiment herein, the transistors include four transistors, 
         [0000]    Transistors of a first pair of the four transistors are identical with one another, and transistors of a second pair of the transistors are identical with one another. 
         [0013]    In still a further example embodiment herein, at least one of temperature coefficients and voltage drops across the transistors essentially cancel out, and the circuit provides minimal loading on the alternating power source. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The teachings claimed and/or described are further described in terms of exemplary embodiments. These exemplary embodiments are described in detail with reference to the drawings. These embodiments are non-limiting exemplary embodiments, in which like reference numerals represent similar structures throughout the several views of the drawings, and wherein: 
           [0015]      FIG. 1  shows a circuit used to capture the peak voltage of an alternating current (AC) source in the prior art. 
           [0016]      FIG. 2  shows a circuit used to capture the peak voltage of an AC source, according to an example embodiment herein. 
           [0017]      FIG. 3  shows a circuit used to capture the peak voltage of an AC source, according to another example embodiment herein. 
       
    
    
       [0018]    It should be noted that different ones of the Figures may include the same reference numerals to identify the same components, and thus a description of each such component may not be provided herein with respect to each particular Figure. 
       DETAILED DESCRIPTION 
       [0019]    The present application presents several novel and inventive example approaches for, among other things, capturing the peak positive voltage of an alternating current (AC) source, with low voltage drop from the AC source and minimal loading on the AC source, and providing a corresponding DC output voltage. 
         [0020]    As described in the Description of the Related Art section above, the circuit  101  of  FIG. 1  has several limitations, including a large voltage drop across the diode  103  which is temperature dependent and a requirement that the internal impedance of the AC source  102  (represented by the impedance  106 , for example), be low. In accordance with an example aspect herein, on the other hand, a circuit is provided having a low voltage drop from an output voltage of the AC source  102  as well as a small overall loading on the AC source  102 . 
         [0021]      FIG. 2  shows a circuit  201  according to an example embodiment herein. As in the circuit  101  in  FIG. 1 , the circuit  201  includes an AC source  102 , a capacitor  105  and a resistor  104 . In one example, the AC source  102  provides a voltage  401  varying between zero and +3 volts as shown in  FIG. 4 , although the voltage need not be referenced to “0” as shown in the example, and also need not be the example values set forth herein. Instead of the diode  103  in  FIG. 1 , circuit  201  includes a positive supply voltage source  202  which is connected with a resistor  205 , which, in turn, is connected with a pair of PNP bipolar junction transistors (BJTs)  203  and  204  through their coupled emitters at a node  206 . In one example, the voltage source  102  supplies a voltage of 5 volts, although this example is not exclusive. In another example embodiment, the resistor  205  can be replaced with a current source. Such a current source preferably provides a constant current, independent of the value of the peak voltage output by voltage source  102 . For example, as the peak voltage output by source  102  decreases, less peak rectification current is needed, but with resistor  205  being a fixed value, it may supply more current than needed for times when the peak voltage is less than its maximum peak voltage. Thus, use of a current source instead may be deemed suitable in such a situation, although it is not required. 
         [0022]    In the illustrated example embodiment, the BJTs  203  and  204  preferably are identical each with a current gain β (although depending on predetermined operating criteria, in other embodiments they do not need to be identical). The base of the BJT  203  is connected with the AC source  102 , while the collector of the BJT  203  is connected to ground via a node  207 . The base and the collector of the BJT  204  are joined together (and thus BJT  204  behaves as a diode with a forward voltage of about 0.7 volt) at a node  208 , which is connected via node  209  with one end of each of the capacitor  105  and resistor  104 . The AC source  102 , capacitor  105  and resistor  104  are also connected to ground at the other ends thereof 
         [0023]    In one example, the preferred RC time constant of the resistor  104  and the capacitor  105  is large compared to the period of the input AC source  102 . This keeps the ripple voltage on the capacitor  105  to an acceptably low value. However, in another example, such as a case where the peak voltage of the AC source  102  slowly varies over many cycles, the RC time constant can be short enough such that the voltage across the capacitor  105  can follow this changing input voltage. 
