Abstract:
An integrated circuit including a vertical power component having a terminal formed by a chip substrate of a first conductivity type, a control circuit thereof, the control circuit isolated from the substrate by an isolation region of a second conductivity type, and a protection structure against polarity inversion of a substrate potential. The protection structure includes a first bipolar transistor with an emitter connected to said isolation region and a collector connected to a reference potential input of the integrated circuit, a bias circuit for biasing the first bipolar transistor in a reverse saturated mode when the substrate potential is higher than the reference potential, and a second bipolar transistor with an emitter connected to the substrate and a base coupled to the isolation region for coupling the isolation region to the substrate through a high-impedance when the substrate potential is lower than the reference potential.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of intergrated circuits. More specifically, the invention relates to a structure for the protection of integrated circuits against polarity inversion of the substrate potential. 
     2. Discussion of the Related Art 
     VIPower (“Vertical Intelligent Power) denotes integrated circuits which, in a same chip, integrate one or more vertical power components (power bipolar transistors) and a circuitry (control circuitry) for controlling the switching of the power components. 
     VIPower integrated circuits typically comprise a common semiconductor substrate forming one electrode of the power component. 
     In VIPower integrated circuits, in order to electrically separate from each other and from the substrate the components of the control circuitry, a P type doped region (called an isolation region) is provided. 
     FIG. 1 shows in cross-section a portion of a control circuitry of a VIPower integrated circuit. On an N+ substrate  1 , an N− layer  2  is epitaxially formed. A P type isolation region  3  is formed inside the N− layer  2 . The P type isolation region  3  defines two isolated N− layer portions  4 ,  5  which are isolated from each other and from the N− layer  2 . Inside N− layer portion  4  a PNP bipolar transistor T 1  of the control circuitry is formed, while in the N− layer portion  5  an NPN bipolar transistor T 2  of the control circuitry is formed. 
     By properly biasing the P type isolation region  3  at the ground potential (or, more generally, at the lowest potential existing in the integrated circuit), the PN junctions formed by the isolation region  3 , the N− layer  2  and the N− layer portions  4 ,  5  are reverse-biased, so that electrical isolation is achieved. This is necessary in order to assure that parasitic bipolar transistors Qn 1 , Qn 2 , Qn 3  are kept off. 
     In VIPower technology, the PN junction formed by the P type isolation region  3  and the N− layer  2  has a structure capable of sustaining high reverse voltages, typically of some hundreds of volts. 
     FIG. 2 is a schematic electrical diagram showing a possible use of a VIPower integrated circuit. Specifically, FIG. 2 depicts a circuit arrangement wherein a VIPower integrated circuit  8  is used for controlling a high-voltage IGBT (Insulated Gate Bipolar Transistor)  7 . The VIPower integrated circuit chip  8  and the IGBT chip  7  are advantageously housed in a same package  6  and constitute a driver for a coil  9 . In this arrangement the common N+ substrate of the VIPower integrated circuit  8  (N+ substrate  1  in FIG. 1) is electrically connected to the collector of the IGBT  7 . In the example shown in FIG. 2, wherein an IGBT is used, the IGBT chip and the VIPower chip are advantageously housed in a same package. This is only an example. If the power device used to drive the coil were a power bipolar transistor, which can be directly integrated in the VIPower chip, the VIPower chip can directly drive the coil, without the need of having an IGBT chip. 
     Externally, the package  6  appears as a three-terminal device having a control terminal  10  (receiving a control or trigger signal TRIGGER, typically a logic signal switching between ground and 5 V) and two drive terminals  11 ,  12 . Terminal  12  is connected to a first battery pole, providing a reference potential (ground). Terminal  11  is connected to a first terminal of the coil  9 , the second terminal of the coil  9  being connected to a second battery pole BAT which, in normal operating conditions, is at a potential higher than that of the first pole. 
     Referring to the circuit arrangement of FIG. 2, it is necessary to guarantee that in case the polarity of the battery is inadvertently inverted the VIPower integrated circuit is not destroyed. Typical battery voltages have values up to 24 V. So, the VIPower integrated circuit must be capable of sustaining reverse voltages of −24 V without being damaged. 
