to the supply voltage of the controller vcc the reference current i 2 is therefore given by from equations 2 and 5 the on state voltage required to turn on the comparator is therefore given by which can be seen to be independent of the on resistance of the second semiconductor device igbt 210 as mentioned above the comparator output is connected to a control circuit controller 204 which can turn the power device 202 t 1 off by pulling the gate drive signal low whenever the collector voltage exceeds the threshold voltage this will protect the power device t 1 from a too high collector emitter voltage as could otherwise occur in saturation devices 210 and 222 igbts t 2 and t 3 are important preferably both are small and substantially identical devices for example high voltage igbts fabricated using the applicant 39 s particularly advantageous u201c powerbrane u201d u2014 trade mark technology mentioned above devices 210 and 222 t 2 and t 3 can be very small in size as the value of r on does not affect the measurement device 210 transistor t 2 is controlled by the control circuit 204 using sat_drive 236 in order to measured the.