Abstract:
A current detector for detecting a main circuit current of an inductor connected to a switching element whose operation generates that current. The inductor includes main and auxiliary windings equal in numbers of turns and connected to the detector so as to cancel out electromotive forces generated by the operation in the windings. The detector includes a voltage detection unit having two input terminals. One end of each winding is connected to a different one of these terminals. The voltage detection unit detects a voltage difference between the connected ends. The detector also includes a temperature detection unit detecting a temperature of the main winding, and a current calculation unit correcting a main winding resistance using the detected temperature and calculating a current as the main circuit current, using the corrected resistance and an average detected voltage calculated from sampled values of the detected voltage using a sampling frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a continuation application of International Application number PCT/JP2014/056108, which was filed on Mar. 10, 2014 and designated the United States. The disclosure of this earlier application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a current detector, which detects a current flowing through an inductor, and a power conversion device which carries out power conversion by controlling a semiconductor switching element on/off using a current detection value detected by the current detector. 
         [0004]    2. Background Art 
         [0005]    As a chopper which boosts or bucks a direct current voltage, there is one which detects a current flowing through an inductor in which to store energy, and controls a semiconductor switching element on/off based on the current detection value of the detected current. 
         [0006]      FIG. 5  shows a common buck chopper including the current detection function of a main circuit. In  FIG. 5 , a semiconductor switching element  12 , such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), and a diode  13  are connected in series to each other, across a direct current power source  11 , in a reverse direction. Also, an inductor  14 , a current detection section  15 , and a smoothing capacitor  16  are connected in series across the diode  13 , and a load  17  is connected across the smoothing capacitor  16 .  12   d  denotes a parasitic diode. 
         [0007]    With the buck chopper, the semiconductor switching element  12  is turned on to store energy in the inductor  14 . Also, the semiconductor switching element  12  is turned off to emit the stored energy of the inductor  14 , and the energy is supplied to the smoothing capacitor  16  via the diode  13 , thereby realizing a buck operation. 
         [0008]    A control circuit  30  carries out a feedback control such that the semiconductor switching element  12  is turned on/off using the detection value of a current (a main circuit current) I L  output from the current detection section  15  and the detection value of an output voltage V out  obtained from the smoothing capacitor  16 , thus matching the output voltage V out  with a command value. 
         [0009]    As the current detection section  15 , there is a circuit using, for example, a shunt resistance and a Hall CT (Current-Transformer), wherein the current I L  is converted to a voltage value by these parts. 
         [0010]    Meanwhile, as a heretofore known technology which detects the current of an inductor, for example, JP-A-3-178555 (at page 3, an upper right column, line 17 to a lower right column, line 20, FIG. 1, and so on) shows a method whereby the inductor includes a main winding and an auxiliary winding, and with one end of the auxiliary winding being connected to one end of the main winding of the inductor, the voltage between the other end of the auxiliary winding and the other end of the main winding is detected. 
         [0011]      FIG. 6  is a circuit diagram when the heretofore known technology described in JP-A-3-178555 is applied to the current detection section  15  in  FIG. 5 , wherein  141  is a main winding of the inductor  14 ,  142  is an auxiliary winding, and  20  is a voltage detection section. The main winding  141  and the auxiliary winding  142  are wound in the same direction and are also equal in the number of turns. a and b are one end and the other end of the main winding  141 , and a′ and b′ are one end and the other end of the auxiliary winding  142 . 
         [0012]    Herein, the main winding  141  is connected in series to a main circuit (between the output side of the semiconductor switching element  12  and one end of the smoothing capacitor  16  in  FIG. 5 ). Also, the one end a′ of the auxiliary winding  142  is connected to the main circuit, and the other end b′ is connected to the voltage detection section  20 , together with the other end b of the main winding  141 . 
         [0013]    In  FIG. 6 , as the current I L  flows through the main winding  141  and does not flow through the auxiliary winding  142  when the input impedance of the voltage detection section  20  is large enough, a voltage drop (R·I L ) caused by winding resistance R of the main winding  141  occurs in only the main winding  141 . 
         [0014]    Also, an alternating voltage of a size of (L·dI L /dt) is generated across the main winding  141  by the switching operation of the unshown semiconductor switching element. L is the inductance of the main winding  141 . 
         [0015]    Herein, as the main winding  141  and the auxiliary winding  142  are in the same relationship as the primary winding and secondary winding of a transformer with a winding turn ratio of 1:1, electromotive forces of a size equal to the alternating voltage (L·dI L /dt) generated across the main winding  141  are generated across the auxiliary winding  142  in the same polarity. 
