Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
1999-05-04
2001-07-10
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S309000, C361S056000
Reexamination Certificate
active
06259297
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
The present application claims priority under 35 USC 119 from Italian Patent Application No. MI98A000962, filed May 4, 1998.
BACKGROUND OF THE INVENTION
1. Technical Field of Invention
The present invention relates to a protection circuit for a bipolar power transistor. More particularly, the invention relates to a protection method and circuit for preventing a bipolar power transistor from leaving an area of safe operation.
2. Background of the Invention
It is known that the safe operative region of active devices is limited by the maximum power that can be dissipated thereby and by voltage and current values which must not be exceeded if one does not wish to damage the active device.
In the case of a power transistor, the limits of safe operation are set by the maximum allowed values in terms of temperature, power dissipation, and voltage and current levels of the power transistor.
Exceeding the temperature limits can cause melting of the materials, dissociations or generation of reaction products which alter the characteristics of the transistor, deterioration of the technical characteristics and of the materials, and increased surface currents.
Taking into account these limits in terms of current, voltage and power, the useful region in which a bipolar power transistor can operate is generally the one shown in
FIG. 1
, which is a chart in which the axis of the abscissa plots the collector voltage and the axis of the ordinate plots the collector current of a bipolar power transistor.
The chart of
FIG. 1
plots various curves which subtend useful operating regions having different areas according to the pulse duration applied to the base terminal of the power transistor being considered.
Outside the indicated useful region, if the current or power limits are exceeded the transistor is typically damaged or destroyed.
The natural positive feedback of a bipolar power transistor with respect to temperature can become critical when working at voltages higher than the ones at which a first breakdown of the bipolar power transistor occurs, i.e., when the current capacity of the device quickly decreases as the applied voltage rises.
Without a carefully designed protection circuit, the operating point of the bipolar power transistor can leave the above-indicated useful region unless a power transistor is used which is oversized with respect to the working points used.
Conventional circuits for protecting bipolar power transistors suffer some drawbacks. In particular, the known protection circuit shown in
FIG. 2
(showing a simplified manner which nonetheless shows its key aspects) in which the power transistor is designated by the reference numeral
1
, uses two current sources
4
and
5
and bipolar transistors
2
and
3
. The protection circuit of
FIG. 2
, however, is unable to precisely set the short-circuit current Isc of the associated power transistor
1
because current source
5
tends to reach saturation.
There are other existing circuit configurations which tend to eliminate the positive feedback problems of the bipolar power transistor with respect to the temperature. In these circuits, as the temperature increases, the circuits respond by forcing a lower Isc current. These circuits, however, cannot be applied in a context which requires the options of eliminating current consumption with a power-off command and setting a constant current Isc for higher voltages, i.e., for voltages up to the maximum voltage considered by the technology of the device.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide a protection circuit for a bipolar power transistor in which the area of useful operation of the power transistor can be utilized to the maximum extent, thereby allowing the device to operate up to its safety limit.
Within the scope of this aim, an object of the present invention is to provide a protection circuit for bipolar power transistors in which it is unnecessary to use a power transistor which is oversized with respect to the operating points used.
Another object of the present invention is to provide a protection circuit for bipolar power transistors in which it is possible to eliminate the current consumption of the device as well as the consumption of the protection circuit by employment of an external power-off command.
Still another object of the present invention is to provide a protection circuit for bipolar power transistors in which the short-circuit current thereof is set precisely without uncertainties due to saturations of bipolar transistors utilized in conventional protection circuits.
It is another object of the present invention to provide a regulator which uses a power transistor in conjunction with the above-described protection circuit.
Another object of the present invention is to provide a protection method and circuit for a bipolar power transistor which is highly reliable, relatively easy to provide and inexpensive to manufacture.
This aim, these objects and others which will become apparent hereinafter are achieved by a protection circuit for a bipolar power transistor for preventing the operating point of the transistor from leaving its area of useful operation. The protection circuit includes a current source which drives the base terminal of the power transistor. The collector terminal of the power transistor is connected to the input supply voltage via a sense resistor. The output of the power transistor is drawn from the emitter terminal thereof.
The present protection circuit includes a first circuit branch which is connected between the collector terminal of the power transistor and a common output node, including a first diode and a first current source. The first diode is connected to the collector terminal of the power transistor. In a preferred embodiment, the common output node coincides with the output of the power transistor.
The present protection circuit further includes a second circuit branch having a second diode and a second current source which is connected between the input supply voltage and the common output node.
A third circuit branch of the protection circuit is connected between the input supply voltage and the common output node, including a first pair of transistors which have common-connected emitter terminals and a second pair of transistors which are respectively series-connected to the transistors of the first pair of transistors. The emitter terminals of the second transistor pair are common-connected and the collector terminals thereof are respectively connected to the collector terminals of the transistors of the first pair of transistors. The transistors of the second pair of transistors are connected in a current mirror configuration.
A fourth circuit branch of the present protection circuit includes a plurality of series-connected diodes which are connected in series to a first resistor and in parallel to a second resistor. The fourth circuit branch includes a current mirror formed by a pair of transistors in which the respective emitter terminals thereof are connected to the common output node. The base terminals of the current mirror transistors are common-connected. The collector terminal of one of the current mirror transistors is connected to the first and second resistors of the fourth circuit branch, and the collector terminal of the other of the current mirror transistors is connected to a third current source.
The present protection circuit further includes a Zener diode which is connected between the first and fourth circuit branches, and a transistor which is connected between the base terminal of the power transistor and the common output node, having a base terminal which is connected to current mirror of the third circuit branch.
REFERENCES:
patent: 4276442 (1981-06-01), Iekana et al.
patent: 4712153 (1987-12-01), Marget et al.
patent: 4714896 (1987-12-01), Addis
patent: 5202811 (1993-04-01), Minks
patent: 5663673 (1997-09-01), Tanaka et al.
patent: 5986832 (1999-11-01), Barnett
Cunningham Terry D.
Galanthay Theodore E.
Jenkens & Gilchrist P.C.
Nguyen Long
STMicroelectronics S.r.l.
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