JP2003121481A - Device and method for measuring inductance of current sensing low resistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inductance measuring device and its method for measuring an effective inductance in an actual usage condition of a current sensing low resistor. SOLUTION: This inductance measuring device is provided with a generation source 21 of a serrate current, a wire line 22 feeding the serrate current to a measured resistor 11, a current detector 23 connected to the wire line 22 for detecting a current, a voltage detector 12 connected to both ends of the measured resistor 11 for detecting a voltage generated by the current, and a waveform display device 26 displaying an output of the current detector 23 and an output of the voltage detector 12 with contrast between them.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for measuring a current detection low resistor suitable for use in a current detection circuit of a switching power supply circuit. In particular, the present invention relates to an apparatus and method for measuring the effective inductance of a current detection low resistor for detecting a large current containing even high frequency components.

[0002]

2. Description of the Related Art In recent years, in electronic equipment such as personal computers, DC / D
A switching power supply such as a C converter is used.
This switching power supply uses a low-current resistor for current detection, is used in a frequency band of several tens to several hundreds of kHz, and has several A
A sawtooth wave current of several tens A flows and the magnitude of the current value is detected from the voltage generated across the resistor. In the current detection low resistance for such an application, the resistance value needs to be as low as possible, several mΩ or less is used, and it is desirable that the parasitic inductance of the resistance itself is as low as possible. This is because the resistance value of the resistor itself is small and the frequency is relatively high, so that even with a small inductance of about 1 nH, the combined impedance seen at both ends of the resistor becomes large, resulting in a voltage detection error.

In order to evaluate the parasitic inductance which influences the detection error of the low resistance for current detection, conventionally, the current detection resistor is individually attached to a test fixture or the like to measure the impedance. However, the inductance value calculated from the impedance thus measured does not make much sense in the design of a current detection circuit such as a DC / DC converter actually used. This is for the following reason.

Usually, the inductance of the low current detecting resistor is very small, about 1 nH or less, from several hundred MHz to G
It can be measured only by an impedance analyzer or the like, which uses a frequency on the order of Hz. However, the low resistance for current detection is actually used in the frequency band of about 10 MHz or less. When the frequency becomes high, the skin effect and so on will appear remarkably,
The inductance measured at a high frequency far from the actual use state is a meaningless value, unlike the inductance in the actual use state of the current detection low resistor.

Further, as described above, a current of several A to several tens of A flows in the current detecting low resistor of the switching power supply such as the DC / DC converter. Even if the resistance value is low, a large current causes a large Joule heat. This heat changes the resistivity in the resistor and changes the current path, so the inductance is a function of the passing current. Conventional measuring machines such as impedance analyzers cannot handle such large currents. Therefore, there is a problem that the inductance measured by the conventional method is different from the inductance in the actual use state of the current detection low resistor.

Further, the impedance between terminals can be measured by a general test fixture, but the inductance measured in this way is different from the inductance in the actual use state. That is, the inductance in the actual use state shows a change in voltage with respect to the sawtooth wave current in the frequency band of several tens to several hundreds kHz. In this frequency band, the skin effect and the effect of the parasitic capacitance are significant, and therefore the value is different from the impedance measured by an ordinary impedance analyzer or the like. Therefore, it is hard to say that this amount reflects the actual usage state.

[0007]

For the above reasons, the inductance value measured by the conventional measuring method is merely a reference to see whether the parasitic inductance is likely to be large or small. It cannot be applied to the design of current detection circuits such as converters.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an inductance measuring apparatus and method capable of measuring the effective inductance of a current detection low resistor in an actual use state. To do.

[0009]

According to the inductance measuring method of the present invention, a sawtooth wave current close to an actual use state is supplied to a resistor to be measured, the current is detected, and both ends of the resistor to be measured are detected. The voltage generated by the current is coupled and detected, and the effective inductance of the measured resistor is calculated from the change in the sawtooth wave current and the corresponding change in the detected voltage. Therefore, the inductance measuring device of the present invention includes a source of a sawtooth current, wiring for supplying the sawtooth current to the resistor to be measured, a current detector for detecting the current, and a resistor to be measured. A voltage detector that detects the voltage generated at both ends and a waveform display device that displays the current detector output and the voltage detector output in comparison are provided.

