On-board electrical system isolating circuit for DC/DC converters, and method for isolating an on-board electrical system from a DC/DC converter

An on-board electrical system supply system (1), comprising:

• a DC/DC converter (11) having at least one semiconductor rectifier element (4);
• two supply terminals (9a, 9b) for an on-board electrical system, which are coupled to the DC/DC converter (11) on the output side;
• an isolating switch (5) coupled between one of the supply terminals (9a, 9b) and the DC/DC converter (11);
• and
• an on-board electrical system isolating circuit (10) configured to determine a reverse voltage (U;
• Ua;
• Ub) at at least one of the semiconductor rectifier elements (4) and to drive the isolating switch (5) for decoupling the on-board electrical system from the DC/DC converter (11) depending on a value of the determined reverse voltage (U;
• Ua;
• Ub),
• wherein the on-board electrical system isolating circuit (10) includes:
• a comparator (12a;
• 12b) configured to determine an undershooting of a first threshold value for the reverse voltage (Ua;
• Ub) of the semiconductor rectifier elements (4);
• and
• a flip-flop (16) configured to output a drive signal for the isolating switch (5) depending on an output signal of the comparator (12a;
• 12b).

A method (20) for isolating an on-board electrical system from a DC/DC converter (11), comprising the following steps:

• converting (21) a supply voltage into an on-board electrical system voltage by means of a DC/DC converter (11);
• measuring (22) a reverse voltage (U;
• Ua;
• Ub) at at least one of the semiconductor rectifier elements (4) of the DC/DC converter (11);
• and
• driving (24) an isolating switch (5) for decoupling the on-board electrical system from the DC/DC converter (11) depending on the value of the measured reverse voltage (U;
• Ua;
• Ub).