The present invention is directed to thermal management, and more specifically to thermal management of an electronic switch.
In many systems, the power dissipation of a given electronic switch (e.g., a field effect transistor (FET)) is set during manufacture. For example, in many systems, a slew rate of a control signal is selected based on a desire to reduce electromagnetic emissions from the electronic switch so as to decrease the noise radiated to other circuitry in the system. Unfortunately, in many applications, reducing a slew rate of the control signal can cause the switch to dissipate excessive power during switching. This is undesirable in that excessive power dissipation can produce switch semiconductor junction temperatures that exceed the rated maximum switch temperature, which can lead to a breakdown of a switch semiconductor junction and ultimately to the destruction of the switch.
In systems that utilize pulse width modulated (PWM) control signals (e.g., systems that include variable assist electromagnets for power steering), the frequency of a PWM current (delivered to a load) must be high enough that audible noise is not excessive. However, utilizing higher frequency control signals can also cause an electronic switch to dissipate excessive power. As above, excessive power dissipation in the switch can lead to breakdown of the semiconductor junctions of the switch, and ultimately to destruction of the switch.
Traditionally, the junction temperature of electronic switches has been controlled through appropriate heat sinking and by selecting an electronic switch that has a junction area that can withstand an expected worse case power dissipation. However, both heat sinking and utilizing switches with larger die areas adds additional cost to a given system.
As such, a technique which thermally manages an electronic switch, thus allowing electronic switches with reduced semiconductor die area to be utilized and decreasing the need for heat sinks, is desirable.
The present invention is directed to a technique for thermal management of an electronic switch that provides power to a load responsive to a pulse width modulated control signal. A switch temperature of the electronic switch is monitored to determine whether the switch temperature is below a first set temperature. When the switch temperature exceeds the first set temperature, the control signal is modified such that an average power dissipated by the electronic switch is reduced. Preferably, the control signal is restored to its original state when the switch temperature no longer exceeds the first set temperature. In one embodiment, the control signal is modified by increasing a slew rate of the control signal when the switch temperature exceeds the first set temperature. In another embodiment, a frequency of the control signal is reduced when the switch temperature exceeds the first set temperature.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.