GB2207571A

GB2207571A - Electrical isolation amplifier

Info

Publication number: GB2207571A
Application number: GB08717665A
Authority: GB
Inventors: Anton Lorenz Vrba
Original assignee: Crystalate Electronics Ltd
Current assignee: Crystalate Electronics Ltd
Priority date: 1987-07-25
Filing date: 1987-07-25
Publication date: 1989-02-01
Also published as: GB8717665D0

Abstract

A core (1) of magnetic material has first and second winding sets (2), (3), each comprising two interconnected windings (2A), (2B), and (3A), (3B), on it and a third winding set (4). A direct current input (I1) is converted into a first alternating current through the first winding set (2) by being alternately swtiched by two FET switches (9), (10), through windings (2A), (2B). A second direct current (I2) is converted into a second alternating current through the second winding set (3) by being alternately switched by two FET switches (18), (19) through the windings (3A), (3B). The second alternating current is arranged to oppose the first alternating current. An error signal, monitored at the third winding set (4), and resulting from an induced third current in this winding set (4) is demodulated (21), amplified and integrated (22), and applied to adjust the second current (I2) until the error signal at the third winding set (4) is reduced substantially to zero. The resulting second direct current output (I2) is directly proportional to, and isolated from, the direct current input (I1). <IMAGE>

Description

ISOLATION AMPLIFIER This invention relates to isolation amplifiers and particularly to an isolation amplifier of simple construction and excellent transfer linearity.

Isolation amplifiers are well known in industrial instrumentation and medical electronics and are essentially characterised by galvanic separation between input circuitry and output and power circuitry. Isolation amplifiers provided hitherto have involved one of three techniques, as follows. A first technique involves amplitude modulating a signal and using a transformer as a transmission medium. The maximum linearity attainable is directly governed by the magnetic properties of the transformer material and typical linearity is 0.01%.

In a second technique, pulse width modulation and demodulation is employed, involving either a transformer or optical coupling. Linearities of 0.005% are achievable but the equipment required is expensive.

In a third technique, differential optocouplers are employed, in which a light-emitting diode illuminates two photosensitive diodes. One of the photosensitive diodes, in an input circuit, is used as a control and the other is in an output circuit. Linearity is dependant upon the matching characteristics of the photosensitive diodes and is, at best, 0.07%.

In these prior art techniques, transfer between input and output circuits is by feed-forward techniques, ie there is no means of providing feedback from an ouput signal to an input signal to correct any errors in the output.

It is an object of the present invention to provide an isolation amplifier including a feedback means, such that improved linearity and stability is achieved using simple and inexpensive circuit arrangements.

The present invention provides an electrical isolation amplifier comprising: a core of magnetic material; a first wire winding set, a second wire winding set and a third wire winding set on said core; means connected to said first wire winding set and also adapted to be connected to input means for receiving a direct current input and arranged to provide, from said direct current input a first alternating current flowing through said first winding set; means to provide a second direct current and to convert this into a second alternating current to flow through said second winding set in such a way as to oppose the first alternating current flowing in the first winding set; means to monitor an electrical error signal at said third winding set resulting from an induced third electric current in said third winding set and to adjust said second direct current and hence said second alternating current such that there is substantially no net magnetic flux in said core and said third current is reduced substantially to zero; output means for the adjusted second direct current, said second direct current at said output means being directly proportional to, while being isolated from, said direct current input at said input means.

Suitably the means to provide from said direct current input a first alternating current flowing through the first winding set comprises one or more switching devices, eg semiconductor switching devices such as field effect transistor (FET) switches. In a particular embodiment, two said switching devices are provided, arranged to switch the direct current input alternately through two interconnected windings constituting the first winding set.

Suitably the means to convert the second direct current into a second alternating current to flow through the second winding set also comprises one or more switching devices, eg semiconductor switching devices such as field effect transistor (FET) switches. In a particular embodiment, two said switching devices are provided, arranged to switch the second direct current alternately through two interconnected windings constituting the second winding set.

Suitably the switching devices are operated by an isolated drive means, preferably comprising a transformer.

Suitably the means to monitor the electrical error signal at the third winding set and to adjust said second current comprises a synchronous demodulator, operating to demodulate the error signal at the third winding set, and a means for amplifying and conditioning the demodulated error signal, the amplified and conditioned error signal being applied to adjust the second current until the error signal at the third winding set is reduced substantially to zero.

If desired, multiple isolated direct current inputs may be provided, each with its own associated first winding set and direct current to alternating current conversion means, the second direct current at the output means representing the sum or difference of the separate inputs. By this means an isolated summing junction may be implemented.

The invention is now described by way of example with reference to the accompanying drawing which represents a schematic diagram of an isolation amplifier according to the invention.

