Amplifiers – With semiconductor amplifying device – Including protection means
Reexamination Certificate
1997-12-23
2001-05-01
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including protection means
C330S20700P, C361S104000
Reexamination Certificate
active
06225867
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the large field of integrated circuit design. More specifically, it relates to resolving problems related to thermal runaway and over-voltage breakdown in integrated circuits involving interconnected power transistors.
BACKGROUND OF THE INVENTION
Power amplifiers are used to amplify electrical signals in a wide variety of applications including cellular telephony, and radio and television broadcasting. For power ranges of up to about 100 watts, the most common power device used in these amplifiers is a bipolar junction transistor (BJT). In many cases, the power device is not a single transistor, but is composed of several transistors connected in a parallel circuit arrangement. This parallel circuit arrangement creates some technical problems for circuit designers since a limited number of weak transistors are responsible for circuit failure. Two of these problems, thermal runaway and over-voltage breakdown, are further explained below.
Thermal runaway: In an amplifier that is made up of several transistors connected in parallel, it is desirable for all the transistors to share the bias current equally. However, when bipolar junction transistors are used, the transistors with the highest temperature tend to carry more current. This can lead to a condition called thermal runaway. It a transistor carries a little more current than the others in the circuit, it will also dissipate more power, which will tend to heat the transistor even more. Since the transistor is now even hotter, it will tend to carry even more current. This self-heating operation can escalate until an over-current condition is reached and the transistor and amplifier circuits fail.
The most common solutions to prevent thermal runaway are: 1. To connect a ballast resistor in series with each of the transistors in the array; 2. To use a fuse to protect the entire circuit from high current conditions. This is, unfortunately, not an optimal solution since it requires field replacement (i.e. substantial costs). Also, for some integrated circuit technologies, a fuse would not protect the amplifier from damage during thermal runaway due to current hogging effects; and 3. To use thermal shutdown circuitry. A disadvantage of this solution is that it would require a reset signal and possibly field maintenance (i.e. substantial costs).
Over-voltage breakdown: Any transistor has a maximum operating voltage, above which the device will cease to function properly and may be damaged, Typically, when the applied voltage exceeds the maximum operating voltage, the transistor enters an operating state where the device current is uncontrolled and extremely high. This can lead to simultaneous over-voltage and over-current conditions. In an integrated circuit, which contains several transistors, the maximum operating voltage of the individual devices will not be uniform. Instead, they will have an operating range. In a mature manufacturing process, this voltage range will be relatively narrow and easily described by a statistical variation. On the other hand, in new or leading edge manufacturing processes, the voltage range will be relatively large due to material and process defects. The problem that this creates is that the circuit's operation is limited by the weakest transistor.
One solution for avoiding over-voltage breakdown is to limit the voltage that is applied to the circuit to a value that is lower than the maximum operating voltage for the weakest transistors in the circuit. For example, a circuit optimally designed to operate at 7 volts will be operated in the 5 volts range. Having to work in a lower design voltage range requires a larger number of power transistors (e.g. 20% to 25% more) to obtain the same power gain. The direct result in Lhis case is an increase in size of the amplifier circuit, hence an increase in costs.
When considering the above background information, it is clear that there is a need for a device which will permit continuous operation of transistor inclusive power devices for longer periods, thereby avoiding unnecessary field maintenance costs. Furthermore, this device should take up less space than those performing similar functions in existing integrated circuits.
OBJECTIVES AND SUMMARY OF THE INVENTION
An object of this invention is to provide an integrated circuit power amplifier which is less prone to circuit damage as a result of power transistor malfunction. To this effect, the invention provides a fusible protection scheme which eliminates, from an integrated circuit amplifier, weak power transistors before any damage has occurred.
As embodied and broadly described herein, the invention provides an integrated circuit power amplifier comprising a series of power transistors interconnected for producing, from an input signal, an output signal of a certain power level, wherein each power transistor is associated with a fusible structure that carries a current passing through the power transistor, said fusible structure having a fusible link for disabling the associated power transistor in the event that the output current of said associated power transistor exceeds a safe level.
In a particular embodiment, the fusible structure consists of a ballast fuse serially connected to the output of the associated power transistor. The ballast fuse consists of a body of resistive material which operates as a ballast resistance under normal operating conditions. The body of resistive material has a fusible portion which defines the fusible link.
This invention proposes the integration of a fusible structure with transistors (e.g. in the ballast resistance) as a protection scheme in multi-transistor power amplifiers. The integrated fuses will limit damage that may occur in parallel transistor configurations due to over-voltage and over-current conditions.
The proposed scheme will improve the reliability of integrated multi-transistor amplifiers where statistical variations in transistor breakdown characteristics are a concern. This will improve the manufacturing yield and reduce the number of field returns for equipment possessing multi-transistor amplifiers.
This scheme will be most successful in cases where it has been determined that a limited number of weak transistors in a parallel-connected amplifier circuit are the cause of the circuit failure. The proposed fuse structure will remove the weak device upon its failure while preserving the operational functionality of the circuit.
The use of a fusible link in an integrated circuit to protect it from over-voltage is common in some memory circuits such as EPROM (Electrically Programmable Read-Only Memory). However, in these circuits the electronic characteristics of the link in the infused state are not utilized. Placing a fuse in series with each transistor in a power amplifier in an integrated circuit, which is made up of several transistors in order to improve the yield and reliability, is a new concept.
Similarly, the use of a ballast resistor to prevent thermal breakdown in a bipolar junction transistor is a well-established technique. However, the integration of the ballast resistance function and the protection fuses function as a single circuit element is a new concept which results in significant area and cost savings in the manufacturing of the amplifier circuit. This protection scheme is applicable to any integrated circuit process where multiple transistors are used to provide the power amplification function, including gallium arsenide heterojunction bipolar transistors, silicone bipolar junction transistors and silicon germanium heterojunction bipolar transistors.
One of the problems outlined earlier is the requirement to limit the voltage that is applied to the circuit to a value that is lower than the maximum operating voltage for the weakest transistors in the circuit in order to avoid over-voltage breakdown. Having to work in a lower design voltage range requires a larger number of power transistors (e.g. 20% to 25% more) to obtain the same gain. On the other hand and as
Ilowski John
Kung William
Nguyen Khanh Van
Nortel Networks Limited
Pascal Robert
LandOfFree
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