AU691936B2

AU691936B2 - Servo-amplifier for controlling a high-ohmic low-voltage source

Info

Publication number: AU691936B2
Application number: AU28798/95A
Authority: AU
Inventors: Norbert Fichtner; Walter Stieglbauer; Hermann Zierhut
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1994-07-21
Filing date: 1995-07-10
Publication date: 1998-05-28
Anticipated expiration: 2015-07-10
Also published as: DE59503721D1; DE4425901A1; EP0771492A1; ATE171575T1; JPH10503068A; CN1088291C; EP0771492B1; CA2195442A1; WO1996003806A1; CN1150505A; AU2879895A

Abstract

A servo-amplifier for controlling a high-ohmic low-voltage source controllable between two boundary values of a pulse-width-modulated control signal, in which the input side has a galvanic separation and there is an operating voltage supply for the servo-amplifier. In an isolating transformer (1) for galvanic isolation, a circuit (with the components 4, 5, 6, 7) for finding the average of the pulse-width-modulated control signal for imaging the control signal is connected to a secondary winding (3) and the latter is also involved in a voltage multiplication circuit (17) to provide the operating voltage for the servo-amplifier. The servo-amplifier can in particular be protected against incorrect connection.

Description

1 Control amplifier for controlling a high-impedence lowvoltage source The invention relates to a control amplifier for controlling a low-voltage source having internal impedence, which low-voltage source is connected to two output terminals and can be controlled between two limit values by means of impedence changing, in detail as claimed in the precharacterizing clause of patent claim 1.

The input side has DC isolation by means of an isolating transformer, and an operating voltage supply for the control amplifier is provided on the secondary side.

There are a very wide range of different fields

S.

15 of application for control amplifiers for controlling a Slow-voltage source. For example, such a control amplifier is required in order to drive electronic ballasts, called EVGs in German, which can be dimmed. High-impedence lowvoltage sources are also spoken of in this context. For 20 example, a digital signal for a dimming process is supplied via a bus from building system engineering, via a bus coupler and the control amplifier, to a switchdimmer actuator. The switch-dimmer actuator converts the digital signal into a pulse-width-modulated signal, and the control amplifier uses this- to provide a constantly present analog signal which is supplied to an electronic ballast which can be dimmed, in order to achieve a specific dimming level for a fluorescent lamp. The control signals for the dimming can also be supplied via an infrared controller, rather than via a bus. In this case, control amplifiers are required which are controlled between two limit values of the pulse-widthmodulated signal.

11111"111 311-~1111 la An amplifier is thus required such that a voltage source having an internal impedence is adjusted in the range of a few volts by changing the impedence. Such a voltage source is present in the case of electronic ballasts which can be dimmed. Furthermore, there is intended to be DC isolation between the control signal and the amplifier to be driven.

In the case of a known control circuit, the control pulses are supplied in a DC-isolated manner (FR-A-2 615 676). In this case, the control signal and operating voltage for controlling an operational amplifier are supplied via a transformer. The secondarywinding of the transformer is used to provide the operating voltage. The information is in this case contained in S 15 the amplitude.

S*e 9 o *9 I 0 The invention is based on thc object of developing a control amplifier which provides a constantly present analog signal at the Output from a pulse-width-inodulated signal and which satisfies stringent insulation requirements, as exist, for example, in the case of the EIBA bus from the European Installation Bus Association.

