EP0986039B1 - Device for supplying power to a current loop transmitter - Google Patents

Description

The invention relates to an arrangement for signal transmission between a receiving station and a transmitting station and to the power supply of the transmitting station.

From the EP-A-0 744 724 Such a circuit arrangement is known in which the two stations are connected by a two-wire line, via which a variable between two thresholds analog signal stream is transmitted, which represents a detected in the transmitting station by a sensor measured value and forms the required for the operation of the transmitting station supply current , The transmitting station in this case has a circuit which generates a constant operating voltage for the transmitting station, and it contains a controllable current source, which determines the current flowing through the two-wire line in dependence on the measured value and which is fed from a supply voltage source in the receiving station. In this case, the transmitting station finds a voltage at its two inputs, which can vary within wide ranges depending on the choice of the supply voltage source. For measured value transmission The transmitting station controls its input current in an ideal way so that it is dependent only on the measured value. The supply of the transmitting station takes place exclusively via the two-wire line, wherein the input voltage is generally greater than the internally required supply voltage. In the transmitting station, therefore, the input voltage is reduced by a linear regulator to the internally required supply voltage. But so that the available supply current is limited by the input current of the transmitting station. However, this limitation limits the flexibility with regard to the use of the sensors and signal evaluation circuits in the transmitting station, since it may well also be desirable to use sensors which require a larger current than can be supplied via the two-wire line.

WO88 / 02528A1 discloses a digital converter for improving the output of a two-wire transducer. Parts of the analog control circuit for controlling the signal current are replaced by digitally operating components.

EP0591926 A2 discloses an arrangement for signal transmission between a receiving station and a transmitting station and to the power supply of the transmitting station, wherein these two stations are interconnected by a two-wire line through which a variable between two thresholds analog signal stream is transmitted, which is detected in the transmitting station by a sensor measured value represents and which forms the supply current required for the operation of the transmitting station, wherein the transmitting station has a circuit which generates a constant operating voltage for the transmitting station, and wherein in the transmitting station, a controllable current source is provided which the flowing over the two-wire line current in dependence of the Measured value and which is fed from a supply voltage source in the receiving station, wherein the current source is a series current regulator, which is fed from the supply voltage source in the receiving station, with the output d he power source is connected to a charge pump, which generates from the voltage occurring at the output of the power source required for the operation of the sensor and a signal processing circuit connected thereto operating voltage, and with the input or the output of the charge pump, a parallel controller for keeping constant the input voltage or the Output voltage of the charge pump is connected. In the described arrangement, during starting of the device, the signal current is in an undefined state when the supply voltage to be provided by the charge pump is not yet stabilized. It is therefore the object of the invention to overcome the described disadvantages of the prior art.

The invention is further based on the object to provide a signal transmission and Stromdorgungsdnordnung which is very flexible in terms of usable sensors and signal processing units in the transmitting station and can be adapted to the respective conditions with respect to the power supply.

To solve this problem, an arrangement for signal transmission between a receiving station and a transmitting station and the power supply of the transmitting station is provided by the invention, in which these two stations are connected to each other by a two-wire line through which a variable between two thresholds analog signal stream is transmitted represents a measured value detected in the transmitting station by a sensor and forms the supply current required for the operation of the transmitting station, wherein the transmitting station has a circuit which generates a constant operating voltage for the transmitting station, and wherein in the transmitting station a controllable current source is provided which the determined over the two-wire line current as a function of the measured value and determined from a supply voltage source in the receiving station is fed, wherein the power source is a series current regulator, which is fed from the supply voltage source in the receiving station, to the output of the power source, a charge pump is connected, from the voltage occurring at the output of the power source for the operation of the sensor and one connected thereto Signaling circuit generates required operating voltage, and to the input or the output of the charge pump, a parallel regulator for keeping constant the input voltage or the output voltage of the charge pump is connected, wherein the current source is bridged by a voltage regulator, which provides an input voltage for the charge pump in a start-up phase the current source is designed so that it delivers an output current in the start-up phase only when the charge pump delivers a sufficient output voltage for its operation, wherein the voltage regulator is designed so that it enters a locked state ht when the output voltage reaches the operating voltage.

According to a first embodiment of the invention, a further parallel regulator is provided in addition to the parallel regulator at the input or at the output of the charge pump on the other side of the charge pump.

According to a second embodiment of the invention, a longitudinal regulator is provided in addition to the parallel regulator at the entrance or at the outlet of the charge pump on the other side of the charge pump.

In a further development, the charge pump has a voltage transfer factor <1.