According to the present invention described above, it is possible to measure the inductance in a state where the low current detecting resistor is actually used. Therefore, it is possible to accurately evaluate the inductance that greatly affects the detection error of a resistor of several mΩ or less, and it is possible to predict the occurrence of an error voltage by using it in a switching power supply such as a DC / DC converter. Becomes Further, since the inductance of the low resistor in the actual use state can be evaluated, it also contributes to manufacture of a low inductance current detecting low resistor in which an error voltage hardly occurs.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1A shows an example of a circuit for detecting a current to be measured of a current detection low resistor. Both ends (electrodes) of the low-current detecting resistor 11 are fixed to the lands 13 and 14, and the resistor 1
A current I to be measured is passed through 1. Then, by extracting the potential difference (voltage) from the voltage lead-out portion (voltage detection terminal) of the land with both ends of the resistor fixed, the current and the voltage are in proportion to the known resistance value. Detect the size of. When detecting the current I to be measured, ideally the potential difference V extracted from between points A and B in FIG.
_AB must simply be the product of the resistance value R of the resistor and the measured current value I. However, in reality, the parasitic inductance L of the resistor generates a voltage L × (dI / dt) corresponding to the time change of the measured current I, and this voltage becomes a detection error. That is, as shown in FIG. 1B, a sawtooth wave current shown by a broken line in the drawing flows through the resistor 11, but a voltage change ΔV occurs at the top D of the current waveform.

The voltage appearing on the voltage detection terminal wirings 16a and 16b is guided to the voltage comparator or the voltage amplifier 12 as shown in FIG. 1 (a), and the voltage is detected. However, the self-inductance L exists in the resistor as described above, and the extraction pattern (voltage detection terminal wiring pattern) and the current I flowing through the measured current path are magnetically coupled, so that the actual potential difference V _AB Is V _AB = R × I + L × (dI / dt) −M × (dI / d
t).

Here, M is the mutual inductance shown in the equivalent circuit of FIG. The current flowing to the voltage comparator or the voltage amplifier 12 is almost negligible with respect to the measured current I, and thus is not added to the above equation. LM is an effective inductance that gives an error in the actual current detection resistor, and when expressed as Le in the meaning of effective inductance, Le = L- _{MVA B} = R × I + Le × (dI / dt) It will be easy. Le × (dI / dt) is substantially the detection error voltage ΔV.

In measurement with an impedance analyzer or the like, only the self-inductance L or the impedance seen from the voltage detection terminal can be measured. The effective inductance Le cannot be calculated from the impedance seen from the voltage detection terminal for the following reason. This includes the self-impedance of the voltage detection terminal wiring itself. The inductance of the low current detecting resistor changes depending on the current value. This is because the energizing current in actual use reaches several to several tens of amperes, so that high Joule heat is generated inside the resistor. When the inside of the resistor pair generates heat, the resistance value changes according to the temperature coefficient of resistance of the substance forming the resistor, and the current path is changed. Since the self-inductance L and the mutual inductance M both have a shape-dependent property, if the current path changes, it changes accordingly. Also, physically, the voltage detection terminal wiring,
The same large current as the resistor current cannot flow. The current detection low resistor is generally used in a frequency band of several tens to several hundreds of kHz. The effective inductance Le has a very small value of several nH or less. When measuring such a value with a conventional measuring device such as an impedance analyzer, it is necessary to use an AC voltage / current with a frequency of several tens to several hundreds of MHz or higher, so accurate measurement is possible due to the skin effect. It will be difficult.

FIG. 3 shows a preferred form of the voltage detection terminal wiring for extracting the voltage generated across the resistor. Both electrodes of the current detection resistor 11 are fixed to the lands 13 and 14, and the voltage detection terminal wiring 16 for voltage extraction is provided to the lands 13 and 14.
It is pulled out from 14. Effective inductance Le
Is determined by the structure of the current detection resistor and the voltage detection terminal wiring pattern on which it is mounted. Therefore, the mutual inductance is subtracted from the self-inductance, and the pattern as shown in FIG. 3 in which the effective inductance Le becomes almost zero is preferable. The reason will be described below.