8 According to one aspect of the present invention there is provided a control amplifier circuit for controlling a low-voltage source said circuit including: an isolating transformer for providing both DC isolation to an input side of said control amplifier circuit, and an operating voltage to said control amplifier circuit; a generating circuit connected to a secondary winding of said isolating transformer, said generating circuit generating a DC voltage mean value from a pulsewidth-modulated control signal, wherein said DC voltage mean value is a reference .6::value; an operational amplifier circuit for producing an impedance change, wherein *w the impedance change is controlled in such a manner that an actual value and said reference value are matched to one another; and :6:6.:two output terminals wherein said control amplifier circuit controls a voltage 66 present at said output terminals and said voltage is said actual value, and said low voltage source is connected to said output terminals and can be controlled between two limit values by means of said impedance changing, In this case, a circuit for forming the mean value from the pulse-widthmodulated control signal is connected to a secondary winding of an isolating transformer for DC isolation, in order to form the control signal. The secondary winding can also be included in a voltage multiplier circuit for providing the operating voltage for the control amplifier. In the case of applications -for the ELBA bus, the control amplifier can be designed for the purpose of processing control signals of the input side in the order of magnitude of one to ten volts. The control amplifier can advantageously operate with, in its output stage, a field-effect transistor, FET, whose off-state voltage is at least 400 volts, An off-state voltage of 700 volts is particularly jNAL1Dppj0091 7:JJP 2o- advantageous, in order to avoid malfunctions. The control amplifier can be designed as a component of a switch-dimmer actuator, According to a development, the control amplifier is protected against overvoltages by means of a measure wherein the FET is driven by a feedback circuit in such a manner that it is switched to the off-state if the nominal output voltage is exceeded. The field-effect transistor in the output stage is in this case driven by a feedback circuit in such a manner that i, is switched to the off state if the normal output voltage is exceeded as a result of a voltage supplied to the control amplifier on the output side. This takes place, for example, if an installation mains voltage of 230 volts is applied, for example, instead of the high-impedence low-voltage source in the order of magnitude of 10 volts. By means of a resistor and/or an inductor in the source line 0@* of the FET, it is easily possible to adjust the maximum permissible drain current in the event of incorrect wiring on an incorrect i* 9 t i 0 IN:\LIBpp)OO17:JJP 94 P 3387 3 connection. The control amplifier is then protected against incorrect connection.

It is advantageous to protect the field-effect transistor against the consequences of its permissible off-state voltage being exceeded. For this purpose, the field-effect transistor can be driven by a feedback circuit in such a manner that, if overvoltages are present on the input side, which lead to the permissible offstate voltage of the field-effect transistor being exceeded, this field-effect transistor is briefly switched on. It is simpler and more advantageous to choose the field-effect transistor to be avalanche-resistant.

It is favorable to connect an impedence, irrespective of whether this is a resistive or an inductive 15 impedence, in series with the field-effect transistor in order to achieve current limiting, until the field-effect transistor is switched to the off state.

The invention will now be explained in more "detail with reference to an exemplary embodiment which is 20 shown schematically and roughly in the drawing: On the input side, the control amplifier has an isolating transformer 1 for DC isolation, in series with which isolating transformer 1 a capacitor 2 for DC voltage isolation is advantageously connected for decoupling DC voltage. A circuit for forming the mean value from the pulse-width-modulated control signal which is present on the input side is connected to the secondary winding 3 of the isolating transformer 1. The formation of the mean value is carried out in the exemplary embodiment by means of a diode 4 and a capacitance 5 as well as the resistors 6 and 7. The mean value which is formed from the pulsewidth-modulated signal is present as the reference value at the input 8 of an operational amplifier 9. A respective actual value is supplied to the negative control input 10 of the operational amplifier 9 from the voltage divider having the resistors 11 and 12, these resistors 11 and 12 being located in a feedback line 14. Via its output 13, the operational amplifier 9 controls the FET 19 in such a manner

~II

94 P 3387 4 that the actual value and reference value are matched to one another. The term formation of a mean value is in this case intended also to include other circuits which act in an identical manner overall.

The operating voltage inputs 15 and 16 of the operational amplifier 9 receive their operating voltage from a voltage multiplier circuit 17, a voltage doubler circuit in the exemplary embodiment.

In the exemplary embodiment, the gate of a fieldeffect transistor 19, also called an FET, is connected to the output 13 of the operational amplifier 9 via a protective resistor 18. A constant analog signal, voltage or current, which is controlled by the FET is then present.

at the output terminals 20 and 21. The FET 19 is driven by a feedback circuit having the components zener diode 22, resistors 23, 24 and 25 as well as the transistor 26 in such a manner that the FET 19 is switched to the off state if the nominal output voltage is exceeded. When 20 the zener diode 22 is forward-biassed, the 'transistor 26 20 draws current via its base-emitter junction so that it is switched to the on state and the gate electrode of FET 19 is connected to ground. As long as the transistor 26 is not drawing any current, the resistor 25 ensures that it is switched off in the stable manner.

