A circuit device, made up of a reduced number of components, for simultaneously turning on a plurality of power transistors

Abstract: A circuit device (1), made up of a reduced number of components, for simultaneously turning on a plurality of power transistors (2), being of a type adapted to be connected between the output (7) of an oscillator (6) and the bootstrap input (5) of a turn-on circuit (4) which are both incorporated to an integrated circuit (10) also incorporating said transistors (2), further comprises: a first capacitor (C1) having one end connected to the output (7) and the other end connected to the input (5) via a diode (D1), a second capacitor (C2) connected between the input (5) and one voltage supply pole (Vs), and a second diode (D2) connected between the first capacitor (C1) and the diode (D1) and the voltage supply pole (Vs).

Description: This invention relates to a circuit device, made up of a reduced number of components, for simultaneously turning on a plurality of power transistors, each connected in a high-side driver configuration between the poles of a voltage supply through a load. As is well known, power transistors are widely employed nowadays for driving electric loads. For the purpose, in some circuits, they are connected between a power supply source and the electric load with the gate, in the instance of MOS transistors, or the base, in the instance of bipolar transistors, connected to a control circuit. In particular, power transistors of the DMOS type are employed in the so-called high-side-driver configuration to drive inductive electric loads. Such DMOS transistors are also used in bridge circuits, e.g. to control one phase of a stepper motor. In applications like this, a DMOS power transistor is also to transfer the supply voltage value Vs to the load with almost nil voltage drop between the drain and source electrodes. This operating condition, under which the transistor will deliver maximum current, is the same as having a voltage drop Vgs of about ten volts between the gate and source electrodes. Between the gate and the source of the DMOS transistor is an inherent capacitance Cgs, and the above-mentioned operating condition is achieved in practice by charging this capacitance to ten volts, thereby the gate electrode is brought up to a voltage exceeding by ten volts that of the supply pole Vs. To accomplish this, the prior art provides a first, so-called charge pump solution which consists of charging said inherent capacitance Cgs by capacitive division. In basic terms, an additional capacitance C is connected between the transistor gate and an oscillator arranged to supply a voltage which varies between zero and the value of the supply voltage Vs. During one half-cycle of the oscillator, the capacitance C is charged at a predetermined voltage value Vc, and during the following half-cycle of the oscillator, this will charge the inherent capacitance Cgs. This prior circuit arrangement, while serving its purpose, still has the drawback of being inherently slow-acting, because it requires several half-cycles of the oscillator to charge the inherent capacitance. A second prior technical solution consists instead of connecting the integrated circuit incorporating the DMOS transistor to an external component, specifically a bootstrap capacitance for each DMOS transistor. This would lead to using an external capacitance for each power transistor integrated to the circuit, which is, of course, a cost-intensive practice. A third prior technical solution provides for an additional supply line at a voltage level which is by ten volts higher than the supply voltage Vs, as well as an external buffer capacitance which is charged from the additional capacitance C associated with the oscillator, in a similar manner to that described in connection with the first-mentioned charge pump approach. This, the third, prior solution, while being beneficial in that it allows connection of plural high-side-driver DMOS transistors to one side of the buffer capacitance, has a limitation in that the size of that external capacitance cannot be increased beyond a finite maximum if the additional capacitance C is to keep constantly charged. This poses practical limitations to the number of the transistors that can be made to conduct at any one time through the external capacitance or, in an equivalent way, on the turning-on rate. An article published in Siemens Components XIX(1984) No. 4, pages 169,170, entitled "Operation of SIPMOS Transistors with Grounded Loads" discloses a gate voltage generator which is connected between the output of an oscillator and the gate of a power MOSFET transistor and comprise a first capacitor and a first diode connected in series between the oscillator output and the transistor gate and a second diode connected between a voltage supply and the junction point of the first capacitor and the first diode. An article published in Siemens Forsch.-u. Entwickl.-Ber.Bd. 17 (1988) Nr. 1, pages 35-42, entitled "Smart SIPMOS Technology" discloses a range of high power FET devices, including DMOS (double - diffused) transistors adapted to the used as high-power switching devices. The technical problem that underlies this invention is to provide a circuit device which has such structural and performance characteristics as to permit a plurality of power transistors to be driven simultaneously to the "on" state, under a condition of maximum delivery to the load, using a uniquely reduced number of components to overcome the cited drawbacks with which the prior art is beset. This problem is solved by a circuit device as defined and characterized in claim 1. The features and advantages of a circuit according to the invention will become apparent from the following detailed description of an exemplary embodiment thereof, given by way of illustration and not of limitation with reference to the accompanying drawing. The drawing figure shows the circuit of this invention in wiring diagram form. With reference to the drawing figure, generally and diagramatically indicated at 1 is a circuit device, made up of a reduced number of components, for simultaneously turning on a plurality of power transistors 2. In particular, in the embodiment under consideration, such transistors 2 would be of the DMOS type with corresponding drain D and source S electrodes connected to a positive voltage supply pole Vs and an electric load 3 to be driven. The transistors 2 are incorporated to an integrated circuit 10 which also comprises a turn-on circuit 4 and an oscillator 6 operative to supply a voltage value varying between zero and a predetermined value Vr. The gate electrode G of the transistor 2 is connected to the output of the turn-on circuit 4 which has its input connected to a bootstrap pin 5 of the integrated circuit 10 including the transistors 2. Between the gate G and the source S of each transistor 2 is an inherent capacitance Cgs to be charged to a predetermined voltage value through the device 1 and the turn-on circuit 4. The device 1 is in essence a charge pump circuit adapted for connection between a pin 7, connected to the output of the oscillator 6, and said bootstrap pin 5. This device 1 comprises a capacitor C1 having one end connected to the pin 7 and the other end connected to the pin 5 via a diode D1 forward biased to said capacitor C1. A second capacity C2 is connected between the bootstrap pin 5 and the supply pole Vs, whilst a second diode D2 has one end connected between said capacitor C1 and the diode D1, and the other end connected to the pole Vs. This diode D2 is forward biased to said pole. The capacitor C1 is pre-charged to a voltage equal to the difference between the supply voltage Vs and the voltage drop across the diode D2. As a result, a voltage will appear at the bootstrap pin 5 which is equal to Vs + Vr - 2*Vdiode. It will be sufficient, therefore, to provide the integrated circuit 10 with a low-voltage oscillator, e.g. between zero and twelve volts, to thereby ensure, at the gate G of the DMOS power transistors, a voltage value which exceeds the supply by ten volts. In fact, the voltage drop across the diodes D1 and D2 has a combined value of two volts, thereby to ensure proper operation of the device, it will suffice for the oscillator 6 to supply a voltage of about twelve volts. The circuit device according to the invention allows for the use of a reduced number of components with attendant savings in silicon area over conventional circuits. Another advantage is the reduced number of the pins involved in the connection between this device and the integrated circuit incorporating the power transistors. The circuit device of this invention is therefore particularly economical both in terms of the components used and of the area occupied on the integrated circuit.

