MOS-TRANSISTOR BRIDGE CIRCUIT

Abstract: In this MOS-transistor bridge circuit, for obtaining a fast flyback conduction of the current after a normal operation of the circuit, instead of the flyback diodes (4-­ 7) associated with each transistor (1-4) of the bridge, the MOS transistors themselves are employed, driven so as to conduct current from the ground to the power supply, that is in the opposite direction with respect to that of normal operation. For this purpose a control section is provided receiving at the input a fast flyback signal and comprising delay means (10,30,35-39) connected to the disable inputs (EN) of the transistors, so as to delay switching off thereof, and inverters (20; 16)) switching on the two transistors (1, 4) flown by the flyback current.

Description: The present invention relates to a MOS-transistor bridge circuit. As is known, MOS-transistor bridge circuits comprise four transistors, each connected in parallel to a respective flyback diode and controlled at the gate terminal by a circuit intended to ensure switching of the transistors according to three input signals, to obtain the desired operation, and powering a load at the desired current and/or voltage levels. (ELECTRONIC DESIGN, vol 30, no 25, Dec.1982, pages 107-114) A known circuit of this type is shown in figure 1. Herein the MOS transistors 1-4, with the respective diodes 5-8, are driven by AND gates 11-14 according to the signals present on the inputs IN1, IN2 and EN. Respective inverter elements 15 and 16 connect the switching inputs of the AND gates driving the transistors of the same branch, so as to prevent their simultaneous switching on. On the transverse arm of the bridge an inductive load 9 is provided, and the bridge is connected between a supply voltage V CC and the ground through a resistor 10. For the operation of the circuit of figure 1, suppose that each individual MOS transistor is in the on state when it receives a high signal at its gate terminal and, vice versa, is off if it receives a low signal. Accordingly, for each branch, the upper transistor is on when both the enable signal EN and the control or switching input IN1, IN2 are high. Vice versa the lower transistor of each branch is on when the control or switching signal is in the low state, but the enable signal is always high. Moreover the enable signal, if in the low state, is capable of switching off all four transistors simultaneously. Figure 1 also shows the diodes 5-8 connected in parallel to each transistor. These diodes may be formed for example by the intrinsic diodes obtained in the manufacture of the power MOS transistors (DMOS). Accordingly, with a high signal on the gate terminal of each MOS transistor, the same is on and behaves like a resistor with value equal to R DS(ON) of the transistor with in parallel the intrinsic drain-source diode which intervenes in the inverse operating region to limit the excursion of the voltage across it. Vice versa, when the MOS transistor is off with V GS =0, the equivalent circuit is constituted only by the diode, which can thus conduct the current in a direction opposite to that of normal operation of the MOS transistor. Figure 2 illustrates the behaviour of the current I DS versus the voltage V DS , wherein the curve I, which is a straight line with slope R DS (ON), plots the resistive behaviour of the MOS transistor in on condition, while the curve II illustrates the behaviour, typical of the diode, when the transistor is off. From the plot of figure 2 it is furthermore apparent that with a negative current I₁ during flyback with the MOS transistor off, an operating voltage V₂ is obtained, entailing therefore a power dissipation equal to P diss1 =V₂.I₁. Bridge circuits of the described type are extensively used for driving loads, in particular inductive loads such as motors and the like; however, due to the illustrated behaviour, in some cases they have an excessive power dissipation which it is desired to reduce. Accordingly, the aim of the present invention is to provide a MOS-transistor bridge circuit capable of providing a fast current flyback with lower dissipation with respect to known bridges, so as to allow its use in an increasingly large number of applications. Within this aim, a particular object of the present invention is to provide a bridge circuit which can be controlled by means of simple, easily producible elements, so as to obtain an overall circuit having a complexity comparable with, or only just greater than, that of known circuits. Not least object of the present invention is to provide a bridge circuit having reliable operation in any condition. The indicated aim, the mentioned objects and others which will become apparent hereinafter are achieved by a MOS-transistor bridge circuit, as defined in the accompanying claims. In practice the invention is based on the observation that the bridge circuit has less dissipation if the MOS transistors are kept on during flyback, so as to conduct current in an opposite direction with respect to that of normal operation at a lower voltage with respect to the switching-on voltage of the associated diodes. Therefore according to the