WO1993026089A1

WO1993026089A1 - Isolated error-amplifying control circuitry

Info

Publication number: WO1993026089A1
Application number: PCT/US1993/005188
Authority: WO
Inventors: James M. Simonelli; Zeljko Arbanas
Original assignee: Digital Equipment Corporation
Priority date: 1992-06-10
Filing date: 1993-06-01
Publication date: 1993-12-23

Abstract

A galvanically isolated error amplifier using Hall effect principles for application in control circuitry. Input currents to be compared are conducted through respective windings around a gapped ferrite toroidal core. A standard Hall effect magnetic field sensor/amplifier positioned in the gap provides a voltage signal proportional to the difference in the current inputs.

Description

ISOLATED ERROR-AMPLIFYING CONTROL CIRCUITRY

Field of the Invention

This invention is related generally to the field of control circuitry in electronic equipment, particularly in electrical power supply equipment.

Background of the Invention

A common problem in control system design is the need to transfer a control signal from a circuit which is referenced at a potential which is different from the potential of the receiving circuit. Particularly in power supplies, and especially in "intelligent" power supplies, this often requires the transfer of a signal from a safe potential to a hazardous potential.

Transformers are often used for the isolation task, but transformers cannot transfer a DC component. Thus, if the signal contains a DC component, as many control signals do. other means must be used for isolated transfer of the control signal. Such means have included modulation systems, which are complex and expensive, and opto-couplers, which are difficult to implement in an open loop transfer system.

Hall effect devices have been used previously to detect and isolate a current signal. Fig. 1 is a simplified drawing of a commercially available Hall effect current sensor, and Fig. 2 is a signal diagram for its application as a current detector. Thus, the current to be sensed is passed through the primary circuit 11. This causes a compensating magnetic field created by the secondary winding 12 that is equal in ampere-turns and opposite in polarity to the primary. The magnetic field is detected at Hall effect magnetic field detector 13, which is amplified and compensated by circuit 14. and output at 16. Power for circuit 14 is supplied at terminals 15 and 17, and the circuit is simplified as the loop shown in Fig. 2. including amplifier 23. The loop relationship is given as:

N_p * I,, = N_s * I, where N_p is the number of primary turns 22 (1 for current sensing), I is the current to be measured at input 21. N_s is the number of secondary turns 25 of secondary winding 12, and I_s is the output current, measured at output 24. The current to be sensed, i.e., I-, is counterbalanced by a controllable secondary current I- so that the net flux sensed by the Hall effect device is zero. I- is controlled to create the zero flux situation for the Hall effect device.

Thus, the prior art Hall effect current sensor shown in Figs,. 1 and 2 is a two-port device with a current sensing input 11 or 21 and an output 16 or 24. The input to secondary winding 12 is from the detection circuitry 14. typically at a low power appropriate to circuitry in a self-contained sensor module, and is not controlled by the user. The input to secondary winding 12 is dedicated to the feedback from the output of amplifier 23 and is varied according to the current in primary circuit 11, which it measures. There is no provision to compare with each other two inputs from outside the prior art module.

In many control circuits, exemplified by those used in power supplies, an error amplifier is used to subtract the difference between two signals. Typically this is done using an operational amplifier connected as a difference amplifier.

Where both isolation and error amplification are required in the same control signal path, a traditional solution would be to use one or more of the isolation devices mentioned previously and the difference amplifier circuit. In the case of a control circuit for a 400 VDC to 120/240 VAC inverter, the implementation would call for the use of two Hall effect current sensors like that shown in Fig. 1 to isolate the control signal and to sense the inductor current. These devices together with differential amplifier circuitry currently cost on the order of $30-40.

Summarv of the Invention

The invention in its broad form resides in a method for comparing a current to be sensed with a reference current to obtain a differential signal, as recited in claim 1. The invention also resides in apparatus as recited in claim 4. Described hereinafter is a novel use of a Hall effect magnetic field sensor/amplifier that is used not only as an isolation device for a control signal referenced to a different potential than the circuit using the information, but as an error amplifier in the process of transmitting that information. In particular, a relatively large current at high voltages may be controlled by an isolated signal referenced to a different voltage.

