Circuit device for driving an AC electric load

Abstract: The invention relates to a circuit device (1) for driving an A.C. electric load (M), which device is of the type incorporating a rectifying bridge (2) having a first input (A) connected to one terminal (H) of the electric load (M) and a second input (A') connected to an plug (5) of an A.C. mains supply, the rectifying bridge (2) having output terminals (B,B') connected to a power switch (3) which is controlled by an electric signal (PWM). The device comprises a first circuit loop-back path (7) connected in parallel to the electric load (M) and a second circuit loop-back path (8) connected in parallel to the electric load (M), the first (7) and second (8) paths being alternately activated by the positive and negative half-waves of the mains supply when the switch (3) is in its "off" state.

Description: Field of the Invention This invention relates to a circuit device for driving an A.C. electric load. More particularly, but not exclusively, the invention relates to a device of a type incorporating a rectifying bridge which has a first input connected to one terminal of the electric load and a second input connected to a plug of an A.C. mains supply, the rectifying bridge having output terminals connected to a power switch which is controlled by an electric signal. As is well recognized in this specific field, the simplest way of varying the supply voltage to an electric motor, or resistive/inductive load, is that of using a Triac device effective to divide the mains voltage being applied to the motor. This technique is cost-efficient, but has a problem in that it introduces harmonics in the current waveform of the mains supply. Stringent European standards have set restrictions on the harmonic contents of mains current, and today's trend favours the use of circuit devices based on bridge structures which produce PWM (Pulse Width Modulation) signals and are less prone to introduce harmonics. Prior Art Several types of drive circuits for A.C. motors are known from the literature. These circuit types use basically D.C./A.C. switches with inverters, whereby a sinusoidal voltage is output which can be varied in amplitude and frequency in quite an independent manner. All these conventional type circuits are based on bridge circuits producing PWM signals. The D.C. voltage is obtained by rectifying and filtering the mains supply voltage according to a scheme which is known per se and essentially as shown in Figure 1 of the accompanying drawings. A rectifier diode is connected in parallel to a filter capacitor, and to a switch having the electric motor, or another electric load such as a lamp or an electric oven, connected across it. These drive circuits have a drawback in that, additionally to introducing harmonics in the waveform of the mains current, they incorporate fairly expensive components. On the other hand, approaches based on the use of Triac devices lead to acoustic noise being generated by A.C. motors, when such motors are regulated by phase control. Such is the case with kitchen hoods, for example, where this technique is often used for supplying and driving the suction fans for cost reasons. The higher order harmonics thus produced cause metal parts and the magnetic circuits associated with the motor to vibrate, thereby producing a rattling noise which is surely objectionable by the house occupants. For varying the voltage applied to an A.C. load, it has been proposed of driving electric motors by a divided mains voltage sinusoid, as achieved by controlling the conduction angle of a thyristor. In this way, the voltage at the load is made to depend on the turn-on angle alpha of the thyristor. However, not even this method can solve the problem of harmonics appearing on the mains current. A drive circuit for A.C. motors comprising a rectifying bridge is known from the European Patent Application No. 0 221 574 filed on 07-11-1986 in the name of Brown, Boveri & CIE. Also known from the European Patent Application No. 0 573 197 filed on 25-05-1993 in the name of Strand Lighting Limited is a dimming system comprising a power distribution means connected to an A.C. mains. The underlying technical problem of this invention is to provide a circuit device for driving A.C. loads, which has constructional and functional features such that the load can be connected directly to the A.C. supply line, thus drastically reducing the probability of current harmonics being introduced on the supply line, and overcoming the limitations and drawbacks of available prior art solutions. Summary of the Invention The concept behind this invention is one of providing a double bi-directional loop-back path connected in parallel to the electric load. Advantageously, the first and second paths are activated alternately by the positive and negative