         [0016]    Consequently, the voltage between the other ends b and b′ of the main winding  141  and auxiliary winding  142 , the one ends a and a′ of which are of the same potential, exhibits only the voltage drop (R·I L ) caused by the current I L  flowing through the main winding  141 , and this voltage is detected by the voltage detection section  20 . Therefore, by measuring the winding resistance R of the main winding  141  in advance, the control circuit can obtain the current I L  from the relationship of a voltage detection value V (=R·I L ) detected by the voltage detection section  20 . 
       SUMMARY OF THE INVENTION 
       [0017]    As well known, as the winding resistance value of an inductor depends on the temperature of a winding material (copper), when the winding temperature reaches a high temperature, the winding resistance also increases, as when power is being supplied to a heavy load. For example, when the temperature rises by 80[K], the winding resistance R becomes as high as 1.3 times, meaning that the heretofore known technology of JP-A-3-178555 on the premise that the winding resistance R is a fixed value is not practical because the margin of error of the current detection value increases significantly. 
         [0018]    Therefore, a problem to be solved by the invention is to provide a current detector, which can accurately detect the size of a main circuit current flowing through an inductor without being affected by the winding temperature of the inductor, and a power conversion device using the current detector. 
         [0019]    In order to solve the problem, the invention relates to a current detector which detects a main circuit current which flows through an inductor by a switching operation of a semiconductor switching element, the inductor including a main winding and an auxiliary winding which are equal in the number of turns, and which is connected in such a way that electromotive forces generated in the main winding and auxiliary winding by the switching operation are cancelled out and to a power conversion device using the current detector. 
         [0020]    Further, a current detector includes a voltage detection section to the input terminal of which are connected the other ends of the main winding and auxiliary winding, one end of each of which is connected to a main circuit line, and which detects only the voltage between the other end of the main winding and the other end of the auxiliary winding. 
         [0021]    Furthermore, the current detector includes a temperature detection section, which detects the temperature of the main winding, and a current computing section which corrects the winding resistance of the main winding based on the temperature detected by the temperature detection section, and computes the main circuit current, which flows through the main winding, using the corrected winding resistance and a voltage detection value detected by the voltage detection section. 
         [0022]    It is desirable that the current computing section computes the main circuit current using the voltage detection value sampled in synchronism with the switching operation of the semiconductor switching element. 
         [0023]    Also, the current detector is such that the main winding is configured by connecting a plurality of winding elements in parallel, and that the number of winding elements of the auxiliary winding is set to be equal to or less than the number of parallel connections of the main winding. 
         [0024]    Furthermore, the diameter of the winding elements of the auxiliary winding can be set to be smaller than the diameter of the winding elements of the main winding. The current detector is such that the auxiliary winding of the inductor is substituted by a secondary winding of a transformer. 
         [0025]    That is, the invention includes a transformer with a primary winding that is connected in parallel to an inductor and a secondary winding having the same turn ratio as the primary winding. The inductor is connected in series to a main circuit line. Also, the invention includes a voltage detection section having first and second input terminals. The first input terminal is connected to one end of the inductor and the second input terminal is connected to one end of the secondary winding. The other end of each of the inductor and the secondary winding is connected to the main circuit line so that electromotive forces generated in the inductor and the secondary winding by the switching operation are cancelled out, and the voltage detection section detects only the voltage between the other end of the inductor and the other end of the secondary winding. 
         [0026]    Furthermore, the invention includes a temperature detection section, which detects the temperature of the inductor, and a current computing section which corrects the winding resistance of the inductor based on the detected temperature, and computes the main circuit current, which flows through the inductor, using the corrected winding resistance and a voltage detection value detected by the voltage detection section. 
         [0027]    Also, a power conversion device converts direct current power or alternating current power by controlling the switching operation of the semiconductor switching element using a current detection value detected by the current detector. 
         [0028]    According to the invention, when the current detector measures the main circuit current flowing through the inductor based on a voltage drop, temperature of the inductor&#39;s main winding is taken into account to avoid an error that would otherwise occur if a resistance of the main winding were assumed to have a fixed value, thereby significantly improving the accuracy of the current detection. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1A  is a configuration diagram showing a first embodiment of the invention. 
           [0030]      FIG. 1B  is a circuit diagram of  FIG. 1A . 
           [0031]      FIG. 2  is a waveform diagram. 
           [0032]      FIG. 3  is a circuit diagram showing a modification example of the first embodiment of the invention. 
           [0033]      FIG. 4  is a circuit diagram showing a second embodiment of the invention. 
