Circuit for controlling the current in inductive loads and control method therefor

Abstract

A circuit for controlling current in an inductive load is provided. The circuit includes a driver circuit for driving a load current in the inductive load. The driver circuit includes a switch, which is switched on to increase the load current and a recirculation diode, which re-circulates the load current when the switch is off. The circuit includes a control module that generates a control signal to switch on and off the switch. The control module includes a PWM current controller comprising a negative feedback closed loop implementing at least a proportional control and an integral control. The PWM current controller receives a target current value and an estimated current flowing in the load during a measurement PWM cycle. The PWM current controller generates the control signal for a control input of the switch based on an error between the target current and the estimated current.

Claims

1. A circuit for controlling current in an inductive load, comprising: a driver circuit configured to drive a load current in said inductive load, said driver circuit including: a low side or high side controllable switch having a control input for receiving a control signal, the controllable switch is configured to be switched on to increase the load current flowing in the inductive load and switched off based on the control signal, wherein a recirculation diode, external to the driver circuit is arranged to re-circulate the load current when the controllable switch is switched off; a negative feedback closed loop configured to implement at least one of proportional control and integral control, the negative feedback closed loop being configured to output a target current value and an estimated value of the current flowing in the inductive load during an immediately preceding measurement PWM cycle, the estimated value of the current flowing in the inductive load during the immediately preceding measurement PWM cycle being determined as a value of the current flowing in the inductive load at a mid-point of an on-time of the immediately preceding measurement PWM cycle; and a PWM current controller module configured to: receive the target current value and the estimated value of the current flowing in the inductive load during the immediately preceding measurement PWM cycle; generate a control signal based on an error between said target value and said estimated value of the current flowing in the load; and output the control signal to the control input of the controllable switch.

2. The circuit according to claim 1 wherein said controllable switch is a low side switch having conductive terminals that are coupled to an output node and a ground node, respectively, and the recirculation diode is coupled in parallel with the inductive load and has a cathode that is coupled to a battery node that provides power supply.

3. The circuit according to claim 1, comprising: a sense circuit coupled to the controllable switch and configured to sense the load current and supply a sense current to the negative feedback closed loop.

4. The circuit according to claim 1, wherein the negative feedback closed loop implements a derivative action.

5. A method for controlling current in an inductive load, comprising: feeding back, by a negative feedback closed loop implementing at least proportional control and integral control, an estimated value of the current flowing in the inductive load during an immediately preceding measurement PWM cycle, the estimated value of the current flowing in the inductive load during the immediately preceding measurement PWM cycle being determined as a value of the current flowing in the inductive load at a mid-point of an on-time of the immediately preceding measurement PWM cycle; generating a control signal to switch on and off a controllable switch based on an error between a target current and said estimated value of the current flowing in the inductive load; and driving the load current in said inductive load based on the control signal by at least: switching on the controllable switch to increase the current flowing in the inductive load; switching off the controllable switch; and in response to switching off the controllable switch, causing a recirculation diode to re-circulate the current in the inductive load when the controllable switch is switched off.

6. The method according to claim 5, wherein the controllable switch is a low side switch having a first conductive terminal coupled to an output node to the inductive load and a second conductive terminal coupled to a ground node, and wherein the recirculation diode is coupled in parallel with the inductive load and has a cathode that is coupled to a battery node.

7. The method according to claim 5, comprising: sensing a current flowing through the controllable switch; and estimating, based on said sensed current flowing through the controllable switch, said estimated value of the current flowing in the inductive load during the immediately preceding measurement PWM cycle.

8. The method according to claim 5 wherein said negative feedback closed loop implements a derivative action.

9. A system comprising: an inductive load; a diode coupled in parallel to the inductive load and having an anode coupled to a controller output node; and a controller including: a controllable switch having a first conductive terminal coupled to the controller output node, a second conductive terminal coupled to a ground node having a control terminal for receiving a control signal, the controllable switch is configured to be switched on to increase a load current flowing in the inductive load and switched off based on the control signal, wherein the diode re-circulates the load current when the controllable switch is switched off; a feedback loop configured to output an estimated value of the load current flowing in the inductive load during an immediately preceding measurement PWM cycle, the estimated value of the current flowing in the inductive load during the immediately preceding measurement PWM cycle being determined as a value of the current flowing in the inductive load at a mid-point of an on-time of the immediately preceding measurement PWM cycle; and a PWM current controller module configured to: receive a target current value and the estimated value of the current flowing in the inductive load during the immediately preceding measurement PWM cycle; generate the control signal based on an error between said target current and said estimated value of the current flowing in the inductive load; and output the control signal to the control terminal of the controllable switch.