     The IGBT inherently has a structure capable of sustaining such reverse voltages. By contrast, as far as the control circuitry is concerned, if the P type isolation region  3  is kept biased at the ground voltage as usual, and if the polarity of the battery were inadvertently inverted, the PN junction between the P type isolation region and the substrate would be forward biased, which would cause the destruction of the VIPower integrated circuit. 
     The same problem is encountered even if the IGBT is not provided, and the power component (power bipolar transistor) directly integrated in the VIPower chip is used to directly drive the coil. 
     It is thus necessary to properly bias the P type isolation region, so as to assure that not only the components of the control circuitry are electrically isolated from each other and from the substrate, but also the possibility of an inversion of the polarity of the substrate potential. 
     A known solution is described in U.S. Pat. No. 5,382,837. FIG. 3 is an electrical equivalent circuit of such a solution. FIG. 4 is a circuit diagram similar to that of FIG. 3, showing a possible practical implementation of the circuit of FIG.  3 . With reference to FIG. 3, the isolation region (ISO) of the control circuitry of the VIPower integrated circuit is connected to the common collectors of two NPN bipolar transistors Q 1 , Q 2 . Transistor Q 1  has the emitter connected to ground, transistor Q 2  has the emitter connected to the substrate (SUB) of the VIPower integrated circuit. The base of transistor Q 1  is connected through a bias resistor R 1  to a voltage supply Vd. The base of transistor Q 2  is kept at a constant pre-set bias voltage by a bias circuit  13 , a possible implementation of which is shown in FIG.  4 . All the transistors that are connected to the substrate of the VIPower integrated circuit, such as Q 2  in FIGS. 3 and 4, are high-voltage vertical transistors whose emitter coincides with the substrate and whose base is a P type doped region similar to but isolated from the P type isolation region of the control circuitry of the VIPower integrated circuit. 
     In the circuits of FIGS. 3 and 4, in normal operating conditions, when the potential of the substrate (SUB) is positive, transistor Q 2  is off and transistor Q 1 , in saturation, biases the isolation region (ISO) at VCE,sat(Q 1 ). If the substrate potential goes negative, transistor Q 2 , whose base current is supplied by transistor Q 3  (FIG.  4 ), goes into saturation, so that the isolation region ISO is biased at a voltage equal to the negative potential of the substrate plus VCE,sat(Q 2 ). 
     The drawback of the circuits shown in FIGS. 3 and 4 is that they require a supply voltage Vd for their operation. On the contrary, in the arrangement of FIG. 2, the control signal TRIGGER which determines the coil charge time, is also used as a supply voltage for the control circuitry of the VIPower integrated circuit. Signal TRIGGER is not activated when an inadvertent inversion of the battery polarity can take place, so that in this condition the VIPower circuit lacks a voltage supply. Consequently, the circuits of FIGS. 3 and 4 cannot be used, because there is no supply voltage Vd. 
     In view of the state of art described, it is an object of the present invention to provide a structure not affected by the above-mentioned drawbacks. 
     SUMMARY OF THE INVENTION 
     According to the present invention, this and other objects are achieved by an integrated circuit including a vertical power component having a terminal formed by a chip substrate of a first conductivity type, a control circuit thereof, the control circuit isolated from the substrate by an isolation region of a second conductivity type, and a protection structure against polarity inversion of a substrate potential, comprising a first bipolar transistor with an emitter connected to said isolation region and a collector connected to a reference potential input of the integrated circuit, a bias circuit for biasing the first bipolar transistor in a reverse saturated mode when the substrate potential is higher than the reference potential, and a second bipolar transistor with an emitter connected to the substrate and a base coupled to the isolation region for coupling the isolation region to the substrate through a high-impedance when the substrate potential is lower than the reference potential. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will be made apparent by the following detailed description of an embodiment thereof, illustrated as a non-limiting example only in the annexed drawings, wherein: 
     FIG. 1 is a cross-section of a control circuitry portion of a VIPower integrated circuit; 
     FIG. 2 is a schematic electrical diagram of a possible use of a VIPower integrated circuit; FIG. 3 shows a conventional circuit suitable for biasing an isolation region of the control circuitry of a VIPower integrated circuit; 
     FIG. 4 shows a possible practical embodiment of the conventional circuit of FIG. 3; 
     FIG. 5 is an electrical diagram of a structure according to the present invention; 
     FIG. 6 is an electrical schematic diagram of the structure of FIG. 5, showing the associated parasitic elements; 
