JP2008249645A - Current detection circuit, and initialization method for current detection circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a current detection circuit capable of restraining fluctuation of an offset voltage generated in the current detection circuit by an external unnecessary magnetic field, even under current detection, and an initialization method for the current detection circuit, in the pulse current detection circuit using a magneto-resistive element. <P>SOLUTION: The current detection circuit 1 has a magneto-resistive part 10 having the magneto-resistive elements 11-14, and for detecting a magnetic field generated in response to a pulse current to generate a detection signal in response to the magnetic field, and an initialization part 40 having an initialization coil 44 for supplying an initialization magnetic field to the magneto-resistive elements 11-14, and for initializing the magneto-resistive elements 11-14 in response to leading-up of the detection signal. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a current detection circuit using a magnetoresistive element and an initialization method for the current detection circuit.

  A circuit using a magnetoresistive element has been devised as a current detection circuit. The current detection circuit changes the resistance value of the magnetoresistive element in accordance with the magnetic field generated by the current, and converts the change in resistance value into a change in voltage value. Known magnetoresistive elements include MR elements and GMR elements. In this type of magnetoresistive element, the resistance value characteristic with respect to the magnetic field exhibits a hysteresis characteristic, and therefore may have a different resistance value (one of the two resistance values is indeterminate) with respect to a certain magnetic field. For this reason, the resistance value of the magnetoresistive element varies, and the offset voltage of the current detection circuit varies.

Patent Document 1 describes a method of initializing a magnetoresistive element for preventing fluctuations in the resistance value of the magnetoresistive element. In this method of initializing the magnetoresistive element, the magnetoresistive element is temporarily saturated by applying a control magnetic field to the magnetoresistive element, and the magnetoresistive element is initialized so as to show only one resistance value for a certain magnetic field. It has become. At this time, in this magnetoresistive element initialization method, the initialization is repeated at a timing synchronized with the reference clock.
Japanese Patent No. 3368964

  However, in the current detection circuit, when the magnetoresistive element initialization method described in Patent Document 1 is used, the current cannot be accurately converted into voltage because the magnetoresistive element is saturated during initialization. Therefore, it has been difficult to use this type of current detection circuit for an apparatus that always requires current detection for feedback control and protection control, such as a switching power supply.

  Accordingly, an object of the present invention is to provide a current detection circuit and an initialization method for the current detection circuit that can suppress fluctuations in offset voltage generated by the current detection circuit due to an external unnecessary magnetic field even during current detection.

  The current detection circuit of the present invention has a magnetoresistive element, detects a magnetic field generated according to a pulse current, and generates a detection signal according to the magnetic field, and an initialization magnetic field in the magnetoresistive element And an initialization unit that initializes the magnetoresistive element in response to the fall of the detection signal.

  According to this current detection circuit, the initialization unit can initialize the magnetoresistive element by supplying the initialization magnetic field from the initialization coil in response to the fall of the detection signal. Synchronously, the magnetoresistive element can be initialized when no pulse current is flowing. Therefore, according to this current detection circuit, it is possible to suppress fluctuations in the offset voltage without losing necessary current information even during current detection.

  The current detection circuit described above preferably further includes a holding unit that holds and outputs the value of the immediately preceding detection signal during the period in which the initialization unit initializes the magnetoresistive element.

  According to this configuration, even when noise is output as a detection signal at the time of initialization, it is possible to prevent the holding unit from being interrupted and erroneous information from leaking to the outside.

  The initialization unit described above preferably initializes the magnetoresistive element when the value of the detection signal is equal to or greater than the threshold value, and does not initialize the magnetoresistive element when the value of the detection signal is less than the threshold value. .

  According to this configuration, even when noise is superimposed on the detection signal, it is possible to prevent the initialization unit from detecting noise and performing erroneous initialization.

  Further, it is preferable that the initialization unit described above continues initialization of the magnetoresistive element until the rising edge of the detection signal is detected.

  According to this configuration, the initialization unit can fully initialize the magnetoresistive element by making maximum use of a period during which no pulse current flows. In addition, since the holding unit can block the detection signal during a period when the pulse current is not flowing, erroneous current information can be prevented from leaking outside.

