A frequency slider circuit for reducing power dissipation in the form of switching losses in a switching device when the switching device is near its desired maximum temperature limit is disclosed herein. The switching device controls an electric load, such as a motor. The frequency slider circuit samples a current of the switching device. The frequency slider circuit then uses a current transducer to convert the current to a signal level. The signal level is then processed by a model of the thermal response of the switching device. The output of the thermal model is compared to a reference signal, and if the output of the thermal model is greater than the reference signal, the switching frequency of the switching device will be lowered gradually to keep the power generated during the switching periods to be below a predetermined value. A minimum frequency clamping circuit is included to ensure that the switching frequency never drops below a minimum value, to ensure stable operation of the switching device. The frequency slider can also operate to sample a voltage across the switching device to determine the estimated temperature of the switching device, or to use both the current and the voltage of the switching device to determine the estimated temperature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method and apparatus for reducing the power 
dissipation, in the form of switching losses, in a switching device. More 
specifically, the invention relates to a method and apparatus for 
estimating the temperature of the switching device controlling an electric 
device, such as a motor, in order to operate the switching device at an 
optimal switching frequency (i.e. controlling or sliding the frequency) to 
reduce switching losses for purposes of thermal protection. 
2. Description of the Related Art 
It is desirable to provide thermal protection for a semiconductor switching 
device by maintaining it at or below a certain maximum temperature limit 
both during normal operation and during the switching operation, when the 
switching device is switched ON or OFF. The thermal protection of a 
switching device, which may have high power dissipation under both stable 
and transitory operating conditions, better ensures temperature stability 
and, therefore, reliable operation. Conventional methods of providing 
thermal protection for a switching device involves measuring the actual 
temperature of the switching device, and adjusting the switching frequency 
to an optimal level based on the measured temperature. 
For such purposes, "switching frequency" refers to the rate or interval at 
which the switching device, such as a transistor, is switched ON or OFF. 
For example, if a transistor is switched ON and OFF continuously at a 1 
millisecond interval, it is said to have a 1 kHz switching rate. During 
each switching interval, power is generated by the switching device as a 
result of slew rate of the voltage and the current supplied to the 
switching device. This generated power is dissipated in the form of losses 
in the switching device. The total power losses of a switching device 
consist of switching loss and conduction loss. The magnitude of the 
switching loss is a function of the switching frequency and the amount of 
the current (I) conducted through the switching device. The magnitude of 
the conduction loss is a function of the amount of current (I) passing 
through the switching device. Power generated by such losses is absorbed 
by the switching device in the form of thermal energy, or heat, which 
typically increases the temperature of the switching device, and in turn 
must be dissipated from the switching device to the environment by heat 
transfer methods, such as radiation, convection and conduction. 
FIG. 1 shows typical voltage and current curves for a typical semiconductor 
switching device. As shown in FIG. 1, when the switching device is turned 
ON, the voltage V drops at a particular slew rate (or slope) between times 
T.sub.1 (the time the switching device is provided the ON signal) and 
T.sub.2 (the time the switching device is in the steady-state of ON), and 
the current I increases at a particular slew rate during this same time 
period. The interval between times T.sub.1 and T.sub.2 corresponds to the 
switching ON time. When the switching device is turned OFF, the voltage 
increases at a particular slew rate, and the current decreases at a 
particular slew rate between times T.sub.3 (the time the switching device 
is provided the OFF signal) and T.sub.4 (the time the switching device is 
in the steady-state of OFF). The interval between times T.sub.3 and 
T.sub.4 corresponds to the switching OFF time. 
In FIG. 1, the slew rates are shown as being the same for the ON and OFF 
times for both the current and voltage levels in the switching device. 
However, this need not necessarily be the case, and all slew rates may 
vary based upon whether the switching device is switched ON or OFF. For 
example, the time it takes the voltage to drop from its high (OFF) level 
to its steady-state low level (ON) between times T.sub.1 and T.sub.2 may 
be less than the time it takes the voltage to increase from the 
steady-state low level to the high level between times T.sub.3 and 
T.sub.4. 