half-waves of the mains supply. Based on this concept, the technical problem is solved by a circuit device as previously indicated and defined in the characterizing portion of the appended Claim 1. The features and advantages of a device according to the invention will be apparent from the following description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings. Brief Description of the Drawings Figure 1 is a diagramatic view of an electric load driver according to the prior art. Figure 2 is a diagramatic view of a circuit device according to the invention. Figure 3 is a voltage vs. time plot of the base-emitter voltage drop through a transistor incorporated to the device of Figure 2. Figure 4 illustrates the behaviour of the current flowing through a power switch of the device of Figure 2 during a full cycle of the mains current. Figure 4A is drawn on the same time base as Figure 4 to illustrates the reversal of the mains negative half-wave as obtained through a circuit portion of the device shown in Figure 2. Figures 5 and 5A illustrate schematically the current and voltage patterns at the switch incorporated to the device in Figure 2. Figure 6 shows diagramatically a modified embodiment of the inventive device. Figure 7 schematically illustrates an exemplary use of a set of devices according to the invention for driving a three-phase electric load. Detailed Description Referring specifically to Figure 2 of the drawings, generally and schematically shown at 1 is a circuit device according to the invention for driving electric loads M which are supplied an A.C. current. These loads could be electric motors, or electric apparatus of any other types presenting a resistive/inductive load and to be driven by varying the mains supply voltage. The 220V A.C. mains supply is represented by a pair of plugs 4, 5 having a filtering capacitor C2 connected across them. The circuit device of this invention includes a bridge type of rectifying structure 2 which is connected between a mains plug 5 and the electric load M. In particular, the bridge 2 has two input terminals A, A' and two output terminals B, B'. Preferably, the bridge 2 comprises diodes D1, D2, D3, D4. The input terminal A is connected to a terminating node H of the electric load M, and the other terminal A' is connected to the mains plug 5. The output B, B' of the bridge 2 is connected to an electronic switch 3 which is controlled by a signal PWM. Associated in parallel with the switch 3 is a series of a resistor R1 and a capacitor C1, which are connected between the output terminals B, B' of the bridge 2. A discrete bipolar transistor Q1, of the NPN power type, receives the signal PWM on its base terminal, and has its conduction terminals connected between the output terminal B of the bridge 2 and a ground reference supply GND. The other output terminal B' is also taken to the ground reference GND. The device of this invention comprises a bi-directional current loop-back arrangement which is controlled by the same current as is flowed through the inductive windings of the electric load M. A double loop-back circuit path is provided, according to the invention, in parallel with the electric load M. Specifically, a first component pair formed of a selection transistor Q2 and a diode D7, are arranged to form, in accordance with the invention, a first circuit path between the mains plug 4 and the nodes H, K at the input of the electric load M. A second pair formed of a selection transistor Q3 and a diode D9 are arranged, according to the invention, to form a second circuit path 8 between the mains plug 4 and the nodes H, K at the electric load M input. The first path 7 comprises the NPN bipolar transistor Q2, acting as a selector and having its base terminal connected to the mains plug 4 via a resistor R. The conduction, or collector and emitter, terminals of this transistor Q2 are connected to the node H of the load M, via a diode D7, and to the node K of the load M, respectively. The second circuit path 8 comprises a bipolar transistor Q3, acting as a selector and having its base terminal connected to the mains plug 4 via a resistor R2. This transistor Q3 is a PNP transistor. The conduction, or collector and emitter, terminals of the transistor Q3 are connected to the node K of the load M, and via a diode D9 to the node H of the load M, respectively. Advantageously, the selection of the two diodes D7 and D9 is differently provided using the positive and negative half-waves of the mains voltage signal. The structure of the device 1 also comprises a double pair 6 of diodes D1, D6, D8 and D10, which diodes are connected in antiparallel and essentially connected between the bases and the emitters of both transistors Q2 and Q3. This double diode pair 6 is further connected between the mains plug 4 and the terminating input node