  The initialization unit described above preferably includes a counter, and initializes the magnetoresistive element when the number of times the detection signal falls is detected reaches a threshold value.

  In an actual use state, since the external magnetic field that is large enough to change the resistance value of the magnetoresistive element is less frequently applied to the magnetoresistive element, it is not necessary to frequently initialize the magnetoresistive element. According to this configuration, the number of initializations can be reduced, so that the current consumption can be reduced. In particular, although a relatively large current is required for the initialization coil, the current consumption of the initialization coil can be reduced.

  An initialization method for a current detection circuit according to the present invention includes a magnetoresistive element that includes a magnetoresistive element, detects a magnetic field generated according to a pulse current, and generates a detection signal according to the magnetic field, and a magnetoresistive element In an electric current detection circuit having an initialization coil for supplying an initialization magnetic field to an initialization unit for initializing the magnetoresistive element in response to a fall of the detection signal, the initialization unit initializes the magnetoresistive element. Turn into.

  In this current detection circuit initialization method, the initialization unit can initialize the magnetoresistive element by supplying an initialization magnetic field from the initialization coil in response to the fall of the detection signal. In synchronization with the current, the magnetoresistive element can be initialized when the pulse current is not flowing. Therefore, according to the initialization method of the current detection circuit, it is possible to suppress fluctuations in the offset voltage without losing necessary current information even during current detection.

  ADVANTAGE OF THE INVENTION According to this invention, the initialization method of the current detection circuit and current detection circuit which can suppress the fluctuation | variation of an offset voltage even during current detection can be obtained.

DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals.
(GMR element)

  First, a GMR element will be described as a magnetoresistive element. FIG. 3 is a perspective view showing the GMR element, and FIG. 4 is a diagram showing resistance value characteristics of the GMR element with respect to the magnetic field.

  As shown in FIG. 3, the GMR element has a characteristic that the resistance value changes according to the magnitude and direction of the magnetic field received. Further, the GMR element has hysteresis characteristics as indicated by the curves X1 and X2 with respect to the magnetic field in the axis X direction (longitudinal direction). Therefore, the GMR element has different resistance characteristics with respect to the magnetic field in the axis Y direction depending on the magnetic field in the axis X direction. For example, when the GMR element exhibits the characteristic of the curve X1 with respect to the magnetic field in the axis X direction, the GMR element exhibits the resistance characteristic of the curve Y1 with respect to the magnetic field in the axis Y direction, and the curve X2 with respect to the magnetic field in the axis X direction. When the characteristic is shown, the resistance value characteristic of the curve Y2 is shown with respect to the magnetic field in the axis Y direction. Therefore, a different resistance value R1 or R2 may be shown for a certain magnetic field H1. As a result, the resistance value of the magnetoresistive element of the GMR element changes, and the offset voltage of the current detection circuit described later changes.

Therefore, in the present invention, a control magnetic field is once applied in the axis X direction to forcibly saturate in the axis X direction. For example, the characteristic with respect to the axis X direction always indicates the curve X1 (or curve X2), and the axis Y The resistance value characteristic with respect to the direction always tries to show the curve Y1 (or curve Y2).
(Current detection circuit)

  Next, the current detection circuit according to the embodiment of the present invention will be described. FIG. 1 is a circuit diagram showing a current detection circuit according to an embodiment of the present invention. The current detection circuit 1 shown in FIG. 1 includes a GMR bridge unit (magnetic resistance unit) 10, an amplification unit 20, a holding unit 30, and an initialization unit 40.

  The GMR bridge section 10 has four GMR elements 11 to 14 constituting a Wheatstone bridge. Specifically, the GMR element 11 is connected between the high power supply line Vdd and the first output terminal 15, and the GMR element 12 is connected between the first output terminal 15 and the ground power supply line G. ing. Similarly, the GMR element 13 is connected between the high power supply line Vdd and the second output terminal 16, and the GMR element 12 is connected between the second output terminal 16 and the ground power supply line G. .

