JP2577800B2 - Automotive DC power supply current detector - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current detection device for an automobile DC power supply having a circuit configuration for accurately and stably measuring a DC current.

[Conventional technology]

FIG. 6 is a drawing showing the most general method of current detection. In FIG. 6, reference numeral 1 denotes a power supply for supplying a current _IL to a power supply load 3, and 2 denotes a current detection resistor, the resistance of which is R _S. 4 is an instrumentation amplifier.
Output voltage V _OUT that is proportional to the voltage V _I between One input terminals 41
Can be obtained at the output terminal 43.

In FIG. 6, since V _I = I _L · R _S , if the amplification degree of the instrumentation amplifier 4 is A, V _OUT = A
The current I _L flowing through the electric load 3 as I _L · _RS can be measured.

In general, the voltage drop across the detection resistor 2 is not desirable, so the resistance value R _S is set as small as possible. For example, if the maximum measured current is 100A,
When the voltage drop at is 10 mV, R _S becomes 0.1 mΩ. At this time, if I _L = 10 A, V _I = 1 mV, which is a very small voltage level handled by the instrumentation amplifier 4. Such current detectors are also used for measuring the output current of an automotive generator, in which case the ambient temperature of the detector varies from -40C to + 100C. Further, in an automobile, since the body is used as an electrical ground (ground), the current detection must be measured on the power supply side (usually on the positive side). Therefore, the potential of the detection resistor 2 is equal to the voltage of the vehicle-mounted battery. Become. The power supply for automobiles usually fluctuates more than 1 volt due to fluctuations of various on-vehicle electric loads. Therefore, the instrumentation amplifier 4 has a common mode noise of 1 volt or more and must amplify any small voltage of 1 mV.

Under such environmental conditions, it is not technically impossible to obtain an instrumentation amplifier capable of stably amplifying a voltage of 1 mV or less as described above. Not a target.

FIG. 7 is a diagram showing the principle of a system used for solving the above-mentioned difficulties.
The details are described in, for example, JP-A-63-228071 and JP-A-1-113674. In FIG. 7, reference numeral 5 denotes a ferromagnetic ring having a slit-shaped notch in a part thereof, and a conductor 20 is provided so as to penetrate the ferromagnetic ring.
The power supply current _IL flows to the power supply. A Hall element 6 is arranged in the cutout of the ferromagnetic ring 5 so as to measure the circumferential magnetic field of the cutout of the ferromagnetic ring 5. Reference numeral 7 denotes a power supply for supplying a current (or voltage) to the Hall element 6, and reference numeral 8 denotes an amplifier for amplifying an output voltage of the Hall element 6, and a voltage obtained at an output terminal 81 thereof is defined as V _OUT . The size of the magnetic field applied to the Hall element 6 is proportional to the current I _L naturally flowing conductors 20, the relationship of I _L and V _OUT is given by the following equation.

V _OUT = K · I _L + V _O (1) In the equation (1), K is a constant, and V is determined by the size of the magnetic ring 5, the sensitivity of the Hall element 6, and the amplification of the amplifier 8. _O is an offset voltage, which is determined by the residual magnetism of the magnetic ring 5, the unbalanced voltage of the Hall element 6, and the offset voltage of the amplifier 8. The method shown in FIG. 7 measures the current after converting the current into a magnetic field.
Is very useful because there is no need to worry about the voltage drop due to the above, but there are many ideas for stably setting K and V _{O in the} equation (1) to constant values. In other words, among the factors for determining the constant K, the sensitivity of the Hall element 6 has a large variation in the characteristics of each component and a large temperature dependency. Further, among the factors that determine the offset voltage V _O , the unbalanced voltage of the Hall element 6 also has large variations among components and has temperature dependence. For this reason, in the current detector according to the method shown in FIG. 7, some adjustment is required in the manufacturing process in order to compensate for variations in sensitivity and unbalanced voltage of the Hall element 6, and it is necessary to perform temperature compensation. There is. The temperature dependency (temperature characteristic) of the sensitivity and unbalanced voltage of the Hall element 6 shows the same tendency when the type (model) of the Hall element 6 is the same, but it is guaranteed that the Hall element 6 has exactly the same temperature coefficient. Therefore, even if the temperature compensation is performed, the temperature dependency cannot be completely eliminated.

[Problems to be solved by the invention]

As described above, in the method of converting a current to be measured into a voltage using a detection resistor, a minute DC voltage must be amplified in the presence of large common mode noise, and
In the method of measuring the current with a Hall element after converting the current into a magnetic field, it is necessary to adjust the sensitivity of the Hall element and reduce the influence of the unbalanced voltage, and to compensate the temperature characteristics of the above two parameters. There were issues such as the necessity and complete compensation being difficult.

The present invention has been made in order to solve the above-described problems, and has an object to obtain a current detection device for a DC power supply for a vehicle that is resistant to common mode noise, that is not adjusted, and that is stable against temperature changes. I do.

[Means for solving the problem]

The current detection device for a DC power supply for an automobile according to the present invention includes:
The current is converted to voltage by a detection resistor with a resistance value small enough to prevent voltage drop from becoming a problem, and this voltage is converted to AC by a switching circuit, and then guided to an AC amplifier via a transformer for AC amplification. Is converted into a direct current again to obtain a direct current signal proportional to the magnitude of the measured current.

