Method for detecting a reverse current in a switching structure supplying an inductive load

1. A method for detecting a reverse current in a switching structure supplying power to an inductive load, the switching structure comprising at least one power switch and being designed to drive a current in the load in accordance with a duty cycle, the switching structure being linked firstly to an electric power source (Vps) and secondly to a ground (mas) in order to control the inductive load, the method comprising a control phase in which the current from the power source (Vps) supplies power to the inductive load in accordance with a given duty cycle and a freewheeling phase in which the induced current from the inductive load is decreasing, the reverse current being liable to be created in the switching structure during a freewheeling phase following a high duty cycle in a previous control phase creating a counter-electromotive force (cemf) in the inductive load t, wherein the counter-electromotive force (cemf) at a given instant is approximated as being substantially proportional to an integration of the duty cycle (integ[Dut cycl]) as a function of time, the reverse current either being calculated as a function of an estimated counter-electromotive force (cemf) or a reversal of a current criterion being established.

2. The method as claimed in claim 1 , wherein the approximation of the counter-electromotive force (cemf) is calibrated with respect to the inductive load, and a coefficient of integration for the duty cycle is defined.

3. The method as claimed in claim 2 , wherein an integration of the duty cycle of rank n integ[Dut cycl(n)] is defined by an integration of the previous duty cycle of rank n−1: integ[Dut cycl(n−1)] and by the duty cycle of rank n dut cycl(n) according to the following equation, a being the coefficient of integration:

Integ[Dut cycl( n )]= a ·integ[Dut cycl( n− 1)]+(1− a )·dut cycl( n ).

4. The method as claimed in claim 3 , wherein, when a reversal of the current criterion is established, a gradient limit p on a curve of the integration of the duty cycle (integ[Dut cycl]) is estimated and a limit value b corresponding to the gradient limit for a temporal granularity g is set according to the equation:

b=p·g

a current reversal criterion in relation to a difference between the integration of the duty cycle of rank n integ·Dut cycl (n) and the integration of the duty cycle of rank n−1: integ[Dut cycl(n−1)] is then defined, and when this difference is less than −b for an integration of the positive duty cycle, that is to say:

Integ[Dut cycl( n )]−integ[Dut cycl( n− 1)]<− b

or when this difference is greater than +b for an integration of the negative duty cycle, that is to say:

Integ[Dut cycl( n )]−integ[Dut cycl( n− 1)]>+ b

it is estimated that a current reversal is effective.

5. The method as claimed in claim 4 , wherein the limit b is able to be calibrated in accordance with the selected temporal granulometry and in accordance with the inductive load that is used as well as the frequency of the signal of a pulse width modulation.

6. The method as claimed in claim 2 , wherein, when the reverse current I is calculated, this calculation is performed based on a current expressed as a function of the estimated counter-electromotive force cemf, of the measured voltage of the power source Vps, of the duty cycle dut cycl and of the resistance of the circuit R according to the following equation:

I =(dut cycl· Vps −cemf)/ R.

7. The method as claimed in claim 3 , wherein, when the reverse current I is calculated, this calculation is performed based on a current expressed as a function of the estimated counter-electromotive force cemf, of the measured voltage of the power source Vps, of the duty cycle dut cycl and of the resistance of the circuit R according to the following equation:

I =(dut cycl· Vps −cemf)/ R.

8. The method as claimed in claim 1 , wherein, when the reverse current I is calculated, this calculation is performed based on a current expressed as a function of the estimated counter-electromotive force cemf, of the measured voltage of the power source Vps, of the duty cycle dut cycl and of the resistance of the circuit R according to the following equation:

I =(dut cycl· Vps −cemf)/ R.

9. An assembly of an inductive load and its electric power supply device, the power supply device comprising a switching structure comprising at least one power switch and being associated with a control unit comprising means for driving a current in the load in accordance with a pulse width modulation duty cycle, the switching structure being linked firstly to an electric power source (Vps) and secondly to a ground (mas), wherein the assembly implements a method for detecting a reverse current in the switching structure as claimed in claim 1 , the control unit comprising means for integrating the duty cycle (integ[Dut cycl]), means for approximating a counter-electromotive force (cemf) as a function of the integration of the duty cycle (integ[Dut cycl]), means for calculating the reverse current from the approximation of the counter-electromotive force (cemf) or means for detecting a current reversal as a function of a reversal criterion kept in memory means of the control unit, the control unit also comprising means for transmitting current reversal information.

10. The assembly as claimed in claim 9 , wherein the switching structure is an H-bridge.