  The GMR bridge unit 10 is mounted on a wiring pattern on a circuit board, for example. At this time, in the GMR elements 11 to 14, the increase and decrease of the resistance value of the GMR element 11 and the increase and decrease of the resistance value of the GMR element 14 are substantially the same with respect to the magnetic field in the same direction. And the increase / decrease of the resistance value of the GMR element 13 are substantially the same. Further, the GMR elements 11 to 14 are arranged so that the increase and decrease of the resistance value of the GMR elements 11 and 14 and the increase and decrease of the resistance value of the GMR elements 13 and 14 are opposite to the magnetic field in the same direction.

  Specifically, the GMR elements 11 to 14 have the same resistance value R in a state where no magnetic field is applied, and the balance of these bridges is maintained. At this time, the potential difference between the first output terminal 15 and the second output terminal 16 becomes zero, and the differential output voltage (differential detection signal) of the GMR bridge unit 10 becomes zero. Next, when a magnetic field in the same direction is applied to the GMR elements 11 to 14, for example, the resistance values of the GMR elements 11 and 14 are decreased by ΔR, and the resistance values of the GMR elements 12 and 13 are increased by ΔR. Therefore, the voltage at the first output terminal 15 increases and the voltage at the second output terminal 16 decreases, and the GMR bridge unit 10 outputs the differential output voltage ΔVo.

  As described above, the GMR bridge unit 10 detects the magnetic field generated according to the current, and converts the change in the resistance value according to the magnetic field into the change in the differential voltage value ΔVo.

  The amplifying unit 20 amplifies the differential output voltage from the GMR bridge unit 10 and outputs it to the holding unit 30. The amplification unit 20 includes, for example, an error amplification amplifier 21. The plus input terminal of the error amplification amplifier 21 is connected to the first output terminal 15 of the GMR bridge section 10 via the resistor element 22, and the resistor element 23 is connected between the plus input terminal and the ground power supply line G. Has been. Further, the negative input terminal of the error amplification amplifier 21 is connected to the second output terminal 16 of the GMR bridge unit 10 via the resistance element 24, and the resistance element 25 is connected between the negative input terminal and the output terminal. Has been. The output terminal of the error amplification amplifier 21 is connected to the holding unit 30.

  The holding unit 30 holds and outputs the output voltage of the amplifying unit 20 immediately before the GMR elements 11 to 14 are initialized. The holding unit 30 performs this operation according to the timing signal from the initialization unit 40. For example, a sampling / hold circuit is used as the holding unit 30.

  The initialization unit 40 detects the fall of the differential output voltage from the GMR bridge unit 10 and initializes the GMR elements 11 to 14 when the fall is detected. The initialization unit 40 includes a fall detection unit 41, a timing generation unit 42, a current supply unit 43, and an initialization coil 44.

  The fall detection unit 41 detects whether or not the value of the differential output voltage ΔVo from the GMR bridge unit 10 is equal to or greater than a threshold value. Further, when the value of the differential output voltage ΔVo is equal to or greater than the threshold value, the falling detection unit 41 detects the falling of the differential output voltage ΔVo, and each time it detects the pulse-like trigger signal Vf. The data is output to the generation unit 42. For example, the fall detection unit 41 can be realized by a plurality of comparators.

  When receiving the trigger signal Vf from the fall detection unit 41, the timing generation unit 42 generates a pulsed timing signal Vt and outputs it to the current supply unit 43 and the holding unit 30. For example, a one-shot multivibrator or a delay circuit is used as the timing generation unit 42.

  The current supply unit 43 supplies a current to the initialization coil 44 in accordance with the timing signal from the timing generation unit 42.

  When the current is supplied from the current supply unit 43, the initialization coil 44 applies a magnetic field in the same direction each time with respect to the axis X direction of the GMR elements 11 to 14, to once saturate the GMR elements 11 to 14, and then , Stop the generation of magnetic field.