invention the bridge circuit, to obtain fast current flyback, is not switched off by acting on the enable signal but is driven so that two of the four transistors remain on following the curve I of figure 2 also in the third quadrant, so that with reverse current I₁ the voltage across them is V₁, and on each MOS transistor in the on state there is a dissipated power P diss2 = V₁ . I₁ obtaining therefore a dissipated power reduction equal to P diss = (V₂-V₁)I₁ Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is an equivalent electric diagram of the known bridge circuit; figure 2 plots the behaviour of the current and of the voltage for a MOS power transistor, in the on and off states; figure 3 is an equivalent electric diagram of the bridge circuit according to the invention, with an example of circuitry capable of driving the bridge circuit according to the desired operation; figure 4 plots the behaviour, versus time, of some signals taken at appropriate points of the circuit of figure 3; and figure 5 is an equivalent diagram of only the bridge circuit, showing parasitic structures existing in integrated bridges. With reference to figure 3, the bridge circuit according to the invention comprises a central part fully similar to known circuits, the elements whereof, for the sake of consistency, have been given the same reference numerals, and a driving part for the operation of the bridge circuit in the desired region. In detail, the circuit of figure 3 again comprises four MOS power transistors 1-4, each associated with a respective diode 5-8 (constituted for example by the intrinsic diodes of DMOS transistors) and controlled at the gate terminal by respective AND gates 11-14. As in known circuits, each AND gate has a first enable input, receiving a signal EN, and a second control or switch input to drive in the on or off state the associated transistor. In detail, the gate 11 receives at the input the signal IN1 which is supplied, inverted by the inverter 15, also to the gate 12, while the gate 13 receives at the input the signal IN2 which is supplied, inverted by the inverter 16, also to the gate 14. As in figure 1, the bridge circuit is connected between a supply voltage V CC and the ground by a resistor 10, while 9 indicates the inductive load. According to the invention, the circuit has been provided with a fast flyback control section intended to drive the bridge circuit in the operation region with lower dissipation. In detail, the control section comprises delay means receiving at the input a fast flyback signal FFW and supplying at the output the enable signal EN so as to switch off the transistors of the bridge only when the current flowing through the load is reduced to zero, and a switching section ensuring switching on of the MOS transistors during flyback. Therefore, with reference to figure 3, the circuit has a fast flyback input, indicated at FFW, which is directly connected to the input IN2 of the AND gate 13 and connected through an inverter 20 and an OR gate 21 to the input IN1 of the gate 11. The fast flyback input is furthermore connected to the delay circuit which generates the enable signal EN. In detail, the input FFW is connected, through line 37, to an OR gate 38 and, through an inverter 36, to a NAND gate 35 the output whereof is brought to the second input of the OR gate 38 and the second input whereof is connected to the output of a zero-detector circuit 30 connected to the resistor 10. The illustrated exemplifying circuit is completed by components intended to control the switching of the gates 11 and 12 and therefore of the transistors 1 and 2 during normal operation so that the current flowing in the load 9 has the waveform shown in figure 4. Said components comprise a comparator 22 connected with its positive input to the resistor 10 so as to detect the voltage drop existing on said resistor, which is proportional to the current flowing in the load, and connected with its negative input terminal to a reference voltage V REF . The output of the comparator 22 is carried to the reset input R of a flip-flop 23 the set input S whereof is connected to the output of an oscillator 24 and the output Q whereof is brought to the second input of the OR gate 21. Of course the components 21-24 are intended to ensure current to flow in the load in the illustrated direction, and may be duplicated and appropriately connected and interposed between the fast flyback input FFW and the input IN2 to obtain a similar control of the current flowing in the load in the direction opposite to the one illustrated. Said circuit may furthermore be replaced with any other driving system or circuit adapted to obtain a load current with a desired value and behaviour, similarly to known circuits. Thus, with reference to the example shown, initially, when the fast flyback signal switches from low to high, it immediately causes switching ON of the enable signal through the line 37 and the output 39 of the OR gate 38. Accordingly the gates 11 to 14 are enabled, while the signals present on the inputs IN1 and IN2 determine their operating state and therefore that of the associated transistors. Initially, therefore, after