An optically isolated circuit for failure detection of a switch in an HV series connected stack

Stacks of series connected high-power semiconductor switches are widely used in High Voltage (HV) and high current pulse generators for accelerator kicker magnets: this is due to advantages such as simpler control, longer life-time, lower rate of erratic turn-on and no pre-heating requirements, compared to traditional gas switches. For reliability reasons, redundant semiconductors are included in the stack to preserve the HV stack functionality in the case of an individual semiconductor failure. In such a case the applied voltage is divided among residual functional semiconductors. Nevertheless, it is important to detect the failure or degradation of an individual semiconductor as soon as possible and to replace the faulty semiconductor. A simple and low cost Voltage Controlled Oscillator (VCO) was developed which is connected in parallel with each semiconductor. Each VCO derives its power from the semiconductor off-state voltage and is optically coupled to its detection circuit. Failure of a semiconductor or an increase of its leakage current leads to voltage reduction across that switch, which will be rapidly detected by a lower VCO frequency compared to that of the other series semiconductors. The VCO reported in this paper has a wide useful dynamic range: depending on the chosen transistor the VCO may operate from less than 100 nA supply current and its output is reasonably linear over almost 3 decades of applied voltage.