Monitoring of current in an inductive load, PWM driven through a bridge stage

Abstract: The monitoring the current lowing through an inductive load driven through a bridge power stage in a PWM mode, by using a pair of complementary periodic reference signals and amplifying by a sensing amplifier the signal existing on of a current sensing resistor functionally connected in series with the load, for producing an amplified signal representative of the current in the load to be fed to an input of an error amplifier driving a power amplifier of said bridge stage, comprises sampling the signal output by the sensing amplifier with a Sample & Hold circuit comprising a sampling switch and a storing capacitor. The average value of the current in the load in monitored by sampling at an instant half way an active driving phase and at an instant half way a current recirculation phase by closing the switch with a synchronizing pulse that coincides with the half instant of said phases of operation. The synchronizing pulse is generated in coincidence with the peak and with the virtual zero crossing of said two reference periodic signals, outphased by 180 degrees. A two-input logic AND gate, combining said synchronizing pulse and a masking signal of a preestablished duration generated at every switching of said bridge stage may also be employed.

Description: Field of application The present invention relates to a system for sensing a current flowing through an inductive load, such as for example a DC motor or a VCM (Voice Coil Motor) or a winding of a brushless motor, and the like, driven by a bridge power stage composed of bipolar or Mosfet transistors in a PWM mode. Technological Background The document US-A-5,341,286 discloses a method of detecting current useful for implementing a digital current mode control of a motor driven in a PWM switching mode. The current during the two phases of a PWM cycle are detected by properly synchronizing a sampling signal by calculating the appropriate sampling instants in function of the switching frequency and duty cycle of the PWM signal. In the European Patent Application EP-A-0 760 552, filed on 31.08.95, in the name of the same applicant, there is described a PWM driving system of an inductive actuator whose block diagram is shown in Figure 1. The system comprises a control loop for the current flowing in an inductive load (R L -L L ) with the power stage functioning in a PWM at a constant frequency. The peculiarity of the illustrated system is that the current through the load is controlled by varying the duty-cycle of the two outputs (OUT+ and OUT-) of the POWER AMPLIFIER stage and more precisely the condition of null current is obtained by driving the two outputs with two signals having the same frequency and a 50% duty-cycle. By increasing the duty cycle of the output OUT+ and decreasing the duty cycle of the other output OUT-, or viceversa, the current through the load will circulate with a direction and intensity dependent on the difference of duty-cycle difference between the two outputs. The current sensing is in this case implemented by way of a sensing resistor Rs in series to the load; the voltage generated on this resistor is adequately amplified by an operational amplifier SENSE AMPL. and is used as feedback input by the ERROR AMPL. to close the control loop. A critical point of this system is indeed the sensing of the current, and more precisely the generation of a voltage signal proportional to the mean value of the current that flows through the load. In fact, in the known system this is simply obtained through an operational amplifier to the inputs of which are fed a differential voltage in the order of hundred of millivolts, represented by the voltage drop caused by the load current on the sensing resistor and a common mode voltage that goes from ground potential to the supply voltage Vp because the power stage functions in a PWM mode. Figure 2 shows the waveforms of the system described in the above cited patent application which show the relationships between the load current, the output voltage of the SENSE AMPL. and the two outputs OUT+ and OUT- of the power stage. It may be noticed that the output voltage of the SENSE AMPL. is perturbed by the switching of the two outputs of the power stage. Wishing to control the mean value of the current through the load, the gain and the passband of the current control loop must be limited in a manner to filter the noise existing on the output voltage of the SENSE AMPL. representing the feedback signal. Object and summary of the invention The object of the present invention is that of sensing the current flowing in an inductive load for generating a voltage signal proportional to the mean value of the current circulating in the load in such a way as to make it substantially immune to disturbances caused the switchings of the outputs of the power stage. Essentially, the method of the present invention is based on a sampling technique synchronous with the PWM frequency of the power stage, in order to sample the current circulating in the load during precisely established instants distant from the switchings of the outputs OUT+ and OUT-, thus generating a clean voltage signal, perfectly usable as feedback signal by the current control loop. The method of the invention consists in sampling the output signal of a sensing amplifier by means of Sample & Hold circuit comprising a sampling switch and a storing capacitance of the sampled value and in monitoring the mean value of the current through the load by effecting, according to a preferred embodiment, the sampling at exactly half way of the active driving phase and at half way of the current recirculation phase through the inductive load by closing the sampling switch by a synchronizing pulse coinciding with the half of these phases of operation. More preferably, the closing of the sampling switch is commanded by the output of a logic AND gate, combining said synchronizing pulse and a masking signal, having a preestablished duration, generated at each switching of said bridge stage, in order to ensure immunity to switching noise even under limit conditions of operation. Alternatively, the closing of the sampling switch may be effected by the same masking signal generated at each switching of the bridge stage, implementing a continuous sampling during the entire phase in which the masking signal remains at a logic zero. Brief description of the drawings The various aspects and advantages of the invention will become even more evident through the following description of several embodiments and by referring to the attached drawings, wherein: Figure 1 shows a block diagram of a PWM current control loop, according to the known technique; Figure 2 shows the waveforms of the known circuit of Figure 1; Figure 3 is a temporal diagram showing the waveforms relative to a PWM driving of a system like that of Figure 1 and the logic signals needed to realize a synchronized sampling of the current through the load according to the method of the present invention; Figure 4 is a block diagram of the PWM current control loop comprising a current sampling block according to the present invention; Figure 5 is a scheme similar to that of Figure 4 wherein