On-board electrical system supply system , comprising: a DC/DC converter having at least one semiconductor rectifier element ; two supply terminals for an on-board electrical system, which are coupled to the DC/DC converter on the output side; and an isolating switch coupled between one of the supply terminals and the DC/DC converter ; characterized by an on-board electrical system isolating circuit designed to determine a reverse voltage (U; Ua; Ub) at at least one of the semiconductor rectifier elements and to drive the isolating switch for decoupling the on-board electrical system from the DC/DC converter depending on a value of the determined reverse voltage (U; Ua; Ub), wherein the on-board electrical system isolating circuit comprises: a comparator (12a; 12b) designed to determine an undershooting of a first threshold value for the reverse voltage (Ua; Ub) of the semiconductor rectifier elements and to determine whether the reverse voltage (Ua; Ub) of the semiconductor rectifier elements exceeds a second threshold value; a flip-flop designed to output a drive signal (T) for the isolating switch depending on an output signal of the comparator (12a; 12b); and a comparison trigger designed to switch the flip-flop when at least one of the determined reverse voltages (Ua; Ub) is between the second threshold value and the first threshold value over a minimum comparison time interval. a DC/DC converter having at least one semiconductor rectifier element ; two supply terminals for an on-board electrical system, which are coupled to the DC/DC converter on the output side; and an isolating switch coupled between one of the supply terminals and the DC/DC converter ; characterized by an on-board electrical system isolating circuit designed to determine a reverse voltage (U; Ua; Ub) at at least one of the semiconductor rectifier elements and to drive the isolating switch for decoupling the on-board electrical system from the DC/DC converter depending on a value of the determined reverse voltage (U; Ua; Ub), wherein the on-board electrical system isolating circuit comprises: a comparator (12a; 12b) designed to determine an undershooting of a first threshold value for the reverse voltage (Ua; Ub) of the semiconductor rectifier elements and to determine whether the reverse voltage (Ua; Ub) of the semiconductor rectifier elements exceeds a second threshold value; a flip-flop designed to output a drive signal (T) for the isolating switch depending on an output signal of the comparator (12a; 12b); and a comparison trigger designed to switch the flip-flop when at least one of the determined reverse voltages (Ua; Ub) is between the second threshold value and the first threshold value over a minimum comparison time interval. a comparator (12a; 12b) designed to determine an undershooting of a first threshold value for the reverse voltage (Ua; Ub) of the semiconductor rectifier elements and to determine whether the reverse voltage (Ua; Ub) of the semiconductor rectifier elements exceeds a second threshold value; a flip-flop designed to output a drive signal (T) for the isolating switch depending on an output signal of the comparator (12a; 12b); and a comparison trigger designed to switch the flip-flop when at least one of the determined reverse voltages (Ua; Ub) is between the second threshold value and the first threshold value over a minimum comparison time interval. <EOS>