By using the combination of current and voltage regulators in conjunction with a charge pump, the current and voltage values required for the operation of the transmitting station can be set within wide limits, so that a high flexibility in terms of usable sensors is achieved. In particular, in the transmitting station circuit units may be used which require a supply current which is greater than the current which may flow as the maximum signal current via the two-wire line to the receiving station. It should be emphasized as an advantage of the arrangement according to the invention also their easy integration. It contains no inductors, but essentially capacitors, which can easily be manufactured with capacities <1nF in integrated form.

Figur 1

eine schematische Übersichtsdarstellung einer Anordnung zur Signalübertragung zwischen einer Sendestation und einer Empfangsstation, in der die Erfindung anwendbar ist,

Figur 2

ein schematisches Blockschaltbild der erfindungsgemäß aufgebauten Spannungsversorgung für die Anordnung von Figur 1,

Figur 3

ein Schaltbild der Spannungsversorgung von Figur 2, wobei die einzelnen Schaltungseinheiten beispielhaft genauer in ihrem Aufbau dargestellt sind,

Figur 4

ein Blockschaltbild einer zweiten Ausführungsform einer erfindungsgemäß aufgebauten Spannungsversorgung für die Verwendung in der Anordnung von Figur 1 und

Figur 5

ein Schaltbild der Spannungsversorgung von Figur 4, wobei die einzelnen Schaltungseinheiten beispielhaft in ihrem Aufbau genauer dargestellt sind.

Embodiments of the invention will now be explained in more detail with reference to the drawing. Show it:

FIG. 1

a schematic overview of an arrangement for signal transmission between a transmitting station and a receiving station, in which the invention is applicable,

FIG. 2

a schematic block diagram of the inventively constructed power supply for the arrangement of FIG. 1 .

FIG. 3

a circuit diagram of the power supply of FIG. 2 wherein the individual circuit units are shown more precisely in their structure by way of example,

FIG. 4

a block diagram of a second embodiment of a voltage supply constructed according to the invention for use in the arrangement of FIG. 1 and

FIG. 5

a circuit diagram of the power supply of FIG. 4 , wherein the individual circuit units are exemplified in more detail in their construction.

In the FIG. 1 illustrated arrangement for signal transmission includes a receiving station 10 and a transmitting station 12, which are connected to each other via a two-wire line 14. In the receiving station 10 is a Signalauswertungsachaltung 16, which is symbolically represented as an ammeter, since the current flowing via the two-wire line 14 to the receiving station 10 is the electrical parameter to be evaluated. Furthermore, the receiving station 10 contains a supply voltage source 18 which provides the energy required for the operation of the signal transmission arrangement both on the receiving side and on the transmitting side.

The transmitting station 12 includes a sensor 20 which detects in a process a process variable, such as a temperature, pressure, level or the like as a measured value. The sensor 20 outputs its output signal, which represents the measured value, to a signal processing circuit 22, which generates a control signal proportional to the measured variable detected by the sensor 20. A circuit 24 included in the transmitting station 12 generates the operating voltage required for the operation of the signal processing circuit 22 and the sensor 20, and at the same time sets the current flowing through the two-wire line 14 under the control of the control signal applied to its input 25 to that from the sensor 20 measured value proportional current value I _in . Their inputs 26 and 28 are connected to the two-wire line 14, while their outputs 30 and 32, at which it outputs the constant operating voltage, are connected to the supply voltage terminals of the signal processing circuit 22 and the sensor 20.

In FIG. 2 the structure of the circuit 24 is shown in a schematic diagram. The circuit includes a current source 34, which is designed as a series current regulator. The current set via the control signal at input 25 is kept constant at the set value by the series current regulator, the reference voltage used being the reference value which is tapped off at a measuring resistor 36 through which the set current flows. At the output of the series current regulator 34 results due to the internal resistance the other circuit parts a voltage U _V. This voltage serves as a supply voltage for a charge pump 38, which supplies at its output a voltage U _out , which represents the supply voltage for the signal processing circuit 22 and the sensor 20. This output voltage U _out is kept constant by means of a voltage regulator 40 designed as a parallel regulator.

The charge pump 38 conventionally consists of a series of switches 38.1-38.4 and capacitors C1, C2, C3 and a control circuit 39 which control (open and close) the switches 38.1-38.4 so that a charging voltage occurs across the capacitor C3 which is the desired one Output voltage corresponds. The structure of the charge pump is in FIG. 3 and in FIG. 5 shown only schematically, since the structure and operation of such circuits to those skilled in various versions are known (for example, "semiconductor circuitry" by U. Tietze and Ch. Schenk, 1991, p 570, 571).