a masking process for the noise existing on the output of the SENSE AMPL. caused by the switchings of the power stage is implemented; Figure 6 is a scheme similar to that of Figure 4 wherein the dosing of the SW switch for sampling the current through the load is effected by means of a masking signal. Description of several embodiments Figure 3 represents a temporal diagram of the signals showing how the PWM driving of a bridge power stage is implemented. The signals driving the outputs of power stage are generated by comparing the output signal (ERR. OUT) of the error amplifier (ERROR AMPL.) of the scheme of Figure 1 with the two reference signals, which, in the considered example, are outphased by 180 degrees among each other: Tria_0 and Tria-180 having a triangular shape. Alternatively, these reference signals may be sinusoidal or of any other periodic form. Of course, there are an active or driving phase during which the current increases when between OUT+ and OUT- there is a voltage difference, and a phase of recirculation when the two outputs are at the same voltage. By comparing the triangular-shaped reference signals with the output voltages of the power stage and the current through the load, it may be observer that the positive and negative peaks of the triangular-shaped reference signals correspond exactly to the half of the duration of the recirculation phase of the current through the load, while the virtual zero crossing (Vref) of the same corresponds to the half of the active drivery phase of the current through the load. Therefore, by realizing a current sensing circuitry of the current through the load capable of sampling the current level at these two instants as just described, a voltage signal that is proportional to the average value of the current through the load is not influenced by noise caused by the switchings of the power stage outputs is obtained. Figure 3 shows a pulse signal Sync that is normally generated every time the reference signals (be them triangular or of any other form) Tria_0 and Tria_180 cross the virtual zero and at every consequent negative or positive peak This Sync signal may thus be used for synchronizing also the circuit that realizes a Sample & Hold sampling of the load current shown in Figure 4. The block diagram of Figure 4 comprises a first stage consisting of an operational amplifier SENSE AMPL. which differentially receives at its inputs the voltage drop determined by the current crossing the sensing resistor Rs. At the output of this operational amplifier a voltage proportional to the value of the current lload that flows through the load, though influenced by spurious voltage peaks caused by the switchings of the power stage. The SW switch of the Sample & Hold block, identified by the dashed line perimeter in the figures, coupled in series between the output of the operational amplifier and the capacitor Csh, is driven by the synchronizing signal Sync. This signal transfers and maintains the voltage level existing at the output of the operational amplifier by charging the capacitor Csh. Hence, by sampling every time at the middle of the active driving phase and at the middle of the recirculation phase of the current load through the load, intrinsically, far from the switching instants of the outputs of the power stage, at the output of the Sample & Hold block a noise free voltage, proportional to the average value of the current through the load is obtained. In these embodiments the system is not immune of the noise generated by the switchings of the power stage under any condition of operation. Indeed, if the active phase or the recirculation phase of the current become exceptionally short, for example when the duty-cycle of the current ripple approaches 0% or 100%, it is possible that the output voltage of the operational amplifier, which senses the current through the load be read, when the transient generated by the switching an output of the power stage has not decoyed completely. Therefore, at the output of the Sample & Hold block it may occur to have a voltage influenced by noise generated by the output switchings. To overcome also these residual inconvenients and ensure immunity to noise also under these extreme conditions of operation, a Mask signal is generated every time a switching of any of the power stage outputs takes place. This Mask signal is also represented in the diagrams of Figure 3 and has a duration that is slightly longer than that of the disturbances superimposed to the output voltage of the SENSE AMPL. caused by switchings of the output stage. In the alternative embodiment scheme of Figure 5 it is shown the way such a masking signal is used to overcome the above noted drawbacks. By ANDing the MASK pulse with the synchronizing pulse Sync of the sampling of the current, an output signal is obtained which will be forced to zero for the whole duration of the masking signal, whereas it will be equal to the synchronizing signal for the remaining time. By using the logic AND output to control the closing of the SW switch the sampling of the current is disabled for a predefined time every time one of the outputs of the power stage switches. In this way, when the active phase of the current through the load has a duration inferior to that of the masking time, the current sampling is not carried out during such a phase, but is effected only during the recirculation phase. Similarly, when the current recirculation phase has a duration inferior to that of the masking time, the sampling is not effected during the recirculation phase, rather it is carried out only during the active phase. In this way, the sampling of the value of the current through the load, by closing the switch of Sample & Hold block is done only at instants when the output voltage of the sense amplifier is not affected by noises caused by the switchings of the PWM output stage, thus attaining a noise free voltage signal proportional to the average value of the current through the load to be used as feedback signal by the control loop of the current. A third alternative embodiment consists of sampling the current through the load by reading the SENSE AMPLIFIER output, by using only the Mask signal to close the SW switch. In this way, with regard to the preceding case in which the current is sampled at exactly half way the active phase and half way the recirculation phase, the SW switch that transfer the output voltage of the sense amplifier onto the capacitor Csh remains closed for the whole duration in which the masking signal is at a high logic level, and is opened only to mask the transient disturbances caused by the switchings of the power stage for a duration in which the same signal is at a low logic level. The voltage signal obtained at the Sample & Hold block output, is a signal transparent to the sense amplifier output for the whole duration in which the masking signal is at a high logic level, following in this way the trend of the current through the load instant by instant. During the phase in which the masking signal is at a low logic level, this signal keeps the voltage level that existed before the opening of the SW switch, ensuring in any case a signal free of disturbances present at the SENSE AMPLIFIER output.

