METHOD FOR CHECKING THE SWITCH-OFF CAPABILITY OF A MOSFET

Abstract

Disclosed is a method for checking the turn-off capability of an electronic fuse in the form of a MOSFET, said electronic fuse being used as an interrupter switch between a voltage supply and a control device, wherein the MOSFET is turned on during operation as intended and is operated in the linear region, wherein, for checking the turn-off capability, the gate-source voltage of the MOSFET is reduced by a predefined value until a predefined threshold value is reached and, after the threshold value has been reached, the gate-source voltage is increased again to the previous value for turning on the MOSFET, and wherein a check is made to ascertain whether the drain-source voltage of the MOSFET increases as the gate-source voltage is reduced.

Claims

1. A method for checking the turn-off capability of an electronic fuse in the form of a MOSFET, said electronic fuse being used as an interrupter switch between a voltage supply and a control device, comprising: turning on the MOSFET during operation as intended, operating the MOSFET in the linear region, reducing the gate-source voltage of the MOSFET by a predefined value until a predefined threshold value is reached, after the threshold value has been reached, increasing the gate-source voltage to a previous value for turning on the MOSFET, and ascertaining whether the drain-source voltage of the MOSFET increases as the gate-source voltage is reduced.

2. The method as claimed in claim 1, wherein the MOSFET is controlled and checked by a control and diagnosis circuit controlled by a control unit.

3. The method as claimed in claim 1, wherein the gate-source voltage is reduced in time ranges in which the control device does not have a high current demand.

4. The method as claimed in claim 2, wherein the gate-source voltage is reduced in time ranges in which the control device does not have a high current demand.

Description

[0018] The invention will be described in greater detail below on the basis of an exemplary embodiment with the aid of figures. In the figures here:

[0019] FIG. 1 shows a basic circuit diagram of a control device which is protected by means of an electronic fuse and which is supplied from a voltage supply source,

[0020] FIG. 2 shows a known family of characteristic curves of a MOSFET for elucidating the operating regions, and

[0021] FIG. 3 shows a profile of the gate-source voltage and of the drain current of a MOSFET controlled according to the invention.

[0022] FIG. 1 schematically shows two connecting terminals 1, 2 for a voltage source, for example a 12-volt battery in a motor vehicle. The connecting terminal 1 for the positive pole of the voltage source is connected to a control device 4 via an electronic fuse 3.

[0023] The electronic fuse 3 is formed by a MOSFET and during normal operation, in which the control device 4 is supplied by the voltage source, is fully turned on in order to minimize the on-state resistance of the MOSFET-usually designated as R.sub.DSon.

[0024] The control device 4 is of a type which must not be completely disconnected from the voltage supply, and so checking the turn-off capability of the MOSFET by completely opening the latter is out of the question.

[0025] In the exemplary embodiment illustrated, the MOSFET is controlled by a control and diagnosis circuit 5, which is in turn controlled by a control unit 6.

[0026] For elucidation, FIG. 2 illustrates the family of characteristic curves of a MOS field effect transistor with a load line. During normal operation, the MOSFET is controlled with a high gate-source voltage V.sub.GS, such that its on-state resistance R.sub.DSon becomes as small as possible. This is elucidated by the point A in FIG. 2. The transistor is operated in the so-called linear region since the voltage drop V.sub.DS across its load path is proportional to the drain current I.sub.D.

[0027] In a manner according to the invention, during a test phase that begins at the point in time t1 in the diagram in FIG. 3, the gate electrode capacitance begins to be discharged at a somewhat delayed point in time t2, this being indicated by a discharge current I_dc. After a certain time, the gate-source voltage V.sub.GS begins to decrease until it reaches a predefined threshold value after a discharge time t_dc, said threshold value being lower than the normal gate-source voltage V.sub.GS by a measure dV. The gate electrode capacitance is then charged again with a charging current I_c, as a result of which the gate-source voltage V.sub.GS rises again until it reaches the initial value again after a charging time t_c.

[0028] As a result of the discharge of the gate electrode capacitance and the attendant reduction of the gate-source voltage V.sub.GS, the operating region of the MOSFET moves into the hatched region of the family of characteristic curves illustrated in FIG. 2. As a result, the drain-source voltage V.sub.DS of the MOSFET increases significantly, which can be detected, which is preferably done by means of the control and diagnosis circuit 5. This increase in the drain-source voltage V.sub.DS is then an indication that the electronic fuse 3 realized as a MOSFET can be opened in a serious situation and is thus functional.

[0029] The turn-off capability test is carried out in a vehicle status with limited current demand at the output of the electronic fuse 3. This may be e.g. in the phase in which the supplied control device 4 is still in operation, but does not activate any current-consuming loads. In this status, the current consumption of the control device 4 provides for a rapid voltage drop phase during the switching capability test and limits the power loss of the MOSFET during the linear load.

[0030] Ideally, communication network-based power management defines events for the turn-off capability test on the basis of the gathered information about the vehicle status and the status of the supplied control devices.