           [0034]      FIG. 5  is a circuit diagram of a common buck chopper. 
           [0035]      FIG. 6  is a circuit diagram of a heretofore known technology described in JP-A-3-178555. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    Hereafter, a description will be given, along the drawings, of embodiments of the invention. 
         [0037]    Firstly,  FIG. 1A  is a configuration diagram of a current detector according to a first embodiment of the invention, and  FIG. 1B  is a circuit diagram thereof. The current detector is connected between the output terminal of a semiconductor switching element  12  and one end of a smoothing capacitor  16 , for example, as shown in  FIG. 5 , and is used to detect a current (a main circuit current) I L  flowing through an inductor, switch the semiconductor switching element  12  with a control circuit  30 , and control an output voltage V out  in accordance with a command value. 
         [0038]    In  FIGS. 1A and 1B , a main winding  1  and an auxiliary winding  2 , which are equal in the number of turns, are wound in the same direction on a core  4  of an inductor  3 . One ends  1   a  and  2   a  of the main winding  1  and auxiliary winding  2 , at which winding starts, are connected to a main circuit line  50 , and the main circuit line  50  is connected to the output side of a power conversion device, as shown in  FIG. 5 . Also, the other ends  1   b  and  2   b  of the main winding  1  and auxiliary winding  2 , at which winding ends, are connected to the input side (first and second input terminals) of a voltage detection section  5  which detects and amplifies the voltage between the other ends  1   b  and  2   b.    
         [0039]    As the main winding  1  and the auxiliary winding  2  are wound in the same direction and are also equal in the number of turns, respective alternating voltages (L·dI L /dt) generated across the main winding  1  and auxiliary winding  2  as a result of a switching operation of the semiconductor switching element are equal in size and polarity, in the same way as in a main winding  141  and auxiliary winding  142  of  FIG. 6 . That is, as is clear from  FIG. 1B , the relationship of connection between the main winding  1  and auxiliary winding  2  and the voltage detection section  5  is the same as the relationship of connection between the main winding  141  and auxiliary winding  142  and a voltage detection section  20  in  FIG. 6 . 
         [0040]    Also,  7  is a temperature detection element, such as a thermistor, which detects the temperature of the main winding  1 , and the output of the temperature detection element  7  is input into a current computing circuit (current calculating circuit)  6 , such as a microcomputer, together with the output of the voltage detection section  5 . 
         [0041]    The current computing circuit  6  includes, as a first function, the function of correcting the winding resistance of the main winding  1  in response to the temperature of the main winding  1  detected by the temperature detection element  7 , and includes, as a second function, the function of correcting a voltage detection value, detected by the voltage detection section  5 , using the corrected winding resistance. With the second function, it is also possible to correct an error resulting from the imbalance, between an increase and decrease in the voltage detection value V, caused due to the leakage inductance of the main winding  1  when in switching operation. 
         [0042]    Next, a description will be given of an operation of the first embodiment. When the current I L  flows by the switching operation of the semiconductor switching element, electromotive forces (L·dI L /dt) generated in the main winding  1  and auxiliary winding  2  are cancelled out by the same principle as in  FIG. 6 , and a voltage V depending on only winding resistance R of the main winding  1  is detected by the voltage detection section  5  and input into the current computing circuit  6 . At the same time as this, the temperature of the main winding  1  is detected by the temperature detection element  7  and input into the current computing circuit  6 . 
         [0043]    Herein, as the temperature-resistance characteristics of the main winding  1  is known, the current computing circuit  6  computes the current I L  from the relationship of V=R·I L  using the winding resistance R corrected in response to the detected temperature of the main winding  1  and the voltage detection value V detected by the voltage detection section  5 . By so doing, it is possible to resolve a measurement error resulting from a difference in winding temperature. 
         [0044]    When using a thermistor as the temperature detection element  7 , an error can also occur due to the non-linearity of the temperature characteristics of the thermistor. However, it is easy for the current computing circuit  6  to correct the winding resistance R, including the non-linearity, and it is possible to significantly reduce a computation error of the current I L . 
         [0045]    However, as an imbalance occurs between the increase and decrease in the voltage detection value V, due to the leakage inductance of the main winding  1 , as a result of the switching operation, the imbalance causes an error in an average value V average  of the voltage detection value V. 
         [0046]      FIG. 2  is a schematic waveform diagram of the current I L  and voltage detection value V in the embodiment, and as areas S 1  and S 2  of the hatched portions in the waveform of the voltage detection value V depend on the ratio of the exciting inductance and leakage inductance of the main winding  1 , on the conduction ratio (on-duty) of the semiconductor switching element, and on the voltage across a reactor  3 , it is possible to infer the areas S 1  and S 2  by way of the computation of the current computing circuit  6 . 
