Half-bridge turn-off slew-rate controller using a single capacitor

Abstract: In a half-bridge output stage employing a complementary pair of output power transistors, each driven through an integrating stage for controlling the slew-rate, a single integration capacitance is conveniently shared by the two integrating stages that drive the power transistors. A pair of switches connect the single integrating capacitance to the input of either one of the two integrating stages and are controlled by a pair of nonoverlapping signals that have a certain advance with respect to the pair of logic signals that drive the half-bridge stage. In the case of a driving system of a multi-phase machine, the two configuring switches of the single integration capacitor may be driven by a pair of control signals that drive a different phase winding of the multi-phase machine, thus eliminating the need of a dedicated circuitry for generating said pair of anticipated signals to control the configuration switches.

Description: This invention relates to a slew-rate controller of power transistors in a half-bridge output stage employing a complementary transistor pair. When driving inductive loads in a switching mode, abrupt changes of current in the windings may cause electromagnetic emissions, besides causing torque discontinuities in electric motors that cause a noisy operation. A typical situation is illustrated in Fig. 1, which shows three separate half-bridge output stages, each driving one winding (L1, L2, L3) of a three-phase, DC, brushless motor. If, for example, it is necessary to switch the current from the power transistor M1 to the power transistor M2, an abrupt turn-off of M1 makes the inductance of the winding L1 discharge on the supply rail through the diode D4. This determines a drop of the current through the sensing resistance Rs of the control system, and a consequent change in the current passing through the excitation winding of the motor. In fact, the current regulating circuit will respond to keep the voltage across the sensing resistance Rs constant, by changing the bias conditions of the output transistors thus altering the current in the motor windings. This causes an undesirable variation of the torque. At the same time, the injection of the discharge current of the winding's inductance on the supply rail may cause disturbances on the line. It is common knowledge that these problems may be considerably reduced by controlling the turn-off time of the power transistor. In doing so, the following should be taken into account: a) the turn-off time must be long enough to allow the inductance to fully discharge; b) an excessively long turn-off delay would affect negatively speed performance, as occurs in driving a motor. Commonly, a turn-off slew-rate control is individually implemented for each output power transistor of the stage (see US-A-5191269). On the other hand, depending on the characteristics of the fabrication process of the integrated circuit, complementary and structurally similar architectures are highly preferable because the advantages provided in terms of a relatively simpler design and layout and higher overall performance. A half-bridge output stage architecture employing a pair of push-pull connected, complementary, power transistors, offers significant advantages as compared to architectures employing transistors of only one type which, in a bridge configuration, require driving circuits that are not "specular" to each other. An object of the invention is to provide a helf-bridge output stage that is itself particularly effective in controlling the turn-off slew-rate of the output transistors of a half-bridge stage and which is extremely simple to realize and cost-efficient. This object is solved by the output stage according to Claim 1. A slew-rate controller has now been devised and is object of the present invention. The output stage employs a pair of push-pull connected, complementary, power transistors. A main advantage of the novel device of the invention is that a single capacitor is shared by two structurally similar integrating stages that are used to drive the two complementary output transistors, respectively, so as to control their slew-rates during a turn-off phase. A single integrating capacitor is switched on either stage by two switches, controlled in phase opposition to each other by a pair of control signals that are suitably out-of-phase with respect to the pair of logic signals that drive the two complementary output transistors, respectively. Another important advantage of the device of the invention, when employed in the driving circuitry of a multi-phase, brushless, DC motor, is that it does not require a dedicated circuit for generating the control (timing) signals for the pair of switches used in the slew-rate controller circuit. In fact, the pair of driving signals of a different excitation phase of the motor can be profitably used for that purpose. They are readily available within the motor drive system itself. The various aspects and advantages of the device of the invention will stand come out more clearly from the following description of one important embodiment, whose description is herein incorporated by express reference. Figure 1 , as already mentioned, shows a driving circuit diagram of the windings of a three-phase, DC, brushless motor. Figure 2 shows the functional circuit diagram of a single, half-bridge stage provided with a turn-off slew-rate controller for the push-pull connected, complementary output transistor pair, according to the present invention. Figure 3 shows the circuit diagram of a driving circuit of a three-phase DC brushless motor, employing three half-bridge stages with turn-off slew-rate controller of the present invention. Figure 4 shows the timing diagram of the signals (states) of the three complementary transistor pairs of the half-bridge output stages that compose the driving circuit shown in Fig. 3. Although the use of MOS transistors is shown in the figures, this remains a matter of design choice and should not be intended as a limitation. Bipolar transistors can be used as well. A better notion of how the invented system of the invention works can be acquired by referring to the timing diagrams shown in Fig. 4 and to the circuit of Fig. 3 and/or to the functional, circuit diagram of a single half-bridge stage, as shown in Fig. 2, wherein the same symbols are used, for facilitating cross-referencing. By convention, to a "low" state ("0" on a potential scale) of the signals in the timing diagrams of Fig. 4, corresponds an OFF condition of the respective output transistor; conversely, to a "high" state (Vdd potential) of a signal corresponds an ON condition of the respective output transistor. To simplify the explanation, only the half-bridge that drives the U-phase winding of the three-phase motor is shown in detail in Fig. 3, whereas the other two half-bridges, of the V and W phase windings, respectively, are depicted as circuit blocks. Naturally, all three half-bridge stages shown in Fig. 3 are identical and the remarks made for U half-bridge also apply to the other two similar half-bridge circuits. With reference to the U half-bridge, the controlled turn-off of the transistor PU must take place at instant t2, whereas the transistor NU will be turned-off at the subsequent instant t5. The timing diagrams of Fig. 4 show how, during the T2 interval that precedes the turn-off instant t2 of the transistor PU, the state of the output transistors is the following: PU=ON, NW=ON. This means that current is passing through the Lu and Lw windings. During the interval T2, the node A of the circuit shown in Fig. 3 is at ground potential, whereas the node C is at the supply (Vdd) potential minus the ohmic voltage drop that occurs through the conducting transistor PU. In order to prevent the CU capacitor from being at a "floating" potential at the switching instant t2 (beginning), in view of the fact that both the switches S1 and S2 are OFF and to ensure conversely that the CU capacitor be correctly polarized at a given potential, according to the working conditions of the output power transistor, S1 must be switched-on with a certain advance with respect to the switching instant t2. For this purpose, existing logic drive signals pertaining to a different phase winding of the motor can be conveniently used. In fact, by controlling the switch S1 with the control signal of the transistor NW, the desired pre-polarization of the CU capacitor, starting from instant time tl, (during the interval T2) is ensured. This also prevents the generation of any further disturbance, since S1 actually switches at the same time as the output stages. At the instant t2, the PU control signal determines the beginning of a turn-off phase of the PU transistor by bringing the output of the logic drive buffer BuffAU to a "tristate" condition (high output impedance). Therefore, the node A will be connected only to the generator I1, which will inject current into the terminal of the CU capacitor at the lowest-potential. The node C will thus achieve the potential that results from the following formula: Vc = Vin - K*t where Vin is the voltage at node C and the time constant K is determined by the ratio: K = I1/CU (V/µsec) In this way, the desired slew-rate control of the PU transistor during a turn-off phase is achieved. In fact, the slew-rate value will be equal to K and will be easily varied by adjusting the I1 current. By applying the signal PW that drives the PW transistor of the W half-bridge of the three-phase driving system shown in Fig. 3 to the control terminal of the switch S2, the same remarks made in connection with a turn-off phase of the PU transistor, beginning at the instant t2, also apply to a turn-off phase of the NU transistor, beginning at instant t5. In this case, a pre-polarization of the CU capacitor takes place during the interval T5 that precedes the switching instant t5, owing to an anticipated transition of the PW signal that drives the S2 switch with respect to the switching instant t5 (turn-off phase of the NU transistor). Naturally, the slew-rate controller of the present invention, as outlined in Fig. 2, is also useful for driving mono-phase loads. Clearly, if no logic signal is available for controlling the two switches S1 and S2 of the circuit of the invention, a dedicated logic circuitry will be required for generating a pair of complementary control signals: sr and sr' for controlling the switches S1 and S2 in phase opposition to each other. Common circuits can produce the required pair of signals sr and sr', having a certain advance with respect to a pair of logic signals sl and sl' that drives the output transistor pair of the half-bridge stage. In particular, the signals sr and sr' may be two non-overlapping signals, that is, they may both be "low" during an interval of the cycle, but in no case they should be both in a "high" logic state. Common logic timing circuits that positively exclude this occurrence are well known in the art and do not require any specific description.