When the switching device is operating in its normal state (i.e., during 
the steady-state period between an ON or an OFF switching time interval), 
there is a current value of I.sub.L and a voltage value of V.sub.L being 
supplied to the switching device, and this is shown between times T.sub.2 
and T.sub.3 of FIG. 1. The switching loss corresponds to the power 
generated during interval from times T.sub.1 to T.sub.2 (when the 
switching device is turning ON) and during the interval from times T.sub.3 
to T.sub.4 (when the switching device is turning OFF), while the 
conduction loss corresponds to the power generated during the interval 
between times T.sub.2 and T.sub.3 (when the switching device is ON). 
The time or interval between consecutive turning ON and turning OFF of the 
switching device defines the switching rate, or switching period. At a 
faster switching rate, the switching device has less time between 
switching intervals to dissipate through heat transfer to the environment, 
the total power generated by the switching operation and absorbed as 
thermal energy within the switching device during each switching period. 
As a result, the switching device will be absorbing more thermal energy 
than it is transferring to the environment and its temperature typically 
will increase as a result of the increasing residual between power 
generated and thermal energy dissipated. (Note that the heat generated 
during the switching periods is typically much greater than the heat 
generated during the conduction periods; however, since the switching 
times (i.e., the interval between times T.sub.1 and T.sub.2 and times 
T.sub.3 and T.sub.4) are typically smaller than the normal operating 
periods of the switching device, this does not present too much of a 
problem.) As a result, when the switching device is operating at a high 
switching rate, the temperature of the switching device may increase 
beyond acceptable limits. 
U.S. Pat. No. 4,727,450, entitled "Temperature Measuring, Protection and 
Safety Device, Thermal Protection Device Using the Temperature Measuring 
Device and Electronic Power Controller Using the Thermal Protection 
Device" (the '450 patent), issued on Feb. 23, 1988, describes a method for 
controlling the temperature of an electronic power controller. 
The device disclosed in the '450 patent utilizes an initialization device 
for setting a thermal model to an operating point representative of the 
actual thermal state of the circuit to be protected. The real (actual) 
temperature of the electronic power controller is measured and, based on 
that temperature, appropriate thermal protection is effected by cutting 
off power to the circuit. The device disclosed in the '450 patent also 
utilizes a thermometer to set the initial conditions in the initialization 
circuit. 
Although the device of the '450 patent may provide thermal protection for a 
switching device, it is desirable to have a thermal protection device that 
does not cut off power to the circuit when the temperature of the 
switching devices gets too high, but instead adjusts the switching 
frequency of the switching device in a manner that allows continued 
operation without risk of thermal damage to the switching device. 
Further, in many applications it is undesirable to have a thermal 
protection device which requires an initialization device in order to set 
up the thermal model for determining if the device is operating at too 
high a temperature, as is disclosed in the '450 patent. 
SUMMARY OF THE INVENTION 
The present invention relates a method for controlling the temperature of 
and thereby providing thermal protection for a switching device operating 
to switch a current within a desired frequency range to control an 
electric device. The method includes the step of generating a signal 
representative of the switching device. The method further includes the 
step of estimating the temperature of the switching device based on the 
signal. The method also includes the step of comparing the estimated 
temperature of the switching device to a predetermined temperature 
reference level. The method also includes the step of reducing a switching 
frequency of the switching device within the desired frequency range when 
the estimated temperature exceeds the predetermined temperature reference 
level. 
The present invention also relates to an apparatus for controlling the 
temperature of and thereby providing thermal protection for a switching 
device operating to switch a current within a desired switching frequency 
range to control an electric device. The apparatus includes a current 
transducer circuit coupled to the switching device. The current transducer 
circuit is configured to monitor the load current and to provide a signal 
level based on the load current. A temperature predictor circuit is 
coupled to the current transducer and configured to estimate the 
temperature of the switching device based on the signal level. The 
apparatus also includes a comparison circuit coupled to the temperature 
predictor circuit and configured to compare the estimated temperature of 
the switching device to a predetermined temperature reference level and to 
provide a comparison signal indicative of the estimated temperature of the 
switching device and whether it exceeds the predetermined temperature 
reference level. The apparatus further includes a frequency clamping 
circuit coupled to the comparison circuit and configured to clamp the 
switching frequency of the switching device at or below a maximum 
frequency level if the comparison signal indicates that the predicted 
temperature of the switching device is greater than the predetermined 
temperature reference level. The apparatus still further includes a 
temperature lowering circuit connected to the frequency clamping circuit 
and configured to reduce the switching frequency by a predetermined amount 
when the comparison signal indicates that the estimated temperature of the 
switching device exceeds the predetermined temperature reference level. 
