GB2432258A

GB2432258A - A switch comprising a relay and a transistor

Info

Publication number: GB2432258A
Application number: GB0523082A
Authority: GB
Inventors: Jolyon Crane
Original assignee: PG Drives Technology Ltd
Current assignee: PG Drives Technology Ltd
Priority date: 2005-11-11
Filing date: 2005-11-11
Publication date: 2007-05-16
Anticipated expiration: 2025-11-11
Also published as: US20070108845A1; US7781918B2; GB0523082D0; GB2432258B

Abstract

A switch comprises a relay 11 and a transistor 12, the relay 11 comprising electrical contacts, the switch being arranged such that in use, electrical current is caused to flow through the transistor 12 while the electrical contacts of the relay 11 are closing or opening, thereby reducing electrical arcing across the contacts.

Description

Switch The present invention is concerned with electronic switching

circuits and particularly but not exclusively to switching circuits comprising electromechanical relays.

Electromechanical relays are a very mature technology.

Despite being replaced by semiconductor devices in many applications, the basic relay still retains many advantages over modern switching systems including an inherently low voltage drop and electrical isolation.

The life of the electrical contacts of the relay is usually the limiting factor in determining their incorporation in modern circuits. This is especially true for direct current (DC) applications, where contact erosion takes place. The rated useful life of the relay contacts is often only 1% of the mechanical life, especially where loads which are controlled are inductive e.g. motor, or have a high in-rush current e.g. tungsten lamps.

Relays are still widely used in automotive applications as the system voltage for automobiles is relatively low e.g. l2V.

At higher voltages, the material from which the relay contacts are made behave differently and contact erosion through arcing during the opening and closing of the contacts becomes more significant. This has for instance been a major hurdle in 42V systems. Thus, there is a requirement for a switching circuit which retains the advantages of a relay operation, but which is capable of accurately and repeatedly switching high power systems whilst obviating contact erosion.

In accordance with this invention as seen from a first aspect there is provided a switch comprising a relay and a transistor, said relay comprising electrical contacts, the switch being arranged such that in use, electrical current is caused to flow through the transistor while the electrical contacts of the relay are closing or opening, thereby reducing electrical arcing across the contacts.

Preferably, the relay and transistor are connected in a parallel configuration.

Preferably, the transistor is operated in accordance with the voltage drop across the relay.

Preferably, the switch further comprises a control circuit for switching the transistor on in response to a voltage drop across the relay reaching a predetermined voltage.

Preferably, the control circuit comprises a pulse generator for generating a pulse for turning the transistor on for a first predetermined period in response to the voltage drop across the relay reaching a predetermined voltage.

Preferably, the control circuit comprises a comparator for producing a signal when the voltage drop across the relay reaches the predetermined voltage.

Preferably, the control circuit further comprises an edge detector for producing an edge detector signal in response to the signal from the comparator, the edge detector signal being used to control the pulse generator.

Preferably, the predetermined voltage is selected to limit the damage causing arcing across the relay contacts.

Preferably, the predetermined voltage is in the range 4V-8V.

Preferably, the control circuit further comprises an inhibitor for inhibiting the pulse generator from generating a pulse during a second predetermined period following the first predetermined period.

Preferably, the switch comprises electrical current overload protection.

Preferably, the current overload protection comprises a fuse.

Preferably, the transistor is a Metal Oxide Semiconductor

Field Effect Transistor (MOSFET)

In accordance with this invention, as seen from a second aspect there is provided an electronic circuit comprising a switch in accordance with the first aspect of this invention.

In accordance with this invention, as seen from a third aspect there is provided a method of switching electrical current, the method comprising diverting current from a relay to a transistor whilst electrical contacts of the relay are closing or opening.

The preferred embodiment of the present invention will now be described by way of example only, and with reference to the accompanying drawings, in which: Figure 1 is a representation of the circuit diagram; and, Figure 2 is a voltage timing diagram.

Referring to the drawings and initially figure 1, there is shown a circuit 10 comprising a relay 11 connected in a parallel configuration with a transistor 12. A voltage comparator 13 is also connected across the relay 11 contacts and the transistor 12. The switching threshold of the comparator 13 is at a voltage greater than the maximum expected relay contact voltage drop, at maximum expected system current, but less than the voltage at which significant arcing can take place. The voltage at which the comparator switches is set by the reference voltage 21, e.g. 6V and must be chosen to be less than the voltage at which significant contact erosion can take place through arcing.

Connected to the output of the comparator 13 is an edge detector 14, which is capable of detecting positive and negative going transitions of the comparator 13 output, such that positive and negative transitions of the comparator are used to trigger the pulse generator 15.

The pulse generator 15 switches on the transistor 12, which may be of the MOSFET (Metal Oxide Semiconductor Field Effect Transistor) type, in accordance with a signal from the edge detector 14 (and thus the comparator 13), for a fixed period of time. A fuse 17 connected in series with the MOSFET 12 protects the MOSFET 12 from current overload, and is designed such that normal circuit current loads will burn the fuse 17, but the very short switch transitions will not.

Referring to figures 1 and 2, upon reducing the voltage at A, the load switches from the off state to the on state, following a short delay due to the rise time of the coil current.

As the voltage at A is reduced the relay contacts begin to close because of V011. As the voltage at A falls, the voltage Vload will begin to drop across the load 20 causing the voltage at B to fall. When the voltage at B falls from Vload to 6V, the comparator 13 will change from a high voltage state to a low voltage state (or vise versa), thereby triggering a pulse from the edge detector 14.

This pulse from the edge detector 14 is subsequently transmitted to the pulse generator 15 where a pulse of duration T is generated at F. The pulse at F is used to switch on the MOSFET 12 and therefore divert the large current flowing through the relay 11 to protect the relay 11 from the damage causing arcing.

