Switching device with a power FET and an inductive load

• US 5,936,439 A
• Filed: 05/13/1997
• Issued: 08/10/1999
• Est. Priority Date: 05/14/1996
• Status: Expired due to Fees

• Current Assignee: Temic Telefunken Microelectronic GmbH (Continental AG)
• Original Assignee: Temic Telefunken Microelectronic GmbH (Continental AG)
• Inventors: Pollersbeck, Werner
• Primary Examiner(s): Callahan, Timothy P.
• Assistant Examiner(s): ADOLPHE, EUNJA
• Application Number:
  US08/855,469
• Time in Patent Office: 819 Days
• Field of Search: 327/427, 327/574, 327/581, 327/110, 327/379, 327/380, 327/381, 327/308, 327/309, 327/310, 327/313, 327/314, 327/327, 327/328
• US Class Current: 327/110
• CPC Class Codes: H03K 17/08122 in field-effect transistor switches
• CPC Class Codes: H03K 17/161 in field-effect transistor switches

Abstract

A switching device with a power FET for switching an inductive load to which a free-wheeling diode is connected in parallel, wherein the terminal of the series resistor facing away from the gate terminal is connected to a driver circuit which is so designed that it connects the specified terminal with a reverse potential in order to block the FET, wherein, at the beginning of the process of making the FET conductive, it connects the specified terminal with a high resistance to a control voltage source that puts the FET into the conductive state, in such a way that the current rise of the current flowing through the FET is slowed down to such an extent that, within a period of time in which the free-wheeling diode is not yet blocking after starting to make the FET conductive, an increase of the current to undesirable high values is prevented, so that damage to the power FET and the free-wheeling diode and/or other circuit elements and/or the occurrence of electromagnetic disturbances is reduced, and wherein, after the point of time at which the free-wheeling diode has achieved its blocking capability, the specified terminal is connected by the driver circuit with low resistance to a control voltage source which puts the FET into the fully conductive state such that the rate of current rise is increased. Advantages are that electromagnetic disturbances can easily be reduced and that the free-wheeling diode does not have to be selected for short reverse recovery time.

Claims

1. A switching circuit arrangement for controlling a current by switching the current to flow through an inductive load, said circuit arrangement comprising;

an inductive load connected in series with a source-drain current path of a power FET between respective terminals for a source of operating potential;

a freewheeling diode connected in parallel with one of the inductive load and the source-drain current path of the power FET;

a low-impedance first resistor having first and second terminals with the first terminal being connected to the gate terminal of the power FET to supply a control voltage to the power FET;

a high-impedance second resistor connected between the second terminal of the first resistor and a control voltage source providing a potential that places the power FET in the fully conductive state; and

,

circuit means, connected to the second terminal of the first resistor, for selectively causing a control voltage having one of three states to be applied to the gate terminal of the power FET via the first resistor, with the three states being (1) a blocking potential for blocking the power FET to render it nonconductive, (2) a low voltage potential that places the FET into the conductive state and slows down a rise in the current flowing through the power FET to such a degree that for a specific time interval during which the low voltage potential is applied as the control voltage, the current controlled by the power FET is prevented from rising to undesirably high values so as to prevent damage to the power FET and other connected switching elements, and (3) the higher voltage potential of the control voltage source which causes the power FET to become fully conductive; and

wherein the circuit means comprises;

first and second series-connected, electronic switches connected between the control voltage source, which places the FET into the completely conductive state, and the blocking potential, with the first electronic switch being connected to the control voltage source and with the common connection of the series connected electronic switches being connected to the second terminal of the first resistor; and

switching control means connected to and controlling the first and second electronic switches for causing;

(a) blocking of the power FET by switching the second electronic switch, which is connected to the blocking potential, to be conductive and the first electronic switch, which is connected to the control voltage source, to be non-conductive; and

,

(b) the power FET to become fully conductive by initially switching the first and second electronic switches to be non-conductive to slow down the current rise, so that the gate terminal of the power FET is connected via the first and second resistors to the control voltage source, for a time interval greater than the reverse recovery time of the free-wheeling diode, wherein the free-wheeling diode is not yet blocking, and thereafter bridging the high-impedance second resistor by switching the first electronic switch to be conductive and thereby connecting the gate terminal of the power FET via a low-impedance of the first electronic switch and the first resistor to the control voltage source, so that the power FET is fully conductive.

2. A switching circuit arrangement in accordance with claim 1 wherein the electronic switches are parts of a programmable output stage of an integrated circuit.

3. A switching circuit arrangement in accordance with claim 2, wherein a parasitic source-drain diode of the power FET is provided as the free-wheeling diode.

4. A switching circuit arrangement in accordance with claim 1 wherein each of the first and second electronic switches is an FET.