10. The system according to claim 9 wherein the diode has a cathode coupled to a voltage supply node that provides a voltage supply.

11. The system according to claim 9, comprising: a current sense circuit coupled to the controllable switch and configured to sense the load current and provide a sensed current to the feedback loop, and wherein the feedback loop is configured to output the value of the load current based on the sensed current.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) Embodiments of the present disclosure will now be described with reference to the annexed drawings, which are provided purely by way of non-limiting example and in which:

(2) FIG. 1, 2, 3, 4 have been already described in the foregoing;

(3) FIG. 5 shows schematically an embodiment of a control circuit of the type here described.

(4) FIG. 6 shows a time diagram representing currents and values used by the control circuit of FIG. 5.

(5) FIG. 7 shows a flow diagram representing an embodiment of a method implemented by the circuit of FIG. 5.

DETAILED DESCRIPTION

(6) In the following description, numerous specific details are given to provide a thorough understanding of embodiments. The embodiments can be practiced without one or several specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the embodiments.

(7) Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

(8) The headings provided herein are for convenience only and do not limit the scope or meaning of the embodiments.

(9) In FIG. 5 described is a solution for controlling the current in inductive loads that obtains a sufficient accuracy and a real time current, using a single switch, in particular a low side switch represented by a current control circuit 30. Such a solution provides in the current control circuit 30 a PWM current controller configured to regulate the load current in real time. Regulation in real time means regulation within a single PWM period, without requiring a high microprocessor work-load. A current feedback value for the PWM current controller to be compared with the target current value is extracted that minimizes the current error with respect to the real average current, taking in account also the real valve current shape. An integrated circuit 30a comprising an integrated current control loop with an external recirculation diode is used. Different possible control strategies are described to compute the feedback current value to be compared with target value, based on the available ON-current measurement.

(10) As shown in FIG. 5, the control circuit 30 presents substantially the same circuit arrangement of the control circuit 20, having an integrated circuit 30a including a single low side switch 11 having the load 50 connected between the output node OUT, which corresponds to the drain electrode of the low side switch 11, and the battery voltage V.sub.BATT. The recirculation diode 22, external with respect to the integrated circuit 30a, is connected between the same nodes, OUT and V.sub.BATT, to conduct directly the current into the battery node V.sub.BATT.

(11) In this case, differently from the circuit 20 shown in FIG. 3, the command signal CMD is issued by the controller 15, which is included in a current controller module 35, included in the integrated circuit 30a, which receives as one of its inputs the PWM period T.sub.PWM. The set-point input of the controller 15, however, receives an error current I.sub.error computed as the difference, which is performed in a specific sum module 31 also included in the current controller module 35, between the target current I.sub.target and a median current I.sub.mid. Such median current I.sub.mid is measured in a median current extraction module 32, which receives the sense current I.sub.sense from the sense circuit 11b and the command signal CMD as inputs.

(12) The integrated circuit 30a, therefore, in the embodiment shown above includes the current controller module 35, the driver 11, 11a, the median current extraction module 32, and the current sensing circuit 11b.

(13) With reference to FIG. 6, shown is a diagram of the current I.sub.sense as a function of time t, the median current I.sub.mid extracted by the median current extraction module 32 can be calculated on the basis of the sense current I.sub.sense, corresponding to the low side current I.sub.LS, preferably as follows. A first PWM cycle, is defined from when the PWM control signal CMD goes from the OFF to the ON state, starting an ON period Ton.sub.i, and from the subsequent OFF period Toff.sub.i. A subsequent PWM cycle i+1 is defined as measurement cycle, since, as better detailed during this cycle the median current I.sub.mid is calculated and feed to the node 31 to calculate the error current I.sub.error updating the input of the controller 15, therefore varying the length of the on period Ton.sub.i+, of the measurement cycle i+1.