Claims: A circuit device (1), made up of a reduced number of components, for simultaneously turning on a plurality of power transistors (2), each connected in a high-side-driver configuration between a first pole (Vs) of a voltage supply and a second pole of said voltage supply through a respective electric load (3), said circuit device (1) being connected between the output (7) of an oscillator (6) and a bootstrap input (5) coupled to the control electrodes of said power transistors (2) and comprising a first capacitor (C1) having one end connected to said output (7) and the other end connected to said bootstrap input (5) via a first diode (D1) and a second diode (D2) connected between the junction point of the first capacitor (C1) with the first diode (D1) and the said first voltage supply pole (Vs) characterized in that it comprises a second capacitor (C2) connected between said bootstrap input (5) and said first voltage supply pole (Vs) and a turn-on circuit (4) having an input connected to said bootstrap input (5) and an output connected to said control electrodes, said oscillator (6) and turn-on circuit (4) being both incorporated to an integrated circuit (10). A device according to Claim 1, characterized in that said power transistors (2) are of the DMOS type. A device according to Claim 1, characterized in that said second diode (D2) is forward biased with respect to said voltage supply pole (Vs). A device according to Claim 1, characterized in that said first diode (D1) is forward biased by the first capacitor (C1).

Inventors: ALZATI ANGELO, BAIOCCHI ANTONELLA, CERATO SANDRO, ALZATI, ANGELO, BAIOCCHI, ANTONELLA, CERATO, SANDRO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1989
Main classification: H03K17
IPC codes: H03K17, H02M3