The voltage detection terminal wiring 16 for voltage measurement is a horizontal central axis along the current of the resistor 11 (central axis of both lands).
Along the center (indicated by B and B'in the figure) and bent at right angles to the central axis (vertical central axis) between the lands (indicated by A and A'in the figure), and then one of them The vias 15 are connected to the back pattern, folded back, and arranged parallel to the front and back surfaces of the substrate along the vertical central axis. That is, the wiring pattern A passing only on the substrate front surface of the voltage detection terminal wiring and the wiring pattern A ′ drawn out to the back surface via the via 15 are overlapped on the front and back surfaces with the insulating layer of the substrate sandwiched therebetween. Is guided vertically. By overlapping the wiring patterns in this manner, the magnetic flux of the current flowing through the resistor 11 and the magnetic flux of the current flowing through the wiring pattern do not interlink in the loop formed by the voltage detection terminal wiring 16 composed of both wiring patterns, so that the mutual flux in FIG. The inductance M is not affected by the length of the extraction pattern. Then, the voltage from both voltage detection terminals is connected to a litz wire (stranded wire), etc. after being pulled out in a state of being overlapped to a position where the influence of the magnetic flux created by the current flowing through the resistor and the energization pattern is sufficiently reduced Is detected by the voltage detector 12.

The voltage detection terminal wiring is 0.2 to 0.3 mm
Use a pattern that is as thin as possible, and make the voltage detection terminal horizontal patterns B and B'line along the horizontal center axis as much as possible.
The voltage detection terminal vertical patterns A and A'are preferably centered on the vertical center axis. As shown in FIG. 4A, when the voltage detection terminal vertical pattern is separated from the vertical center axis, the mutual inductance M decreases and the effective inductance L decreases.
e increases. On the contrary, when the voltage detection terminal vertical pattern exists beyond the vertical center axis as shown in FIG. 4B, when the mutual inductance M increases and becomes larger than the self-inductance L, the effective inductance Le becomes It decreases and becomes negative.

Therefore, the voltage detection terminal vertical pattern A,
It is important to superimpose A ′ on the vertical central axis in order to reduce the effective inductance Le and prevent excessive coupling as shown in FIG. That is, the wiring length of the voltage detection terminal wirings B and B'is substantially equal to the length of the self-inductance of the resistor. Therefore, the mutual inductance due to the voltage detection terminal wiring can be substantially subtracted from the self-inductance. If you want to universally measure and compare the effective inductance Le of the resistor, the voltage detection terminal pattern must be such that the reproducibility of the effective inductance Le is ensured under the minimum conditions. The effects of noise must be eliminated.
The above pattern exactly meets that requirement.

The effective inductance Le measured with such a wiring pattern is an index showing how the low resistance for current detection can be used for current detection without correction by other circuits. Further, it is desirable that the insulating layers sandwiched by the voltage detection terminal wirings 16 stacked on the front and back surfaces of the printed circuit board are as thin as possible in order to reduce an error. Land 1
Since the shapes of 3 and 14 differ depending on the electrode shape of the current detecting low resistance resistor to be measured, it is desirable to use a shape that matches the electrode shape of the current detecting low resistance resistor.

The value of the effective inductance Le obtained by such a wiring pattern is a universal and practical design index showing the goodness of the current detecting resistor itself. That is, if the current path of the low resistor is linear and the patterns B and B ′ extend to the vertical central axis and are bent as shown in FIG. 3, the self-inductance and mutual inductance of the resistor are almost the same. They become equal, and the effective inductance Le becomes almost zero. When the current path of the resistor is curved due to trimming or the like, the self-inductance and the mutual inductance are not equal to each other, and the effective inductance Le has a value of, for example, 1 nH. Of course, if the pattern for mounting the current detection resistor is specified, a specified pattern different from the universal pattern can be used. However, in that case, the universality will be lost from the obtained value, but it will be a value that can be immediately reflected in the design.