the time t₀ (see fig. 4), the input IN2 is high, the input IN1 is low (the output of the flip-flop 23 being low at this moment) and current is supplied to the load through the transistor 3, said load 9, and the transistor 2 as well as the resistor 10. During this phase the current in the load grows linearly and the voltage across the resistor 10 increases. This phase lasts until the instant t₁ when the comparator 22 detects the equality between the voltage V S on the resistor 10 and the reference voltage V REF and therefore sends a reset signal to the flip-flop 23 which thus switches, sending a high signal to the OR gate 21. Accordingly the signal IN1 switches to the high state, causing switching on of the transistor 1 and switching off of the transistor 2, with current flyback through the same transistor 1. During this phase thus the current flowing through the load decreases linearly, as shown in figure 4 in the time interval comprised between t₁ and t₂. Subsequently, at the instant t₂, the oscillator 24 feeds a set pulse to the flip-flop 23 which therefore switches again and sends a low signal to the input of the OR gate 21 which again sends a low signal to the gate 11, causing therefore switching off of the transistor 1 and switching on of the transistor 2. This behaviour continues as long as a high signal is present on the input FFW. Suppose now that at the instant t₇ the fast flyback input signal switches to the low state, to obtain a fast current flyback in the load and switching off of the circuit. As explained above, this phase was obtained, according to the prior art, by directly acting on the enable signal EN which simultaneously switched off the four gates 11-14 and then the associated transistors 1-4. This control allowed the power supply voltage to be fed to the load with appropriate sign to obtain a rapid decrease of the current according to the law in which L indicates the inductance of the load. In fact, switching off the MOS power transistors 1-4 with current flowing in the load in the indicated direction caused switching on of the intrinsic diodes 5 and 8, with flyback of the current through the resistor 10, the diode 8, the load 9 and the diode 5 up to the complete reduction of the current with no need for further controls or synchronizations. With said known control, a dissipation linked to the drop existing on the two diodes 5 and 8 was therefore obtained which however, as previously indicated, could entail problems in particular applications. According to the invention, instead, rapid current flyback is no longer obtained by switching off the driving transistors and conduction through the respective diodes, but by appropriately switching the MOS transistors so that they themselves flyback or conduct the flyback current in the opposite direction with respect to that of normal operation, allowing therefore operation of each MOS transistor at a lower voltage with respect to the operating one, with equal current, of the respective intrinsic diode, as illustrated in figure 2 for a current I₁. To obtain this driving operation, in the example according to figure 3 the fast flyback signal is sent directly to the gate 13 which therefore, on the input IN2, has a low signal, and, negated, to the gate 21 which therefore, for all this operating phase, keeps the output, defining the input IN1, in the high state, regardless of the operation of the current control circuit 22-24. During this phase, the enable delay circuit is active and switches off the bridge circuit with a delay with respect to the fast flyback signal. In fact, after switching of the signal FFW, the enable signal EN remains in the high state since the output of the zero-detector 30 (which switches from the low state to the high state upon detection of the passage through zero of the voltage existing on the resistor 10) is still low, such as to keep the output of the NAND gate 35 in the high state, thereby maintaining the output 39 of the OR gate 38 in the high state. Accordingly all the gates 11-14 are enabled, though only the gates 11 and 14 are operative and keep the respective transistors 1 and 4 on. Thus the current flows through the resistor 10, the transistor 4, the load 9, the transistor 1, with the bridge operating at lower voltages and therefore lower dissipation with respect to the prior art. This flyback phase (in which the current in the load and the voltage detected by the resistor 10 have the behaviour shown in figure 4 in the interval comprised between the instants t₇ and t₈) ends when no more current flows in the load. In fact at this instant (instant t₈) the voltage on the resistor 10 (with negative value) is zero and tends to reverse its sign. This situation is detected by the detector circuit 30 which therefore switches with its output to the high state. Accordingly the NAND gate 35, receiving at its inputs two high signals, switches to the low state, causing therefore also switching of the OR gate 38 and of the enable signal EN to the low state. Accordingly the driving gates 11-14 and therefore the related transistors 1-4 are simultaneously disabled, with switching off of the circuit. The operation of the