(Operation)

The current detecting apparatus for a DC power supply for an automobile according to the present invention converts a DC voltage proportional to a measured current into an AC having an amplitude proportional thereto, and uses an AC amplifier which is essentially insensitive to temperature and power supply voltage fluctuations. After being amplified to a sufficiently large level, it is converted to DC by an AC / DC conversion circuit.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, reference numerals 1, 2, and 3 denote a DC power supply, a detection resistor, and a DC power supply connected in series so as to form one closed loop as in FIG.
An electrical load, current I _L that flows through the sense resistor 2 connected to the ungrounded side of the power supply 1 is a current to be measured. Reference numeral 9 denotes a switching circuit for generating an AC signal. The input terminal 91 is connected to the high potential side of the detection resistor 2, and the common terminal 92 is connected to the low potential side. Reference numeral 10 denotes a transformer, and primary-side input terminals 100 and 101 are connected to an output terminal 94 and a common terminal 92 of the switching circuit 9. The output terminal 103 at one end on the secondary side of the transformer 10 is grounded, and the output terminal 102 at the other end is connected to the input terminal 110 of the AC amplifier 11. An AC / DC conversion circuit 12 has an input terminal 121 connected to the output terminal 111 of the AC amplifier 11, and a grounded common terminal 122 and an output terminal.
Has 123. An input terminal 131 is connected to an output terminal 123 of the AC / DC conversion circuit 12, and has a common terminal 132 and an output terminal 133 which are grounded. 14 is an oscillator having an output terminal 141, 15 is an input terminal 151 is an output terminal 141
The output terminal 152 of the driving circuit is connected to the control input terminal 93 of the switching circuit 9.

Next, the operation of one embodiment of the present invention will be described with reference to FIG. Current I _L flows through the sense resistor 2 and electric load 3 from the power source 1, current I _L that flows through the sense resistor 2 is a current to be measured. The switching circuit 9, synchronously switching the pulse signal applied to the DC voltage V _S to the control input terminal 93 between the applied sense resistor 2 across between the common terminal 92 and the input terminal 91, an output terminal 94 the common terminal It generates an AC signal (pulse train) e _m with amplitude proportional to V _S between 92. The pulse train e _m is applied between input terminals 100 and 101 of the primary side of the transformer 10, the output terminal 102 of the secondary side of the transformer 10 in the form of a direct current component has been removed,
Guided between 103 and the signal e _S. This signal e _S is applied to the input terminal 110 of the AC amplifier 11 and its output terminal 111
Obtaining an output e _O amplified to. And AC / DC conversion circuit
12 get to the output terminal 123 the ripple signal e _P having an average value proportional to the amplitude of an AC signal (pulse train) e _O from the AC amplifier 11 is applied between the common terminal 122 and the input terminal 121. The smoothing circuit 13 includes an AC / DC conversion circuit 1 applied to the input terminal 131.
From pulsating signal e _P from 2 except AC components, and outputs a DC signal V _OUT which is proportional input signal to the average value of (pulsating signal) e _P terminal
Get to 133.

On the other hand, the oscillator 14 applies to the input terminal 151 of the drive circuit 15 via the pulse signal output terminal 141 of the repetition frequency f.
The drive circuit 15 generates a pulse signal e _X having a repetition frequency f at the output terminal 152 and having a DC level and an amplitude necessary for switching the switching circuit 9, and applying the pulse signal e _X to the control input terminal 93 of the switching circuit 9. I do.

The case where the current _IL is flowing through the electric load 3 will be described in more detail with reference to the signal waveform of each part. FIGS. 2 (a) to 2 (f) show signal waveforms at various parts.
(A) is the voltage V _S across the detection resistor 2 and at the same time, is the input signal of the switching circuit 9, whose value is _IL ·
R _S. (B) is the input signal e _m of the output voltage or the transformer 10 of the switching circuit 9, the repetition frequency f, amplitude
It is a pulse train of K _I , I _L, and R _S. Here, K _I is a constant Kimare the input impedance of the structure and the impedance of the transformer 10, the AC amplifier 11 of switching circuit 9. (C) is the output voltage of the transformer 10, that is, the input signal e _S of the AC amplifier 11, and the amplitude (peak-to-peak)
There is a pulse train of a _{K I · I L · R S} . (D) is an AC amplifier 11
Output voltage, that is, the input signal of the AC / DC conversion circuit 12.
In e _O, its amplitude is _{G · K I · I L ·} R S. Here, G is the amplification degree of the AC amplifier 11. (E) is the input signal e _P of output or smoothing circuit 13 of the AC-DC converter circuit 12, a low level is zero, the high level is a pulse train of _{K 2 · G · K 1 ·} I L · R S. Here, K ₂ is a constant related to the efficiency of the AC / DC conversion circuit 12. (F) is an output signal of the smoothing circuit 13
V _OUT , whose value is K ₃ · K ₂ · G · K ₁ · I _L · R _S.
K ₃ is a constant given as a product of the pulse rate (duty factor) of the signal e _P and the amplification of the smoothing circuit 13.
If the duty factor of e _P is 50% and the smoothing circuit 13 is a simple CR low-pass filter, K ₃ = 0.5.