Next, with reference to FIGS. 5 and 6, an example of the construction of a measuring circuit for actually measuring the effective inductance Le using the voltage detection terminal wiring pattern will be described. FIG. 5 shows a device for measuring the effective inductance Le of the current detecting low resistor of the present invention. A sawtooth wave current is supplied to the measured resistor 11 from a sawtooth wave current generation source 21 according to an actual use state. This sawtooth wave current generator 21
From, the sawtooth wave current of several A to several tens A is supplied to the measured resistor 11 of several mΩ or less in a cycle of 2.5 μS, for example. Wiring 2 for supplying the sawtooth current to the measured resistor 11
2 includes a current detector 23, such as a current probe, which is connected to the wiring to detect the current. The measured resistor 11 is mounted on the dedicated substrate 24 having the above-described voltage detection terminal wiring, and the voltage generated by the sawtooth wave current is taken out from the voltage detection terminal wiring 16 coupled to both ends of the measured resistance. The end of the voltage detection terminal wiring 16 of the substrate 24 is connected to the voltage detector 12 including a differential amplifier or the like via the twisted wire 25 or the like, and the voltage generated across the resistor to be measured is detected. The output of the current detector 23 and the output of the voltage detector 12 are the waveform display device 2 such as an oscilloscope.
6 is input and displayed in contrast to the unit of current or voltage.

FIG. 6 shows an example of the measuring circuit. For example,
A + 12V DC power source is alternately switched by using the switching elements 32 and 33, and a positive and negative current is alternately supplied to the integrating circuit including the choke coil 34 and the capacitor 35. By adjusting the inductance value of the choke coil, the capacitance value of the capacitor, the switching frequency, the duty factor, etc., the measured resistor 11
The sawtooth current shown by the broken line in FIG.
By adjusting the resistance value of the DC load 36, which is a resistor load, a DC current of several to several tens of amperes is supplied at, for example, DC 1V.

The condition that the measuring circuit should have is that, in addition to the above-mentioned adjustment, when a constant voltage having different polarities is alternately applied to a choke coil that does not saturate, a sawtooth wave current with good linearity and constant amplitude is obtained. Is to be generated. In addition, it is desirable that the common mode voltage rejection ratio of the differential amplifier that detects the voltage across the voltage detection terminal lead-out wiring pattern of the current detection low resistor is as high as possible. In addition, the choke coil and the low resistance for measuring the measured current should be arranged with a sufficient distance so that the leakage flux of the choke coil does not affect the low resistance for measuring the measured current within the measurement error range. It is desirable that magnetic shielding is provided between them so that the same effect can be obtained. In addition, the voltage detection terminal wiring pattern 16 which is overlapped on the front and back surfaces of the substrate shown in FIG.
The path is extended to the point where it is not affected by the external magnetic flux while overlapping, or the litz wire (stranded wire) is used so that the external magnetic flux does not interlink when extending with a wire rod. ) Is preferred. The current waveform is observed using a current probe, current transformer, etc. that has a sufficient bandwidth even for harmonics of the switching frequency.

Next, the procedure for calculating the effective inductance Le will be described with reference to FIG. To measure the effective inductance of a low-current detecting resistor, a sawtooth wave current is supplied to the measured resistor 11 to detect the current, and the voltage generated by the current is coupled to both ends of the measured resistor. To do. Then, the effective inductance of the measured resistor is calculated from the change in the sawtooth wave current and the corresponding change in the voltage.

First, the voltage change ΔV at the changing point (top) of the sawtooth wave current is ΔV = V1-V2, where R is the resistance value of the measured resistor, D is the duty, T is the period, and i1 is the current. = i1 × R + Le (di1 / dt) -i2 × R-Le (di2 / dt) At t = D × T, i1 = i2, so ΔV = Le (di1 / dt) -Le (di2 / dt) Becomes Here, if the current change value (peak value) is Vip_p, (di1 / dt) = Vip_p / (R × D × T) (di2 / dt) =-Vip_p / {R × (D-1) × T} Therefore, by substituting these and organizing,

[Equation 2]

[Equation 3] Since Vip_p and ΔV are obtained by a waveform display device or the like, it is possible to calculate the effective inductance of the current detecting low resistor by this.