The present invention still further relates to an apparatus for controlling 
the temperature of and thereby providing thermal protection for a 
switching device operating to switch a current within a desired switching 
frequency range to control an electric device. The apparatus includes a 
voltage transducer circuit coupled to the switching device. The voltage 
transducer circuit is configured to monitor a voltage level across the 
switching device and to provide a signal level based on the voltage level. 
A temperature predictor circuit is coupled to the current transducer and 
configured to estimate the temperature of the switching device based on 
the signal level. The apparatus also includes a comparison circuit coupled 
to the temperature predictor circuit and configured to compare the 
estimated temperature of the switching device to a predetermined 
temperature reference level and to provide a comparison signal indicative 
of the estimated temperature of the switching device and whether it 
exceeds the predetermined temperature reference level. The apparatus 
further includes a frequency clamping circuit coupled to the comparison 
circuit and configured to clamp the switching frequency of the switching 
device at or below a maximum frequency level if the comparison signal 
indicates that the predicted temperature of the switching device is 
greater than the predetermined temperature reference level. The apparatus 
still further includes a temperature lowering circuit connected to the 
frequency clamping circuit and configured to reduce the switching 
frequency by a predetermined amount when the comparison signal indicates 
that the estimated temperature of the switching device exceeds the 
predetermined temperature reference level. 
The present invention provides an apparatus for controlling the temperature 
of and thereby providing thermal protection for a switching device 
operating to switch a current at a desired frequency to control an 
electric device. The apparatus includes means for generating a signal 
representative of the load current supplied to the electric device by the 
switching device. The signal can be based either on the current output 
from the switching device, a level of voltage measured across the 
switching device, or a combination of the two. The apparatus further 
includes means for estimating the temperature for the switching device 
based on the signal. The apparatus also includes means for comparing the 
predicted temperature to a predetermined temperature reference level. The 
apparatus still further includes means for reducing the switching 
frequency of the switching device below the operating frequency (within a 
desired frequency range for the application) when the estimated 
temperature exceeds the predetermined temperature reference level.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The frequency slider as described herein regulates the switching frequency 
of the switching device based on a level of current measured from the 
electric device that it controls, i.e., the load current. If the current 
is determined to be above a predetermined value then, based on the 
switching frequency of the switching device, the switching frequency may 
be reduced so that the total power dissipated and resultant thermal energy 
generated during the switching operation and thereby absorbed by the 
switching device is likewise reduced to protect the switching device from 
thermal damage such as overheating, burn out or other harm. 
Referring back to FIG. 1, the power dissipated between times T.sub.1 and 
T.sub.2, plus the power dissipated between times T.sub.3 and T.sub.4 is 
considered the switching loss, and for a fixed current in the switching 
device, this power loss is only dependent on the switching interval (i.e., 
the switching frequency). The power loss between times T.sub.2 and 
T.sub.3, i.e., the conduction loss, is dependent upon how much current is 
being switched by the switching device. 
Referring now to FIG. 2, when the switching device has a switching rate of 
F.sub.1 switches per second, a current level of I.sub.1 amps corresponds 
to W.sub.0 watts of power generated of the switching device, a current 
level of I.sub.2 amps corresponds to W.sub.1 watts of power generated by 
the switching device, and a current level of I.sub.3 amps corresponds to 
W.sub.3 watts of power generated by the switching device. 
