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A hybrid powertrain system has a high-voltage electric circuit including a high-voltage battery and a DC link coupled to first and second inverters electrically connected to first and second torque machines. A method for operating the hybrid powertrain system includes receiving a motor torque comman

A hybrid powertrain system has a high-voltage electric circuit including a high-voltage battery and a DC link coupled to first and second inverters electrically connected to first and second torque machines. A method for operating the hybrid powertrain system includes receiving a motor torque command for the second torque machine, determining a preferred DC link voltage for achieving the motor torque commanded from the second torque machine, selectively interrupting electric power flow between the high-voltage battery and the DC link to achieve the preferred DC link voltage.

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1. Method for operating a hybrid powertrain system including a high-voltage electric circuit comprising a high-voltage battery electrically connected to a high-voltage bus including first and second sides, and a DC link including first and second DC link rails coupled to first and second inverters e

1. Method for operating a hybrid powertrain system including a high-voltage electric circuit comprising a high-voltage battery electrically connected to a high-voltage bus including first and second sides, and a DC link including first and second DC link rails coupled to first and second inverters electrically connected to first and second torque machines, the method comprising: receiving a motor torque command for the second torque machine;determining a preferred DC link voltage set for achieving the motor torque commanded from the second torque machine; andwhen the preferred DC link voltage is greater than an output voltage of the high-voltage battery: selectively decoupling the high-voltage battery and the DC link using a switch device comprising at least one pair of switch elements connected in parallel, wherein no electrical power flows between the high-voltage battery and the DC link when the high-voltage battery and the DC link are decoupled, anddriving voltage of the DC link to achieve the preferred DC link voltage with voltage solely provided from the first inverter to the second inverter while the high-voltage battery and the DC link are decoupled, the voltage provided from the first inverter is generated using mechanical power transferred from an engine to the first torque machine independent of the output voltage of the high-voltage battery;wherein the switch device is placed between the first side of the high-voltage bus and the first DC link rail while the second side of the high-voltage bus is always directly coupled to the second DC link rail, the high-voltage battery and the DC link selectively decoupled by controlling the switch device to decouple the first side of the high-voltage bus from the first DC link rail;wherein the first side of the high-voltage bus and the first DC link rail both comprise one of a positive polarity and a negative polarity and the second side of the high-voltage bus and the second DC link rail both comprise the other one of the positive polarity and the negative polarity. 2. The method of claim 1, further comprising: periodically coupling and decoupling the high-voltage battery and the DC link when a monitored DC link voltage is greater than a threshold voltage. 3. The method of claim 2, wherein periodically coupling and decoupling the high-voltage battery and the DC link comprises controlling the switch device to selectively couple and decouple the high-voltage battery and the DC link to achieve the preferred DC link voltage. 4. The method of claim 3, wherein controlling the switch device to selectively couple and decouple the high-voltage battery and the DC link comprises executing a pulsewidth-modulated control of the switch device placed between the first side of the high-voltage bus and the first DC link rail to selectively couple and decouple the high-voltage battery and the DC link to achieve the preferred DC link voltage. 5. The method of claim 2, wherein periodically coupling and decoupling the high-voltage battery and the DC link comprises controlling a second switch device coupled across the DC link to selectively couple and decouple the high-voltage battery and the DC link to achieve the preferred DC link voltage. 6. The method of claim 5, wherein controlling the second switch to selectively couple and decouple the high-voltage battery and the DC link comprises executing a pulsewidth-modulated control of the switch coupled across the DC link to selectively couple and decouple the high-voltage battery and the DC link to achieve the preferred DC link voltage. 7. The method of claim 1, further comprising: selectively coupling the high-voltage battery and the DC link when the preferred DC link voltage is less than an output voltage of the high-voltage battery. 8. The method of claim 1, further comprising commanding a motor torque output from the first torque machine based on a motor torque output from the second torque machine. 