Claims: A method of monitoring the current flowing through an inductive load (LOAD) driven in a PWM mode through a bridge power stage (POWER AMPL.), the two half-bridges of which are controlled by respective first and second amplifiers, to a first input of which is respectively applied one of a pair of complementary periodic reference signals (TRIA_0, TRIA_180), the method comprising the steps of amplifying by a sensing amplifier (SENSE AMPL.) a signal existing on a current sensing resistor (R S ) functionally connected in series with the inductive load (LOAD), for producing an amplified signal representative of the current in the inductive load; feeding the amplified signal to an input of an error amplifier (ERROR AMPL.), the output of which (Err. Out) is connected in common to a second input of said first and second amptifiers, and characterized in that it comprises the steps of: sampling the amplified signal from the sensing amplifier using a sample and hold circuit comprising a sampling switch (SW) and a storage capacitor (Csh), monitoring an average value of current in the inductive load by sampling at a half way instant of an active driving phase and at half way instant of a current recirculation phase by closing said sampling switch (SW) based up a synchronizing pulse (SYNC) that coincides with the virtual zero crossings of said two reference periodic signals out of phase by 180 degrees from each other (TRIA_0, TRIA_180) of the PWM driving system. The method according to claim 1, in which the closing of the sampling switch is controlled by the output of a two-input logic AND gate (A), combining said synchronizing pulse (SYNC) and a masking signal (MASK) of a preestablished duration generated at every switching of said power bridge stage. The method according to claim 1, characterized in that the closing of the sampling switch is controlled by a masking signal (MASK), having a preestablished duration and being generated at every switching of. said bridge stage. A system for monitoring the current flowing through an inductive load (LOAD) driven in a PWM mode through a bridge power stage (POWER AMPL.), the two haft-bridges of which are controlled by respective first and second amplifiers, to a first input of which is respectively applied one of a pair of complementary periodic reference signals (TRIA_0, TRIA_180), the system comprising a sensing amplifier (SENSE AMPL.) receiving a signal present on a sensing resistor (R S ) functionally connected in series to the load for outputting a signal representative of the current in the load; an error amplifier (ERROR AMPL.) having a first input coupled to the output of said sensing amplifier (SENSE AMPL.) and a second input coupled to a reference voltage (Vref) and having an output coupled in common to a second input of said first and second amplifiers of said bridge power stage, and characterized in that it further comprises: a sample and hold circuit functionally connected to the output of said sensing amplifier (SENSE AMPL.) and comprising at least a sampling switch (SW) and a storage capacitor (Csh); means generating a synchronization signal (SYNC) for closing said sampling switch (SW) during an active driving phase of forcing a driving current through the inductive load (LOAD) and during a recirculation phase of a discharge current in the inductive load (LOAD), temporally coinciding with the virtual zero crossings of said two reference periodic signals out of phase by 180 degrees from each other (TRIA_0, TRIA_180) of the PWM driving system. The system according to claim 4, characterized in that it includes a two input AND gate (A) combining said synchronization pulses (SYNC) and a masking signal (MASK) of a preestablished duration generated at every switching of said bridge stage and having an output coupled to the control node of said sampling switch (SW). The system of claim 4, characterized in that the said sampling switch (SW) is controlled by a masking signal (MASK) of a preestablished duration generated at every switching of said bridge stage.

Inventors: GALBIATI EZIO, SALINA ALBERTO, GALBIATI, EZIO, SALINA, ALBERTO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: H02M7
IPC codes: H02P25, H02P6, H02M7, G01R19