When switching device has a higher switching rate of F.sub.2 switches per 
second, a current level of I.sub.l amps corresponds to W.sub.0 watts of 
power generated by the switching device, a current level of I.sub.2 amps 
corresponds to W.sub.2 watts of power generated by the switching device, 
and a current level of I.sub.3 amps corresponds to W.sub.4 watts of power 
generated by the switching device. A current level of zero amps may not 
necessarily correspond to a voltage level of zero volts, due to 
voltage-only losses in the switching device (e.g., I.sub.1 =zero amps does 
not necessarily imply that W.sub.0 =zero watts). 
More power is generated as the switching frequency of the switching device 
increases, since more power is generated during switching intervals (i.e., 
between times T.sub.1 and T.sub.2 of FIG. 1) than during normal operating 
intervals (i.e., between times T.sub.2 and T.sub.3 of FIG. 1). 
Referring back to FIG. 2, assume that a power level of W.sub.3 watts is the 
maximum amount of power that the switching device can reliably dissipate 
(by heat transfer to the environment) during operation, and that if any 
additional power is generated, such as W.sub.4 watts, the switching device 
will absorb the power in the form of thermal energy which will result in 
an increase in the operating temperature of the switching device and may 
cause damage to the switching device. The frequency slider utilizes the 
measured operating current (i.e. the load current) supplied to the 
electric device, such as a motor or other type of load, that is controlled 
by the switching device, to estimate the operating temperature of the 
switching device. Since the operating (or load) current supplied to the 
electric device is typically the parameter that is being controlled, its 
value is readily measurable and continuously available for use by the 
frequency slider. 
FIG. 3 shows a block diagram of a direct current (DC) source 100 supplying 
direct current to a switching device 150. The switching device 150 
receives a switching frequency signal 180 from a frequency slider 50, and 
based on that switching frequency signal 180, the switching device 150 
converts the DC current received from the DC source 100 into an 
alternating current (AC). The alternating current output from the 
switching device 150 is supplied to an electric load, such as an AC motor 
190. The type of load 190 that may be controlled by switching device 150 
include, but are not limited to, a motor, a transducer, a solenoid, and a 
resistive load. 
The switching device 150 also receives a control signal from a current 
regulator circuit 175, which determines an appropriate duty cycle of the 
alternating current (i.e., load current) supplied to the electric load 
190, based on a command received from the load command circuit 155. The 
load command circuit 155 determines an appropriate amount of current to be 
supplied to the electric load 190 based on a current command signal 
received from a load controller circuit 179, which is directly connected 
to the electric load 190. 
The current regulator circuit 175 controls the current loop of the electric 
load 190, while the frequency slider 50 controls the switching frequency 
applied to the electric load 190. Accordingly, the switching frequency of 
the switching device 150 can be set to not exceed the maximum power level 
(e.g., W.sub.3 watts) of the switching device 150, based on a particular 
level of measured load current. 
In the frequency slider 50, a threshold power level below the maximum power 
level W.sub.3 is preset externally, and in FIG. 2, this corresponds to the 
threshold power level of W.sub.2 watts. If the switching device 150 is 
operating at a switching frequency of F.sub.2 switches per second, then 
the measured current level of I.sub.2 amps will result in a predicted 
power level of W.sub.2 watts. When the measured load current level 
indicates that the threshold power level of W.sub.2 watts is reached, then 
the switching frequency of the switching device 150 will be gradually 
reduced to an appropriate switching rate by the frequency slider 50 as the 
load current is likewise adjusted in order to keep the power generated by 
the switching device 150 at or below the maximum power level of W.sub.3 
watts, as will be described more fully below. 
In FIG. 2, the frequency slider 50 reduces the switching frequency of the 
switching device 150 from F.sub.2 switches per second down to F.sub.1 
switches per second as the current increases from I.sub.2 amps to I.sub.3 
amps. During the lowering of the switching rate, the switching device 150 
operates approximately along the power curve 52, which is shown as a line 
between power curves F.sub.1 and F.sub.2 of FIG. 2. This way, the power 
dissipation of the switching device 150 is maintained at a somewhat 
constant level; i.e., the total loss in watts remains somewhat constant. 
That is, as the current of the switching device 150 is increased, the 
frequency of the switching device 150 is decreased. A constant level of 
power is thereby lost in the switching device 150. 