The use of the invention in the control circuit mentioned above costs less than $10. less than a third of the traditional approach.

Brief Description of the Drawings

A more detailed understanding of the invention can be had from the following description of preferred embodiments, given by way of example and to be understood in conjunction with the accompanying drawing wherein:

Fig. 1 is a simplified diagram of a commercial Hall effect current sensor/amplifier, including a Hall effect magnetic field sensor.

Fig. 2 is a block signal diagram of a commercial Hall effect current sensor.

Fig. 3 is a small signal diagram of an exemplary use of an embodiment of the invention in a DC/AC inverter circuit. Fig. 4 is a block diagram of an embodiment of the invention.

Fig. 5 is a representation of the embodiment.

Detailed Description of an Illustrative Embodiment

Fig. 3 shows a small signal block diagram of the control system used to control the 400 VDC to 120/240 VAC inverter mentioned above. The notations i, and i-._ef respectively refer to the inductor current and reference current. Block 31 is a voltage sensing transformer, and block 32 is a voltage difference sensing amplifier. Block 33 is a load current sense transformer, and block 34 is a current difference sensing transformer. Block 35 is a power operational amplifier for voltage to current conversion. Block 36 shows the number of turns on the Hall sense core, and block 37 is the Hall sense gain. Block 38 shows the inverter gain, and block 39 shows the PWM ramp voltage. The difference between these i, and i_ref, the error, is required for the operation of the circuit. It should be noted that i-._ef is at a safe potential level while i, is at a hazardous potential. The invention is used in this application to perform a required subtraction and isolate the reference signal i-._ef from i,,

Fig. 4 is a block diagram of a preferred embodiment of the invention. The isolated error amplifier 40 is a three-port device, with input 1-,.-,^ at port 41. input I_ref at port 42. and output V_crτ0. at port 47. The physical implementation is shown in Fig. 5. A gapped ferrite toroid 53 is wound with a sense winding 51 and a control or reference winding 52. The difference between the ampere-turns of current flowing in these windings is detected and amplified at the Hall effect magnetic field sensor/amplifier 54, which may be of the commercial type corresponding to Hall effect sensor 13 and circuit 14 (without the feedback to the secondary winding 12) shown in Fig. 1 for a prior art Hall effect current sensor/amplifier.

The Hall effect magnetic field sensor/amplifier 54 is a transducer which puts out a voltage that is proportional to the strength of the magnetic field which is sensed at its input sensor, typically on the order of 3-7 mV/Gauss. The gapped ferrite toroid 53 functions as a magnetic toroidal core and is used to concentrate the magnetic flux to the Hall effect sensor/amplifier input which is placed in the flux concentrating gap of the toroid. The size of the toroid also plays a role in determining the transfer function.

In the case of prior an apparatus, in as much as the apparatus of Fig. 1 is controlled to operate around a zero flux state, the cross sectional area of the magnetic toroidal core (on which the secondary winding 12 is wound) can be minimal. Depending on the permeability of the magnetic material of the toroidal core, the cross section of the toroidal core can be chosen to handle practically a no flux or zero flux state.

The control winding 52 is a multi-turn winding which is wound on the toroid (250 turns in the example of block 36 in Fig. 3). Current through this winding will produce a flux in the toroid which can be determined by the following approximate relationship:

H ~~ N * and

B = μ_eH where H is the magnetic field intensity (Orstead), B is the magnetic flux density

(Gauss), N is the number of turns of the winding, and μ_e is the effective permeability of the core.

The sense winding 51 is typically a single turn winding on the toroid which, like the control winding, produces a flux in the core propoπional to the current through the winding. In this invention, the sense winding must be wound on the toroid in such a way as to cancel the flux produced by the control winding, that is, to produce a flux in the toroid which is proportional to the difference between the flux generated by the control and sense windings, to be detected bv the Hall effect sensor. Typically, if the sense current is to follow the reference current (same direction at a given input), their respective windings should be in an opposite helical sense.