         [0024]    The output current I 1  of the resistor  205  is set to be the maximum peak current of the AC source  102  during the rectified peak input voltage. In one example embodiment herein, the peak rectified current is estimated to be that value, assuming that the source impedance  106  is zero and that an ideal diode is used. If the current supplied by element  205  is slightly less than the assumed ideal peak current, then in one example this circuit conducts current to the element  105  slightly longer during the input peak voltage, and the peak output voltage of capacitor  105  is slightly less. 
         [0025]    When the base voltage Vb 1  of the BJT  203  is approximately less than the base voltage Vb 2  of the BJT  204 , the base-emitter junction of the BJT  203  becomes forward biased and that of the BJT  204  becomes reverse biased (the BJT  204  turns off). A small current  12  equal to I 1 /β then flows back towards the AC source  102  through the base of the BJT  203 . When the voltage Vb 1  is equal to the voltage Vb 2 , the current I 1  generally gets split evenly between the BJTs  203  and  204 . When the voltage Vb 1  is greater than the voltage Vb 2  or approximately exceeds it, the base-emitter junction of the BJT  203  becomes reverse biased (the BJT  203  turns off) and that of the BJT  204  becomes forward biased. As a result, essentially the entire current I 1  is then provided to the “diode-connected” BJT  204  and used to charge the capacitor  105 . 
         [0026]    In one example embodiment, the off-to-on voltage ratio of a BJT (i.e, the ratio of the base voltage at 5% of the peak current from the emitter to the base voltage at 95% of the peak current from the emitter) is about 75 mV, and thus an absolute difference between Vb 1  and Vb 2  of at least 75 mV causes the current I 1  to go in largely one of the two directions described above. For example, as voltage Vb 1  rises above Vb 2  by about only 75 mV, 95% or essentially the entire current current I 1  is used to charge the capacitor  105 . Thus, in one example the dual transistor configuration behaves like a diode having a voltage drop of nominally 0 volts and an off-to-on voltage of only about 150 mV (as compared with a normal diode that has an ON voltage of 0.7 volts and an OFF voltage (where current has dropped to just 5%) which is about a 75 mV drop. 
         [0027]    As can be appreciated in view of  FIG. 2 , a voltage supplied by the AC source  102  can be reduced somewhat as result of the source impedance  106  to Vb 1 . When the output of the AC source  102  is such that Vb 1  is greater than Vb 2 , then as described above essentially the entire current I 1  is used to charge the capacitor  105 . When the output of the AC source  102  is such that Vb 1  is less than Vb 2  and the BJT  204  turns off, a small current flows back to the AC source  102  while the capacitor  105  discharges to the resistor  104 . In this manner, the resistor  104  discharges the capacitor  105  between the peak voltages represented by voltage  401  in  FIG. 4 . The capacitor  105  outputs to resistor  104  a DC voltage (which may have a small amount of ripple)  403  such as in the example represented in  FIG. 4 , and thus the circuit  201  rectifies the output voltage  401  of the AC source  102  in a manner so as to capture the peak of that voltage  401 . In other words, when the BJT  203  is off and the BJT  204  is on, the current I 1  is used (“shunted”) to charge the capacitor  105  during the peak of the voltage  401  output by AC source  102  (e.g., during 5% to 10% of the period of that voltage), thereby enabling the circuit  201  to “capture” the peak of that voltage  401  (whereas at other times, such as when the BJT  203  is on and the BJT  204  is off, that current I 1  is “shunted to” ground). 
         [0028]    While the BJTs  203  and  204  each have an emitter-base voltage of about 0.7V, these voltages tend to cancel each other out because the BJTs  203  and  204  are identical. Therefore, the sum voltage drop across the pair of BJTs  203  and  204  is essentially zero, as opposed to the voltage drop across the diode  103  in  FIG. 1  being a nominal 0.7V. Therefore, the BJTs  203  and  204  behave as a diode but with a voltage drop of nominally zero. In addition, owing to the presence of the voltage source  202  and the manner in which current flows from the voltage source  202  to the BJTs  203  and  204 , as discussed above, the capacitor  105  can remain charged at least until it captures the peak output voltage  401  of the AC source  102 . While the BJTs  203  and  204  in one example embodiment each may have a temperature coefficient of −2 mv/° C. or similar values, the temperature dependencies similarly tend to cancel each other, thereby minimizing or substantially avoiding temperature-sensitivity of the circuit. 
