Method for driving an output stage by pulse-width modulation

#The invention relates to a method for driving an output stage by pulse-width modulation, which driving serves to supply a unit cooling the output stage during operation.

#If electric motors are used to drive cooling units, in particular refrigerating machines of air conditioners or fans in motor vehicles, the cooling can also be used to cool the output stage. Various requirements have to be considered in the case of driving the output stage by pulse-width modulation (PWM)—inter alia with regard to electromagnetic compatibility and noise intrusions at frequencies below the auditory threshold. These requirements are, however, frequently opposed to a high edge steepness or to a low frequency of the pulses, which are desirable in order to achieve the lowest possible power loss in the output stage.

#It is the object of the present invention to design a style of driving an output stage, which driving serves to supply a unit cooling the output stage during operation, in such a way that both the said ambient conditions are generally satisfied, and overloading of the output stage at high ambient temperatures owing to a high power loss is avoided.

#The invention permits numerous embodiments. One of these is illustrated diagrammatically in the drawing with the aid of a plurality of figures and is described below. In the drawing:

#FIG. 1 shows an arrangement for carrying out the method according to the invention,

#FIG. 2 shows pulse diagrams in the case of a first embodiment of the method according to the invention,

#FIG. 1 shows as a block diagram in a greatly simplified form an output stage 1 which is driven by a control device 2 with pulse-width-modulated signals. Various signals required for controlling purposes, for example input/output signals or speed/temperature stipulations can be fed to the control device via the inputs 3 and 4. The output stage 1 supplies a motor 5, which drives a refrigerating unit 6. The refrigerating unit is connected in a thermally conducting fashion to the output stage 1, and this is symbolized by arrows. A temperature sensor 7 measures the temperature of the output stage 1 or of another point in the system which has a good correlation with the output stage temperature, and passes the measurement result to control device 2.

#In the first embodiment of the method according to the invention, according to FIG. 2a) the output stage 1 receives pulses with steep edges such that the power loss in the output stage 1 is so low that even at a higher temperature the permissible maximum temperature of the output stage transistors is not exceeded. Such high temperatures can also occur, for example, in the engine compartment when a motor vehicle is parked after being driven quickly and/or at high ambient temperatures. After a run-up phase, the output stage 1 is cooled by the refrigerating unit 6. A changeover is then made to the pulses shown in FIG. 2b), although, because of their lower edge steepness, the said pulses result in a higher power loss in the output stage, but exhibit a weaker interfering effect in higher frequency ranges of the electromagnetic emission.

#FIG. 3 shows an alternative, specifically switching over the carrier frequency, a lower frequency being provided in the run-up phase shown in FIG. 3a), with the result that the power loss caused by the finite edges is lower. Short-term noise emission is possible in return when the frequency is below 16 kHz. After the lower operating temperature has been reached, a switchover is made to a high frequency above the range of audibility with edges which are not excessively steep (FIG. 3b)).