Three-Terminal Insulated-Gate Power Electronic Device With a Variable-Slope Saturated Output Characterisitic Depending in a Discontinuous Way on the Output Current

1. Field of the Invention

The present invention relates to a three-terminal insulated-gate power electronic device with a variable-slope saturated output characteristic depending in a discontinuous way on the output current.

2. Discussion of the Related Art

Modern Lamp ballast circuits make use of Power MOS transistors, which combine fast switching performance with ease of driving. In such circuits, the required value for the Power MOS transistor "on" resistance (Ron) in saturation condition is about 1.2 ohm. Since in steady-state operating conditions the current flowing through the Power MOS transistor is in the range of 200-400 mA, the power dissipated by the Power MOS transistor is not of concern. The problem arises during the circuit start-up, when currents of the order of 5 A are necessary. During startup, the power dissipation in the Power MOS transistor becomes significant, as the voltage drop across the power MOS transistor can reach 6 V.

This problem is related to the linearity of the output current-voltage characteristic of the Power MOS transistor, wherein the output resistance is substantially constant and independent of the output current.

Another well-known power electronic device includes a MOSFET designed for high voltages (e.g. 650 V) and a bipolar power transistor connected in a darlington configuration, with the MOSFET drain and source electrodes connected respectively to the bipolar transistor collector and base electrodes. This device, called an "Insulated Gate Darlington" (IGD), exhibits a saturation output resistance which, for high collector currents, is much lower than that of a MOS field effect transistor, so that even when currents of some Amperes flow, the power dissipation and the voltage drop across the power device are kept low. For low collector currents, however, the output resistance of the IGD is higher than that of a MOS field effect transistor, resulting in excessive power dissipation during the steady state operation.

A possible solution to the problem of excessive power dissipation during steady state operation in the IGD includes the connection of a low value resistor (e.g. 1 ohm) in parallel with the base-emitter junction of the power bipolar transistor. In this manner, the saturated output characteristic of the overall device is MOSFET-type for low output currents and bipolar-type for high output currents. The output resistance in the MOSFET-type region of the output characteristic is given by the sum of the MOSFET on-resistance plus 1 ohm. If the ballast circuit is designed for working with a resistance value of 1-2 ohm, the MOSFET on-resistance must be 0.2 ohm. Such a low value is not common or practical to achieve in high-voltage MOSFETS. On the other hand, the resistance value of the base-emitter resistor cannot be significantly reduced, since this would result in an excessively high current flowing through the MOSFET when the power bipolar transistor is on (such current is in fact given by V.sub.BE /R, where V.sub.BE is the voltage across the base-emitter junction of the power bipolar transistor in saturation, typically 650 mV, and R is the resistance value of the base-emitter resistor). A high current flowing through the MOSFET is again responsible for a high voltage drop across it, and therefore is responsible for an excessive power dissipation.

In view of the state of the prior art described, an object of the present invention is to provide a three-terminal power electronic device which does not causes excessive power dissipation either for low output currents or for high output currents. The three-terminal device may be suitable for use in ballast circuits without being affected by the above mentioned drawbacks.

The foregoing object is attained in one illustrative embodiment of the invention wherein a three-terminal insulated-gate power electronic device is provided comprising a bipolar power transistor and an insulated-gate transistor forming a darlington pair. The bipolar power transistor has a first electrode, a second electrode and a control electrode respectively connected to a first electrode of the insulated-gate transistor and a first external terminal of the three-terminal device, to a second external terminal of the three-terminal device, and to a second electrode of the insulated-gate transistor. The three-terminal insulated-gate power transistor further includes switching means connected between the control electrode and the second electrode of the bipolar power transistor, and control circuit means connected to another second electrode of the insulated-gate transistor and controlling said switching means to switch them from a highly-conducting condition for low values of a current flowing through the first and second external terminals to a non-conducting condition for high values of the current flowing through the first and second external terminals.

The present invention makes it possible to realize a three-terminal power electronic device with an output resistance which, for low currents is dominated by the on-resistance of the insulated-gate transistor, while for high currents is dominated by the output resistance of the bipolar power device.