The arrangement automatically triggers the MOSFET 12 conduction pulse to coincide with the actual point where the contacts physically close. The point at which this takes place is not dependent on the relay coil current rise and decay times that will vary with factors such as construction, drive current and temperature. This means that the MOSFET 12 "on" period can be kept very short, e.g. <0.Sms, which is just long enough for the contacts to be made when closing.

The very short MOSFET 12 conduction time means that the pulse rating of the MOSFET 12 becomes applicable i.e. the device will be capable of being used at very high currents, which reduces costs and provides an overload protection.

The pulse generated at F is also passed back to the pulse generator 15 via the timer inhibit 16, a NOT gate 19, and an AND gate 18, the latter of which also has an input from the edge detector 14. The inhibit timer 16 increases the duration of the pulse at F from T to T+AT thereby locking out further trigger inputs into the pulse generator for a period T+LIT, which is longer than the MOSFET 12 gate pulse. This prevents unwanted oscillations and false triggering of the pulse generator 15.

When the voltage at A is increased, so as to switch off the load 20, the relay contacts 11 will begin to open. The voltage at B will then begin to rise and upon passing through 6V, the comparator 13 will undergo a change of state at C. The predetermined 6V voltage can be set to any desired voltage, however, 6V is found to cause minimal arcing damage to the electrical contacts of the relay while the contacts are opening or closing. The change of state of the comparator 13 will cause the edge detector 14 to generate a pulse at D, which causes the pulse generator 15 to generate a pulse at F of duration T. This pulse of width T, will switch the MOSFET 12 on for a period T which is just long enough for the relay contacts 11 to open to produce an air gap sufficient to sustain isolation, and thus, protect the relay 11 from the damage causing arcing. As the NOSFET 12 switches on, the voltage at B will fall again. This "edge" will trigger a further pulse at D. However, the AND 18 and NOT 19 gates in conjunction with the signal generated at G, ensures that the pulses are not transferred to E, which would affect the operation of the MOSFET 12, i.e. switch it off.

The signal at C will change again when the pulse at F has expired. However, this edge detected by the edge detector is prevented from generating a further signal to the MOSFET 12 due to the pulse at G and the associated logic gates 18, 19.

A relay arrangement as described above thus protects the relay contacts from the large currents during the opening and closing of the relay contacts, by diverting the current at these times through a MOSFET 12. This technique increases the electrical life of the relay compared with the mechanical life, and is capable of switching many loads that are outside of the capabilities of the relay itself.

In an alternative embodiment, the circuit further comprises a monitoring arrangement (not shown) to detect a failure of the fuse or relay. The arrangement is placed between the NOSFET 12 and the fuse 17 and is used to verify that the voltage across the MOSFET 12 is high before the relay coil is energised or after the pulse generator has finished transmitting the pulse. If the voltage is low then either the fuse has blown, the load is open circuit or the relay is faulty, i.e. the contacts have become stuck. In this case, a serious system failure is detected.

Furthermore, whilst the arrangement shown in figure 1 illustrates the situation whereby the relay and transistor are arranged on the low voltage side of the load, the invention applies equally to the situation in which the relay and transistor are situated on the high voltage side of the load.

It should be appreciated by the skilled reader that whilst the invention has been described herein with references to the circuit shown in figure 1, other circuit arrangements could equally incorporate the relay-MOSFET operation, or the use of other transistors such as the Insulated Gate Bipolar Transistor (IGBT), without departing from the spirit and scope of the invention.

Claims

Claims 1. A switch comprising a relay and a transistor, said relay

comprising electrical contacts, the switch being arranged such that in use, electrical current is caused to flow through the transistor while the electrical contacts of the relay are closing or opening, thereby reducing electrical arcing across the contacts.

2. A switch as claimed in claim 1, wherein the relay and transistor are connected in a parallel configuration.

3. A switch as claimed in any preceding claim wherein the transistor is operated in accordance with the voltage drop across the relay.

4. A switch as claimed in any preceding claim, further comprising a control circuit for switching the transistor on in response to a voltage drop across the relay reaching a predetermined voltage.

5. A switch as claimed in claim 4, wherein the control circuit comprises a pulse generator for generating a pulse for turning the transistor on for a first predetermined period in response to the voltage drop across the relay reaching a predetermined voltage.

6. A switch as claimed in claims 4 or 5, wherein the control circuit comprises a comparator for producing a signal when the voltage drop across the relay reaches the predetermined voltage.

7. A switch as claimed in claims 5 or 6, wherein the control circuit further comprises an edge detector for producing an edge detector signal in response to the signal from the comparator, the edge detector signal being used to control the pulse generator.

8. A switch as claimed in any of claims 4 to 7, wherein the predetermined voltage is selected to limit the damage causing arcing across the relay contacts.

9. A switch as claimed in any of claims 4 to 8, wherein the predetermined voltage is in the range 4V-8V.

10. A switch as claimed in claim 7, wherein the control circuit further comprises an inhibitor for inhibiting the pulse generator from generating a pulse during a second predetermined period following the first predetermined period.

11. A switch as claimed in any preceding claim, wherein the switch comprises electrical current overload protection.

12. A switch as claimed in claim 10, wherein the current overload protection comprises a fuse.

13. A switch as claimed in any preceding claim wherein the transistor is a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) 14. An electronic circuit comprising a switch as claimed in claim 1.

15. A method of switching electrical current, the method comprising diverting current from a relay to a transistor while electrical contacts of the relay are closing or opening.

16. A switch substantially as herein described with reference to the accompanying drawings.