25 If no control voltage is present on the FET 19, the resistor 27 ensures that the gate is at reference Sground. potential. An impedence 28 in the source line of the FET 19 ensures, in particular in conjunction with a resistor 29 and the transistor 26, current limiting by the FET until it can be protected against overloading as a result of overvoltages by means of its feedback circuit, which acts as a protection circuit and has the resistors 23, 24 and 25 as well as the transistor 26. In this case, the transistor 26 carries out a double function. If, for example, 0.7 volts, the threshold voltage of the base-emitter junction of the transmitter 26, is exceeded across the resistor 28, the transistor 26

I

94 P 3387 5 drives the gate of the FET 19 closer to reference ground potential and drives the FET increasingly into the off state.

In the exemplary embodiment, a diode 30 protects the control amplifier against accidental connection to installation voltage, that is to say for example 230 volts, in that only a DC voltage which is compatible with it is supplied to the FET 19, and this DC voltage causes the protection mechanisms described above to act.

The control amplifier is based on the principle of obtaining its control voltage and its operating voltage from a control voltage supplied on the input side.

It is favorable to select an avalanche-resistant 15 FET for protection against the off-state voltage of the FET 19 being exceeded. On the other hand, the FET can also be driven by a further feedback circuit, which is not shown in the exemplary embodiment, in such a manner .that, if overvoltages which would lead to the permissible 20 off-state voltage of the FET being exceeded are present on the output side of the control amplifier, this FET is briefly switched on again.

Claims

1. A control amplifier circuit for controlling a low-voltage source said circuit including: an isolating transformer for providing both DC isolation to an input side of said control amplifier circuit, and an operating voltage to said control amplifier circuit; a generating circuit connected to a secondary winding of said isolating transformer, said generating circuit generating a DC voltage mean value from a pulse- width-modulated control signal, wherein said DC voltage mean value is a reference o1 value; an operational amplifier circuit for producing an impedance change, wherein the impedance change is controlled in such a manner that an actual value and said reference value are matched to one another; and two output terminals wherein said control amplifier circuit controls a voltage present at said output terminals and said voltage is said actual value, and said low voltage source is connected to said output terminals and can be controlled between two limit values by means of said impedance changing.

2. The control amplifier as claimed in claim 1, wherein, for the actual value in the order of magnitude from 1 to 10 volts, the operational amplifier circuit has as the output stage a field-effect transistor FET, whose off-state voltage is at least 400 volts.

3. The control amplifier as claimed in claim 2, wherein the FET is driven by a feedback circuit in such a manner that it is switched to the off-state if the nominal output voltage is exceeded.

4. The control amplifier as claimed in claim 2 or 3, wherein the FET is driven by a feedback circuit in such a manner that, in the event of overvoltages being applied on the input side to the control amplifier which would lead to the permissible off-state voltage of the FET being exceeded this FET is switched on briefly. 'N:\LIBppjOO917:JJP IILI~ IIICIIIII~II~~---~ 1- The control amplifier as claimed in 2 or 3, wherein the FET is selected to be avalanche-resistant.

6. The control amplifier as claimed in one of claims 2 to 5, wherein the FET has an impedance connected in its source line.

7. A control amplifier, substantially as herein described with reference to the accompanying drawing. DATED this Thirtieth Day of March 1998 Siemens Aktiengesllschaft Patent Attorneys for the Applicant SPRUSON FERGUSON l0 *ft o [N:\LIBppl00917:JJP r II L 94 P 3387 Abstract Control amplifier for controlling a high-impedence low- voltage source Control amplifier for controlling a high-imped- ence low-voltage source which can be controlled between two limit values of a pulse-width-modulated control signal, the input side having DC isolation and an operat- ing voltage supply being provided for the control ampli- fier. It is envisaged that a circuit for forming the mean value (having the components 4, 5, 6, 7) from the pulse- width-modulated control signal is connected to a secondary winding of the isolating transformer (1) for DC isolation, in order to form the control signal, and in that the secondary winding is also included in a voltage multiplier circuit (17) for providing the operating voltage for the control amplifier. The control amplifier can, in particular, be protected against incorrect connection. FIG 1 I