A CIRCUIT FOR HOLDING A MOS TRANSISTOR IN A CONDUCTION STATE IN A VOLTAGE SUPPLY OUTAGE SITUATION

Abstract: A circuit (1) for holding a MOS power transistor (MP) in a conduction state on the occurrence of an outage in the voltage supply (VA), being of a type which comprises a first MOS transistor (M1) having its source (S1) connected to a line (2) of the voltage supply and its drain (D2) connected to the gate (GP) of the power transistor (MP), further comprises a diode (D) connected between the drain (D1) of the first transistor (M1) and the gate (GP) of the power transistor (MP), and a second transistor (M2) of the MOS type having its gate (G2) connected to the gate (G1) of the first transistor (M1) and its drain connected to the gate (GP) of the power transistor (MP). The circuit (1) prevents the gate capacitance (GP) of the power transistor from becoming discharged on a failure of the voltage supply (VA), thus holding that transistor in a conducting state.

Description: This invention relates to a circuit for holding a MOS power transistor in a conduction state on the occurrence of an outage in the voltage supply, comprising a first P-channel MOS transistor having its source connected to a first terminal of said voltage supply, its drain coupled to the gate of said power transistor and its gate connected to a first input terminal and being effective to operatively connect the gate of the power transistor to said first terminal of the voltage supply when it is rendered conductive by a signal applied to said first input terminal, said power transistor having its gate coupled to a second input terminal for receiving driving signals, its source connected to a second terminal of said voltage supply and its drain connected to a load. As is known, MOS power transistors are widely employed to connect and control electric loads, in particular electric motors. Should a drop off occur in the voltage supply, the phases of the electric motor must be connected to ground by closing switches. Such switches may comprise the above-noted MOS power transistors which, in order to operate as closed switches, are to be held in a conducting state even if the electric power supply has failed. To meet that demand, the prior art has proposed a circuit arrangement as described herein below. Each power transistor is connected to a make circuit which comprises a MOS transistor having its source terminal connected to an electric supply line via a diode, and its drain terminal connected to the gate of the power transistor. The gate terminal of that MOS transistor is intended to receive a make signal for the power transistor, which transistor receives, in turn, on its gate terminal a signal from the output of a so-called driver. Also provided is a large capacitance connected between the source of the make MOS transistor and ground. This capacitance makes up for current leakouts from the make circuit on the occurrence of a drop-off in the electric supply, as well as for current leakouts from the driver which, when receiving no power, behaves at its output as a circuit shorting out to ground. This prior technical approach, while substantially achieving its objective, has the serious drawback that the value of said capacitance must be high, thereby its incorporation to the integrated circuit forming the make circuit is unthinkable, on account of enhanced costs and space requirements. The technical problem that underlies this invention is to provide a circuit which has such design and performance characteristics as to allow of a MOS power transistor to be held in a conducting state on a failure of the electric power supply to thereby obviate the drawback with which the prior art is beset. This problem is solved by a circuit of the kind specified hereinabove being characterized in that it comprises a diode connected in a forward biased direction between the drain of the first transistor and the gate of the power transistor, and a second transistor of the MOS type having its gate connected to the gate of the first transistor, its drain connected to the gate of the power transistor and its source connected to said second input terminal and operative to be non-conductive by said signal being applied to the first input terminal. The features and advantages of a circuit according to the invention will be more clearly apparent from the following detailed description of an embodiment thereof, to be taken by way of illustration only in conjunction with the accompanying drawing. The drawing shows a schematical view of a holding circuit according to this invention. With reference to the drawing, the numeral 1 comprehensively and schematically designates a circuit for holding a field-effect power transistor MP of the MOS (Metal-Oxide Semiconductor) type in a conduction stated on the occurrence of a power outage. That transistor MP is of the n-channel type and conventionally includes a source terminal SP connected to ground, and a drain terminal DP which forms an output terminal OUT for the circuit 1 and intended for connection to an external load, such as an electric motor not shown because of conventional design. The power transistor MP further includes a gate terminal GP having an inherent gate capacitance CGS. The circuit 1 is connected to an electric power supply line 2, in turn connected to a positive supply pole VA. In detail, the circuit 1 comprises a first transistor M1 of the p-channel MOS type having its source terminal S1 connected to the line 2, and its drain terminal D1 connected to the gate GP of the transistor MP via a diode D. The transistor M1 further includes an intrinsic diode DM1 connected between the drain D1 and the source S1, as well as a gate terminal C1 connected to an input terminal A of the circuit 1 and intended for receiving a signal commanding the make state of the transistor MP. Advantageously, it is also contemplated that a second transistor M2 of the n-channel MOS type be arranged to have its gate terminal G2 connected to the gate G1 of the transistor M1 and its drain terminal D2 connected to the gate GP of the power transistor MP. That second transistor M2 also has the source S2 and substrate B2 terminals connected to each other and and to an input terminal IN which is connected to the output of a driver circuit 3 external of the circuit 1 and operative to drive the transistor MP. The transistor M2 is in turn provided with an intrinsic diode DM2 connected between the source S2 and drain D2 terminals; in addition, the transistor M2 should be sized to have a negligible serial resistance compared to the serial resistance of the driver 3. The circuit 1 of this invention operates as follows. Assuming in operation a starting condition whereby power is supplied on the line 2, there may occur either of two cases. CASE 1 The voltage V at the input terminal A is equal to the supply voltage VA (V = VA). It follows that the voltage drop VGS between the gate and the source of the transistor M1 will be 0 Volts, and accordingly that transistor will be in an off state. The transistor M2 is instead conducting if the supply voltage VA is higher than the voltage VIN at the input terminal IN plus the threshold voltage VTH2 of the transistor M2. Even if the threshold voltage VTH2 is greater than a diode voltage drop, the driver 3 would drive the gate GP of the transistor MP and all the output voltage VIN is delivered to that gate less the voltage drop across the intrinsic diode DM2. CASE 2 The voltage V at the terminal A is nought (V = 0 Volts), the transistor M1 is conducting and, through the diode D, charges the gate capacitor of the transistor MP, thereby the latter will be in turn conducting. At the same time, the transistor M2 is in an off state in that there is a positive or zero voltage VIN at the source and zero voltage at the gate G2. Now, a third possible operational case presents itself. CASE 3 This is the case in which there is no supply voltage on the line 2 (VA = 0), and the voltage V is zero also at the terminal A. Inasmuch as that operating condition follows in time the Case 2 just discussed, the gate capacitor of the transistor MP will be charged, and that transistor keep conducting. The transistor M2 is still in a non-conducting state, and the transistor M1 could provide a route for discharging the capacitor CGS of the power transistor through the diode DM1. Actually, since the voltage VA is lower than the voltage VGP at the gate of the transistor MP, the diode D would be reverse biased and inhibit discharge of the gate capacitor of the power transistor MP. To summarize, diode D isolates a current leakout through transistor M1, having the intrinsic diode DM1, and transistor M2 is off and isolates the driver 3 from the gate capacitor of the power transistor. Accordingly, on the occurrence of the supply voltage VA drop, the gate of the power transistor is automatically isolated from all the components of the circuit 1, and holds that transistor in a permanently conducting state. The circuit of this invention solves, therefore, the technical problem and affords a number of advantages, an outstanding one being that the very inherent gate capacitance of the power transistor is utilized to hold it in conduction on the occurrence of a drop in the voltage supply. The circuit of this invention is made up of peculiarly low number of components, and makes provisions unnecessary to cancel leakage currents of the drivers in the absence of power supply. The circuit can be driven, in the absence of power supply, in an extremely simple way, since the input impedance of the make terminal A is almost infinite and it receives under that condition a signal having a low logic value.

