Buck-boost converter for an electric drive

An electric drive system for an electric machine, comprising:

• positive and negative bus rails carrying a direct current (DC) bus voltage;
• an energy storage system (ESS) connected to the positive and negative bus rails, and having battery cells and a first capacitor arranged in parallel with the battery cells to provide a battery output voltage having a battery polarity;
• a power inverter having a first plurality of semiconductor switches operable for inverting the DC bus voltage into an alternating current (AC) bus voltage;
• and
• a DC-DC converter connected to the positive and negative bus rails between the capacitor and the power inverter, and having a converter semiconductor switch disposed in the positive bus rail, an inductor coil connected to the positive bus rail and receiving current flowing through the converter semiconductor switch, at least one diode configured to direct current flowing through the power inverter via the inductor coil, and a bypass switch connected to the positive bus rail and configured to allow current to bypass the converter and flow through the power inverter when the bypass switch is closed, and a second capacitor arranged across the positive and negative bus rails, the DC-DC converter configured to output a DC bus voltage to the power inverter with a same polarity as the battery polarity when the bypass switch is opened and when the bypass switch is closed;
• wherein the electric drive system is configured to regulate operation of the DC-DC converter based on power, torque, and speed values of the electric machine, to adjust the DC bus voltage until the DC bus voltage is equal to the battery output voltage, to selectively bypass the DC-DC converter by closing the bypass switch under predetermined operating conditions of the electric machine when the DC bus voltage is equal to the battery output voltage, and to selectively open the bypass switch and thereafter regulate the DC bus voltage to a predetermined voltage.

A vehicle, comprising:

• an electric drive system configured to provide output torque to at least one vehicle wheel, the electric drive system including:
• positive and negative bus rails carrying a direct current (DC) bus voltage;
• an energy storage system (ESS) connected to the positive and negative bus rails, and having battery cells and a first capacitor arranged in parallel with the battery cells to provide a battery output voltage having a battery polarity;
• a power inverter having a first plurality of semiconductor switches operable for inverting the DC bus voltage into an alternating current (AC) bus voltage;
• an electric machine having phase windings that are electrically connected to the power inverter;
• a DC-DC converter connected to the positive and negative bus rails between the capacitor and the power inverter, and having a converter semiconductor switch disposed in the positive bus rail, an inductor coil connected to the positive bus rail and receiving current flowing through the converter semiconductor switch, at least one diode configured to direct current flowing through the power inverter and the electric machine via the inductor coil, and a bypass switch connected to the positive bus rail and configured to allow current to bypass the converter and flow through the power inverter and electric machine when the bypass switch is closed, and a second capacitor arranged across the positive and negative bus rails, the DC-DC converter configured to output a DC bus voltage to the power inverter with a same polarity as the battery polarity;
• and
• a controller programmed to regulate operation of the DC-DC converter based on power, torque, and speed values of the electric machine, to adjust the DC bus voltage until the DC bus voltage is equal to the battery output voltage, to selectively bypass the DC-DC converter by closing the bypass switch under predetermined operating conditions of the electric machine when the DC bus voltage is equal to the battery output voltage, and to selectively open the bypass switch and thereafter regulate the DC bus voltage to a predetermined voltage, wherein the DC-DC converter outputs the DC bus voltage with the same polarity as the battery polarity when the bypass switch is opened, and wherein the DC-DC converter outputs the DC bus voltage with the same polarity as the battery polarity when the bypass switch is closed.