FIG. 4 shows a block diagram of the frequency slider 50 according to a 
first embodiment of the invention. A source of current supplied by the 
switching device 150 is measured by a current measuring device 10. As seen 
in FIG. 3, that current measuring device may be a circuit receiving a 
level of current from the electric load 190. The current regulator 175 
sends a command to the switching device 150 to supply an appropriate duty 
cycle for the electric load 190. 
Referring now to FIG. 4, a current transducer circuit 20 converts the 
received current into a voltage value. A temperature predictor circuit 30 
is provided with the voltage value, and based on that voltage value, 
estimates the temperature of the switching device 150. The estimation of 
the temperature of the switching device 150 may be implemented in either 
software or hardware. For example, a second or higher order differential 
equation may be utilized in order to estimate the temperature of the 
switching device 150 as is known by one of ordinary skill in the art. 
Based on several input parameters, such as ambient air temperature and 
other thermal characteristics of the switching device 150, an accurate 
estimation of the temperature of the switching device 150 may be obtained 
based upon the current of the switching device 150. 
Once the temperature is estimated by the temperature predictor circuit 30, 
an excess temperature detector 40 compares that estimated temperature to a 
temperature reference signal RS.sub.1. The excess temperature detector 40 
has a minimum temperature threshold level RS.sub.0 stored within the 
excess temperature detector 40 upon detection of which the excess 
temperature detector 40 turns itself on. The excess temperature detector 
40 also stores a maximum temperature threshold level RS.sub.1, which 
corresponds to a preset temperature at or above which the switching device 
cannot go beyond for purposes of thermal stability. 
If a voltage value is input to the temperature predictor circuit 30 
corresponding to an estimated temperature level of the switching device 
150 below the minimum temperature threshold level RS.sub.0, then there is 
no need to perform frequency sliding by the frequency slider 50, and the 
switching device 150 continues to operate at the selected frequency 
F.sub.2, thereby following the F.sub.2 curve as shown in FIG. 2. Referring 
back to FIG. 4, if the estimated temperature is determined to be above the 
temperature reference signal RS.sub.1, then the switching frequency of the 
switching device 150 is reduced by a predetermined amount which can be 
either a preset amount, such as 10 Hz, or be determined based upon the 
difference between the predicted temperature and the temperature reference 
signal RS.sub.1. In such a configuration, a larger temperature difference 
would result in a greater reduction of the switching frequency of the 
switching device 150. 
A minimum frequency clamping circuit 120 is used to ensure that the 
switching frequency of the switching device 150 never drops below a 
minimum switching frequency level, F.sub.1. If the switching frequency of 
the switching device 150 was allowed to drop below the minimum switching 
frequency level F.sub.1, then the current loop of the switching device 150 
would begin to run into the load control loop (555 of FIG. 3) of the 
electric load 190 applied to the switching device 150, which would cause 
an unstable condition. The minimum frequency clamping circuit 120 prevents 
this condition by ensuring that the switching frequency of the switching 
device 150 is always above the critical frequency of the load control loop 
555. 
In FIG. 3, the current regulator 175 controls the current supplied to the 
electric load 190. The current regulator 175 controls the amount of ON and 
OFF time for each switching cycle, i.e., the duty cycle, while the 
frequency slider 50 controls the rate of the switching cycles. 
Referring back to FIG. 2, assume that I.sub.2 is 70% of the maximum current 
carrying capacity of the switching device 150, and that I.sub.3 is 100% of 
the maximum current carrying capacity of the switching device 150. As the 
current increases in the switching device 150 while the switching device 
150 is switching at a rate F.sub.2 times per second, the power curve 
follows the F.sub.2 curve from I.sub.1 to I.sub.2 amps. At a current level 
of I.sub.2 amps, the current transducer circuit 20 outputs a signal level 
corresponding to that amount of current, I.sub.2 amps. That signal level 
is converted to a voltage level and used by the temperature predictor 
circuit 30 of FIG. 4. The switching frequency of the switching device 150 
is gradually reduced so that it follows the power curve 52 to thereby 
operate the switching device 150 along the power curve 52 instead of along 
the F.sub.2 curve. 
