Patent ID: 7477089

Claim:
A device comprising: a power insulated gate field effect transistor, having main cells controlled by a main cell insulated gate and sense cells controlled by a sense cell insulated gate; a feedback sample and hold circuit connected in series with the sense cells and arranged to operate in a plurality of states including at least one sample state and a hold state; wherein the sample and hold circuit is arranged to sense the current flowing through the sense cells when in the at least one sample state but not in the hold state, wherein the sample and hold circuit is arranged to operate in the sample state to sense the current passing through the sense cells and having an output arranged to drive the sense cell gate towards a voltage in which a target current passes through the sense cells, and to operate in the hold state to hold its output voltage and to drive the main cell insulated gate with that output voltage, and wherein the output of the feedback sample and hold circuit is connected to a gate drive node connected to the sense cell insulated gate and connected to the main cell insulated gate through a first switch, the first switch being held open in the sample state to isolate the main cell insulated gate from the gate drive node and closed in the hold state to drive the main cell insulated gate from the gate drive nodes, and wherein the feedback sample and hold circuit comprises: a sample and hold voltage amplifier connected to drive the gate drive node; an input summing node connected to the input of the voltage amplifier; a current supply supplying a reference current to the input summing nodes; and a current mirror circuit connected to the input summing node, the current mirror circuit being arranged in the sample state to pass a mirror current from the input summing node mirroring the current passing through the sense cells; wherein the current mirror circuit and sample and hold voltage amplifier are arranged to act in the sample state as a feedback loop to tend to drive the gate drive node towards a voltage in which a target current matching the reference current passes through the sense cells.