FIG. 8 and FIG. 9 show examples of actual measurement of the effective inductance Le of the low resistor by the above measuring device and wiring pattern. FIG. 8 is intended for a resistor having a linear current path with a resistance value of 2 mΩ and a short distance from the printed circuit board to be mounted to the current path to have a structure that minimizes the effective inductance. . Although there is self-inductance of the resistor itself, the mutual inductance with the voltage detection terminal wiring cancels it out, so that the current waveform and the voltage waveform are substantially the same as shown in (a). Then, as shown in (b), as a result of measuring 10 samples, it is found that the effective inductance Le is almost zero. On the other hand, FIG. 9 shows the measurement result of a low resistance device having a resistance value of 3 mΩ and a current path bent by a vertical trimming cut. As shown in (a), it can be seen that a large error voltage is generated with respect to the sawtooth current waveform. In this case, as shown in (b), it can be seen that the effective inductance Le is about 0.9 nH in this low resistance device.

It should be noted that the inductance measurement of the current detecting low resistance device of the present invention is not limited to the above illustrated example, and various modifications can be made without departing from the scope of the present invention. Of course.

[0029]

The effective inductance of the current detecting low resistor is determined by the interaction between the structure of the current detecting low resistor and the wiring pattern for mounting the current detecting low resistor. It is difficult to measure with a general method for measuring high frequency characteristics using an impedance analyzer.
The inductance of the low resistor measured by the measuring apparatus and method provided by the present invention accurately reflects the inductance of the low resistor in the actual operating state of the DC / DC converter. As a result, in a switching power supply circuit such as a DC / DC converter, it is possible to correctly measure and evaluate the generation of an error voltage due to the inductance of a resistor having a small resistance value.

[Brief description of drawings]

FIG. 1A is a diagram showing an example of a detection circuit of a measured current of a current detection low resistor, and FIG. 1B is a sawtooth-shaped current waveform flowing through the current detection low resistor and a voltage detection terminal. It is a figure which shows the voltage waveform taken out from the wiring as an example.

FIG. 2 is an equivalent circuit diagram of a resistor to be measured and voltage detection terminal wiring.

FIG. 3 is a pattern diagram showing a configuration example of voltage detection terminal wiring.

FIG. 4 is a pattern diagram showing another configuration example of the voltage detection terminal wiring as a comparative example with respect to FIG.

FIG. 5 is a block diagram showing a device for measuring the effective inductance of the current detecting low resistor according to the embodiment of the present invention.

6 is a circuit diagram showing a specific circuit example of FIG.

FIG. 7 is a waveform diagram of current and voltage for calculating the effective inductance Le.

FIG. 8 (a) shows an example of voltage and current waveforms in a low resistor in which an effective inductance hardly exists,
(B) shows a measurement example of the effective inductance value.

9A and 9B show examples of voltage and current waveforms, and FIG. 9B shows an example of measurement of an effective inductance value in a low resistor having an effective inductance.

[Explanation of symbols]

11 Resistor to be measured 12 Voltage detector 13,14 Land 15 beer 16 Voltage detection terminal wiring 21 Sawtooth current generator 22 wiring 23 Current detector 24 Low-resistor mounting board 25 Litz wire (stranded wire) 26 Waveform display device 31 DC power supply 32, 33 switching elements 34 choke coil 35 capacitor 36 DC load

Claims (4)

[Claims] 1. A source of a sawtooth wave current, a wire for supplying the sawtooth wave current to a resistor to be measured, a current detector coupled to the wire for detecting the current, and the resistor to be measured. A voltage detector coupled to both ends of the detector to detect a voltage generated by the current, and a waveform display device for displaying the current detector output and the voltage detector output for comparison. Inductance measuring device for low resistance detector. 2. A sawtooth wave current is supplied to a resistor to be measured,
Along with detecting the current, the voltage generated by the current is coupled to both ends of the measured resistor, the change in the sawtooth current and the corresponding change in the voltage of the measured resistor A method for measuring the inductance of a current detection low resistor, which is characterized by calculating an effective inductance. 3. The effective inductance Le of the resistor under test is calculated from the sawtooth current and the corresponding change in the voltage by However, D: duty T: period R: resistance value of low resistor Vip_p: current change value ΔV: voltage change value at the top of the current waveform. Inductance measurement of the low resistor for current detection according to claim 2. Method. 4. The current according to claim 3, wherein the effective inductance is the self-inductance of the resistor itself minus the mutual inductance of the voltage extraction path wiring with which the magnetic flux links. Measuring method of inductance of low resistance detector.