These and other features and advantages of the present invention will become apparent by the following detailed description of two embodiments, illustrated as non-limiting examples in the annexed drawings, in which:

FIG. 1 is a schematic circuit diagram of a lamp ballast circuit making use of two three-terminal devices according to the present invention;

FIG. 2 is a circuit diagram of a first embodiment of a three-terminal device according to the present invention;

FIG. 3 shows an output current-voltage characteristic curve for the device of FIG. 2;

FIG. 4 is a circuit diagram of a second embodiment of the three-terminal device according to the present invention;

FIG. 5A is a structural diagram of an illustrative power MOSFET, having multiple source cells, for use in the device of the present invention;

FIG. 5B is a cross-sectional diagram of a portion of an illustrative power MOSFET with multiple source cells for use in the device of the present invention.

FIG. 1 illustrates a ballast circuit for driving a lamp L. The ballast circuit includes two three-terminal power electronic devices 1, each having two power terminals C and E and a driving terminal G. A first one of the two three-terminal devices 1 has one power terminal C connected to a power supply line V+, and another power terminal E connected to one end 3 of a primary winding 2 of a transformer, that is connected in series with the lamp L. The second three-terminal device 1 has one power terminal C also connected to the end 3 of the primary winding 2, and another power terminal E connected to a ground line GND. The first three-terminal device 1 has the driving terminal G connected, through a first secondary winding 2' of the transformer, to the end 3 of the primary winding 2, while the second three-terminal device 1 has a driving terminal G connected, through a second secondary winding 2", to the ground line GND. The ballast circuit is also provided with a starter circuit (not shown). The circuit thus obtained is self-oscillating, i.e., once the power supply line V+ has been powered, a sinusoidal current starts flowing through the lamp L, and the three-terminal devices 1 alternatively connect the end 3 of the primary winding 2 of the transformer to the power supply Line V+ and to the ground Line GND.

As shown in FIG. 2, a three-terminal electronic device 1 according to the present invention comprises a high-voltage MOS field effect transistor T1, a low-voltage MOS field effect transistor T3, and a bipolar power transistor T2. The high-voltage MOS transistor T1 is composed, in a conventional manner, by a plurality of identical elementary source cells, and has a drain electrode D1 connected to a collector electrode C2 of The bipolar power transistor T2 and to an external collector terminal C. FIGS. 5A and 5B illustrate exemplary arrangements of multiple source cells for forming a high-voltage MOS transistor. The structure of FIG. 5A includes an N type substrate 11 in contact with a drain metallization 9, and a plurality of source cells 7 interconnected by a metallization 5. Each of the source cells 7 has an N+ source region 8 in contact with a P type region 10 and a silicon gate 6. FIG. 5B is a cross-sectional view of a portion of another illustrative power MOSFET having a plurality of source cells. The structure of FIG. 5B includes an N+ substrate 19 in contact with a N- drain layer 20 and a drain metallization 9. A gate oxide layer 12 is formed over the N- drain layer 20 and in contact with a polysilicon gate layer 13, and a dielectric layer 14 is formed on top of the polysilicon gate layer 13. The structure further includes a plurality of source cells 16 each having a body region 18 and two source regions 17. The plurality of source cells 16 is interconnected by source metallization 15. Each of the plurality of source cells in a high-voltage MOSFET as shown in FIGS. 5A and 5B contributes a fraction of the total current of the device. The high-voltage MOSFET of FIG. 2, which has multiple source electrodes, includes a first plurality of source cells (e.g., 7 in FIG. 5A and 16 in FIG. 5B) connected to a first source electrode, and a second plurality of source cells connected to a second source electrode. The high-voltage MOS transistor T1 has a first source electrode S11, to which a first subset of said plurality of elementary source cells is connected, which is connected to a base electrode B2 of the bipolar power transistor T2 and to a drain electrode D3 of the low-voltage MOS transistor T3. A source electrode S3 of the low-voltage MOS transistor T3 and an emitter electrode E2 of the bipolar power transistor T2 are commonly connected to an external emitter terminal E. The high-voltage MOS transistor T1 has a second source electrode S12, to which a second subset of the plurality of elementary source cells is connected. The second source electrode S12 is connected to a first, inverting input of a comparator H, whose second, non-inverting input is connected to a reference voltage supply VREF. The output of the comparator H is connected to a gate electrode G3 of the low-voltage MOS transistor T3.

A gate electrode of the high-voltage MOS transistor T1 is connected to an external gate terminal G. A diode D is back-connected between the external gate terminal G and the base electrode B2 of the bipolar power transistor T2.