         [0029]    Furthermore, since the current gain β is of the order of 50 to 100, the small current that flows through the base of the BJT  203  when the voltage Vb 1  is less than the voltage Vb 2  is only 1% or 2% of the peak rectified current that is drawn by the diode  103  in  FIG. 1 . The pair of BJTs  203  and  204  therefore causes a smaller loading effect than does the diode  103 . For example, in the case of  FIG. 1 , all of the peak current that charges capacitor  105  must come from the source  102 , and a large voltage drop is experienced through source impedance  106 . As a result, the rectified voltage across capacitor  105  is reduced by the amount of voltage drop across impedance  106 , and capacitor  105  thus does not capture the true peak voltage of the source  102 . Owing to the configuration of  FIG. 2 , on the other hand, in which VCC source  202  is used to provide peak rectification current I 1  for charging capacitor  105  during the peak voltage of source  102 , the circuit  201  draws only about 1% or 2% of the peak rectified current, and thus the voltage drop owing to the impedance  106  is a factor of about 50 to 100 less than that in the case of the  FIG. 1  circuit. Hence, the circuit of  FIG. 2  suffers essentially no penalty in the voltage output at node  209 , for example, from losses in source impedance  106 , even in cases where that impedance  106  may be high. 
         [0030]    Therefore, this embodiment provides a circuit  201  where capacitor  105  largely tracks the positive voltage peaks of AC source  102  to provide a corresponding DC, rectified voltage, and is able to capture the peak voltage of the voltage source  102 , without much loading on the voltage source  102 , as compared to the case of the circuit  101  of  FIG. 1 . 
         [0031]    As can be appreciated in view of the above description, the BJTs  203  and  204  of the circuit  201  described above preferably operate such that, other than during a small transition region in which both devices may be “on” at the same time, at other times only one of those BJTs is “on” (and operating in a linear region) at a time while the other BJT is “off”. This feature is unlike at least some conventional devices having transistors connected at their emitters, because in such devices both transistors typically operate continuously in a linear region. 
         [0032]    Because the transistor  204  operates as a “diode-connected” transistor, in another example embodiment that transistor  204  can be replaced with a diode that preferably has a substantially similar voltage drop as does the BJT  203 . In that example, an input of the diode is connected to node  206 , and an output of the diode is connected to node  208 . 
         [0033]    Referring now to  FIG. 3 , a circuit  301  according to another example embodiment herein will now be described. As in the circuit  201  in  FIG. 2 , there are an AC source  102 , a capacitor  105 , a resistor  104 , a positive-supply voltage source  202 , a resistor (or current source)  205 , and PNP BJTs  203  and  204 . Circuit  301  also includes resistors  304  and  305  and PNP BJTs  302  and  303 . The resistors  304  and  305 , in one example, have substantially equal resistance values, and are selected to provide a smaller current (in one example, 10% to 20% of I 1 ) than that provided through resistor  205 . This arrangement maintains BJTs  302  and  303  on at all times, and thus, in one example, does not significantly increase, if at all, the approximate 150 mV off-to-on characteristic of BJTs  203  and  204 . Like resistor  205 , the resistors  304  and  305  can also be current sources instead. 
         [0034]    In one example, two or more of the BJTs  203 ,  204 ,  302  and  303  are identical each with a current gain β, although they do not have to be identical. Preferably, the BJTs  203  and  204  (referred to for convenience as a “first pair”) are identical with one another, and the BJTs  302  and  303  (referred to for convenience as a “second pair”) are identical with one another, but the BJTs of the first pair need not be identical with the BJTs of the second pair. The BJTs of the second pair can be smaller than those of the first pair, in one example. 