circuit may therefore be restored by returning the fast flyback signal FFW to the high state. As is apparent from the previous description, the invention fully achieves the intended aim and objects. In fact, by driving power MOS transistors of the bridge circuit so that these appropriately conduct the fast flyback current instead of the respective associated diodes, an operation with lower power dissipation is obtained, allowing use of this circuit even in those cases in which it is desired to keep the dissipated power low for reasons of excessive heating or of reduction of power demand of the circuit. In fact, by appropriately switching on the flyback transistors, these behave like resistors, following the current curve indicated as I in figure 2 and therefore obtaining an improvement with respect to the conventional operation using the associated diodes with operation along the curve II of figure 2 with greater voltage. The driving concept according to the invention, which entails a reduction of the dissipated power for bridge circuits of any type, both integrated and discrete, is even more advantageous in the case of integrated bridges. In fact, in the case of these last, each MOS transistor is associated to a parasitic bipolar transistor, always susceptible to switching on and intervening during the flyback phase, entailing in itself a dissipation of power. For a greater understanding of this advantage, reference should be made to figure 5, illustrating the parasitic transistors, indicated at 45 and 46, typical of an integrated structure, and associated with the power MOS transistors 1 and 4, wherein, for the sake of greater clarity, the parasitic transistors of the MOS transistors 2 and 3 have been ignored. As can be seen, the upper MOS transistor 1 is associated to a bipolar PNP transistor 45 having the base-emitter junction in parallel with the drain-source diode 5. In the case of the lower power MOS transistor 4, an NPN-type bipolar transistor 46 is instead formed, also having the base-emitter junction in parallel with the diode 8. These parasitic transistors, in the case of fast flyback through the diodes 5 and 8, according to the prior art, could be switched on and contribute to the power dissipated in the circuit, therefore aggravating the above described problems. In fact, with a current I L circulating through the structure formed by the diode and by the base-emitter junction of the parasitic transistor, since and since generally for parasitic transistors the relation β « 1 is valid, the collector current is I C ≃βI E =βI L . Consequently in each individual parasitic transistor a further power is dissipated equal to P′ diss = 2V cc .I L .β which, though the fraction of current flowing through said parasitic transistor is low, is normally a non-negligible amount since the collector current is multiplied by the entire voltage supply. This further dissipation is therefore eliminated with the driving operation according to the invention since, by virtue of the current flyback through the power MOS transistors which, as has been shown, operate at a lower voltage than the switching-on voltage of the diode and therefore of the parasitic transistor, prevent the operation of said parasitic transistors and therefore eliminate a cause of further dissipation. The invention thus conceived is susceptible to numerous modifications and variations, all of which are within the scope of the inventive concept. In particular the fact is stressed that the delay circuit for the generation of the disable signal delayed with respect to the fast flyback signal, shown in figure 3, is merely exemplifying and may be replaced with any circuit capable of providing the same operation, for example a flip-flop connected with its set input to the fast flyback signal and with its reset input to the zero detector. However, other solutions are conceivable, for example without closed-loop control of the current circulating in the load. In fact, if, during stable operation, the load is made to work at a specific current, it is possible to approximately determine beforehand the time required in order that the current drops to zero, and therefore to cause the enable signal to switch to the low state with a delay equal to that time, without monitoring of the current flowing in the load. Furthermore, the control circuit for switching the gates 11-14 may be completed symmetrically for switching on the transistors 2 and 3 and a fast current flyback through these in the case of current flowing in the opposite direction. Furthermore the driving circuit itself may be structured in any manner adapted to provide the desired adjustment. Moreover, all the details may be replaced with other technically equivalent ones. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: A MOS-transistor bridge circuit, comprising a first branch including a first and a second MOS transistors (3,4) mutually connected in series, a second branch including a third and a fourth MOS transistors (1,2) connected mutually in series and in parallel to said first branch, said first and third transistors (3,1) being furthermore connected to a first higher reference potential line (V CC ) and said second and fourth transistors (4,2) being connected to a second lower reference potential line, each of said branches defining an intermediate tap for connection to a load (9), and four driving gate circuits (11,14), each associated with a respective transistor and comprising a first enable input terminal, a second switch input terminal and an output, said enable terminals being mutually connected and defining a common enable input (EN), said switch input terminals of said gate circuits associated with said transistors (3,4) of said first branch being mutually connected through a first inverter element (16), said switch input terminals of said gate circuits associated with said transistors (1,2) of said second branch being mutually connected through a second inverter element (15), each of said outputs of said gate circuits being connected to a gate terminal of the respective associated transistor, said common input and said switch input terminals receiving a circuit enable signal as well as control signals for switching on at least said first transistor (3) and, selectively, said third and fourth transistors (1,2) and supplying current to said load, characterized in that it comprises a fast flyback control section (10,30,35-39) having a flyback input and comprising delay means (10,30,35-39) connected between said flyback input (FFW) and said common enable input (EN), and switching means (20) connected between said flyback input and said switch input terminals (IN1, IN2), said delay means receiving a fast flyback signal switching between a first and a second logical states and maintaining said enable signal at said common enable input as long as said fast flyback signal is in said first logical state and, with said fast flyback signal in said second logical state, until the current in said load decreases to zero, and said switching means switching on two diagonally opposed MOS transistors (1,4) in said branches for conducting current from said lower to said higher reference potential line through said load (9) with said fast flyback signal in said second operative state until the current in said load decreases to zero. A circuit according to claim 1, characterized in that said switching means comprise a line connected between said flyback input (FFW) and the control input (IN2) of said gate circuit (13) connected to said first transistor (3) and a further inverter element (20) connected between said flyback input (FFW) and the control input (IN1) of said gate circuit (11) connected to said first transistor (1). A circuit according to claim 1, characterized in that said delay means comprise a current sensor element (10) connected to said load (9) for detecting the current flowing through the load, a zero detector circuit (30) connected to said sensor element for generating a zero signal upon detection of zero current, and an enable gate (38) connected to said flyback input and said zero detector circuit (30) for generating said common enable signal (EN) with said fast flyback signal in said first logical state or with said fast flyback signal in said second state until reception of said zero signal. A circuit according to claims 1 and 3, characterized in that said delay means also comprise a further inverter element (36) having an input connected to said flyback input (FFW) and a NAND gate (35) having a first and a second inputs respectively connected to said further inverter element (36) and to said zero detector (30) and an output connected to said enable gate (39). A bridge according to any of the preceding claims, characterized in that said enable gate (38) is an OR gate having a first input connected to said flyback input (FFW) and a second input connected to said NAND gate (35). A process for fast current flyback in a MOS-transistor bridge having a first (3,4) and a second (1,2) branches in parallel, each comprising a pair of MOS transistors (3,4;1,2) connected in series between a lower and a higher reference potential lines, comprising the steps of switching on a first transistor (3) of the first branch (1,2) and selectively one (2) of the transistors of the second branch for supplying current to a load (9), characterized in that for obtaining a fast current flyback said first transistor (3) of the first branch (3,4) is switched off and a second transistor (4) of the first branch and a transistor (1) of said second branch diametrally opposed to said second transistor (4) in said first branch are switched on for conducting current from said lower to said higher reference potential line through said load (9) and, upon the current in the load has decreased to zero, the bridge is disabled. A process according to claim 6, characterized by the step of measuring the current in the load and sending a switch-off signal to said bridge upon detection of zero current. A process according to claim 6, characterized by the step of disabling said bridge with a preset delay after reception of a fast flyback signal (FFW).

Inventors: CINI, CARLO, DIAZZI, CLAUDIO, ROSSI, DOMENICO
Applicants: SGS-THOMSON MICROELECTRONICS S.P.A.
Application year: 1988
Main classification: H03K17
IPC codes: H03K17, H02P7