This invention relates to an overheating protection system for a switching module for driving a load, and particularly, although the invention is not limited to this, to a system for protecting switching devices inside a switching device module for chopping current through phase coils of an electric motor.
For example, a switching circuit for driving a switched reluctance motor (hereinafter abbreviated to SR motor) will be described. A switched reluctance motor generally has a rotor which is constructed with poles projecting outward and a stator constructed with poles projecting inward, and the rotor is simply an iron core while the stator has coils wound in concentration around each pole. In this kind of SR motor, the poles of the stator act as electromagnets and the rotor is rotated by its poles being attracted by magnetic forces of the stator. Accordingly, by the state of excitement of the coils wound on the poles of the stator being successively switched according to the rotational positions of the poles of the rotor, the rotor can be rotated in a desired direction. This type of SR motor is disclosed for example in Japanese Unexamined Patent Publication No. H.1-298940.
In an SR motor, when the poles of the rotor are in a certain rotational positions, the currents being passed through the stator coils are switched. In technology disclosed in Japanese Unexamined Patent Publication No. H.1-298940, a rotational position signal having gentle rises and falls is generated and using this rotational position signal the rises and falls in current of when the currents to the coils are switched on and off are made gentle. Also, in Japanese Unexamined Patent Publication Nos. H.7-274569, H.298669 and H.8-172793, to make the rises and falls in current smooth, the currents passed through a motor are controlled by means of a PWM using an H-type switching circuit, and to lessen shortages of torque the switching mode is controlled.
For example, an SR motor driving system of an electric vehicle in which this kind of SR motor is used has phase drivers (in the case of a three-phase motor, three) including switching devices and gate drivers for passing currents through phase coils, phase current controllers for generating current patterns for the phases (time series current distributions) having phase differences between the phases and passing phase currents according to these patterns through the phase coils, and a system controller for generating target phase currents corresponding with torque motor driving commands and torque commands and supplying these to the phase controllers. The switching devices for passing currents through the phases are provided in a module, and to prevent thermal breakdown of the switching devices a temperature sensor is mounted on the switching device module and each phase driver is provided with a gate signal cutoff circuit for cutting of an on-command signal to the respective gate driver when the temperature detected by this sensor rises above a set value. Current sensors for detecting the currents in the phase coils and an over-current detecting circuit are also provided, and when a coil current becomes excessive a cutoff command signal is fed to the gate signal cutoff circuit and the currents of all the coils are cut off.
However, the temperature sensor is in a position away from the switching devices inside the module, and it takes time for heat produced by the switching devices to reach the temperature sensor, and at times when the amount of heat produced by the devices is large, such as during high-current excitation and at low motor speeds (when the excitation time of each phase is long), there is a risk of the temperature of a device reaching a limit temperature exceeding a threshold value at which the gate signal cutoff circuit operates even though the detected temperature of the temperature sensor has not reached the threshold value, and the switching device consequently breaking before the gate signal cutoff circuit turns the gate off in response to the detected temperature. To avoid this the temperature threshold value at which the gate signal cutoff circuit operates is set low and the motor drive is inhibited.