When the three-terminal device 1 is used in the ballast circuit of FIG. 1 and the voltage applied to the external gate terminal G is positive and sufficient to drive T1 into saturation (e.g., 10 V), T1 and T3 are both on, and they sink a current I from the external collector terminal C. Since T3 is a low-voltage MOS transistor, its on resistance is negligible with respect to that of T1, which is a high-voltage MOS transistor. For this reason, the voltage drop across T3 is negligible, and the voltage drop across T1, i.e., the voltage between its drain electrode D1 and its first source electrode S11, is almost equal to the voltage V.sub.CE applied across the external collector and emitter terminals C and E. Since no significant current is drained by the inputs of comparator H, the voltage on the second source electrode S12 of T1 is almost equal to the voltage on the drain electrode D1; the voltage on the second source electrode S12 of T1 substantially follows the voltage on the external collector terminal C. The current I starts flowing in a sinusoidal way. In this phase the output resistance of the three-terminal device 1 is dominated by the on-resistance of the MOS transistor T1.

When the voltage drop across T1 and T3, i.e., V.sub.CE, exceeds the value V of the reference voltage generated by VREF, the comparator H switches and turns T3 off, so that the voltage on the first source electrode S11 of T1 is pulled toward the voltage on the collector terminal C, if the reference voltage supply VREF is designed to generate a reference voltage V equal or greater to the turn-on base-emitter voltage V.sub.BEON of T2 (typically 650 mV), T1 can drive the base electrode B2 of T2 to turn it on. From now on, the three-terminal device works as an IGD, and its operating point trips into a low-resistance region, dominated by the output resistance of T2.

When the current I flowing into the collector terminal C of the three-terminal device starts decreasing, so does the voltage V.sub.CE. When V.sub.CE falls below the voltage value V, the comparator H switches and turns T3 on, which, in turn, by virtue of its much lower on-resistance than that of T1, pulls the base voltage of T2 toward the voltage on the emitter terminal E and thus turns T2 off. The output characteristic of the three-terminal device is again dominated by the on-resistance of T1.

The presence of the diode D increases the switching speed of the three-terminal device, as it helps to turn T2 off when the voltage applied to the gate terminal G with respect to the emitter terminal E goes negative.

The output current-voltage characteristic of the three-terminal device of FIG. 2 is shown in FIG. 3. It is possible to see that for V.sub.CE values below the reference voltage V, i.e., for low currents, the three-terminal device shows an output resistance higher than that shown for V.sub.CE values above V, i.e., for high currents. Such resistance is however lower than that shown by a common IGD (shown in dash-and-dot line in FIG. 3). As compared to the above-described IGD with base-emitter resistor device according to the prior art, the three-terminal device according to the present invention exhibits a much lower voltage drop across it for low currents. Furthermore, the bipolar power transistor T2 is better driven, since after T3 has been turned off, the whole drain current of T1 flows into the base of T2, instead of being divided between the base of the bipolar power transistor and the base-emitter resistor.

FIG. 4 shows another embodiment of a three-terminal device according to the invention. The comparator H and the voltage reference VREF have been replaced by an NPN bipolar transistor T4 having a base electrode B4 connected to the second source electrode S12 of T1, and an emitter electrode E4 connected to the external emitter electrode E. A collector electrode C4 of T4 is connected to the gate electrode G3 of T3 and, through a bias resistor R1, to the external gate terminal G. In this embodiment, when a positive voltage is applied to the external gate terminal G of the device, and when the voltage applied to the external collector terminal C exceeds the base-emitter turn-on voltage V.sub.BEON of the transistor B4 (approximately 650 mV), T4 turns on and turns T3 off. In fact, as long as the voltage V.sub.CE is lower than V.sub.BEON, no current is supplied by the second source electrode S12 of T1, and the voltage on S12 is equal to the voltage on C. When the voltage V.sub.CE equals V.sub.BEON, T4 is turned on. As in the previous embodiment, T1 can now drive the base electrode B2 of T2 to turn it on, and the operating point trips into the bipolar-type, low-resistance region of FIG. 3.

The three-terminal device according to both of the above-described embodiments of the present invention can be realized both with discrete components, and, preferably, as a monolithic device integrated on a single silicon chip, using any known manufacturing process adopted for the fabrication of an IGD device. One suitable process has, for example, been described in the European Patent Application No. 93830255.1.

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.