A core 1 of magnetic material is provided with a first wire winding set 2, a second wire winding set 3 and a third wire winding set 4. The first wire winding set 2 comprises two windings 2A, 2B of equal numbers of turns, electrically connected in series-aiding manner with a junction 5 there between. The second wire winding set 3 comprises two windings 3A, 3B of equal numbers of turns, electrically connected in series-aiding manner with a junction 6 therebetween. Input terminals 7, 8 are provided for receiving a direct current input 11 from a source (not shown). Input terminal 7 is connected to the junction 5 between the two windings 2A, 2B of the first winding set 2. Input terminal 8 is connected through FET switches 9 and 10 to respective ends of windings 2A, 2B of the first winding set 2.A transformer 20 is provided comprising a core 11 with a primary winding 12, connected to an oscillator 13, and secondary windings 14, 15, 16 and 17. This transformer serves as an isolated gate drive for the FET switches 9 and 10. By means of the FET switches 9 and 10, the direct current input 11 is converted into an alternating current by being alternately switched through the windings 2A, 2B of the first winding set 2. A second direct current is applied to the junction 6 between the two windings 3A, 3B of the second winding set 3 and also to FET switches 18 and 19 which are connected to respective ends of windings 3A, 3B of the second winding set 3. The FET switches 18 and 19 are connected respectively to secondary windings 16 and 17 on the transformer 20.

By means of the FET switches 18 and 19 the second direct current is converted into an alternating current by being alternately switched through the windings 3A, 3B of the second winding set 3, but in such a way that it opposes the alternating current through the first winding set 2, derived from the direct current input Il It is required that the current through the second winding set 3 should oppose the current through the first winding set 2 in such a way that there is substantially no net magnetic flux in the core 1. This is achieved by detecting whether an error current is induced in the third winding set 4 provided on the core 1.Electrical error signals at the third winding set 4 are continuously monitored and if an error current is flowing in the third winding set 4, these signals are synchronously demodulated by a demodulator 21 and amplified and conditioned by a high gain amplifier 22, configured as an integrator, the output of which in turn controls the current through the second winding set by means of an FET 26 until no error current is induced in the third winding set 4. A continuous feedback process is therefore carried out.In the resulting condition in which there is no current induced in the third winding set 4, there is no net magnetic flux in the core 1 and the product of the current through the first winding set 2 and the number of turns of wire on the first winding set 2 is equal and opposite to the product of the current through the second winding set 3 and the number of turns of wire on the second winding set 3. The resulting second direct current 12 at output terminals 23 and 24 is directly proportional to, while being isolated from, the direct current input I1 at the input terminals 7 and 8.

A power supply 25 provides the necessary power for the system.

A resistor 27 is provided to compensate for the effects of non-ideal magnetic coupling between the first and second winding sets 2,3.

By means of the present invention, transfer non-linearity from current I1 to current I2 of better than 0.001% can be readily achieved while maintaining galvanic isolation between these currents.

If, required, multiple isolated direct current inputs Il (not shown) may be provided, each with its own associated first winding set and direct current to alternating current conversion means, the second direct current 12 at the output 23, 24 representing the sum or difference of the separate inputs. By this means an isolated summing junction is implemented.

Claims

1. An electrical isolation amplifier comprising: a core of magnetic material: a first wire winding set, a second wire winding set and a third wire winding set on said core; means connected to said first wire winding set and also adapted to be connected to input means for receiving a direct current input and arranged to provide, from said direct current input a first alternating current flowing through said first winding set; means to provide a second direct current and to convert this into a second alternating current to flow through said second winding set in such a way as to oppose the first alternating current flowing in the first winding set; means to monitor on electrical error signal at said third winding set resulting from an induced third electric current in said third winding set and to adjust said second direct current and hence said second alternating current such that there is substantially no net magnetic flux in said core and said third current is reduced substantially to zero; output means for the adjusted second direct current, said second direct current at said output means being directly proportional to, while being isolated from, said direct current input at said input means,

2. An isolation amplifier according to Claim 1 in which the means to provide from said direct current input a first alternating current flowing through the first winding set comprises one or more switching devices.

3. An isolation amplifier according to Claim 2 in which two said switching devices are provided, arranged to switch the direct current input alternately through two interconnected windings constituting the first winding set.

4. An isolation amplifier according to Claim 1, 2 or 3 in which the means to convert the second direct current into a second alternating current to flow through the second winding set comprises one or more switching devices.

5. An isolation amplifier according to Claim 4 in which two said switching devices are provided, arranged to switch the second direct current alternately through two interconnected windings constituting the second winding set.

6. An isolation amplifier according to any of Claims 2 to 5 in which said switching devices comprise semiconductor switching devices.

7. An isolation amplifier according to Claim 6 in which said semiconductor switching devices comprise field effect transistor (FET) switches.

8. An isolation amplifier according to any of Claims 2 to 8 in which said switching devices are operated by an isolated drive means.

9. An isolation amplifier according to Claim 8 in which said isolated drive means comprises a transformer.

10. An isolation amplifier according to any preceding Claim in which the means to monitor the electrical error signal at the third winding set and to adjust the second current comprises a synchronous demodulator, operating to demodulate the error signal at the third winding set, and a means for amplifying and conditioning the demodulated error signal, the amplified and conditioned error signal being applied to adjust the second current until the error signal at the third winding set is reduced substantially to zero.

11. An isolation amplifier according to any preceding Claim in which multiple isolated direct current inputs are provided, each with its own associated first winding set and direct current to alternating current conversion means, the second direct current at the output means representing the sum or difference of the separate inputs.

12. An electrical isolation amplifier constructed and arranged substantially as hereinbefore described with reference to the accompanying drawing.