As shown in the block diagram of Fig. 4, SSPCs basically consist of the following main parts: • the power section which contains the FET and thus, performs on/off switching and current limitation in the overload case; • a thermal control circuit (self protection of the switch) which determines the FETs junction temperature and initiates switch-off when a critical value is exceeded; • the signal interface which receives on/off commands and provides signals with respect to status and current flow; • the internal power supply Regarding the power section, two different switches are presently under development (ESTeC contracts): I. ‘Linear Switch'. In the failure case the FET is driven in the ‘constant current mode'. A current control loop is activated which limits the overcurrent to a fixed value such that the switch acts as constant current source. Because the dissipation of the switch is drastically increased in this limiting mode, the thermal control circuit will interrupt its operation after a short time interval (some milli-seconds). II. ‘PWM switch'. In order to handle the overload case without excessive dissipation, the FET is operated in the switched mode. When an overload occurs, a current detector switches off the FET which remains off until the output filter current drops to a preset value which results in switch-on, once again, of the FET. Hence, if the overload is maintained, the filter current will limit cycle between the high and lower current limits, resulting in an effective output current equal to their mean value. This is the so-called ‘Limit Cycling Conductance Control' (LC3) principle which is described in more detail in reference [1]. When comparing both ‘switches', it becomes evident that the PWM switch has the following additional feature: in the failure case an output current, close to the limitation value, is maintained at all output voltages, whilst the input current reduces as the output voltage drops (input current foldback characteristic). The switch remains in this status until it is tripped off by an external signal.
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