(14) The median current I.sub.mid is obtained by computing an average value between a high current value I.sub.hc measured when it occurs that the PWM control signal CMD goes from the ON to the OFF state, marking in FIG. 6 the beginning of the first PWM cycle i. The ON period Ton.sub.i of the first PWM cycle i is shown with its area in slanted lines. A low current value I.sub.lc is sampled or acquired when it the PWM control signal CMD goes from the OFF to the ON state at the end of the first PWM cycle i. The low current value I.sub.lc of the current I.sub.sense is acquired as soon as the switch 11 is fully ON and the current sense amplifier embodying the sense circuit 11b is settled to guarantee correct current measurement. The median current I.sub.mid is, therefore, obtained in the measurement cycle i+1, as soon as the low current value I.sub.lc is measured at the beginning of the cycle i+1, averaging it with the high current I.sub.hc which is read at the end of the cycle i using immediately the median current I.sub.mid to update the PI controller 15 computing the value of the ON time Ton.sub.i+1 of the measurement cycle i+1 itself.

(15) In FIG. 6, where it shown a line corresponding to the average current value I.sub.average, i.e., the real average value of the periodic current in the load, are indicated other quantities that can be computed by the module 32 as median current I.sub.mid value. A possible choice is the average ON current value I.sub.avgon, i.e., the average value of the ON current. Another possible choice is the sense current value at T.sub.on/2, I(T.sub.on/2).

(16) In typical driving conditions and with an RL series valve model, where the current ripple shape is very similar to a triangular wave (T.sub.ON and T.sub.OFF being lower than load time constant), these three alternative choices (the median current I.sub.mid, the average ON current value I.sub.avgon, and the sense current value at T.sub.on/2, I(T.sub.on/2)) are almost equivalent since all these three choices supply values that are very near to the true average current I.sub.average. Current ripple measurements on a real valve installed in the complete application environment and driven at different target currents show a current ripple shape that is not fitting with the single time constant behavior provided by a simple R-L series model. In particular, the real current behavior shows a high slope as soon as the driver is turned on or off, while the current slope decreases as the on/off state is kept—it has to be fitted using therefore different time constants. The reason for this behavior seems to be the ferromagnetic characteristics of the valve core with its hysteresis and saturation.

(17) Due to this non-linear behavior of the current ripple, the method based on the median current I.sub.mid provides the lowest error in in a wide target current range.

(18) It has to be underscored that in FIG. 6, for the median current I.sub.mid, the point placed at half the OFF period of cycle i simply indicates the value of the median current I.sub.mid on the vertical axis so that such value of the median current I.sub.mid can be also graphically compared to the other estimates of the current and with respect to the average current I.sub.average. However, the median current I.sub.mid is computed at time t2 in the cycle i+1, and the solid line passing through the median current I.sub.mid point is just drawn to facilitate illustration.

(19) FIG. 7 shows a flow diagram representative of an embodiment of a method.

(20) In step 110, the driver circuit 30 is enabled.

(21) In step 120, variables corresponding to the low current value I.sub.lc and the high current value I.sub.hc are initialized to the zero value.

(22) In step 130 it is evaluated if the command signal CMD is logic one, i.e., the switch 11 is closed and the current I.sub.LS is passing through the load 50. In the negative, i.e., command signal CMD=0, control is brought back to step 120, i.e., the method cycles around the evaluation step 130 until the command signal CMD closes the switch 11.

(23) Therefore if a positive determination is made, step 140 of waiting for the settling of the sense current I.sub.sense after the switching on of switch 11 is performed. This can be performed by setting a settling time of the current sense 11b.

(24) After the step 140, at step 150 the low current value I.sub.lc is sampled.

(25) Then in step 160 the median current I.sub.mid value is calculated, in particular as the sum of the low current I.sub.lc and the high current I.sub.hc divided by two, i.e.,
I.sub.mid=(I.sub.lc+I.sub.hc)/2

(26) In step 160, the updating of the median current I.sub.mid value includes updating the input corresponding to the error current I.sub.error of the controller 15, which determines the calculation of a new ON time (T.sub.ON) for the PWM cycle taking place.

(27) Then in step 170, it is evaluated if the command signal CMD is logic zero, i.e., the switch 11 is open and the load 50 current is no longer passing through the low-side driver.

(28) In the affirmative, the high current I.sub.hc value is sampled in step 190 and control returns to step 120. Since the digital control is much faster than the low-side driver, the acquired or sampled value of high current I.sub.hc is still the current in the load passing in the low-side driver 11, before this really enters an off state. The current value I.sub.hc is acquired at the time instant during which the driver receives the control command CMD to go off, CMD=0. Such high current value I.sub.hc, thus, is acquired just before or synchronously with the issuance of an OFF command signal evaluating when the controllable switch 11 is still close.