Claims: A half-bridge output stage comprising a pair of, push-pull connected, complementary power transistors (PU,NU), each transistor being driven through a logic driving buffer and an integrating stage that controls the slew-rate of the driven power transistor by a pair of driving signals (s1,s1'), characterized by the fact that, said integrating stages controlling the slew-rate share a single integration capacitor (C), a terminal (8) of which is switched-on an input of either integrator stage by two switches (S1,S2), that are controlled in phase opposition to each other by a pair of control signals (sr,sr') that have a certain advance with respect to the transitions of said pair of driving signals (s1,s1'). An output stage as defined in claim 1, as part of a driving circuit of a multi-phase machine, characterized by the fact that said pair of signals for controlling said two switches (S1,S2) is the pair of signals (S1,S1') that drives the power transistors of a different half-bridge output stage of said multi-phase driving circuit. An output stage as defined in claim 1, characterized by the fact that said pair of signals that controls said switches (S1,S2) of the shared integration capacitor (C) are nonoverlapping, phase opposed signals that assume a state for keeping in an OFF condition both switches during an interval of a control cycle. An output stage as defined in claim 1, characterized by the fact that said complementary transistors are field effect transistors. An output stage as defined in claim 1, characterized by the fact that said complementary transistors are bipolar transistors. An output stage as defined in claim 1, characterized by the fact that each of said integrating stages that controls the slew-rate is composed of a buffer whose output is connected to a control terminal of a respective power transistor and has an input node (A) to which a logic driving signal (S1) is fed and which is connected to a charging current generator (Ip) operatively switched on said input of said integration capacitor.

Inventors: FUCILI GIONA, NESSI MAURIZIO, FUCILI, GIONA, NESSI, MAURIZIO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H02P6
IPC codes: H02P6