The net result is shown in block diagram Fig. 4. The number of sense winding turns 43 is shown as N₂, and the number of control winding turns 44 is shown as N,. The balance of magnetic flux in toroid 53 as detected by the Hall effect sensor portion of sensor/amplifier 54 produces the difference signal at functional summer 45. The signal is then amplified with a gain A of the sensor/amplifier 45 (this may be the combined gain of blocks 37 and 38 in Fig. 3) to result in output V_c--._or at port 47. The Hall effect device is not intended to operate continuously at a zero flux state in the context of the present invention. The range of flux in gauss which the

Hall effect device shown in Fig. 4 will be exposed to is determined by the maximum ampere-turn differential resulting from the difference between the reference current I._ef and the sense current I_sense. The cross section of the toroidal core (which may be of any other nontoroidal shape, e.g., square configuration or rectangular configuration) will be determined by the maximum flux which the core is expected to handle, to ensure linearity of current-differential measurement.

Output W_em. is isolated from both sense winding 51 and control winding

52 and is propoπional to the difference between the reference current I-._ef and the sense current I_sense. The control system feedback loop in Fig. 3 uses this output error to determine what the sense current must be to negate the flux generated by the reference current. Therefore it is possible to control the sense current (which may be relatively high in a power supply context) by applying a control current through the control winding.

The foregoing description has been limited to a specific embodiment of this invention. It will be apparent, however, that variations and modifications may be made to the invention, with the attainment of some or all of the advantages of the invention.

Claims

1 1. A method of comparing a current to be sensed I_S(-_n-_e at a relatively

2 high potential, with a reference current I,_ef at a relatively lower potential to obtain

3 a differential signal, said method comprising the steps of: 4

5 providing a core of magnetic material shaped to have a closed

6 loop and including an air gap. and providing a closed magnetic flux curcuit; 7

8 providing a cross sectional area for said core to be sufficient

9 to accommodate without saturation, magnetic flux caused by a predetermined 10 maximum of said differential signal;

11

12 winding the core of magnetic material with a first winding

13 electrically insulated from the core and to be connected to carry said current

14^ L -Sense ^•'

15 winding the core of magnetic material with an insulated second

16 winding in an opposite sense to the first winding and to be connected to

17 carry said reference current I-.._f; 18

19 connecting said first and second windings so as to produce in

20 said core, opposing magnetic fields to result in a differential magnetic flux

21 in said air gap: and

-~

providing a Hall effect magnetic sensor having a sensor portion in said air gap, said Hall effect magnetic sensor having a signal output linearly proportional to magnetic flux present in said air gap.

2. A method as in claim 1 including the step of amplifying said signal output by circuitry of said Hall effect magnetic sensor.

3. A method as in claim 2 including the step of adjusting said current I-_en-_c by feeding back said differential signal as an error signal.

4. Apparatus for comparing sensed current to a reference current. both external to said apparatus, said apparatus comprising:

A) a gapped ferrite toroid;

B) a first winding around said toroid connected to a first port to receive said reference current:

C) a second winding around said toroid connected to a second port to said sensed current; and

D) a Hall effect magnetic sensor having a sensor portion located in the gap of said toroid and providing at a third poπ an output proportional to magnetic flux present in said gap.

5. The apparatus of Claim 4 including means to amplify said output of said Hall effect magnetic sensor by using circuitry of said sensor.

6. The apparatus of Claim 5 wherein sensed current flowing in said second winding produces magnetic flux in a direction within said toroid opposite to magnetic flux produced by reference current flowing in said first winding.

7. An apparatus for galvanically isolated control of a sensed current by a reference current, wherein said sensed current is used outside said apparatus, said apparatus comprising:

A) a core of magnetic flux-concentrating material shaped to have two ends oppose to define a gap and form a closed magnetic flux circuit;

B) a first winding around said core connected to a first port for receiving said reference current;

C) a second winding around said core connected to a second port for receiving said sensed current; and

D) a Hall effect magnetic sensor having a sensor portion located in said gap and providing an output propoπional to magnetic flux present in said gap;

E) an amplifier for amplifying said Hall effect output to provide an amplified error signal: and

F) feedback circuitry for adjusting said sensed current in response to said amplified error signal.