In order to maintain a somewhat constant amount of power to the switching 
device 150, the switching frequency must be decreased from at the same 
time the current is increased, as is shown in FIG. 2. The F.sub.1 curve 
corresponds to a reduced switching rate of F.sub.1 switches per second. 
The time when the switching device 150 is operating along the power curve 
52 corresponds to a gradual reduction of the switching rate until a 
minimum switching frequency of F.sub.1 switches per second is reached. 
The foregoing example assumes that the switching frequency F.sub.1 
corresponds to the minimum frequency reference FM.sub.1. When the 
switching frequency reaches the minimum switching frequency F.sub.1, the 
switching rate of the switching device 150 is clamped at the rate F.sub.1, 
and the switching device 150 follows the F.sub.1 curve up to the maximum 
current output, I.sub.3 amps. This prevents the maximum power generated by 
the switching device 150 from exceeding W.sub.3 watts at any time, even at 
the maximum current for the switching device 150. 
It is important to note that the thermal energy, or heat, generated by 
power losses during the ON and OFF switching periods must be reduced 
during the switching transitions that occur due to the slew rate of the 
voltage and current values during these switching periods. One way to 
reduce this generated heat is to reduce the switching frequency of the 
switching device 150. A reduced switching frequency provides the switching 
device 150 with the additional time to dissipate thermal energy (or heat) 
generated by power losses during the longer operational periods that 
result between switching intervals. 
Referring back to FIG. 4, buffer amplifier 54 and inverting amplifier 60 
together act as a voltage controller for an oscillator circuit 200 of the 
frequency slider 50. The oscillator circuit 200 includes a hysteresis 
switch 70, an amplitude clamping circuit 80, an amplitude clamping circuit 
90, a buffer amplifier 100, and an integrator amplifier 110. The 
oscillator circuit 200 includes a hysteresis feedback signal that is fed 
back from the amplitude clamping circuit 80 to the hysteresis switch 70, 
and a triangle frequency feedback signal that is fed back from the 
integrator amplifier 110 to the hysteresis switch 70. 
The oscillator circuit 200 corresponds to a free-running oscillator that 
generates a triangular wave, although it can generate any type of periodic 
wave, such as a sinusoid, and still fulfill the function of the invention. 
Most every type of switching device 150, such as a pulse width modulator 
device or a variable voltage-variable frequency device, requires some kind 
of clocking oscillator that outputs a fixed frequency, such as that shown 
as the oscillator circuit 200 in FIG. 4. 
As the voltage supplied by the inverting amplifier 60 to the amplitude 
clamping circuit 90 increases, the frequency of the triangular wave output 
from the integrator amplifier 110 of the oscillator circuit 200 decreases. 
Conversely, as the voltage supplied by the inverting amplifier 60 to the 
amplitude clamping circuit 90 increases, the frequency of the triangular 
wave output from the integrator amplifier 110 of the oscillator circuit 
200 decreases. The output of the oscillator circuit 200 is used to control 
the switching frequency of the switching device 150 and to reduce the 
switching frequency to an appropriate rate to avoid overheating of the 
switching device 150. 
FIG. 3 shows an oscillating signal 180 supplied to the switching device 150 
from the frequency slider 50. A higher switching frequency typically 
results in a better response from the electric load 190 under control of 
the switching device 150, and operates the electric load 190 at a 
frequency where there is not normally undesirable audible noise. 
When the switching frequency supplied to the electric load 190 is lowered, 
or slid down from a higher frequency F.sub.2 to a lower frequency F.sub.1, 
audible noise may issue from the electric load 190. However, the 
alternative to audible noise may be thermal damage to the switching device 
150, which may operate unreliably or fail due to the amount of power 
generated during the switching intervals if the switching frequency is not 
reduced to an appropriate level during this time. The temperature 
predictor circuit 30 of FIG. 4 allows operation at optimal switching 
frequency for a given current of the switching device 150. In addition, 
there is no need to measure the actual temperature of the switching device 
150, which would require a separate temperature sensor as is utilized in 
conventional devices as described earlier. Furthermore, there is no 
requirement to preset or load any initial values for the frequency slider 
50. 
