Voltage control circuit for a dual voltage bus computer system

A computer system comprising:

• a circuit element powered by both a first steady state voltage and a second steady state voltage different from the first steady state voltage;
• a first bus coupled to the circuit element which provides the first steady state voltage;
• a second bus coupled to the circuit element which provides the second steady state voltage;
• and
• a voltage control circuit coupled to the first and second buses which prevents damage to the circuit element by preventing an instantaneous voltage on the first bus from exceeding another instantaneous voltage on the second bus during power removal from the computer system and during a short circuit on the second bus, including
• a comparison circuit coupled to the first and second buses which compares the instantaneous voltage on the first bus with the instantaneous voltage on the second bus;
• a switch controlled by the comparison circuit for drawing current from the first bus to reduce the voltage on the first bus to remain below the voltage on the second bus when the voltage on second bus decreases;
• and
• a voltage gap determining circuit coupled between the second bus and the comparison circuit which provides a reference voltage to the comparison circuit that is a predetermined fraction of the instantaneous voltage on the second bus.

A bus voltage control circuit comprising:

• a voltage reducing circuit coupled to a first bus and a second bus, which prevents damage to a circuit element coupled to the first and second buses by preventing an instantaneous voltage on the first bus from exceeding another instantaneous voltage on the second bus during power removal from the second bus and during a short circuit on the second bus, and which includes
• a comparison circuit coupled to the first and second buses which compares the instantaneous voltage on the first bus with the instantaneous voltage on the second bus;
• a switch controlled by the comparison circuit for drawing current from the first bus to reduce the voltage on the first bus to remain below the voltage on the second bus when the voltage on second bus decreases;
• and
• a voltage gap determining circuit coupled between the second bus and the comparison circuit which provides a reference voltage to the comparison circuit that is a predetermined fraction of the instantaneous voltage on the second bus.

A method for preventing damage to a circuit element coupled to first and second buses caused by an instantaneous voltage on the first bus exceeding another instantaneous voltage on the second bus during power removal from the second bus and during a short circuit on the second bus, comprising the steps of:

• (a) comparing the instantaneous voltages on the first and second buses by a comparison circuit coupled to the first and second buses;
• and
• (b) if the instantaneous voltage on the first bus is within a predetermined voltage difference of the instantaneous voltage on the second bus, decreasing the instantaneous voltage on the first bus, including the substep of
• (b-1) providing current flow from the first bus, including the substeps of
• (b-1-A) providing a switch between the first bus and a ground;
• and
• (b-1-B) closing the switch by the comparison circuit.

A method for preventing damage to a circuit element coupled to first and second buses caused by an instantaneous voltage the first bus exceeding another instantaneous voltage the second bus during power removal from the second bus and during a short circuit on the second bus, comprising the steps of:

• (a) comparing the instantaneous voltages on the first and second buses by a comparison circuit coupled to the first and second buses;
• and
• (b) if the instantaneous voltage on the first bus is within a predetermined voltage difference of the instantaneous voltage on the second bus, decreasing the instantaneous voltage on the first bus including the substeps of
• (b-1) providing current flow from the first bus, including the substeps of
• (b-1-A) providing a switch between the first bus and a ground;
• and
• (a-3-B) (b-1-B) closing the switch by the comparison circuit.