K of the load M. A capacitor C3 is connected between the input and the output of the double diode pair 6. The operation of the circuit device according to the invention will now be described. During the time period when the power transistor Q1 is brought to its "on" state, the load M is supplied the mains voltage 220V directly. Upon the transistor Q1 being turned off, when it is acting as an open switch, the current would disappear at once, and the voltage might attain very high values at all the circuit nodes. In the circuit of the invention this is prevented from occurring, however, by that the double loop-back path 7, 8 provided enables the current to flow bidirectionally and gradually from the opposite ends of the electric load M. In particular, the first 7 and second 8 paths connected in parallel to the electric load are alternately activated, according to whether the positive half-wave or the negative half-wave happens to be present on the supply line. In accordance with the invention, perfect synchronization with the zero-crossing of the current flowing through the circuit is achieved. For the purpose, the voltage drop across the series of the two diodes, which are connected in series with the main path and forward biased, is utilized. In fact, with an incoming current to the circuit, the voltage drop across the series of the diodes D1, D6 in the double pair 6 would be positive with respect to the emitters of both transistors Q2 and Q3. As a result, the transistor Q2 would be brought to its saturation range, and the transistor Q3 would be blocked. The capacitor C3 is effective to filter the voltage drop of this series of diodes D1, D6, preventing the transistors from being turned off when the power switch 3 is turned off. Under this condition, the current is allowed to flow through the path comprised of the transistor Q2, diode D7 and load M. On the other hand, with an outgoing current from the circuit, the voltage drop across the series of the diodes D8, D10 in the double pair 6 would be the opposite from the previous case, and force the transistor Q2 to the "off" state and the transistor Q3 to saturation. Figure 3 shows schematically a voltage vs. time plot of the base-emitter voltage drop through a full cycle of the mains supply, and the corresponding change in the loop-back current during one of the mains half-waves. Shown in Figure 4 is the pattern of the current through the transistor Q1, over a full cycle of the mains supply. In Figure 4A, which has the same time base as Figure 4, the reversal of the mains negative half-wave, as provided by the rectifying bridge 2 formed of the diodes D2, D3, D4 and D5, is shown. The series of the two diodes D1, D8 connected in antiparallel ensures saturation of the transistors even at low values of the current being flowed therethrough, and hence low operating power values of the load M. The series of the resistor R1 and the capacitor C1, in parallel with the power switch 3, allows the overvoltage peaks to be kept low, along with the voltage differential and the voltage fluctuations in the upper region of the mains sinusoid. The size of the components R1 and C1 will be selected at the designing stage to suit the type of the electric load and the type of the switch used. Figures 5 and 5A illustrate schematically the patterns for the switch 3 current and voltage. As can be seen, the voltage peak has been well flattened in the upper region. This is made possible by the loop-back paths provided by the invention. Figure 5A shows, in particular, the mains sinusoid envelope. In essence, a major advantage of the circuit according to the invention is that the overvoltage problems of the power switch 3 have now been significantly attenuated by the use of devices which can operate on a lower voltage, besides being more cost-efficient. Similar results are to be obtained with a second embodiment, depicted in Figure 6, wherein an NPN bipolar transistor Q4 is substituted for the PNP transistor Q3 in the path 8. Figure 6 shows schematically a modified embodiment, denoted by the numeral 10, of the circuit device according to the invention. In this embodiment, the second path 8 also includes an NPN transistor Q4. In this case, however, a different configuration is provided for that circuit portion which comprises the antiparallel diodes. In particular, two discrete portions 6', 6'' are provided, each comprising a single diode and a series of two diodes connected in antiparallel to the single diode, as shown in Figure 6. During the positive half-wave, the voltage drop is positive with respect to the potential at the base of the transistor Q2. The voltage drop across the diode series D6, D8 is sufficient to bias the transistor Q2. Under this condition, the current is allowed to flow through the diode D7 while the switch 3 is in the "off" state. The