Sharing of external components for the control of the switching dead-time of a plurality of integrated devices cooperating in driving and R-L multiphase actuator

Abstract: In multiphase driving systems of electric motors in a switching mode employing distinct integrated half-bridge output stages, the use of multiple external components for each integrated half-bridge stage to implement functions of adjustment of the switching dead-time and of disabling/enabling of the half-bridge stage can be avoided and an improved matching obtained by using single external components connected in common to the dedicated pins of all the distinct integrated half-bridge output stages. This is achieved by duplicating the disabling comparator, respectfully for the power transistor of the pull-up branch and for the power transistor of the pull-down branch in each integrated half-bridge stage.

Description: Field of application of the invention The present invention relates in general to integrated circuits for driving external components through half-bridge power stages, and in particular to driving systems for multiphase motors. Background of the invention Integrated circuits for driving systems through half-bridge power stages are widely used in industrial applications as for driving three-phase motors, stepper motors, brushless motors or a generic R-L actuator. When these integrated devices possess only one logic input of command, it becomes necessary to implement a circuit that regulates the switching dead-time to avoid a simultaneous state of conduction of the two branches, that is of the two power devices (HIGH SIDE and LOW SIDE) of the half-bridge stage. In the majority of cases it is necessary to program (adjust) the value of the switching dead-time by taking into account the electric characteristics of the driven R-L actuator. Fig. 1 shows a classical way of realizing such a dead-time programming function, using a dedicated pin PIN of the integrated device lC1. This dedicated pin serves to implement both the programming function of the switching dead-time by suitably dimensioning the external resistor R ex , and the disabling/enabling logic function of the half-bridge power stage of the integrated device through a dedicated external switch S1, controlled by a specific logic command SHUTDOWN. By referring to the conventional scheme of Fig. 1, the amplifier (buffer) A1 restores a certain reference voltage Vbg (typically a band-gap voltage) on the dedicated PIN. Through the external resistor R ex it is then possible to regulate the switching dead-time (Tdt) generated by the homonymous block, by considering that: Tdt = C * V I R ; IR = V bg R ex ; Tdt = C * R ex where C is the delay capacitance within the DEAD-TIME circuit block, V is the voltage existing on such capacitance C at the switching instant and I R is the current that charges the capacitance C. The comparator COMP1 compares the voltage on the dedicated pin with ceratin reference voltage REF1, and produces a shutdown or enabling signal of the output power stage HALF-BRIDGE. In practice, the logic function of "shutdown" is implemented by forcing a low logic stage on the pin PIN as referred to the threshold REF1 established on the inverting input node of COMP1. According to this typical circuit implementation of a programming function of the switching dead-time and an enabling/disabling function of the half-bridge power stage, the output impedance of the operational amplifier (buffer) A1 is necessarily low to allow a good degree of precision for the circuit as well as immunity to noise. These characteristics of known circuits do not allow, especially when they include several output stages, as in the case for driving multiphase motors, to connect in common all the respective dedicated pins (PIN) and the use of a unique set of external components to implement the function of programming the switching dead-time and of enabling/disabling of all the integrated power devices of the multiphase driving system. In practice, according to the prior art, it is necessary to duplicate the external components and the respective lines for each integrated device (IC1, lC2, IC3), as depicted in Fig. 2. Scope and summary of the invention These drawbacks and limitations of known systems are overcome by the circuit of the present invention which consents the use of a unique set of external components for any number of integrated half-bridge stages, cooperatively driving a multiphase R-L actuator. Essentially, the circuit of the invention employs two distinct disabling comparators respectively for the power device of the pull-up branch and the power device of the pull-down branch of the integrated half-bridge stage. The two comparators have their non-inverting inputs coupled in common with the pin dedicated to implement a function of adjustment the switching dead-time and of enabling/disabling of the half-bridge power stage. A first current generator forces a current on the node of the dedicated pin to which is connected an external resistor, which will be eventually connected to the respective dedicated pins of all the integrated devices, used in driving a multiphase actuator. The inverting input of each of the two comparators of each integrated device is coupled to a capacitor that is charged by a second current generator and the voltage at the terminals of which is forced to ground potential by a switch, driven in phase opposition to the similar switch of the other comparator of the pair, by the logic signal input to the integrated half-bridge stage. Also the disabling command, common to all integrated devices cooperating in driving a multiphase actuator, is implemented by the use of a single switch, connected to the common node of the dedicated pins of the distinct integrated circuits. Therefore, the circuit of the invention allows for a saving of external components and interconnections in all applications wherein it is envisaged the use of two or more integrated circuits (multiphase systems). Moreover, the circuit of the invention ensures a better match among the switching dead-times of both switching fronts of the distinct integrated circuit than was obtained with a known circuit. Brief description of the drawings The various aspects and advantages of the invention will become even clearer through the following description of some preferred embodiments and by referring to the attached drawings wherein: Figure 1 , as already mentioned, is a circuit diagram of a half-bridge integrated device according to the prior art; Figure 2 shows a half-bridge driving scheme of the windings of a three-phase motor according to the prior art; Figure 3 shows a circuit for adjusting the switching dead-time and for enabling/disabling a half-bridge device, according to the present invention and the relevant diagrams; Figure 4 shows how two or more integrated half-bridge devices may share the same external components for regulating the switching dead-time and for enabling/disabling the half-bridge stages; Description of preferred embodiments By referring to Fig. 3, the functioning of the circuit of the present invention may be illustrated as follows: The comparators COMP1 and COMP2 compare the reference voltage Iref applied to the noninverting input thereof with respective voltages V1 and V2 of the respective inverting inputs. The voltage Vref depends on the value of the external resistor R ex connected to the dedicated pin of the device, while the voltages V1 and V2 correspond to the voltages existing on the terminals of identical capacitors C1 and C2, respectively connected to the inverting inputs of the two comparators COMP1 and COMP2. Identical current generators I charge the two capacitors C1 and C2, while a current generator A*I (where A is a constant) forces a certain constant current on the external resistor R ex (in the example of Fig. 3, A is equal to 2). The logic input signal IN of the device and its negate IN drive complementarily the switches connected in parallel to the relative capacitors C1 and C2. While one of said capacitors is kept discharged by the logic input signal, the other capacitor is charged with a constant current uo to reach the respective voltage V1 or V2. During this charging phase the voltage on the capacitance grows linearly. By modifying the value of the external resistor R ex and as a consequence the value of the reference voltage Vref, it is possible to program the desired switching dead-time, because: Tdt = C * V ref I and V ref = R ex * A * I thereby: Tdt = R ex * A * I From the above equation it may be said that the circuit of the invention allows to fix a certain switching dead-time Tdt, which still depends on the value of the internal capacitance (C1=C2=C) and on the value of the external resistor R ex , but the circuit permits to connect in common the dedicated pins of a plurality of devices while maintaining a good accuracy of regulation of the switching dead-time Tdt for all the integrated circuits. An example of such a possibility of parallel (common) connection of several integrated circuits is shown in Fig. 4. If for a single integrated circuit (not connected in parallel) the switching dead-time is: Tdt = C*A* R ex , when there are n integrated circuits connected in parallel, according to the invention, the switching dead-time for all of them will be given by: Tdtp = C * A*n*R' ex , because V ref = n * R' ex *A*I. In order to obtain the same switching dead-time for the single integrated circuit (Tdp=Tdtp), the value of the external resistance R' ex must be equal to: R' ex = R ex / n . An example with n=2 is depicted in Fig. 4. Therefore, the circuit of the invention fully satisfies the stated objective without aggravating the circuit complexity and has also the effect of improving the performance of the single integrated device.