         [0035]    The resistor  304  is connected with the pair of BJTs  203  and  302  via a node  306 , where the base of the BJT  203  and the emitter of the BJT  302  are connected with each other via node  306 . The collector of the BJT  203  is connected to ground via node  207  as in  FIG. 2 . The base of the BJT  302  is connected with the AC source  102 , and the collector of the BJT  302  is connected to ground via a node  310 . Similarly, the resistor  305  is connected with the pair of BJTs  204  and  303  via a node  307 , where the base of the BJT  204  and the emitter of the BJT  303  are connected with each other via node  307 . The collector of the BJT  204  and base of the BJT  303  are connected together at a node  308 , which is connected via a node  309  with one end of each of capacitor  105  and resistor  104 . The collector of the BJT  303  is connected to ground via a node  311 . The resistor  104  and capacitor  105  also are connected at other ends thereof to ground, as in  FIG. 2 . Also, the resistors  304 ,  205  and  305  are connected to Vcc source  202 . 
         [0036]    The output current I 1  of the resistor  205  is set to be the maximum peak current of the AC source  102  during the rectified peak input voltage. The output current  14  of the resistor  304  and the output current  15  of the resistor  305  are each set to be between 10% and 20% of the current I 1 . Because the BJTs  302  and  203  have a collector commonly connected to ground, the base-emitter junction of the BJT  302  is always forward biased. The output current  15  is set to also keep the base-emitter junction of the BJT  303  forward biased. When the base voltage Vb 3  of the BJT  302  is less than the base voltage Vb 4  of the BJT  303 , the base-emitter junction of the BJT  203  becomes forward biased and that of the BJT  204  becomes reverse biased (the BJT  204  turns off). As a result, BJTs  302  and  203  are both on, and BJT  303  also is on. Therefore, a small current  16  equal to (I 1 /β/β+I 4 /β) flows back through the base of the BJT  302  back to the AC source  102  (that current is smaller than current which may flow back to source  102  in  FIG. 2 ). When the voltage Vb 3  is equal to the voltage Vb 4 , all BJTs  302 ,  203 ,  204 , and  303  are on, and the current I 1  generally gets split evenly between the BJTs  203  and  204 . As voltage Vb 3  continues to rise by, for example, another 75 mV (and is greater than voltage Vb 4 ) the base-emitter junction of the BJT  203  becomes reverse biased such that BJT  203  turns off, and the BJT  204  becomes forward biased. Therefore, essentially the entire current I 1  is provided to the BJT  204  and that current plus current  15  are provided to the base of the BJT  303  to charge the capacitor  105 . 
         [0037]    As can be appreciated in view of  FIG. 3 , a voltage supplied by the AC source  102  is reduced somewhat as result of the source impedance  106  to Vb 3 . When the output of the AC source  102  is such that Vb 3  is approximately greater than Vb 4 , then as described above essentially the entire current I 1  is used to charge the capacitor  105 . When the output of the AC source  102  is such that Vb 3  is approximately less than Vb 4  and the BJT  204  turns off, a small current flows back to the AC source  102  and the capacitor  105  discharges to the resistor  104 . In this manner, the resistor  104  discharges the capacitor  105  between the peak voltages represented by voltage  401  in  FIG. 4 . The capacitor  105  outputs to resistor  104  a DC voltage (which may have a small amount of ripple)  403  such as in the example represented in  FIG. 4 , and thus the circuit  301  rectifies the output voltage  401  of the AC source  102  in a manner so as to capture the peak of that voltage  401 . In other words, when the BJT  203  is off and the BJT  204  is on, the current I 1  is used to charge the capacitor  105  during the peak of the voltage  401  output by AC source  102  (e.g., during 5% to 10% of the period of that voltage), thereby enabling the circuit  301  to “capture” the peak of that voltage. 
         [0038]    Since the BJTs  302  and  303  preferably are always on, the circuit  301  does not suffer additional losses from their off-to-on voltage ratios on top of the normal off-to-on losses of the BJTs  203  and  204 . Preferably, to minimize loading on the source  102 , the BJTs  302  and  303  do not carry too much current which can cause loading. In one example embodiment, the off-to-on voltage ratio of each of the BJTs  203  and  204  is about 75 mV, and thus an absolute difference between Vb 3  and Vb 4  of at least 75 mV causes the current I 1  to go in largely one of the two directions in a similar manner as described above. 