(29) In the negative, step 180 of evaluation of the duty cycle of the PWM signal is performed: if it is found that the duty cycle is 100%, then the high current I.sub.hc value is sampled in a step 195 and control returns to step 120. The condition verified at step 180, i.e., the duty-cycle of the PWM control signal CMD reaching 100% may occur for instance if the target current I.sub.target cannot be reached for some reason, or during a transient. Otherwise the high current I.sub.hc value is not sampled and control returns to step 160 where the median current I.sub.mid value was calculated, waiting for high current I.sub.hc sampling by CMD=0 or by PWM=100%.

(30) Therefore, generally the method described herein provides for generating at a current controller module 35 the control signal CMD to switch on and off the controllable switch 11, including performing a PWM (Pulse Width Modulation) current control by a negative feedback closed loop, implementing at least a proportional control action and an integral control action and a PWM modulation, using as feedback loop set-point the target current value I.sub.target and as feedback measured value of the loop an estimated value I.sub.mid of the current flowing in the load I.sub.LS during a measurement PWM cycle, generating the control signal CMD for the control input of the controllable switch 11 on the basis of the error current I.sub.error between said target current I.sub.target and said estimate I.sub.mid of the current I.sub.LS flowing in the load 50. In the preferred embodiment described with reference to FIG. 5 the controllable switch is a low side switch.

(31) As described with reference to FIG. 6, the method includes sensing by the sense circuit 11b the current in the controllable switch 11 and estimating in the module 32 on the basis of the sense current I.sub.sense the estimated value I.sub.mid of the current flowing in the load I.sub.LS during a measurement PWM cycle. Although alternate estimated values are possible, such as average ON current Iavgon or the sense current value at T.sub.on/2, I(T.sub.on/2), in a preferred embodiment, which allows real time control, the estimation at module 32 includes in general calculating as the half of the sum of a low current value I.sub.lc acquired after the issuance of an ON command signal CMD in the measurement PWM cycle and a high current value I.sub.hc acquired 170 just before or synchronously with the issuance of an OFF command signal. It is noted that in preferred calculation of the median current Imid, a high current value I.sub.hc acquired before the low current value I.sub.lc is used.

(32) In the embodiment specifically described with reference to FIG. 6 the low current value I.sub.lc is acquired in step 150 from the sense current I.sub.sense after the issuance of an ON command signal CMD in the measurement PWM cycle evaluating when the controllable switch 11 becomes closed, in particular after the settling of the sense current I.sub.sense, the high current value I.sub.hc is acquired in a step 190 before the end of the previous OFF command signal evaluating when the controllable switch 11 becomes open. Then in step 160 the median current I.sub.mid value is obtained as the sum of the low current I.sub.lc and the high current I.sub.hc divided by two.

(33) It is noted that if at step 180 the duty-cycle is found to be 100%, in that PWM cycle the control signal CMD does not return to zero, i.e., the driver is not switched off. Therefore, at the end of the PWM cycle, both I.sub.hc and I.sub.lc are sampled at the same time, thus taking the same value. In the subsequent PWM cycle, starting immediately after this sampling operation, the value of the median current I.sub.mid is calculated as equal to the value of these two samples, and the error I.sub.error input of the controller 15 is updated accordingly.

(34) Therefore the solution here disclosed allows for implementing a fully integrated real-time PWM controller for the inductive load, where real time means that the controller is updated at each PWM cycle, with a latency lower than one PWM cycle. This solves therefore the problem that such a circuit topology in which the load current, when flowing through the external diode as diode current, since the low-side switch is off, cannot be measured by the control circuit and therefore the real average current with respect to the load current cannot be measured, since only the low side current is measurable.

(35) A real time current control loop of this type is faster to react to system variations (such as battery voltage change), to recover to the correct steady state average current value, in particular with respect to solution such as the one depicted in FIG. 3, where the current flowing in the load is read only occasionally. As discussed, implementing a current-control algorithm operating on a PWM-period basis in that context would require quite a heavy SPI traffic load and significant work-load for the microprocessor.

(36) The above advantages are obtained in a low-cost driving topology, using an external recirculation diode, which, as indicated, however allows real time current control, and, by the proportional integral control, allows better current control accuracy at low target current values. The better current control accuracy is particularly ensured by the use of the median current.

(37) Of course, without prejudice to the principle of the invention, the details of construction and the embodiments may vary widely with respect to what has been described and illustrated herein purely by way of example, without thereby departing from the scope of the present invention, as defined by the ensuing claims.

(38) In variant embodiments the controllable switch can be a high side switch and the recirculation diode connected accordingly to recirculate the current in the ground node.

(39) The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.