To summarize the above, the output voltage V _OUT is given by V _OUT = K ₁ · K ₂ · K ₃ · G · R _S · I _L = I _L · R _X (2) Here, R _X = K ₁ · K ₂ · K ₃ · G · R _S. As is clear from the above description, the constants K ₁ , K ₂ , K ₃ , G, and R _S that determine the conversion parameter R _X can be uniquely determined at the circuit design stage, and errors due to temperature changes and component variations can be determined. Can be within the range defined as the design specification.

FIG. 3 shows a specific example of the switching circuit 9, and the input terminal
Two resistors 95, 96 connected in series between 91 and the output terminal 94
And a FET 97 having a drain and a source connected between the connection point of the resistors 95 and 96 and the common terminal 92, and having one terminal of the capacitor 99 connected to the gate. A resistor 98 connected between the gate terminal of the FET 97 and the common terminal 92 fixes the DC potential of the gate. The other terminal of the capacitor 99 is connected to the output terminal 152 of the drive circuit 15.
When positive pulse to the gate of FET97 through the capacitor 99 is applied FET97 is between on to the drain and the source becomes nearly short-circuited state, the output voltage e _m is zero. When a negative pulse is applied to the gate of the FET 97 via the capacitor 99, the FET 97 is turned off, and K is applied to the output terminal 94 as described above.
_Generates a voltage of ₁ · I _L · _RS .

FIG. 4 shows another embodiment of the switching circuit 9.
A PNP transistor 971 is used as a switching element. This method is suitable for automotive applications. That is, in the case of an automobile, since all electric loads are operated by a single DC power supply (battery) 1, the switching circuit 9
Of the common terminal 92 and the potential of the power supply for the operation of the current detection device are almost the same.

Accordingly, when the voltage applied to the control input terminal 93 in FIG.
When the transistor 971 is turned on or the control input terminal 93 is opened, the transistor 971 is turned off, and the third
The same operation as in the figure is performed. In the case of the switching circuit shown in FIG. 4, the output circuit (not shown) connected to the output terminal 152 of the drive circuit 15 is suitably an open collector type of an NPN transistor.

FIG. 5 shows a specific embodiment of the AC / DC conversion circuit 12, which is a circuit known as a phase detection circuit. Fifth
In the figure, a resistor 12 is connected between an input terminal 121 and an output terminal 123.
4 and a series circuit of the capacitor 125 are connected, and the drain / source of the FET 126 is connected between the output terminal 123 and the ground. 1, the circuit shown in FIG. 5 has a control input terminal 127, and this control input terminal 127
Connected to the gate of FET 126 and to output terminal 141 of oscillator 14.

According to the circuit of FIG. 5, the DC component of the signal obtained at the input terminal 121 is completely removed by the capacitor 125.
Since a signal applied to the control input terminal 127 causes the FET 126 to be turned on / off synchronously with the signal frequency, a noise component is effectively removed from the average value of the signal obtained at the output terminal 123. This is a great feature of the phase detection circuit.

〔The invention's effect〕

As described above, according to the present invention, a minute DC voltage is generated by flowing the current to be measured through the detection resistor connected between the positive power supply side of the vehicle DC power supply and the vehicle-mounted electric load. The DC voltage is converted to an AC signal by a switching circuit, guided to an AC amplifier via a transformer, amplified, and then converted to a DC signal to obtain an output. The current detection must be measured on the power supply side, and the detection resistance connected between the positive power supply side of the automatic DC power supply and the on-board Even when detecting the measured current flowing in the current, it is resistant to common mode noise, not affected by temperature changes and component variations, and without causing excessive voltage drop in the measured circuit. There is an effect that can be measured direct current to a constant.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG.
3 and 4 are circuit diagrams showing specific examples of a switching circuit, FIG. 5 is a circuit diagram showing a specific example of an AC / DC converter, Sixth
FIG. 7 is a diagram showing a circuit for explaining the prior art. In the figure, 2 ... detection resistor, 9 ... switching circuit, 10 ...
... Transformer, 11 ... AC amplifier, 12 ... AC / DC conversion circuit, 13 ... Smoothing circuit, 14 ... Oscillation circuit, 15 ... Drive circuit. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

(57) [Claims] 1. A detection resistor connected between a positive power supply side and a negative power supply side of an automotive DC power supply and a vehicle-mounted electric load which forms a common feedback path with a single ground via a vehicle body or the like, and through which a measured current flows, A switching circuit that converts a voltage generated at both ends of the detection resistor into one input, switches the input signal according to a signal applied to a control input terminal, and converts the input signal into a pulse signal;
A transformer having the output of the switching circuit as an input, an AC amplifier having an output of the transformer as an input, an AC / DC conversion circuit for converting the output of the AC amplifier to DC, and an input terminal having the AC / DC conversion circuit And a drive circuit that receives a signal from an oscillator and supplies a pulse signal to a control input terminal of the switching circuit.