Claims: A driving system for a plurality of half-bridge power stages connected to respective phase windings of a multiphase R-L actuator using a plurality of distinct integrated devices (IC A , IC B ), each driving a phase winding of said multiphase actuator, and comprising a circuit for adjusting the switching dead-time for preventing simultaneous conduction of the two branches that constitute a half-bridge power stage (HALF-BRIDGE) and using an external resistor R ex and an external switch for fixing said dead-time and for enabling/disabling the respective half-bridge stage, characterized in that said circuit comprises two different comparators (COMP1 and C0MP2) each controlling the shutdown of a pull-up transistor device and of a pull-down transistor, respectively, of the respective half-bridge power stage, each comparator having a noninverting input (+) connected in common to a dedicated pin (dtp) of the device onto which a first current generator (2l) forces a current through an external resistor (R ex ) connected to said dedicated pin (dtp), and an inverting input (-) connected to a capacitor (C1, C2) charged by a respective constant current generator (I) and a switch (IN, IN ) in parallel to said capacitor (C1, C2), forcing to ground potential the voltage existing on said capacitor (C1, C2), driven by an input logic signal of the integrated power stage complementarily to the switch of the other comparator; said plurality of integrated devices (IC A and IC B ) having said dedicated pins (dtp A , dtp B ) connected in common and to a single external resistor for fixing the switching dead-time and to a single external switch connected in parallel to said resistor for enabling/disabling the half-bridge stages. A driving system according to claim 1, characterized in that it utilizes three distinct devices, each driving a respective one of three phase windings of a three-phase motor.

Inventors: DE MARCO ALBERTO, GENOVA ANGELO, NOVELLI ALDO, PAPPALARDO SALVATORE, DE MARCO, ALBERTO, GENOVA, ANGELO, NOVELLI, ALDO, PAPPALARDO, SALVATORE
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: H02M7
IPC codes: H02P8, H02M1