         [0047]    Herein, the information on the conduction ratio of the semiconductor switching element and the voltage across the reactor  3  can be obtained from the control circuit, but as for the inductance component of the main winding  1 , there is fear that as the variability among the individuals exists with respect to a design value, it is not possible to obtain any high-precision value. 
         [0048]    Therefore, in the embodiment, the waveform of the voltage detection value V is observed while being sampled at a frequency twice a switching frequency, and the average value V average  is computed, with the conduction ratio of the semiconductor switching element taken into account, utilizing the fact that the areas S 1  and S 2  become equal in the observed waveform. By so doing, it is possible to obtain the average value V average  with no error without being affected by the leakage inductance of the main winding  1 . 
         [0049]    t s  in  FIG. 2  denotes a sampling timing, and the timing corresponds to the midpoint of each of an on-period Δt on  and off-period Δt off  of the semiconductor switching element. 
         [0050]    The current computing circuit  6  only has to compute the current I L  (an average value I average  thereof), from the relationship of V=R·I L , using the thus detected average value V average  and temperature-corrected winding resistance R. 
         [0051]    When providing the control circuit (a microcomputer), which controls the semiconductor switching element in accordance with a predetermined conduction ratio, with the function of the current computing circuit  6 , software simply has to be added, and it is not necessary to separately provide, for example, a dedicated circuit which takes in the conduction ratio. 
         [0052]    In this way, according to the embodiment, it is possible to detect the size of the main circuit current I L , with high precision and at high speed, in order to control the semiconductor switching element of the power conversion device. 
         [0053]    When using a current detection value for only a low-speed control and current monitoring of the power conversion device, a lowpass filter only has to be connected to the output side of the voltage detection section  5 , thus eliminating the effect of the leakage inductance of the main winding  1 . 
         [0054]    In the embodiment, as no main circuit current flows through the auxiliary winding  2  when the input impedance of the voltage detection section  5  is large, it is possible to suppress an increase in cost by using a wire, smaller in diameter than the main winding  1 , as the auxiliary winding  2 . 
         [0055]    Also, it is often the case that an inductor for large current is configured by connecting a plurality of main windings in parallel. In this case, as a modification example, with one of a plurality of winding elements of a main winding  1 A, which are connected in parallel to each other, as the auxiliary winding  2 , the other end of the auxiliary winding  2  may be connected to the voltage detection portion  5 , as in a one-turn inductor  3 A shown in  FIG. 3 . In this case, the auxiliary winding  2  may be configured by connecting a plurality of winding elements in parallel, and in any case, there only has to be the relationship of a parallel number N of winding elements of the main winding  1 A&gt;a parallel number M of winding elements of the auxiliary winding  2 . 
         [0056]    Next,  FIG. 4  is a circuit diagram showing a second embodiment of the invention. In  FIG. 4 , the same signs are given to component portions the same as those of  FIGS. 1A, 1B, and 3 , and hereafter, a description will be given centering on the differences. 
         [0057]    In the first embodiment, the altered inductors  3  and  3 A are used, but in the second embodiment, the need to alter the inductor itself is eliminated. 
         [0058]    That is, in  FIG. 4 , the inductor connected in series to the main circuit line  50  is configured of only the main winding  1 . Also,  8  is a transformer with a winding turn ratio of 1:1, wherein a primary winding  8 A of the transformer  8  is connected in parallel to the main winding  1 . Furthermore, one end of a secondary winding  8 B is connected to the one end of the main winding  1 , and the other end of the secondary winding  8 B is connected to one input terminal of the voltage detection section  5 . The other end of the main winding  1  is connected to the other input terminal of the voltage detection section  5 , in the same way as in the first embodiment. 
         [0059]    According to the second embodiment, by utilizing the secondary winding  8 B of the transformer  8 , which is connected in parallel to the inductor, as the auxiliary winding, it is possible to obtain the same working effects as in the first embodiment even without altering the inductor itself. 
       INDUSTRIAL APPLICABILITY 
       [0060]    The invention can be utilized for various power conversion devices, such as a boost chopper, a buck chopper, an inverter, and a converter, which convert direct current power or alternating current power by controlling the semiconductor switching element on/off using the current detection value obtained by the current detector according to each embodiment. Also, the phase type (a single phase or multiple phases) of the power conversion devices is not particularly limited either.