transistor Q3 is held blocked instead. On the other hand, during the negative half-wave, the transistor Q2 is held blocked, and the transistor Q3 brought to the saturation range, enabling the diode D9 to loop back the current. The device of this invention can be readily adapted for driving three-phase electric loads, as shown schematically in Figure 7. By using three identical drive devices 1 for each phase, the three discrete loads of a three-phase system M3 can be driven. The wiring diagram of Figure 7 shows this clearly. The circuit device according to the invention does solve the technical problem, and affords a number of advantages, foremost among which is that the electric load can be connected directly to the A.C. mains supply. Furthermore, a single power switch device is employed, instead of a bridge of power devices as in the prior art. The power regulation provided by this device is linear throughout the regulated range. Additional advantages are the absence of torque pulses, and reduced current harmonics on the mains compared to the regulation provided by the phase control method. The reduction in the harmonics obviously results in the vibrations from the physical structure of the motor being also reduced. Accordingly, the device of this invention can make the operation of electric motors effectively less noisy in many applications. Finally, tests carried out by the Applicant have shown that a high level of efficiency can be attained by the drive system, with small overall losses. The easily implemented filter arrangement made possible by the high operating frequency, and the small number of components used, make the device of this invention attractive for many applications. Changes and modifications may be made in the device within the scope of the invention as defined by the appended claims.

Claims: A circuit device (1) for driving an A.C. electric load (M), which device is of the type incorporating a bi-directional switch (2, 3) including a rectifying bridge (2) having a first input (A) connected to a first terminal (H) of the electric load (M) and a second input (A') connected to a first plug (5) of an A.C. mains supply as well as a power switch (3) which is controlled by an electric signal (PWM) and connected to the output terminals (B,B') of said rectifying bridge (2)a, and is characterized in that it comprises: a first circuit loop-back path (7) connected in parallel to the electric load (M); a second circuit loop-back path (8) connected in parallel to the electric load (M), said first (7) and second (8) paths being alternately activated by the positive and negative half-waves of the mains supply and carrying the freewheeling load current when the bi-directional switch (2, 3) is in its "off" state, and a double pair (6) of diodes (D1,D6;D8,D10) connected in an antiparallel configuration between a second terminal (K) of the electric load (M) and a second plug (4) of the A.C. mains supply as well as to said first and second paths (7, 8). A device according to Claim 1, characterized in that the current loop-back provided by the first (7) and second (8) paths is bi-directional. A device according to Claim 1, characterized in that said first path (7) comprises a selection transistor (Q2) and a diode (D7) connected between said first and second terminals (H,K) of the electric load (M). A device according to Claim 3, characterized in that said transistor (Q2) is an NPN bipolar transistor. A device according to Claim 3, characterized it further comprises a resistor (R) inserted between the base terminal of said transistor (Q2) and said second plug (4) of the A.C. mains supply. A device according to Claim 1, characterized in that said second path (8) comprises a selection transistor (Q3) and a diode (D9) connected between said first and second terminals (H,K) of the electric load (M). A device according to Claim 6, characterized in that said transistor is a PNP bipolar transistor. A device according to Claim 7, characterized it further comprises a resistor (R2) inserted between the base terminal of said transistor (Q3) and said second plug (4) of the A.C. mains supply. A device according to Claim 1, characterized in that it comprises a capacitor (C3) connected in parallel to said double diode pair (6). A device according to Claim 1, characterized in that it is associated with each load of a three-phase load. A device according to Claim 1, characterized in that said double diode pair (6) comprises two discrete portions (6', 6'') each comprising a single diode and a series of two diodes connected in antiparallel to said single diode.

Inventors: CUCUCCIO ANTONINO, DI GUARDO MARIO, MILAZZOTTO ANTONINO, NICOSIA NICOLA, CUCUCCIO, ANTONINO, DI GUARDO, MARIO, MILAZZOTTO, ANTONINO, NICOSIA, NICOLA
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: H02M5
IPC codes: H02M5, H02P25