The invention will be better understood from a reading of the following detailed description of the drawing figures in which like reference designations are utilized to identify like elements, and in which:
To obtain higher current switching capability, power transistors may be comprised of a plurality of individual power transistor cells fabricated on a substrate and connected in parallel.
Turning now to one illustrative embodiment of the invention shown in
Although two transistor cells are shown, it will be appreciated by those skilled in the art that the power transistor structure may have fewer or more transistor cells.
In addition to the two power transistor cells T1, T2, power transistor structure 1 includes a breakdown voltage sensing transistor cell T3 is formed on substrate 201. Transistor cell T3 is fabricated at the same time as transistor cells T1, T2 but is designed to have a drain source characteristic breakdown voltage Vds that is less than the drain-source characteristic breakdown voltage of the power transistor formed by transistor cells T1, T2. Voltage sensing transistor cell T3 has its drain and gate connected to the drain D and gate G, respectively, of transistor cells T1, T2. The source terminal of transistor cell T3 is coupled to a current limiting device or resistor. In the illustrative embodiment of the invention, resistor R is formed on the same substrate 201, but may in some embodiments be separate from substrate 201.
In operation, when the drain-source voltage Vds across transistor cells T1, T2 reaches the second element characteristic breakdown voltage of sensing transistor cell T3, transistor cell T3 conducts current. The current through transistor T3 is limited by resistor R which prevents damage to transistor cell T3. A voltage is produced across resistor R at sense terminal S′. The voltage at sense terminal S′ may be utilized to control the gate of the power transistor formed by transistor cells T1, T2 such that the combined power transistor device formed on substrate 201 makes power transistor 1 self-protecting against breakdown voltages applied across its switching path.
The drain-source breakdown voltage of sense transistor cell T3 tracks the drain-source power transistor cells T1, T2 over process variations since it is an integral part of the power transistor structure 1.
In the illustrative embodiment shown in
Turning now to
The specific structure of amplifier 303 may be any one of a number of known feedback amplifiers. In addition a gate circuit may also be included to assure that the power transistor structure 1 does not turn on as power is applied.
In the illustrative embodiments of the over-voltage protected structures, the breakdown voltage sensing cell or element T3 is shown with a common drain to the power MOSFET cells or elements T1, T2, and the sense output is derived from the source of the over-voltage sensing element T3. In other embodiments, the voltage sensing element source is in common with the sources of the power MOSFET elements, and the sense output is derived from the drain of the breakdown voltage sensing element.
Monolithic power devices such as structure 1 shown in
As shown and described above one or more sense cells or transistors T3 embedded in an integrated multi-celled power device 1 are used to sense that the main power device is nearing breakdown its characteristic breakdown voltage.
Each sense cell T3 is configured to have a predefined breakdown voltage that is lower and tracks the breakdown voltage of the main power device 1.
In many types of semiconductor devices, for example CMOS and lateral diffused MOS (LDMOS), the geometric design layout of the device is one parameter that determines breakdown voltage.
In accordance with the principles of the invention a power device comprises a plurality of identical cells T1, T2, T3. One or more of the cells is designated as sense cell T1. The physical dimensions of the sense cell are modified from that of the other cells.
Turning now to
The invention has been described in terms of specific embodiments. It will be appreciated by those skilled in the art that various changes and modifications may be made to the embodiments without departing from the spirit or scope of the invention. It is intended that the scope of the invention not be limited to the specific embodiments shown and described, but that the scope of the invention be limited only by the claims appended hereto.
This application is related to co-pending application Serial No. ______ filed on even date herewith and assigned to a common assignee.