Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
2002-02-07
2004-04-06
Riley, Shawn (Department: 2838)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
C327S309000
Reexamination Certificate
active
06717787
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to power semiconductor devices, particularly but not exclusively a power semiconductor switch, having a main current carrying transistor section and a sense current carrying transistor section.
The invention also relates to circuit configurations comprised with such a device. The circuit may be used for measuring or modifying or controlling the conduction of the main current carrying section of the device, in relation to an output from the sense current carrying section.
The invention further relates to switching circuits, switching arrangements and switching schemes comprising such a semiconductor device and/or control circuit, particularly but not exclusively as a low-side switch.
The device is typically an insulated-gate field-effect transistor (MOSFET) or an insulated-gate bipolar transistor (IGBT). The device is usually cellular with, for example a close-packed hexagonal, or square or stripe geometry, and so the main current carrying section and the sense current carrying section are generally referred to in terms of main cells and sense cells. The control circuit may be integrated monolithically in the same device body (chip) as the transistor, or it may be in a separate circuit body (IC chip) mounted beside the transistor chip, for example in the same device package (encapsulation).
BACKGROUND OF THE INVENTION
Power MOSFET devices with monolithically integrated control circuits are known. Thus, such devices commercially available from Philips Semiconductors under the trade mark TOPFET include temperature and overload protection functions. TOPFET devices are designed for either high-side or low-side operation, depending on whether used between a power line and the load or the ground line and the load. Such devices are described in the Philips Semiconductors Power MOS Transistors Data Handbook 1997, for example, and on the web-site http://www.semiconductors.philips.com.
In low-side configured MOSFET designs, current limit and current measure functions have difficulty performing well because of the poor tracking between the MOSFET cells that perform the sensing function (sense cells) and the majority of the MOSFET cells (main cells) that are not monitored. This results from the connection of a small measurement resistor to the source of the sense cells, which thereby makes the sense cells non-representative of the main population.
In high side technologies this problem can be resolved by feeding the sense cells into a “virtual earth” node, and hence reducing the difference in source voltages between sense cells and non-sensing main cells of the MOSFET. However, the use of a virtual earth technique in a fast current limit control application is particularly challenging from a stability standpoint.
A virtual earth solution cannot be implemented for low-side MOSFET devices, because there is no means to sink the significant sense current, below source potential. The only strategy believed to be available in low-side technology was to reduce the magnitude of the signal developed across the sense resistor.
U.S. Pat. No. 5,621,601 discloses apparatus designed to prevent the oscillation which often occurs in an over-current protection apparatus for an insulated gate controlled transistor. The apparatus improves the response in current detection, to prevent oscillation, and improves protection speed against over-current. This is accomplished by separating the gates of the main transistor and the current detector transistor, by setting a shorter time constant for the gate circuit of the current detector transistor than that of the gate circuit of the main transistor; by feeding the detection signal obtained from a current detecting resistor which detects the current of the current detector transistor to a control circuit; and by controlling the gate potentials of both transistors to protect the main transistor from the over-current by feeding the comparison output from a comparator circuit, which compares the voltage of the detection signal with a reference voltage, to the control circuit.
One disadvantage of the apparatus disclosed in U.S. Pat. No. 5,621,601 is that it does not take account of the drive impedance presented by the drive circuit acting on the main transistor. For slow switching time devices where the real value of the gate resistor for the main transistor is high compared to the source impedance that can be released in the drive circuit, this arrangement will produce a useful benefit in stability; but for fast switching time devices where this is not the case, the stability advantages will be limited.
However, more important in relation to the present invention is that the disclosure of U.S. Pat. No. 5,621,601 in no way addresses the problem that as a result of connection of the current detecting resistor (the measurement resistor) to the current detector transistor (the sense cells), this detector transistor is not fully representative of the main transistor (the main cells).
SUMMARY OF THE INVENTION
According to the present invention there is provided a power semiconductor device having a main current carrying transistor section integrated with a sense current carrying transistor section for carrying a current which is smaller than and indicative of the current carried by the main current carrying section, wherein the main transistor section and the sense transistor section have a common first main electrode connected to a first main terminal of the device, wherein the main transistor section and the sense transistor section have separate second main electrodes with the second main electrode of the main transistor section connected to a second main terminal of the device and the second main electrode of the sense transistor section connected through a current sensing resistance to said second main terminal of the device, wherein the main transistor section and the sense transistor section have separate control electrodes, wherein control means includes comparison means for comparing a reference voltage defining a current limit value with the voltage across the current sensing resistance and providing a first control signal, wherein the first control signal is coupled to the control electrode of the sense transistor section, and wherein the first control signal is coupled to further control means which provides a second control signal coupled to the control electrode of the main transistor section. In accordance with the present invention, such a device is characterised in that the further control means comprises adjustment circuit means coupled to the first control signal and to the voltage across the current sensing resistance and arranged to provide the second control signal such that the second control signal is effective to maintain the voltage between the control electrode and the second main electrode of the main transistor section equal to the voltage between the control electrode and the second main electrode of the sense transistor section.
The characterising features of the present invention, as defined above, provide correction of the drive to the control electrode of the main transistor section for the reduction in drive voltage applied to the sense transistor section as a result of the current sensing resistance.
Thus, the control electrode of the sense transistor section is connected to the control means so as to define the voltage on this control electrode such that the current in the sense transistor section never exceeds the intended maximum value, and the maximum voltage applied to the sense transistor section control electrode is defined, this nodal connection being the first control signal. The control means compares the reference voltage (defining an intended current limit) with the voltage across the current sensing resistance and so controls the voltage across the current sensing resistance such that it never exceeds the reference voltage. The control means also defines the maximum voltage that is to be applied to the control electrode of the sense transistor section when the external condition a
Koninklijke Philips Electronics , N.V.
Riley Shawn
Waxler Aaron
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