In the event that the control signal at the input 25 sets the series current regulator 34 to a higher current value than can be derived from the charge pump 38, this current can be dissipated via an additional circuit unit 42 which acts as a voltage limiting circuit. Namely, the higher current supplied from the series current regulator 34 results in a higher voltage U _V , and the voltage limiting circuit 42 can be designed to respond when a predetermined voltage value is exceeded, and to dissipate the excess current while achieving a voltage limit.

The circuit of FIG. 2 allows within wide limits the setting of current and voltage values for the operation of the signal processing circuit 22 and the sensor 20 in the transmitting station. Subsequently, an estimation of the operating limits of the in FIG. 2 set forth circuit.

By the voltage _translation v _{U of} the charge pump 38 can be determined which voltage U _V = U _{Vmin is} at least necessary so that the output voltage U _out = U _{OutSoll is} reached at the output: $${\mathrm{U}}_{\mathrm{Vmin}}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{v}}_{\mathrm{U}}}\mathrm{\cdot }{\mathrm{U}}_{\mathrm{OutSoll}}$$

With a limitation of the bias voltage by the voltage _{limiting circuit} 42, the minimum required voltage U _{Vmin should} be achieved safely: $${\mathrm{U}}_{\mathrm{VBypass}}\mathrm{\ge }{\mathrm{U}}_{\mathrm{Vmin}}$$

The lower limit U _{InMin of} the input voltage range results from the maximum possible bias voltage U _VBypass plus the voltage drop U _IReg required for the operation of the current regulator 34: $${\mathrm{U}}_{\mathrm{Inmin}}\mathrm{=}{\mathrm{U}}_{\mathrm{VBypass}}\mathrm{+}{\mathrm{U}}_{\mathrm{ireg}}$$

If it is assumed that no current losses occur in the series current regulator 34 and in the voltage _{limiting circuit} 42, the maximum _extractable current I _OutMax results at the output from the current transformation ratio V _{I of} the charge pump 38 and the input current I _In dependent on the detected measured value. $${\mathrm{I}}_{\mathrm{OutMax}}\mathrm{=}{\mathrm{v}}_{\mathrm{I}}\mathrm{\cdot }{\mathrm{I}}_{\mathrm{In}}$$

Conventional charge pumps achieve a power efficiency of approximately 100%. For the ratios v _U and v _I then: $$\frac{\mathrm{1}}{{\mathrm{v}}_{\mathrm{U}}}\mathrm{=}{\mathrm{v}}_{\mathrm{I}}$$

The series current regulator 34 can be operated with the output voltage U _Out . But then special precautions must be taken so that the circuit 24 starts and supplies the required output voltage. For this purpose, it is possible to design the series current regulator 34 so that it supplies a possibly unregulated current to the charge pump 38 without its own supply voltage. The charge pump 38 is then able to generate an output voltage U _Out . With this output voltage, the series current regulator 34 can then be operated.

In FIG. 3 a circuit diagram is shown in which the basic structure of the series current regulator 34, the voltage limiting circuit 42, the charge pump 38 and the voltage regulator 40 are shown. It should be noted, however, that the structure of the respective circuit units is given only as an example. On the particular structure, it does not matter for the invention. Decisive for the invention are only the function of the individual circuit units and their interaction with the other circuit units.

The series current regulator 34 is according to FIG. 3 a simple series regulator, which keeps the current flowing through the transistor T to a constant, via the operational amplifier OP by means of the control signal at the input 25 adjustable value. The resistance R in the current regulator 34 between the emitter and the collector of the transistor T has the purpose of enabling the starting of the circuit. About this resistor R, a small current can flow even when the transistor T is locked, which suffices as a starting current for the circuit. The voltage limiting circuit 42 is in the simplest case, only a Zener diode, the voltage occurring at the output of the current regulator 34 to a limited constant value. The charge pump can achieve almost any voltage and current ratios depending on its design. The illustrated circuit of the charge pump is just one example; the structure and function of such charge pumps is known in the art and can be taken from numerous references. Also, the voltage regulator circuit 40 is in the simplest case, only a Zener diode, which keeps the value of the output voltage U _Out constant.

If the series current regulator 34 is designed so that it does not work without its own supply voltage, special precautions must be taken so that the current regulator can start its operation and can supply a current to the charge pump 38. In FIG. 4 is shown in a block diagram, as in such a case, the circuit can be put into operation. This circuit includes a current regulator 44, which is initially disabled without its own operating voltage, so can not deliver power to the charge pump 38. How out FIG. 4 can be seen, the series current regulator 44 is bridged by a voltage regulator 46, which generates a voltage U _V1 when commissioning the circuit, which acts as a supply voltage for the charge pump 38, so that they then deliver at its output a voltage U _Out = U _Out1 can. It must be ensured that this voltage is sufficient to put the series current regulator 44 in operation. As soon as the series-current regulator 44 comes into operation, it emits a larger current, so that accordingly the voltage U _V1 rises until the limiting action of the voltage-limiting circuit 42 starts. The voltage U _V1 then has the value U _V. The voltage regulator 46 is designed so that it, when the voltage value U _{V is} reached at the output of the series current regulator 44, no longer effective, but enters a locked state in which it no longer bridges the current regulator 44.

In FIG. 5 is shown a more detailed circuit diagram that shows how the individual components of the circuit of FIG. 4 can be constructed. It can be seen that the series current regulator 44 except for a difference yet explained, the voltage limiting circuit 42, the charge pump 38 and the voltage regulator circuit 40 as well as in the circuit of FIG. 3 are constructed. Only the voltage regulator 46 has been added, which, as the diagram shows, is constructed as a series voltage regulator. In the series-current regulator 44, the transistor T is not bridged by a resistor, as in the case of the series-current regulator 34. This resistance is not required in this case, since here the voltage regulator 46 makes it possible to start the circuit.

In the described embodiments, both the input voltage and the output voltage of the charge pump 38 are kept constant. On the input side, the aforementioned voltage limiting circuit 42 is used, which is nothing more than a parallel regulator. Also, the circuit used on the output side for keeping constant the output voltage of the charge pump 38 is a parallel regulator. But it is also possible to dispense with keeping constant the input voltage of the charge pump 38, which only requires to use a charge pump that can work with larger input voltages or input currents. When the input voltage of the charge pump is kept constant, it is possible to dispense with keeping the output voltage of the charge pump constant, if a load dependency of the output voltage can be tolerated. When using two voltage regulators at the input and the output of the charge pump 38, it is possible to form one of the two controllers as a longitudinal regulator. The desired effect of the entire circuit remains unaffected. The in the Figures 3 and 5 shown charge pump has a voltage transfer factor of 1/2, which means that it causes a halving of the voltage and a doubling of the current. Of course you can also charge pumps with others Voltage transfer factors are used, if other voltage and current conditions are desired. In the case of the arrangement described here, however, a transmission factor <1 is used in each case, since this can provide an increased current at the output of the charge pump.

The in the FIGS. 2 to 5 The circuits shown in two embodiments have the advantage that they can be constructed as integrated circuits and that they allow in a highly flexible manner to supply the various currents and voltages present in the transmitting station for the operation of the respective sensor and the processing circuit receiving its output signal needed. This excellent integrability of all variants is mainly due to the fact that in the circuits no inductors, but essentially only well integrable capacitors (<1nF) are used.

Claims (4)

1. Arrangement for signal transmission between a receiver station (10) and a transmitter station (12) and for the supply of power to the transmitter station (12), where these two stations are connected to one another by a two-wire cable (14) via which an analog signal current, which can alternate between two limit values, is transmitted and this signal current represents a measured value recorded by a sensor (20) in the transmitter station (12) and forms the power supply necessary to operate the transmitter station, whereby the transmitter station exhibits a circuit (24) that generates a constant operating voltage for the transmitter station (12) and where a controllable power source (34) is provided in the transmitter station (12) which determines the current flowing through the two-wire cable (14) depending on the measured value and which is powered from a supply voltage source in the receiver station, whereby the power source (34) is a serial current regulator which is powered from the supply voltage source (18) in the receiver station (10) and where the output of the power source (34) is connected to a charging pump (38) which generates - from the voltage occurring at the output of the power source (34) - the operating voltage which is necessary to operate the sensor (22) and a signal processing circuit (22) connected to the sensor, and where a parallel regulator (40, 42) is connected to the input or output of the charging pump (38) and is used to keep the input voltage or the output voltage of the charging pump (38) constant, characterized in that
   the power source (44) is bridged by a voltage regulator (46) which provides an input voltage (U_v1) for the charging pump (38) in a startup phase, where the power source (44) is designed in such a way that it does not deliver an output current in the startup phase until the charging pump (38) delivers an output voltage sufficient for its operation, where the voltage regulator (46) is designed in such a way that it changes to a locked state as soon as the output voltage (U_out) reaches the operating voltage.
2. Arrangement as per Claim 1, where another parallel regulator is provided in addition to the parallel regulator at the input or the output of the charging pump (38) on the other side of the charging pump (38).
3. Arrangement as per Claim 1, where a linear regulator is provided in addition to the parallel regulator at the input or the output of the charging pump (38) on the other side of the charging pump (38).
4. Arrangement as per one of the previous Claims, where the charging pump (38) has a transmission factor < 1.

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