JP3479061B2 - Mounting structure and method of current detection resistor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mounting structure of a current detecting resistor suitable for use in a current detecting circuit of a switching power supply circuit. In particular, the present invention relates to a mounting technique of a low resistor for detecting a large current containing a high frequency component on which a parasitic inductance becomes a problem, on a circuit board.

[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 in the power supply circuit. A current detection resistor is used in this switching power supply, and it is used in a frequency band of several tens to several hundreds of kHz, and a sawtooth wave current of several A to several tens A flows, and a current is generated from a voltage generated across the resistor. The magnitude of the value is detected. In the current detection resistor for such an application, the resistance value needs to be as low as possible, and several mΩ or less is used.
It is also desirable that the parasitic inductance of the resistor itself be 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 that influences the detection error of the current detection resistor, 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 current detecting resistor is very small, about 1 nH or less, and is several hundred MHz to GH.
It can be measured only by an impedance analyzer or the like, which measures using a z-order frequency. However, the current detection resistor is actually used in a frequency band of about 10 MHz or less. Since the skin effect and the like become prominent at high frequencies, the inductance measured at high frequencies far from the actual use state is a meaningless value unlike the inductance of the current detection resistor in the actual use state. is there.

Further, as described above, the current detection resistor of the switching power supply such as the DC / DC converter has several amperes.
A current of several tens of amperes flows. 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 when the current detection resistor is actually used.

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 error voltage appears in the inductance in the actual use state with respect to the sawtooth wave current in the frequency band of several tens to several hundreds of 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 self-inductance of the resistor itself is large or small, and Cannot be applied to the design of current detection circuits such as DC / DC converters. Further, as described above, since a detection error in current detection occurs due to the parasitic inductance, a mounting structure of a current detection resistor that can minimize this effect has been desired.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a mounting structure and method of a current detecting resistor in which the influence of parasitic inductance is reduced as much as possible.

[0009]

A mounting structure of a current detecting resistor according to the present invention has a pair of lands for fixing both ends of the current detecting resistor and a resistor connected between the lands to be measured. A voltage detection terminal wiring for supplying a current and detecting a voltage generated at both ends of the resistor, the voltage detection terminal wiring extending inward along respective central axes of the lands, Bends vertically in opposite directions along the center axis between them, and one wiring wraps around the back side through the via of the substrate and extends in the direction of the other wiring, and the one wiring and the other wiring of the substrate It is characterized in that it extends parallel to the front and back surfaces.

According to the above wiring pattern of the present invention,
By applying a known sawtooth wave current to the low resistor and measuring the detected voltage waveform that appears on the voltage detection terminal wiring, the inductance that correctly reflects the waveform distortion in the operating state of the DC / DC converter of the current detection resistor. Can be measured.

Further, by using the mounting structure of the present invention, it is possible to construct a current detecting mechanism for a low resistor which is not affected by an external magnetic field. Since the mutual inductance component of the voltage detection terminal wiring can be substantially subtracted from the self-inductance component, it is possible to detect the current to be measured while minimizing the detection error due to the parasitic inductance of the resistor.

[0012]

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 the measured current of the current detection resistor. Both ends (electrodes) of the current detecting resistor 11 are fixed to the lands 13 and 14, and a current I to be measured is passed through the resistor 11. Then, the potential difference (voltage) from the voltage lead-out part (voltage detection terminal) of the land with both ends of the resistor fixed
By taking out, since the current and the voltage are in proportion to the known resistance value, the magnitude of the measured current I is detected. 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 is Le in terms of effective inductance.
If expressed, Le = L- _{MVA B} = R × I + Le × (dI / dt) becomes simple. Le × (dI / dt) is substantially the detection error voltage ΔV.

In the measurement using 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 current detection 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. Further, the same large current as the current of the resistor cannot physically flow through the voltage detection terminal wiring. The current detecting resistor is generally used in a frequency band of tens to hundreds of kHz. The effective inductance Le has a very small value of several nH or less. If you want to measure such a value with a conventional measurement device such as an impedance analyzer, dozens of
Since it is necessary to use an AC voltage / current with a frequency of several hundred MHz or more, accurate measurement becomes difficult due to the skin effect.

FIG. 3 shows an embodiment of the voltage detection terminal wiring for extracting the voltage generated across the resistor. Both electrodes of the current detecting resistor 11 are fixed to the lands 13 and 14, and a voltage detecting terminal wiring 16 for extracting voltage is drawn from the lands 13 and 14. The 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 center axis along the current of the resistor 11 (center 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 Will be 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 current detection resistor 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 a voltage error. Land 1
Since the shapes of 3 and 14 differ depending on the electrode shape of the current detection resistor to be measured, it is desirable to use those that match the electrode shape of the current detection resistor.

