JP4423472B2 - Impedance measurement method - Google Patents

Description

  The present invention relates to an inductance impedance measuring method applied to, for example, a switching power supply.

Inductance and loss resistance related to an inductor that operates in a state where a direct current is superimposed, such as a choke coil used in a switching power supply such as a switching DC / DC converter, is measured by superimposing the direct current on a minute excitation current. It is normal. The outline is shown in FIG.
In FIG. 6, 100 is an impedance measuring device, and 110 is an inductor for measuring impedance. The equivalent circuit of the inductor 110 can be represented as a series circuit of an ideal inductor having only an inductance L and a loss resistance R as shown in FIG. The impedance measuring instrument 100 uses an internal constant current circuit 101 and a signal generation circuit 102 to cause a small amplitude AC signal IAC superimposed with a DC current IDC to flow through the inductor 110, and measures the current flowing through the inductor 110 and the voltage at both ends. L and loss resistance R are calculated. The measurement of the current flowing through the inductor 110 and the voltage at both ends and the calculation of the inductance L and the loss resistance R are performed by an impedance measuring device. FIG. 7 shows an outline of the DC superposition characteristics (changes of the inductance L and the loss resistance R with respect to the DC current IDL) of the inductance L and the loss resistance R thus measured. As shown in FIG. 7, the inductance L and the loss resistance R generally have maximum values with respect to the direct current IDL.

  On the other hand, in a switching power supply aiming at low power output or miniaturization, a small-sized coil is applied as a choke coil, so that the proportion of the alternating current in the direct current and alternating current flowing through the choke coil increases. FIG. 8 shows the magnetic characteristics of a general magnetic element including an inductor. In FIG. 8, Φ is an interlinkage magnetic flux of the inductor, and I is an exciting current. The inductor in the large power or large switching power supply performs the operation of minor loop 1 (small amplitude) in FIG. 8 (since the direct current IDC is large, the alternating current IAC is relatively small). A small power or small switching power supply performs minor loop 2 (large amplitude) operation. Since the inductance L is given by L = ΔΦ / IAC from the change ΔΦ of the linkage flux Φ with respect to the AC currents IAC and IAC in FIG. Indicates the value.

When the ratio of the AC current in the total current flowing through the inductor is small as in a large power or large converter, the impedance of the inductor can be measured by a conventional measurement method using an impedance measuring instrument with a small AC excitation current. However, when the ratio of the AC current in the total current flowing through the inductor is large as in a small current or small converter, the conventional measurement method using an impedance measuring instrument with a small AC excitation current cannot be applied to the impedance measurement of the inductor. . In addition, since the choke coil applied to the switching power supply is in an actual operating environment in which a square wave voltage is applied and a triangular wave current flows, impedance in the actual operation of the inductor is not achieved with sine wave self-excitation as in the conventional measurement method. Cannot be measured accurately.
On the other hand, Patent Document 1 proposes a measuring apparatus capable of measuring the impedance of the inductor under measurement conditions close to actual use conditions. An outline of the measuring apparatus disclosed in Patent Document 1 is shown in FIG. In the measuring apparatus shown in FIG. 9, a signal obtained by amplifying the pulse signal output from the pulse oscillator 120 by the amplifier 130 with respect to the closed circuit including the inductor L1, the inductor L, the load resistor R0, and the diode D1 is transmitted via the diode D2. To be applied. The pulse oscillation circuit 120 outputs a high frequency pulse close to the actual use state. The inductor L is an inductor to be measured, and the voltage V across the inductor L, the pulse width Δt, and the maximum amplitude value (pp value) ΔI of the current flowing through the inductor L are obtained, and from these, the inductance of the inductor L is ( It is calculated by Δt / ΔI) × V. The inductor DC current component can be changed by changing the amplification factor of the amplifier 130 and the resistance value of the load resistor R0, and the DC superposition characteristics of the inductor L can be measured. When the DC component is reduced, the closed circuit composed of the inductor L1, the inductor L, the load resistor R0, and the diode D1 can only flow current in one direction due to the presence of the diode D1, so that the current flowing through the inductor L is discontinuous. The measurement becomes inaccurate. It is the role of the inductor L1 to prevent this. By making the inductance of the inductor L1 larger than that of the inductor L and making the combined inductor of the two inductors larger, the DC current of the inductor L can be reduced without making the current discontinuous.
JP 2002-324716 A (Page 3, FIG. 1)

As described above, the measuring apparatus described in Patent Document 1 requires an inductor having an inductance larger than the inductor L to be measured, and has a problem of causing an increase in measurement cost and an increase in the size of the apparatus. Further, no measurement method is shown for the loss resistance component R of the inductor shown in FIG.
The object of the present invention is to solve the above-mentioned problems, and does not require an inductor other than the inductor to be measured in the measurement circuit, and accordingly, the characteristics of the inductor are measured under measurement conditions close to actual operating conditions without being affected by other inductors. An object of the present invention is to provide a measuring method capable of measuring not only inductance but also loss resistance as a characteristic of an inductor.

Accordingly, in order to solve the above-described problem, the invention according to claim 1 includes a reference power supply having a reference potential terminal and an output terminal, first switch means having one end connected to the output terminal of the reference power supply, Second switch means having one end connected to the other end of the first switch means, and a resistor and a capacitor connected in parallel between the other end of the second switch means and the reference potential terminal of the reference power supply An inductor is connected between the other end of the first switch means and a reference potential terminal of the reference power source of the circuit, and the first switch means and the second switch means are alternately switched at conduction times T1 and T2, respectively. while conducting the said resistor and the voltage of the inductor, measuring at least a portion of the current and power consumption, measuring impedance to calculate the impedance of the inductor from the result Characterized in that it is a method.
According to a second aspect of the present invention, in the first aspect of the present invention, the current of the inductor is determined from the current flowing through the resistor, the power consumed by the resistor, the current flowing through the inductor, and the conduction time T2 of the second switch means. It is an impedance measurement method for calculating inductance.

  According to a third aspect of the present invention, in the second aspect of the present invention, the current IR flowing through the resistor, the power PR consumed by the resistor, and the difference ILpp between the maximum value and the minimum value of the current flowing through the inductor are obtained. The impedance measurement method is characterized in that the inductance L of the inductor is calculated by the following equation (I) from these values and the conduction time T2 of the second switch means.

The invention according to claim 4 is the impedance measuring method according to any one of claims 1 to 3, wherein the loss resistance of the inductor is calculated from the power consumed by the inductor and the current flowing through the inductor. It is characterized by.
The invention according to claim 5 is the invention according to claim 4, wherein the power consumed by the inductor is PL, and the effective value of the current flowing through the inductor is ILrms. The impedance measurement method is calculated by the formula (1).

  The invention according to claim 6 is the invention according to claim 5, wherein T = T1 + T2, the current flowing through the inductor is iL, and the voltage across the inductor is vL. The impedance measurement method is calculated by the formula (1).

  In the measuring method of the present invention, an inductor whose impedance is to be measured is connected to a measuring circuit including a reference power source, a resistor, a capacitor, and switching means, and the operation is close to the actual use state (a large amplitude due to a square wave voltage and a triangular wave current By measuring the current flowing through the inductor and the voltage at both ends while performing the excitation operation), the inductance L of the inductor to be measured can be obtained without using another inductor. Further, the loss resistance R of the inductor can be obtained from the effective value of the inductor current. Further, if the impedance required by the present invention is used, a circuit using the inductor can be designed more accurately.

  Hereinafter, the measurement method of the present invention will be described with reference to the drawings.