  Next, the operation of the current detection circuit 1 will be described with reference to FIGS. 1 and 2. FIG. 2 is a timing chart showing signal waveforms at various parts of the current detection circuit shown in FIG. FIG. 2 shows an operation waveform of the current detection circuit 1 that detects a primary-side pulse current (switching current) in the switching power supply.

  First, when a magnetic field corresponding to the pulse current is detected by the GMR bridge unit 10, for example, the resistance values of the GMR elements 11 and 14 are decreased, and the resistance values of the GMR elements 12 and 13 are increased. A pulse-like differential output voltage ΔVo is generated between the second output terminals 15 and 16 (FIG. 2A). Then, the fall detection unit 41 in the initialization unit 40 compares the differential output voltage ΔVo with the threshold value Vth. If the differential output voltage ΔVo is equal to or higher than the threshold value Vth, the fall of the differential output voltage ΔVo is detected and the fall is detected. The trigger signal Vf is output every time the signal is detected (FIG. 2B). When the trigger signal Vf is input to the timing generator 42, the timing generator 42 generates a timing signal Vt (FIG. 2C).

  Then, current is supplied to the initialization coil 44 by the current supply unit 43, and a magnetic field is generated by the initialization coil 44. This magnetic field is applied in the same direction each time with respect to the axis X direction of the GMR elements 11 to 14, and the GMR elements 11 to 14 are once saturated. Here, since the pulse width Tt of the timing signal Vt generated by the timing generation unit 42 is shorter than the off period Toff in which no pulse current flows, the current supply time from the current supply unit 43 to the initialization coil 44 is also short. The application of the magnetic field to the GMR elements 11 to 14 by the initialization coil 44 is stopped by the on period Ton in which the pulse current flows again.

  On the other hand, when the differential output voltage ΔVo is less than the threshold value Vth, the falling detection unit 41 does not output the trigger signal Vf (time ta in FIG. 2B), and initializes the GMR elements 11 to 14. It can be suppressed.

  The holding unit 30 holds and outputs the previous low-level differential output voltage ΔVo during the period Tt during which the timing signal Vt is output from the timing generation unit 42 (FIG. 2D).

  Thus, according to the current detection circuit 1 and the initialization method of the current detection circuit 1 of the present embodiment, when the initialization unit 40 detects the falling of the differential output voltage ΔVo of the GMR bridge unit 10. Since the initialization magnetic field is supplied from the initialization coil 44, the GMR elements 11 to 14 can be initialized. Therefore, the GMR element 11 is synchronized with the pulse current during the off period Toff in which no pulse current flows. ~ 14 can be initialized. Therefore, according to the current detection circuit 1 of the present embodiment, it is possible to suppress fluctuations in the offset voltage without losing necessary current information (pulse current in the on period Ton) even during current detection.

  Further, according to the current detection circuit 1 of the present embodiment, during the initialization of the GMR elements 11 to 14, the holding unit 30 holds and outputs the differential output voltage ΔVo at a low level immediately before initialization. Even when noise is sometimes output as the differential output voltage ΔVo, it is possible to prevent the holding unit 30 from being blocked and leaking outside.

  Further, according to the current detection circuit 1 of the present embodiment, when the differential output voltage ΔVo is less than the threshold value Vth, the falling detection unit 41 in the initialization unit 40 outputs the trigger signal Vf for initialization. Since no output is made, even if noise is superimposed on the differential output voltage ΔVo, it is possible to prevent the initialization unit 40 from detecting noise and performing erroneous initialization.

  Heretofore, a current transformer has been generally used as a current detection circuit of a switching power supply. Since a current transformer is a kind of transformer, a core using a magnetic material having a high magnetic permeability is required to increase the efficiency of signal transmission, resulting in a large size. On the other hand, GMR elements have high element sensitivity to magnetic fields, so they can detect magnetic flux generated from current even in air, and do not need to amplify signals using a magnetic material such as a current transformer, making them suitable for miniaturization. ing. In the current detection circuit 1 of the present embodiment, since the GMR element is used, the size can be reduced.

  The present invention is not limited to the above-described embodiment, and various modifications can be made.