In certain applications, if the switching frequency of the switching device 
150 is reduced below a particular minimum frequency, a high amount of 
ripple current may be supplied to the electric load 190, which may cause 
thermal damage or other harm to the electric load 190. Thus, a minimum 
switching frequency F.sub.1 is set in the frequency slider 50 in order to 
ensure that the switching device 150 never reduces below a switching 
frequency that may damage the electric load 190 that the switching device 
150 controls. 
Although the above description of the frequency slider 50 uses a reduction 
in switching frequency in order to reduce heat generated by the switching 
device 150 during switching frequency intervals, the frequency slider 50 
can also be utilized to increase the switching frequency of the switching 
device 150 if thermal conditions permit. Once the estimated temperature of 
the switching device 150 reaches the maximum temperature threshold level 
RS.sub.1, and operation is transferred by the frequency slider from along 
power curve F.sub.2 to along power curve F.sub.1 (i.e. along either power 
curve, however, for each operating current there is a corresponding 
switching frequency). Under such operating conditions, the estimated 
temperature of the switching device 150 may drop. Accordingly, referring 
back to FIG. 2, if the switching device 150 is operating at a current 
level between I.sub.1 and I.sub.2 amps, and if the switching device 150 is 
also operating at a switching frequency of F.sub.1, then the frequency 
slider 50 can be configured to monitor and to gradually increase the 
switching frequency of the switching device 150 to F.sub.2. The switching 
device 150 will then operate at the F.sub.2, the increased switching rate, 
as long as the measured current is below I.sub.2 amps. Operation at the 
increased frequency, F.sub.2, will result in a smoother signal supplied to 
the electric load 190 under control of the switching device 150. Of 
course, if the current does increase to I.sub.2 amps, the frequency slider 
50 will then gradually reduce the switching frequency of the switching 
device 150 from a switching rate of F.sub.2 to a switching rate of 
F.sub.1, as is shown along power curve 52 in FIG. 2. 
Similarly, the frequency slider 50 can operate so as to allow the switching 
device 150 to exceed the maximum power level, W.sub.3 watts, for a short 
period of time, in order to achieve a desired speed of the electric load 
190 without harming the switching device 150. The frequency slider 50 is 
configured to allow more output current for the same temperature, due to a 
lowering of the switching frequency of the switching device 150 under 
control of the frequency slider 50. Essentially, the system acts as if the 
temperature of the switching device 150 is held constant as the current 
increases or decreases. 
FIG. 5 shows a second embodiment of the system according to the invention. 
In FIG. 5, instead of using a current output from the switching device to 
determine the temperature of the switching device, a source of voltage 
across the switching device is used instead, as shown as block 11. This 
source of voltage 11 is supplied to a voltage transducer 21, which outputs 
a voltage signal to the temperature predictor circuit 30a. The temperature 
predictor circuit 30a predicts the temperature of the switching device 
based on the voltage signal indicative of the voltage across the switching 
device. The rest of the system according to the second embodiment is 
similar to the system according to the first embodiment as shown in FIG. 4 
and as described earlier. 
FIG. 6 shows a third embodiment of the system according to the invention. 
In FIG. 6, a signal corresponding to a current of the switching device and 
a signal corresponding to a voltage across the switching device are 
supplied to a multiplier 23, which outputs a control signal to the 
temperature predictor circuit 30b that is based on both the current and 
the voltage of the switching device. The system according to the third 
embodiment allows for a more accurate estimate of the temperature of the 
switching device since the temperature estimate can be based on two 
parameters (voltage and current) instead of just one parameter (voltage or 
current) in the systems according to the first and second embodiments, but 
the system according to the third embodiment also requires more elements. 
While a preferred embodiment of the invention has been described herein, 
other modifications to the invention may become apparent to one of 
ordinary skill in the art without departing from the scope of the 
invention as described herein. For example, the circuits of the frequency 
slider 50 can be implemented in appropriately programmed discrete or 
integrated digital circuits, such as discrete logic circuits, including a 
microprocessor or digital signal processor.