FIG. 1 shows an example of a measurement circuit used in the measurement method of the present invention. In FIG. 1, L is an inductor whose impedance is to be measured, and is connected in series with the DC voltage source Vi and the switch S1 to form a closed circuit. A circuit in which a parallel circuit of a variable resistor RL and a capacitor C and a switch S2 are connected in series is connected in parallel with the inductor L. The measurement circuit of FIG. 1 is a model of a step-up / step-down DC / DC converter, and the inductor L is excited or excited with a large amplitude by a square wave voltage and a triangular wave current in the same or as close state as the actual switching power supply. Thus, inductance and loss resistance are measured.
Next, a measurement method according to the present invention using the measurement circuit shown in FIG. 1 will be described. First, the measurement is performed while the switches S1 and S2 are alternately turned on (conductive) and turned off (cut off). FIG. 2 shows waveforms of a current (inductor current) iL flowing through the inductor L during measurement and a voltage (inductor voltage) vL across the inductor L. The current IR flowing through the variable resistor RL is smoothed by the capacitor C, and the direct current becomes dominant, and becomes equal to the average value IL of IL (IL = IR).

The excitation frequency for measuring the impedance of the inductor L is determined by the on / off frequencies of the switches S1 and S2. The maximum value Vi of the inductor voltage vL is adjusted by the DC voltage power supply Vi, and the minimum value VR is adjusted by the on / off ratio of the switches S1 and S2. When the on-time ratio of S1 is D1 and the on-time ratio of S2 is D2, D2 is approximately Vi · D1 / D2. As shown in FIG. 2, when the on time of the switch S1 is T1 and the on time of the switch S2 is T2, D1 = T1 / (T1 + T2) and D2 = T2 / (T1 + T2). The DC superimposed current IL (= IR) of the inductor L is adjusted by the resistance value of the variable resistor RL. With the above-described setting, the operating state of the inductor L whose characteristics are to be grasped is adjusted to simulate the actual operation.
After adjusting the operating state of the inductor L in this way, the waveform of the inductor current iL, the waveform of the inductor voltage vL, the average value IL (= IR) of the inductor current iL, the effective value ILrms of the inductor current iL ( The root mean square of iL), the amplitude ILpp of the inductor current iL = It−Ib (It is the maximum value of iL, Ib is the minimum value of iL), and the voltage VR across the variable resistor RL is measured. From these measurement results, the impedance of the inductance L is calculated as follows.

  First, an inductance calculation method will be described. The energy stored in the inductor L in the steady state is equal to the energy consumed by the resistor RL. When the exciting current of the inductor L is I, the energy WL stored in the inductor in one cycle T (= T1 + T2) is expressed by equation (1). Here, the pure inductance component of the inductor L is also L for convenience.

On the other hand, the energy WR consumed in one cycle T by the resistor RL is expressed by equation (2).

Since WL = WR as described above, the inductance L is expressed by the following equation (3) from the equations (1) and (2).

If the loss of the inductor is ignored, the following equation (4) is established with respect to the change of the inductor current iL in the period of T2 = D2 · T (S1: OFF, S2: ON) in FIG.

When the above equation (4) is modified, the inductor L is expressed by the following equation (5).

From the equations (3) and (5), I ² is represented by the following equation.

From the equations (3) and (6), the inductor L can be finally expressed by the following equation (7). Here, PR = VR · IR, and PR is energy (power consumption) consumed by the resistor RL per unit time.

From equation (7), the inductance L can be calculated from the power PR consumed by the resistor RL and the above measurement result.
Next, calculation of the loss resistance R will be described. The loss resistance R is calculated by the following equation from the inductor loss (power consumed by the inductor) PL and the effective value ILrms of the inductor current.

Here, the loss PL of the inductor is obtained by the following equation (9).

In the equation (9), 0 to T in the integration range is one cycle of the on / off operation of the switches S1 and S2, and becomes zero if the inductor is an ideal inductor. Actually, since there is a loss, it does not become zero, and the loss resistance R is calculated from the value by the equations (8) and (9).

