DC converter for a motor vehicle

1. A DC/DC voltage converter for a motor vehicle having a battery providing a battery voltage, the DC/DC voltage converter comprising:

a step-up converter for converting the battery voltage to an intermediate voltage, with the intermediate voltage being greater than or equal to the battery voltage, said step-up converter configured for open-loop control, at least in one operating mode, of the intermediate voltage in dependence on the battery voltage at a value which is not equal to the battery voltage; and

a step-down converter for receiving the intermediate voltage output from said step-up converter and for regulating of an output voltage being less than or equal to the intermediate voltage;

said step-up converter including a switch and a control circuit configured to open and close said switch to step-up said battery voltage when the battery voltage is less than a first predetermined value, and to permanently open said switch when the battery voltage is greater than the first predetermined value.

2. The DC/DC voltage converter according to claim 1 , wherein:

said step-up converter has an output outputting the intermediate voltage; and

in the at least one operating mode, in which said step-up converter is configured for open-loop control of the intermediate voltage in dependence on the battery voltage, the battery voltage is less than the first predetermined value, and a load which is connected to said output of said step-up converter exceeds a minimum load.

3. The DC/DC voltage converter according to claim 2 , wherein said control circuit is configured to control said switch with a pulse-width-modulated signal when the battery voltage is less than the first predetermined value.

4. The DC/DC voltage converter according to claim 3 , wherein a duty ratio of the pulse-width-modulated signal increases as the battery voltage rises, at least when the battery voltage is less than the first predetermined value.

5. The DC/DC voltage converter according to claim 4 , wherein said control circuit is set such that the duty ratio which is less than 100%, is not exceeded at a minimum battery voltage.

6. The DC/DC voltage converter according to claim 1 ,

wherein said step-up converter has an input; and

further comprising an electromagnetic interference filter disposed between one connection of the battery and said input of said step-up converter.

7. The DC/DC voltage converter according to claim 3 , wherein said control circuit has a subtractor for subtraction of the battery voltage from a reference voltage, and a comparator for setting a duty ratio such that the duty ratio is proportional to a signal which is output from said subtractor.

8. The DC/DC voltage converter according to claim 7 , wherein said comparator is configured to compare an output signal from said subtractor with the reference voltage which has a triangular profile.

9. The DC/DC voltage converter according to claim 1 , wherein the intermediate voltage is set such that it is substantially constant in a battery voltage range which extends from a predetermined minimum value to a nominal battery voltage.

10. The DC/DC voltage converter according to claim 2 , wherein said step-up converter has a diode for switching said step-up converter off in a situation when the intermediate voltage exceeds the first predetermined value.

11. An assembly, comprising:

a battery outputting a battery voltage; and

a DC/DC voltage converter connected to said battery, the DC/DC voltage converter having:

a step-up converter for converting the battery voltage to an intermediate voltage, with the intermediate voltage being greater than or equal to the battery voltage, said step-up converter configured for open-loop control, at least in one operating mode, of the intermediate voltage in dependence on the battery voltage at a value which is not equal to the battery voltage; and

a step-down converter for receiving the intermediate voltage output from said step-up converter and for regulating of an output voltage being less than or equal to the intermediate voltage;

said step-up converter including a switch and a control circuit configured to open and close said switch to step-up said battery voltage when the battery voltage is less than a first predetermined value, and to permanently open said switch when the battery voltage is greater than the first predetermined value.

12. An operating method, which comprises the steps of:

providing an assembly including a battery outputting a battery voltage and a DC/DC voltage converter connected to the battery, the DC/DC voltage converter having a step-up converter for converting the battery voltage to an intermediate voltage, with the intermediate voltage being greater than or equal to the battery voltage, the step-up converter configured for open-loop control, at least in one operating mode, of the intermediate voltage in dependence on the battery voltage at a value which is not equal to the battery voltage, and a step-down converter for receiving the intermediate voltage output from the step-up converter and for regulating of an output voltage being less than or equal to the intermediate voltage, said step-up converter including a switch and a control circuit configured to open and close said switch to step-up said battery voltage when the battery voltage is less than a first predetermined value, and to permanently open said switch when the battery voltage is greater than the first predetermined value; and

using the assembly in a motor vehicle.

13. A method of operating a DC/DC voltage converter including a step-up converter for converting a battery voltage to an intermediate voltage, with the intermediate voltage being greater than or equal to the battery voltage, the step-up converter configured for open-loop control, at least in one operating mode, of the intermediate voltage in dependence on the battery voltage at a value which is not equal to the battery voltage and a step-down converter for receiving the intermediate voltage output from the step-up converter and for regulating of an output voltage being less than or equal to the intermediate voltage, said step-up converter including a switch and a control circuit configured to open and close said switch to step-up said battery voltage when the battery voltage is less than a first predetermined value, and to permanently open said switch when the battery voltage is greater than the first predetermined value, which comprises the steps of:

controlling the step-up converter in the open loop, such that the step-up converter provides the intermediate voltage in dependence on the battery voltage; and

regulating an output voltage by means of voltage regulation by the step-down converter.