  In this embodiment, the initialization unit initializes the GMR element only for the short period Tf in the off period Toff in which no current flows. However, the initialization unit initializes the GMR element by using all of the off period Toff in which no current flows. You may go. In this case, the falling detection unit may include, for example, an SR latch circuit that is set by the falling of the differential output voltage ΔVo and reset by the rising. According to this configuration, the initialization unit can fully initialize the GMR element by making maximum use of the period Toff during which no pulse current flows. In addition, since the holding unit can cut off the differential output voltage ΔVo during the period Toff in which no pulse current flows, it is possible to prevent erroneous current information from leaking to the outside.

  In the present embodiment, the initialization unit initializes the GMR element every time the falling of the differential output voltage ΔVo is detected, but the number of initializations may be reduced. For example, the fall detection unit has a counter, and the GMR element may be initialized when the number of times the fall of the differential output voltage ΔVo is detected reaches a certain value. According to this configuration, the number of initializations can be reduced, so that the current consumption can be reduced. In particular, although a relatively large current is required for the initialization coil, the current consumption of the initialization coil can be reduced. In an actual use state, since the external magnetic field that is large enough to change the resistance value of the magnetoresistive element is less frequently applied to the magnetoresistive element, it is not necessary to frequently initialize the magnetoresistive element.

  In the present embodiment, the Wheatstone bridge type GMR bridge unit 10 is used as the magnetoresistive unit. However, the configuration of the magnetoresistive unit is not limited to this embodiment. For example, as shown in FIG. 5, the magnetoresistive section may be composed of GMR elements 51 and 52 and current sources 53 and 54 that supply constant currents to these GMR elements 51 and 52, respectively.

In the present embodiment, the GMR element is exemplified as the magnetoresistive element. However, the present invention is not limited to the GMR element, and various magnetoresistive elements such as an MR element can be used.
Can be used.

It is a circuit diagram showing a current detection circuit concerning an embodiment of the present invention. It is a timing chart which shows each part signal waveform of the current detection circuit shown in FIG. It is a perspective view which shows a GMR element. It is a figure which shows the resistance value characteristic with respect to the magnetic field of a GMR element. It is a circuit diagram which shows the magnetoresistive part which concerns on the modification of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11-14 ... GMR element (magnetoresistance element), 1 ... Current detection circuit, 10 ... GMR bridge part (magnetoresistance part), 20 ... Amplification part, 21 ... Error amplification amplifier, 30 ... Holding part, 40 ... Initialization part 41 ... Falling detection unit, 42 ... Timing generation unit, 43 ... Current supply unit, 44 ... Initialization coil.

Claims (6)

A magnetoresistive unit that includes a magnetoresistive element, detects a magnetic field generated according to a pulse current, and generates a detection signal according to the magnetic field;
An initialization coil that supplies an initialization magnetic field to the magnetoresistive element, and an initialization unit that initializes the magnetoresistive element in response to a fall of the detection signal;
A current detection circuit.   The current detection circuit according to claim 1, further comprising a holding unit that holds and outputs the value of the immediately preceding detection signal during a period in which the initialization unit initializes the magnetoresistive element.   The initialization unit initializes the magnetoresistive element when the value of the detection signal is equal to or greater than a threshold value, and does not initialize the magnetoresistive element when the value of the detection signal is less than the threshold value. The current detection circuit according to claim 1 or 2.   The current detection circuit according to claim 1, wherein the initialization unit continues the initialization of the magnetoresistive element until the rising edge of the detection signal is detected.   5. The current detection circuit according to claim 1, wherein the initialization unit includes a counter and performs initialization of the magnetoresistive element when the number of times of detecting the falling edge of the detection signal reaches a threshold value. A magnetoresistive element that includes a magnetoresistive unit that detects a magnetic field generated according to a pulse current and generates a detection signal according to the magnetic field, and an initialization coil that supplies an initialization magnetic field to the magnetoresistive element. And an initialization unit that initializes the magnetoresistive element in response to a fall of the detection signal,
An initialization method for a current detection circuit, wherein the magnetoresistive element is initialized by the initialization unit.

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