The value of the effective inductance Le obtained by such a wiring pattern is a universal and practical practical 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 configuration 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 resistor. 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. From the sawtooth wave current generator 21, a sawtooth wave current of several A to several tens of A is supplied to the measured resistor 11 of several mΩ or less at a cycle of 2.5 μS, for example. The wiring 22 for supplying the sawtooth wave current to the resistor 11 to be measured is provided with a current detector 23 such as a current probe which is connected to the wiring and detects the current. Measured resistor 11
Is a dedicated board 24 provided with the voltage detection terminal wiring described above.
The voltage generated by the sawtooth wave current is taken out from the voltage detection terminal wiring 16 attached to the both ends of the resistor to be measured. 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. Output of current detector 23 and voltage detector 12
Is output to the waveform display device 26 such as an oscilloscope, and is displayed by comparing the unit with 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. Further, 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 resistor is as high as possible. In addition, the choke coil and the measured current detection resistor should be arranged with a sufficient distance such that the leakage flux of the choke coil does not affect the measured current detection resistor within the measurement error range, or between them. It is desirable to arrange so that the same effect can be obtained by magnetically shielding. Further, the voltage detection terminal wiring pattern 16 overlapped on the front and back surfaces of the substrate shown in FIG. 3 is guided to the input of the differential amplifier 12, but is its path extended to a point where it is not affected by external magnetic flux while being overlapped? When extending with a wire rod, it is desirable to use a litz wire (stranded wire) so that the external magnetic flux does not interlink between the wires. 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 the 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. . 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 change point (top) of the sawtooth wave current is ΔV = V1-V2, where R is the resistance value of the resistor to be measured, duty D is the period T, and currents i1 and i2 are = 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 1]

[Equation 2] Since Vip_p and ΔV are obtained by a waveform display device or the like, the effective inductance of the current detecting resistor can be calculated from this.

FIGS. 8 and 9 show examples of actual measurement of the effective inductance Le of the low resistor by the above-mentioned 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.

In the above embodiment, an example in which the mounting structure of the low resistor of the present invention is used for measuring the effective inductance of the current detecting resistor has been described, but the wiring pattern and mounting structure are used for the switching power supply circuit. Of course, it can be directly used for a current detection circuit or the like. This makes it possible to measure the measured current stably without being affected by the external magnetic field, and because there is no inductance due to the mounting board itself, the detection error due to only the parasitic inductance of the resistor is minimized. The measured current can be detected.

The mounting structure of the current detecting resistor according to the present invention is not limited to the above-mentioned illustrated example, and various modifications can be made without departing from the scope of the present invention. is there.

[0031]

The effective inductance of the current detecting resistor is determined by the interaction between the structure of the current detecting resistor and the wiring pattern for mounting the current detecting resistor. It is difficult to measure with the general method of measuring high frequency characteristics used. According to the mounting structure and method of the current detecting resistor provided by the present invention, it is possible to correctly measure the effective inductance of the low resistor in the actual operating state of the DC / DC converter.

Further, by using the mounting structure of the current detecting resistor of the present invention in the current detecting circuit of the switching power supply circuit, the mutual inductance due to the voltage detecting terminal wiring can be substantially subtracted from the self-inductance. Therefore, it is possible to detect the current to be measured while minimizing the detection error due to the parasitic inductance of the mounting board.

[Brief description of drawings]

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

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Claims (4)

(57) [Claims] 1. A pair of lands for fixing both ends of a current detecting resistor, a current to be measured is supplied to the resistor connected between the lands, and a voltage generated at both ends of the resistor is detected. In the mounting board of the current detection resistor having the voltage detection terminal wiring, the voltage detection terminal wiring extends inward along the central axes of the lands, and extends along the central axis between the lands. Bend vertically in opposite directions, one wiring wraps around the back side via the via of the substrate and extends in the direction of the other wiring, and the one wiring and the other wiring overlap and are parallel to the front and back surfaces of the substrate. A wiring pattern for mounting a current detection resistor, which is characterized by extending. 2. A substrate for mounting a current detection resistor, comprising the mounting wiring pattern according to claim 1. 3. A mounting structure of a current detecting resistor, wherein the current detecting resistor is mounted on the mounting substrate according to claim 2. 4. The current detecting resistor is fixed at both ends thereof, and a pair of lands for supplying a current to be measured is provided to the resistor, and a voltage generated at both ends of the resistor from the lands is detected. In a method for mounting a current detection resistor in which voltage detection terminal wiring is provided, the voltage detection terminal wiring extends inward along the central axes of the lands, and respectively along the central axis between the lands. Bends perpendicularly in the opposite direction, one wiring wraps around the back side via the via of the substrate and extends in the direction of the other wiring, and the one wiring and the other wiring overlap the front and back surfaces of the substrate and extend in parallel. A method for mounting a resistor for current detection, which is characterized in that: