EP4078793A1

EP4078793A1 - Circuit assembly for connecting to a current source

Info

Publication number: EP4078793A1
Application number: EP20811590.7A
Authority: EP
Inventors: Ralf Lobenstein; Harald SCHÄUBLE; Julian Bockstaller
Original assignee: Endress and Hauser SE and Co KG
Current assignee: Endress and Hauser SE and Co KG
Priority date: 2019-12-16
Filing date: 2020-11-20
Publication date: 2022-10-26
Also published as: DE102019134598A1; CN114762232A; WO2021121865A1; US20230023908A1

Abstract

The invention relates to a circuit assembly for connecting to a current source, preferably a 4-20 mA current loop or a high-ohmic voltage source, preferably a voltage source which has an internal resistance of more than 100 ohm, having: - at least one step-up converter with a coil (L1), a diode (D2), in particular a flyback diode, which is serially connected to the coil (L1), an output-side storage capacitor (C4) for adding an output voltage, and a switch element (V1) for switching the coil (L1) against ground; and - a circuit part for dynamically actuating the switch element of the step-up converter, wherein the circuit part is at least designed to actuate the circuit element of the step-up converter in a start-up phase such that the current source directly charges the storage capacitor via the coil (L1) until a specifiable reference value is reached.

Description

Circuit arrangement for connection to a power source

The invention relates to a circuit arrangement for connection to a power source and a field device in automation technology.

In process automation technology, field devices are often used that are used to record and / or influence process variables. To record process variables, sensors such as level measuring devices, flow measuring devices, pressure and temperature measuring devices, pFI redox potential measuring devices, conductivity measuring devices, etc., which record the corresponding process variables level, flow rate, pressure, temperature, pH value or conductivity, are used. In principle, all devices that are used close to the process and that supply or process process-relevant information are referred to as field devices.

A large number of such field devices are manufactured and sold by Endress + Hauser.

Currently, two-wire field devices are still common in a large number of existing automation systems, which are connected to a higher-level unit, for example a control unit PLC, via a two-wire line or loop. The two-wire field devices are designed in such a way that power is supplied via the two-wire line using a 4-20 mA signal. Due to the energy supply from the 4-20 mA signal, the field device has only very limited internal energy available. The result of this is that the individual electrical components of the field device are coordinated with one another in such a way that the total energy requirement of the field device just does not exceed the energy made available via the two-wire line. In order to still be able to operate electrical or electronic components that briefly have a higher energy requirement than is actually available, (storage) capacitors with corresponding capacities are usually used. In order to have enough energy available to charge such capacitors, the smallest possible voltage drop across the two-wire line is used as an energy source and set the tension high. So-called boost converters (also called boost converters or step-up converters) are usually used here. The start-up phase is particularly problematic when operating such boost converters on a current source, such as a two-wire line, for example, or a high-resistance voltage source. If an operating voltage reaches a switch-on threshold of the step-up converter, the result is an increased current flow, which in turn causes the operating voltage to drop below the switch-on threshold and can lead to the step-up converter switching off. A safe switch-on or start-up of the boost converter is therefore at risk.

It is therefore an object of the invention to propose a circuit arrangement which enables a step-up converter to be switched on or started up reliably.

The object is achieved according to the invention by the circuit arrangement according to claim 1 and the field device according to claim 13.

The circuit arrangement according to the invention for connection to a current source, preferably a 4-20 mA current loop and / or a voltage source, preferably one comprising an internal resistance greater than or equal to 100 ohms (Ri> 100 ohms), comprises:

- At least one step-up converter with a coil, a diode, in particular a freewheeling diode, which is connected in series with the coil, an output-side storage capacitor for adding up an output voltage and a switching element for switching the coil to a ground;

- A circuit part for dynamic control of the switching element of the step-up converter, the circuit part being designed at least to control the circuit element of the step-up converter in a start-up phase in such a way that the current source charges the storage capacitor directly via the coil until a predeterminable or adjustable switching threshold is reached. According to the invention, a circuit arrangement is proposed which is designed for dynamic operation. This means that the circuit starts up safely even when the capacitor is completely discharged and can then easily switch to the actual boost mode in an operating phase following a start-up phase. The storage capacity is charged directly from the power source during the start-up phase.

According to an advantageous embodiment of the circuit arrangement according to the invention, the circuit part is also designed to control the circuit element of the step-up converter after reaching the switching threshold in an operating phase following the start-up phase so that the coil is cyclically switched to ground. According to the embodiment, the step-up converter is only activated or active when the reference voltage at TP1 is reached.

According to a further advantageous embodiment of the circuit arrangement according to the invention, the circuit part comprises a comparator and a capacitor, the comparator for dynamic control being connected with an output to the switching element and to which a reference voltage for specifying or setting an inverting input.

Setting the switching threshold is applied and the capacitor for voltage stabilization is connected in parallel to the current loop.

In particular, the configuration of the circuit arrangement according to the invention can provide that the circuit part further comprises a voltage divider which is connected to the current source with a resistor and to ground with a further resistor, the comparator having a non-inverting input to the voltage divider between the two Resistance is connected, so that a substantially constant voltage drops across the power source in the operating phase. The voltage drop across the current source or the 4-20 mA current loop can be set using the voltage divider or manipulated using an optional additional circuit. Additionally or alternatively, the embodiment can provide that the comparator has an internal reference module which provides the reference voltage. According to a As an alternative embodiment, the circuit arrangement can have a microcontroller that provides the reference voltage.

According to a further advantageous embodiment of the circuit arrangement according to the invention, the step-up converter comprises a further capacitor which is connected in parallel to the storage capacitor. In particular, the embodiment can provide that the further capacitor has a capacitance value between 10 and 500 pF, preferably between 50 and 250 pF, particularly preferably between 80 and 150 pF, very particularly preferably 100 pF.

According to a further advantageous embodiment of the circuit arrangement according to the invention, the circuit part further comprises a Zener diode which is connected in parallel to the current source.

According to a further advantageous embodiment of the circuit arrangement according to the invention, the circuit part further comprises a capacitor and a further resistor, the capacitor being connected in parallel to the circuit source and the further resistor in series with the coil and the diode.

According to a further advantageous embodiment of the circuit arrangement according to the invention, a further Zener diode is provided which is connected in parallel to the storage capacitor.

According to a further advantageous embodiment of the circuit arrangement according to the invention, the comparator according to FIG. 2 is connected to a positive supply voltage pin to the power source and to a negative supply voltage pin to ground, a low-pass filter preferably being connected to the positive supply voltage pin to stabilize the voltage supply of the comparator according to FIG. 2.

The invention further relates to a field device in automation technology comprising a pair of connecting terminals for connecting the field device to a 4-20 mA current loop, a radio module for wireless communication with the field device and a circuit arrangement according to at least one of the preceding claims, wherein the circuit arrangement is connected on the input side to the connection terminal pair and on the output side to the radio module. The radio module is preferably an NB-loT (Narrowband Internet of Things) radio module. NB-loT is a low power wide area network radio technology standard developed by 3GPP that enables a wide range of mobile radio devices and services. The specification was frozen in 3GPP Release 13 in June 2016.

The invention is explained in more detail with reference to the following drawings.

It shows:

Fig. 1: a first embodiment of a circuit arrangement according to the invention, and

Fig. 2: a second embodiment of the circuit arrangement according to the invention.

Figure 1 shows a first embodiment of a circuit arrangement according to the invention. The circuit arrangement is fed on the input side from a 4-20 mA current loop (represented symbolically in FIG. 1 by a circuit symbol for a current source). In particular, the circuit arrangement can be used in a field device in automation technology. In particular, the circuit arrangement can be used in a field device to supply a radio module, for example an NB-loT module, with energy from the current loop. Such an NB-loT module has a very high energy requirement for a short time, which can only be absorbed by a large storage capacitor, for example a storage capacitor with a capacity of approx. 5 F. In order to have enough energy available to charge the storage capacitor, the smallest possible voltage drop from the 4-20 mA current loop is used as an energy source by the circuit arrangement and the voltage is increased by a step-up converter. Alternatively, the circuit arrangement can also be connected to a current source or a (high-resistance) voltage source having an internal resistance greater than or equal to 100 ohms (Ri> 100 ohms). Another possibility is to connect the circuit arrangement to a voltage source and a current source connected in series therewith.

In Fig. 1, a basic variant of the circuit arrangement according to the invention is shown, which is required for operating the circuit. In addition, as shown in FIG. 2, the circuit arrangement can have further electronic circuit elements / components which bring about advantageous effects.

The circuit arrangement according to the invention shown in FIG. 1 comprises at least one step-up converter 1 and a circuit part for dynamic control 2 of the step-up converter 1 in order to overcome the problematic start-up phase mentioned at the beginning.

The step-up converter 1 consists of a coil L1 connected in series with a diode D2 with an inductance and a suitable switching element V1 which switches the coil to ground. The switching element V1 can for example be a MOSFET, which is dynamically switched at its gate by the circuit part for driving 2 of the boost converter 1. The boost converter 1 also has a storage capacitor C4 on the output side for adding up the storage energy at the output UA.

In addition, as shown in FIG. 2, a third capacitor C3 can be provided, which is connected in parallel with the storage capacitor C4. The third capacitor C3 can in particular have a capacitance of a few 10 pF to a few 100 pF, preferably approx. 100 pF (microfarads). As a result, output voltage peaks at the storage capacitor C4, due to its low internal inductance, small equivalent series resistance and short electrical connection, can be reduced. In addition or as an alternative, as also shown in FIG. 2, a third Zener diode D3 can also be used in parallel with the third capacitor C3 Storage capacitor C4 must be connected in order to protect it from overvoltage.

The circuit part for dynamic control 2 of the boost converter 1 comprises, according to the basic variant, a comparator N1, whose output is connected to the gate of the MOSFET, a voltage divider R2, R3 for switching the comparator N1 and a second capacitor C2 for voltage stabilization, which is parallel to the Current loop is switched. The MOSFET has a drain connected to an anode of the second diode D2 and a source connected to the ground. The voltage divider R2, R3 is connected to the second resistor R2 on the current loop and connected to the third resistor R3 to ground. The voltage tap for a plus input (non-inverting input) of the comparator takes place between the second and third resistor. The voltage divider also defines a voltage drop in the current loop. For example, the voltage divider R2, R3 can be dimensioned in such a way that approx. 1.8 V drop across it.

The actual switching threshold of the comparator is determined by applying a reference voltage to a minus input (inverting input) of the comparator. The reference voltage U _R e _f can be provided, for example, by an internal reference voltage module of the comparator N1. Alternatively, the reference voltage U _R e _{f can} also be provided externally, for example by a microcontroller integrated in field device electronics. The reference voltage can be 1.2 V, for example. The comparator N1 is also contacted with a positive supply voltage pin towards the current loop and with a negative supply voltage pin towards ground. To stabilize the voltage supply for the comparator, a fourth resistor R4 and a fifth capacitor C5 can also be connected to the positive supply voltage pin as a low-pass filter.

A voltage drop across the power source can be set by manipulating the voltage divider R2, R3 or the reference voltage at the comparator N1. This is particularly the case that the circuit arrangement is connected to a 4-20 mA current loop, advantageous since the voltage drop can thus be kept small, preferably less than 2.5V, particularly preferably less than 2 V, very particularly preferably approx. 1.8V. This can be done, for example, using a microcontroller port, pulse width modulation with a low-pass filter or a digital-to-analog converter. This allows the charging time of the storage capacitor C4 to be influenced.

In addition, as shown in FIG. 2, a first Zener diode D1 can be connected in parallel with the second capacitor C2 in order to protect the circuit arrangement. In addition or as an alternative to this, a further first capacitor C1 and a first resistor R1 can also be provided on the input side, which serve as filters. The first capacitor C1 can be connected in parallel with the first Zener diode D1 and the first resistor R1 can be connected in series with the coil L1 and the diode D2. Both circuit elements serve to reduce possible interference to the current loop. The first and second capacitors are preferably dimensioned such that the second capacitor C2 has a higher capacitance than the first capacitor C1. The second capacitor C2 is charged up to the switching threshold (switch-on threshold) of the comparator N1 and, after switching on the MOSFET, is discharged again to the switching threshold (switch-off threshold) of the comparator N1.

After switching on, a loop current flows through the circuit arrangement in a start-up phase and slowly charges the capacitors C1 to C4. The voltage at the output capacitors C3, C4 is lower by a forward voltage of the second diode (Schottky diode) D2 and the voltage drops across the series impedances of the coil L1 and the first resistor R1. At the moment of switch-on, the supply voltage at the comparator N1 is initially still below the switching threshold (switch-on threshold), so that the MOSFET located at the output of the comparator is initially switched to non-conductive. If the minimum operating voltage of the comparator N1 is reached, it becomes active, but the reference voltage has not yet been reached at the non-inverting input, so that the output remains on "Low" and the MOSFET is still non-conductive. If the reference voltage at the non-inverting input is reached or exceeded, the MOSFET V1 is switched on by the comparator and the inductance L1 is charged to ground, but only until the voltage at the non-inverting input of the comparator N1 falls below the reference voltage again. This leads to the MOSFET switching nonconductively again.

In order to achieve the highest possible switching frequency, a flysteresis of the comparator should be as small as possible. For example, the comparator should be chosen in such a way that the hysteresis is only a few millivolts (mV), e.g. approx. 40 mV. In this way, the storage capacitor C4 is slowly charged. The ripple behind the resistor R1 corresponds to the switching hysteresis of the comparator N1. The influence on the current loop can optionally be reduced by the resistor R1 and the capacitor C1. The charging process of the capacitors C3 / C4 can optionally be limited by the Zener diode D3 and thus protected against overvoltage.

If a high load is connected in parallel to C4, it discharges slowly because the power from the current loop is not sufficient to maintain the charge at C4. After switching off the high load, C4 is charged again. The output voltage of the circuit is therefore subject to fluctuations. A stabilization of the output voltage UA could be necessary. For this purpose, the circuit arrangement can have a linear regulator or a DC / DC converter, for example.

List of reference symbols

1 boost converter

2 Circuit part for dynamic control of the

Boost converter

D1 Zener diode

R1 resistor

C1 capacitor

C2 capacitor

R2 resistor

R3 resistance

N1 comparator

V1 switching element, for example MOSFET

L1 coil

D2 Zener diode

C3 capacitor

C4 storage capacitor

U _A output voltage

UE input voltage or drop over 4-20 mA current loop

R4 Fourth resistance

C5 Fifth capacitor

URef reference voltage

Claims

1.Circuit arrangement for connection to a current source, preferably a 4-20 mA current loop and / or a voltage source, preferably one comprising an internal resistance greater than or equal to 100 ohms:

- At least one step-up converter with a coil (L1), a diode (D2), in particular a freewheeling diode, which is connected in series with the coil (L1), an output-side storage capacitor (C4) for adding up an output voltage and a switching element (V1) for Switching the coil (L1) to a ground;

- A circuit part for dynamic control of the switching element of the step-up converter, the circuit part being designed at least to control the circuit element of the step-up converter in a start-up phase in such a way that the current source directs the storage capacitor via the coil (L1) until a predeterminable or adjustable switching threshold is reached loads.

2. Circuit arrangement according to claim 1, wherein the circuit part is further designed to control the circuit element of the step-up converter after reaching the switching threshold in an operating phase following the start-up phase so that the coil is cyclically switched to ground.

3. Circuit arrangement according to at least one of the preceding claims, wherein the circuit part comprises a comparator and a capacitor (C2), wherein the comparator (N1) is connected for dynamic control with an output to the switching element and to which at an inverting input a reference voltage for Presetting or setting the switching threshold is applied and the capacitor (C2) is connected in parallel to the current loop for voltage stabilization.

4. Circuit arrangement according to claim 2 and 3, wherein the circuit part further comprises a voltage divider with a resistor (R2) to the Current source and with a further resistor (R3) is connected to ground, the comparator being connected with a non-inverting input to the voltage divider between the two resistors, so that an essentially constant voltage drops across the current source in the operating phase.

5. Circuit arrangement according to one of claims 3 or 4, wherein the comparator has an internal reference module which provides the reference voltage.

6. Circuit arrangement according to one of claims 3 or 4, further comprising a microcontroller which provides the reference voltage.

7. Circuit arrangement according to at least one of the preceding claims, wherein the step-up converter comprises a further capacitor (C3) which is connected in parallel to the storage capacitor (C4).

8. Circuit arrangement according to the preceding claim, wherein the further capacitor (C3) has a capacitance value between 10 and 500 pF, preferably between 50 and 250 pF, particularly preferably between 80 and 150 pF, very particularly preferably 100 pF.

9. Circuit arrangement according to at least one of the preceding claims, wherein the circuit part further comprises a Zener diode (D1) which is connected in parallel to the current source.

10. Circuit arrangement according to at least one of the preceding claims, wherein the circuit part further comprises a capacitor (C1) and a further resistor (R1), the capacitor in parallel with the circuit source and the further resistor in series with the coil (L1) and the diode (D2) is switched.

11. Circuit arrangement according to at least one of the preceding claims, wherein a further Zener diode (D3) is provided, which is connected in parallel to the storage capacitor (C4).

12. Circuit arrangement according to at least one of the preceding claims, wherein the comparator according to Figure 2 is connected to a positive supply voltage pin to the power source and with a negative supply voltage pin to ground, preferably a low-pass filter to the positive supply voltage pin to stabilize the voltage supply of the comparator according to Figure 2 connected.

13. Field device of automation technology comprising a pair of connecting terminals for connecting the field device to a 4-20 mA current loop, a radio module for wireless communication with the field device, in particular an NB-loT radio module and a circuit arrangement according to at least one of the preceding claims, the circuit arrangement on the input side is connected to the pair of connection terminals and to the radio module on the output side.