         [0039]    The base-emitter voltages of the first pair of BJTs  302  and  203  and the second pair of BJTs  204  and  303  tend to cancel each other out because the BJTs  203  and  204  are identical, and the BJTs  302  and  303  are identical. Therefore, the sum of the voltage drops across the first pair of BJTs  302  and  203  and the second pair of BJTs  204  and  303  is essentially zero. In addition, owing to the presence of the voltage source  202  and the manner in which the current flows from the voltage source  202  to the two pairs of BJTs, as discussed above, the capacitor  105  can remain charged until it captures the peak positive output voltage of the AC source  102 . In one example embodiment, the BJTs  203 ,  204 ,  302  and  303  (or the BJTs from a same pair) each have a same temperature coefficient, and, as a result, the temperature dependencies similarly tend to cancel each other, thereby minimizing or substantially avoiding temperature sensitivity of the circuit. Furthermore, the circuit  301  of  FIG. 3  operates substantially similarly as the circuit  201  of  FIG. 2 , although the circuit  301  presents even less loading to the source  102  than does circuit  201 . That is because the base current from the BJT  302  that flows back to the AC source  102  is even smaller than the current that flows back from the base  203  in this circuit  301  and in the circuit  201 . The first pair of BJTs  302  and  203  and the second pair of BJTs  204  and  303  therefore cause an even smaller loading effect than the single pair of BJTs  203  and  204  of  FIG. 2 . Owing to the presence of BJT  302 , any current that may flow back towards the source  102  is reduced relative to, for example, that in the circuit of  FIG. 2 , thereby enabling the circuit  301  of  FIG. 3  to provide a more accurate representation of the peak voltage via capacitor  105 . 
         [0040]    Moreover, whereas the BJTs  203  and  204  operate from full on-to-off, the BJTs  302  and  303  always remain ON and hence there is very little change in their Vbe&#39;s during rectification. Moreover, since there are many transistors in the circuit  301  of  FIG. 3 , that circuit is well suited for an integrated circuit, rather than separate discrete transistors, although it can be either. In an integrated circuit in at least some cases, it can be easier to assure that the transistors are identical and that the temperature of all transistors is the same. 
         [0041]    In one example, at least part of the circuit  301  behaves like a diode having a voltage drop of nominally 0 volts and an off-to-on voltage of only about 150 mV (as compared with a normal diode that has an ON voltage of 0.7 volts and an OFF voltage of about a 75 mV drop). 
         [0042]    As for the circuit  201  of  FIG. 2 , because the transistor  204  of  FIG. 3  operates as a diode-connected transistor, in another example embodiment that transistor  204  can be replaced with a diode that preferably has a substantially similar voltage drop as does the BJT  203 . In this example, an input of the diode is connected to node  206 , and an output of the diode is connected to node  308 . 
         [0043]    Although the above description is described in the context of employing BJT-type transistors, in other embodiments other types of transistors may be employed to carry out the overall functionalities described herein. 
         [0044]    In the above descriptions, various aspects of the invention have been described with reference to specific example embodiments. The specification and drawings are accordingly to be regarded in an illustrative rather than in a restrictive sense. It will, however, be evident that various modifications and changes may be made without departing from the broader spirit and scope of the present invention. 
         [0045]    In addition, it should be understood that the figures illustrated in the attachments, which highlight the functionality and advantages of the present invention, are presented for example purposes only. The architecture of the example aspect of the present invention is sufficiently flexible and configurable such that it may be utilized (and navigated) in ways other than that shown in the accompanying figures. 
         [0046]    Although example aspects of this invention have been described in certain specific embodiments, many additional modifications and variations would be apparent to those skilled in the art. It is therefore to be understood that this invention may be practiced otherwise than as specifically described. Thus, the present example embodiments, again, should be considered in all respects as illustrative and not restrictive.