A device for protecting a circuit having a primary current path from an overcurrent condition, the device being configured to be imposed in the primary current path and the device comprising: a first terminal and a second terminal configured to route the primary current path through the device; a first transistor comprising a first gate, a first drain (151/152), and a first source (151/152); wherein the first transistor is a depletion mode, normally-on transistor; wherein the first transistor is arranged in series in the primary current path between the first terminal and the second terminal; a driver circuitry; the driver circuitry comprising: a voltage converter circuitry adapted to receive an input voltage derived solely from a voltage between the first terminal and the second terminal; wherein the voltage converter circuitry comprises a switched capacitor network configured to convert the input voltage to a releasably-stored voltage, wherein the driver circuitry is configured to apply the releasably-stored voltage or a derivative voltage thereof as a gate voltage at the first gate relative to the first source, wherein the driver circuitry is configured to convert the input voltage to the releasably-stored voltage by multiplying and inverting the input voltage, wherein the multiplication is by a number greater than 1 or more negative than -1; a current monitoring circuitry; wherein, when a normal current condition exists from the first terminal to the second terminal, the first transistor is configured to pass current between the first terminal and the second terminal; and wherein, when the overcurrent condition exists from the first terminal to the second terminal, the driver circuitry is configured to drive the first transistor into blocking depletion mode by applying the releasably-stored voltage or the derivative voltage thereof as the gate voltage; and wherein the device is configured to pass current during normal current conditions, and to substantially block current during the overcurrent condition. a first terminal and a second terminal configured to route the primary current path through the device; a first transistor comprising a first gate, a first drain (151/152), and a first source (151/152); wherein the first transistor is a depletion mode, normally-on transistor; wherein the first transistor is arranged in series in the primary current path between the first terminal and the second terminal; a driver circuitry; the driver circuitry comprising: a voltage converter circuitry adapted to receive an input voltage derived solely from a voltage between the first terminal and the second terminal; wherein the voltage converter circuitry comprises a switched capacitor network configured to convert the input voltage to a releasably-stored voltage, wherein the driver circuitry is configured to apply the releasably-stored voltage or a derivative voltage thereof as a gate voltage at the first gate relative to the first source, wherein the driver circuitry is configured to convert the input voltage to the releasably-stored voltage by multiplying and inverting the input voltage, wherein the multiplication is by a number greater than 1 or more negative than -1; the driver circuitry comprising: a voltage converter circuitry adapted to receive an input voltage derived solely from a voltage between the first terminal and the second terminal; wherein the voltage converter circuitry comprises a switched capacitor network configured to convert the input voltage to a releasably-stored voltage, wherein the driver circuitry is configured to apply the releasably-stored voltage or a derivative voltage thereof as a gate voltage at the first gate relative to the first source, wherein the driver circuitry is configured to convert the input voltage to the releasably-stored voltage by multiplying and inverting the input voltage, wherein the multiplication is by a number greater than 1 or more negative than -1; a current monitoring circuitry; wherein, when a normal current condition exists from the first terminal to the second terminal, the first transistor is configured to pass current between the first terminal and the second terminal; and wherein, when the overcurrent condition exists from the first terminal to the second terminal, the driver circuitry is configured to drive the first transistor into blocking depletion mode by applying the releasably-stored voltage or the derivative voltage thereof as the gate voltage; and wherein the device is configured to pass current during normal current conditions, and to substantially block current during the overcurrent condition.