One of the major problems in the design of a semiconductor device such as power amplifiers is the protection of the semiconductor active devices in the power amplifier from overheating. An active device such as a transistor is not 100% efficient in utilizing electrical energy. As a result, some of the electrical energy in the active device is converted into thermal energy. The generation of thermal energy results in a rise in the junction temperature of the active device. If the thermal energy generated by the active device is not transferred away from the active device, the junction temperature of the active device will eventually rise above a safe operating temperature range. As is well known in the art, the performance of a semiconductor active device deteriorates when the junction temperature exceeds a certain level. In addition, if the junction temperature deviation from the safe operating temperature range is too large, the active device could be permanently destroyed.
Heat generated by an active device is transferred to the ambient through the device's outer enclosure or case. The rate of heat transfer is proportional to the surface area of the case. However, the surface area of the case is generally small. Thus, where heat dissipation is of concern a heat sink is typically attached to the active device to increase the rate of heat transfer from the active device to the ambient.
Although the use of a properly designed heat sink generally keeps an active device below the maximum safe operating temperature, in many cases it is still necessary to constantly monitor the temperature of the active device. This is because, for example, the power amplifier might be inadvertently used outside of its specification and thereby drive the active device above the maximum safe operating temperature.
One of the prior art methods designed to prevent an active device from operating above the maximum safe operating temperature is to attach a temperature sensitive device to either the heat sink or the case of the active device. Preventive steps are taken when the temperature measured by the heat sensitive device reaches a predetermined value. An example of such an approach is illustrated in U.S. Pat. No. 4,054,845 issued to Glogolja, et al. which teaches the use of a temperature sensitive switch. The switch is thermally coupled to a transistor. When the temperature detected by the temperature sensitive switch rises above a predetermined value, the temperature sensitive switch causes the gain of the circuit to become essentially zero. Since the transistor no longer amplifies a signal, heat production by the transistor is reduced. Thus, the temperature of the transistor is prevented from rising above the maximum safe operating temperature.
One problem of using the temperature sensitive device is that there is always a delay between the measured temperature and the actual temperature because the temperature sensitive device takes time to warm up to the actual temperature. If there is a pulse with a large amplitude passing through the active device, the junction temperature of the active device would rise above the safe operating temperature before the temperature sensitive device reaches the predetermined temperature. In this case, the active device would be destroyed.
A second problem of using the temperature sensitive device is that the temperature sensed by the device depends on mechanical factors which typically have loose manufacturing tolerance. Examples of such factors are the position of the temperature sensitive device and the amount of physical contact between the temperature sensitive device and the active device. As a result, there is a large variation in the temperatures sensed by the temperature sensing devices in different amplifiers. Thus, to be safe, it is necessary to provide a larger safety margin. This is accomplished by lowering the predetermined temperature at which preventive measures are triggered. The result is that many amplifiers using this method for thermal protection are shut down prematurely, and the amplifiers are not used to their full capacity.
A second prior art method for the thermal protection of an active device is to incorporate the temperature sensitive device inside the active device. An example of this approach is illustrated in U.S. Pat. No 4,903,106 issued to Fukunaga, et al. It teaches the fabrication on the same substrate of an active device and a temperature sensitive device. An example of such a device is the fabrication of an n-channel enhancement field effect transistor as the active device and a bipolar transistor as the temperature sensitive device. Since the two devices are in close proximity with each other, the temperature at the bipolar transistor is essentially the same as the temperature at the field effect transistor. The bipolar transistor is used in a separate thermal protection circuit which utilizes the linear relationship between the base-to-emitter voltage and the temperature. The thermal protection circuit is designed to turn off the field-effect transistor when the base-to-emitter voltage of the bipolar transistor reaches a predetermined value.
One problem with the device taught by Fukunaga, et al. is that only a small number of active devices incorporate such a design. Thus, an engineer who desires to use an active device with a certain characteristic may not be able to find a suitable device which incorporates the design taught by Fukunaga, et al.
Therefore, it is an object of the present invention to maintain the junction temperature of an active device below the maximum safe operating temperature while allowing optimal use of the active device.
It is another object of the present invention to reduce the time lag between the rise in the junction temperature of the active device and the preventive measures that need to be taken to prevent an active device from rising above the safe operating temperature range.
It is a further object of the present invention to protect a large variety of active devices from rising above the maximum safe operating temperature.
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description of the invention in conjunction with the accompanying drawings.