     FIG. 7 is a device cross-section of the structure of FIG. 6; 
     FIG. 8 is a voltage-time diagram resulting from simulation of the circuit according to the invention; 
     FIG. 9 is a current-time diagram resulting from simulation of the circuit of the invention; 
     FIG. 10 is a voltage-voltage diagram resulting from simulation of the circuit of the invention; 
     FIG. 11 is a current-voltage diagram resulting from simulation of the circuit of the invention; 
     FIG. 12 is a voltage-time diagram resulting from measurement of the circuit according to the invention; and 
     FIG. 13 is a current-voltage diagram resulting from measurement of the circuit according to the invention, in a condition of polarity inversion of the substrate voltage. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 5, a schematic of a structure according to the invention is shown. The structure comprises an NPN bipolar transistor Q 33  with collector connected to ground, emitter connected to the isolation region ISO of the control circuitry of a VIPower integrated circuit (e.g., P type region  3  in FIG.  1 ), and base connected to the isolation region ISO through a resistor R 44 . The base of transistor Q 33  is also connected, through a resistor R 33 , to the collector of a PNP bipolar transistor Q 11  having emitter connected to the signal TRIGGER (the control input to the VIPower, FIG. 2) through a resistor R 11 , and base connected to ground through a resistor R 22 . The collector of transistor Q 11  is further connected to the base of an NPN transistor Q 22  having collector connected to the emitter of transistor Q 11 , and emitter connected to the substrate of the VIPower integrated circuit. 
     The circuit of FIG. 5 operates in the following way. 
     In normal operating conditions, with the substrate at a positive potential, when signal TRIGGER is at the high logic level (5 V), bipolar transistor Q 11  biases bipolar transistor Q 33  in reverse saturation condition. The isolation region ISO is thus kept at VCEsat(Q 33 ), the emitter-collector saturation voltage of transistor Q 33 . Since in this condition the substrate voltage (SUB) is positive, bipolar transistor Q 22  is off. Bipolar transistor Q 22  is a vertical transistor whose emitter is formed by the N+ substrate of the VIPower integrated circuit (FIG.  7 ). 
     It is now assumed the substrate (SUB) is biased at a negative voltage, for example in consequence of an inversion of polarity of the battery in the circuit of FIG.  2 . Signal TRIGGER can either be at ground (0 V) or at high impedance (as mentioned in the introductory part of the description, when battery polarity is inverted signal TRIGGER is not active being an abnormal condition). In these conditions transistor Q 11  is off and does not furnish base current to transistor Q 33 . Transistor Q 33  is thus also off. Transistor Q 22  is on, and the base electrode thereof is at one VBE above the negative voltage applied to the substrate SUB. The isolation region ISO is consequently held substantially at the negative potential of the substrate plus one VBE and it is in a high impedance condition. In this condition, the voltage across the base-collector junction of transistor Q 33  is substantially equal to the voltage applied to the substrate, minus the base-emitter voltage (VBE) of transistor Q 22 . The base-collector junction of transistor Q 33  must be capable of sustaining the voltage applied thereacross. Also, the breakdown voltage between the collector and the emitter of transistor Q 33  must be higher than the voltage applied to the substrate, otherwise the P type isolation region ISO and the N type substrate would form a forward biased diode. These two conditions are satisfied by operating bipolar transistor Q 33  is the inverse region, with the collector connected to ground and the emitter connected to the isolation region ISO, as shown in FIG. 5, so that the breakdown voltages are higher than the maximum negative voltage that can be applied to the substrate. 
     FIG. 7 is a device cross-section showing the structure of transistors Q 11 , Q 22  and Q 33 . As visible, transistors Q 11  and Q 33  are formed inside the P type isolation region  3  which also contains the control circuitry of the VIPower integrated circuit. Transistor Q 11  is formed inside an isolated N− layer portion  33  delimited by the isolation region  3 ; the emitter of transistor Q 11  is a P type region  34  formed in the N− layer portion  33 ; the collector of transistor Q 11  is a P type region  35  surrounding region  34 ; the base is the N− layer portion  33 . Transistor Q 33  is formed inside an isolated N− layer portion  36  delimited by the isolation region  3 ; the base of transistor Q 33  is a P type region  37  formed inside the N− layer portion  36 ; the emitter of transistor Q 33  is an N+ region  38  formed inside region  37 ; the collector of transistor Q 33  is the N− layer portion  36 . Transistor Q 22  is a vertical transistor and has a base formed by a P type region  30  isolated from the P type isolation region  3 . The collector of transistor Q 22  is an N+ region  31  formed inside an N− layer portion  32  delimited by P type region  30 . The emitter of transistor Q 22  is the N+ substrate  1 . 
     Another important advantage of operating transistor Q 33  in the inverse region is the improved control of parasitic components, as will be now discussed. FIG. 6 is an electrical schematic diagram of the circuit of FIG. 5, also showing parasitic components which were not depicted in FIG.  5 . Reference is also made to FIG. 7, where the physical origin of the parasitic components can be clearly understood. When a negative voltage is applied to the substrate SUB, the ground voltage is the highest voltage in the integrated circuit. Since the collector of transistor Q 33  is connected to ground, the parasitic bipolar transistor Qn 11  associated with transistor Q 33  is off and cannot supply base current to the parasitic bipolar transistor Qn 22 . Transistor Q 22  is a vertical bipolar transistor with a structure similar to that of the parasitic bipolar transistors. Transistor Q 22  allows for controlling the parasitic transistors. If transistor Q 22  were absent, the parasitic bipolar transistor Qp 3  associated with transistor Q 11 , which turns on when the potential of substrate goes negative, would turn transistor Q 11  on. Transistor Q 11  would drive transistor Q 33  into saturation. Transistor Q 33  would bias the isolation region ISO at the ground potential, thus creating a conductive path between ground and the substrate through the diode formed by the isolation region and the substrate. Transistor Q 22  is designed to have a gain higher than that of the parasitic bipolar transistors. Transistor Q 22  turns on and subtracts current from the emitter of transistor Q 11 , preventing this current from being supplied to the base of transistor Q 33 . Transistor Q 22  thus forms a regulation loop that allows for controlling the effect of parasitic transistors, which could otherwise bring transistors Q 11  and Q 33  into conduction. By using polysilicon resistors, no further parasitic elements are introduced in the structure. 
     FIG. 8 is a voltage-time diagram of the signal TRIGGER and of node N 1  (FIG. 5) resulting from a simulation of the circuit. This simulation refers to a normal operating condition, with the substrate SUB biased at a positive potential. It is visible that when signal TRIGGER switches to the high logic level (5 V) node N 1  (base of transistor Q 33 ) goes to VCEsat(Q 33 ). Referring to FIG. 9, which is current-time diagram resulting from the same simulation, it is visible that current I 1  (collector current of transistor Q 11 , supplied to the base of transistor Q 33 ) is approximately equal to 1.7 mA, sufficient for saturating transistor Q 33 . This high current value is motivated by the fact that since transistor Q 33  is in the inverse region, it has a low gain. 
     FIGS. 10 and 11 are diagrams resulting from a simulation of the circuit when the substrate is biased at a negative potential. From FIG. 10 it can be seen that the potential of the isolation region ISO follows that of the substrate for negative values of the latter. FIG. 11 shows that when a negative potential is applied to the substrate the substrate current Isub is negligible. 
     FIG. 12 is a time diagram obtain from measurements, in a circuit arrangement corresponding to that of FIG.  2 . It is noted than when signal TRIGGER switches to the high loci level, the isolation region ISO is forced at the potential VCEsat(Q 33 ), while when signal TRIGGER is at the low logic level (Q 11  off) the isolation region is in an high impedance condition and is substantially floating. The spike appearing in the voltage of the isolation region at the high-to-low transition of signal TRIGGER is due to the overvoltage across the coil  9 . 
     FIG. 13 is a diagram resulting from measurement showing the current absorption of the VIPower integrated circuit when the potential of the substrate SUB is negatively biased. It is possible to see that when the polarity of the potential applied to the substrate is inverted, the absorbed current is very small (approximately 233 μA @−25V of substrate to ground voltage); substantially, the current is the one resulting from leakage. 
     Compared to the known solutions, the circuit of the present invention can be used even when no supply voltage is present. The circuit guarantees that, when the polarity of the substrate potential is inverted, the current consumption, and thus the power dissipated, is negligible. The circuit is simple, being formed by only three transistors and four resistors. The circuit thus occupies a small area of the integrated circuit chip. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.