9. Method for operating a hybrid powertrain system including an engine and first and second torque machines coupled to a transmission device to transfer torque to a driveline and a high-voltage electric circuit including a high-voltage battery electrically connected to a high-voltage bus including first and second sides, and a DC link including first and second DC link rails coupled to first and second inverters electrically connected to the first and second torque machines, and a switch configured to decouple the high-voltage battery and the first and second inverters when controlled to an open state, the method comprising: commanding a motor torque output from the second torque machine in response to an operator torque request;determining a preferred DC link voltage set to achieve the commanded motor torque output from the second torque machine; andwhen the preferred DC link voltage is greater than an output voltage of the high-voltage battery: controlling the switch, comprising at least one pair of switch elements connected in parallel, to selectively decouple the high-voltage battery from the first and second inverters, wherein no electrical power flows between the high-voltage battery and the first and second inverters when the high-voltage battery and the first and second inverters are decoupled, anddriving voltage of the DC link to achieve the preferred DC link voltage with voltage solely provided from the first inverter to the second inverter while the high-voltage battery and the first and second inverters are decoupled, the voltage provided from the first inverter is generated using mechanical power transferred from the engine to the first torque machine independent of the output voltage of the high-voltage battery;wherein the switch is placed between the first side of the high-voltage bus and the first DC link rail while the second side of the high-voltage bus is always directly coupled to the second DC link rail, the high-voltage battery selectively decoupled from the first and second inverters by controlling the switch to decouple the first side of the high-voltage bus from the first DC link rail. 10. The method of claim 9, further comprising: selectively coupling the high-voltage battery and the first and second inverters by controlling the switch to a closed state when an output voltage of the high-voltage battery is greater than the preferred DC link voltage to achieve the commanded motor torque output from the second torque machine. 11. The method of claim 9, further comprising: executing pulsewidth-modulated control of the switch when the preferred DC link voltage to achieve the commanded motor torque output from the second torque machine is greater than an output voltage of the high-voltage battery and a monitored DC link voltage is equal to or greater than a threshold voltage. 12. The method of claim 9, wherein the mechanical power transferred from the engine to the first torque machine is responsive to the commanded motor torque output from the second torque machine. 13. The method of claim 12, comprising operating the engine to transfer torque to the first torque machine to achieve the commanded motor torque output from the first torque machine responsive to the commanded motor torque output from the second torque machine. 14. A hybrid powertrain system, comprising an engine and first and second torque machines coupled to a transmission to transfer torque to a driveline;a high-voltage electric circuit comprising a high-voltage battery electrically connected to a high-voltage bus including first and second sides, an inverter module, and a switch comprising at least one pair of switch elements connected in parallel, the high-voltage battery electrically connected via first and second DC link rails of a DC link to the inverter module, the inverter module electrically operatively connected to the first and second torque machines, and the switch configured to decouple the high-voltage battery and the inverter module when controlled to an open state wherein no electrical power flows between the high-voltage battery and the inverter module when the high-voltage battery and the inverter module are decoupled; anda controller monitoring an output voltage of the high-voltage battery and a voltage across the DC link, determining a preferred voltage across the DC link set for achieving a motor torque commanded from the second torque machine, and when the preferred voltage across the DC link is greater than the output voltage of the high-voltage battery: controlling the switch to selectively decouple the high-voltage battery and the inverter module, anddriving voltage of the DC link to achieve the preferred voltage across the DC link with voltage solely provided from the first torque machine to the second torque machine while the high-voltage battery and the inverter module are decoupled, the voltage provided from the first torque machine is generated using mechanical power transferred from the engine independent of the output voltage of the high-voltage battery;wherein the switch device is placed between the first side of the high-voltage bus and the first DC link rail while the second side of the high-voltage bus is always directly coupled to the second DC link rail, the high-voltage battery and the inverter module selectively decoupled by controlling the switch device to decouple the first side of the high-voltage bus from the first DC link rail;wherein the first side of the high-voltage bus and the first DC link rail both comprise one of a positive polarity and a negative polarity and the second side of the high-voltage bus and the second DC link rail both comprise the other one of the positive polarity and the negative polarity. 15. The hybrid powertrain system of claim 14, wherein a second switch is electrically coupled across the DC link. 16. The hybrid powertrain system of claim 14, further comprises executing pulsewidth-modulated control of the switch to selectively couple and decouple the high-voltage battery and the inverter module in response to the voltage of the high-voltage battery when the voltage across the DC link exceeds a threshold.