  An example of the impedance measurement result of the inductor by the measurement method of the first embodiment is shown in FIG. Further, regarding the step-down DC / DC converter of FIG. 4 to which the inductor L showing the measurement result in FIG. 3 is applied, the actual measurement value, the simulation result using the measurement value shown in FIG. 3 and the measurement value by the conventional measurement method are used. The simulation results are shown in FIG. The step-down DC / DC converter shown in FIG. 4 supplies an output voltage Vo and an output current Io. In the figure, PMOS is a switching transistor, NMOS is a synchronous rectification transistor, and L is an inductor whose measurement results are shown in FIG. , Co are output capacitors. The operating conditions of the step-down DC / DC converter shown in FIG. 4 were a switching frequency of 1 MHz, an input voltage of 4V, and an output voltage of 2V. As shown in FIG. 5, the simulation result using the impedance value of the inductor measured by the measurement method of the present invention is closer to the actual measurement than the simulation result using the measurement value by the conventional measurement method. That is, according to the present invention, the impedance of the inductor during actual operation can be accurately grasped.

It is an example of the measurement circuit used for the measuring method of the present invention. It is a waveform of the current iL flowing through the inductor L during measurement and the voltage (inductor voltage) vL across the inductor L. It is an example of the measurement result of the impedance by the measuring method of the present invention. FIG. 3 is a circuit diagram of a step-down DC / DC converter to which an inductor L whose measurement results are shown in FIG. 3 is applied. FIG. 5 shows actual measurement values and simulation results for the step-down DC / DC converter of FIG. 4. It is a figure for demonstrating the conventional measuring method regarding the impedance of the inductor which operate | moves in the state which DC current superimposed. It is a figure for demonstrating the direct current superimposition characteristic of the inductance L and the loss resistance R which were measured with the conventional measuring method. It is a figure which shows the magnetic characteristic of the general magnetic element containing an inductor. It is a figure which shows the outline | summary of the measuring apparatus shown by patent document 1. FIG.

Explanation of symbols

C Capacitor D1 On-time ratio of switch means S1 D2 On-time ratio of switch means S2 L Inductor and its inductance iL Inductor current IL Average value of inductor current iL ILrms Effective value of inductor current iL ILpp Amplitude of inductor current iL It Inductor current iL Ib Minimum value of inductor current iL IR Current flowing through variable resistance RL PL Inductor loss (power consumed by inductor)
PR Power consumption at variable resistor RL R Loss resistance of inductor RL Variable resistor S1, S2 Switch means T Period of ON / OFF operation of switching means S1, S2 (= T1 + T2)
T1 period when the switch means S1 is on and switch means S2 is off T2 period when the switch means S1 is off and switch means S2 is on vL Inductor voltage VR Voltage across the variable resistor RL

Claims (6)

A reference power supply having a reference potential terminal and an output terminal, a first switch means having one end connected to the output terminal of the reference power supply, and a second switch having one end connected to the other end of the first switch means And the other end of the first switch means and the reference power supply in a circuit in which a resistor and a capacitor are connected in parallel between the other end of the second switch means and the reference potential terminal of the reference power supply. An inductor is connected between the reference potential terminals, and the first switch means and the second switch means are turned on alternately at conduction times T1 and T2, respectively, and the voltage, current and power consumption of the resistor and the inductor. And measuring the impedance of the inductor from the result. 2. The impedance according to claim 1, wherein the inductance of the inductor is calculated from the current flowing through the resistor, the power consumed by the resistor, the current flowing through the inductor, and the conduction time T2 of the second switch means. Measuring method. The current IR flowing through the resistor, the power PR consumed by the resistor, and the difference ILpp between the maximum value and the minimum value of the current flowing through the inductor are obtained, respectively, and further from these values and the conduction time T2 of the second switch means, 3. The impedance measuring method according to claim 2, wherein the inductance L of the inductor is calculated by the following equation (I).

4. The impedance measuring method according to claim 1, wherein a loss resistance of the inductor is calculated from electric power consumed by the inductor and a current flowing through the inductor. 5. The loss resistance R of the inductor is calculated by the following equation (II), where PL is the power consumed by the inductor and ILrms is the effective value of the current flowing through the inductor. Impedance measurement method.

6. The power PL consumed by the inductor is calculated by the following equation (III), where T = T1 + T2, the current flowing through the inductor is iL, and the voltage across the inductor is vL. Impedance measurement method.

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