Claims: A circuit (1) for holding a MOS power transistor (MP) in a conduction state on the occurrence of an outage in the voltage supply (VA) comprising a first p-channel MOS transistor (M1) having its source (S1) connected to a first terminal (2) of said voltage supply (VA), its drain (D1) coupled to the gate (GP) of said power transistor (MP) and its gate (G1) connected to a first input terminal (A) and being effective to operatively connect the gate (GP) of the power-transistor (MP) to said first terminal (2) of the voltage supply when it is rendered conductive by a signal applied to said first input terminal (A), said power transistor (MP) having its gate (GP) coupled to a second input terminal (IN) for receiving driving signals, its source (SP) connected to a second terminal of said voltage supply and its drain (DP) connected to a load, said circuit (1) being characterized in that it comprises a diode (D), connected in a forward-biased direction between the drain (D1) of the first transistor (M1) and the gate (GP) of the power transistor, and a second transistor (M2) of the MOS type having its gate (G2) connected to the gate (G1) of the first transistor (M1), its drain connected to the gate (GP) of the power transistor (MP) and its source (S2) connected to said second input terminal (IN) and operative to be non-conductive by said signal being applied to the first input terminal (A).

Inventors: CINI, CARLO, PIETROBON, GIOVANNI, ROSSI, DOMENICO, STORTI, SANDRO
Applicants: SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1988
Main classification: H03K17
IPC codes: H03K17, H02H7