Electrified vehicle power conversion for low voltage bus
An electrified vehicle high voltage battery pack has series-connected battery units or cells combining to provide the high voltage. To power a low voltage bus (e.g., for low voltage accessories or charging a low voltage battery) in a balanced manner, a plurality of DC/DC converters each has an input coupled to a respective battery unit and the converters have respective outputs coupled in parallel to the low voltage bus. A first loop controller receives an actual bus voltage. The first controller generates a target current in response to the bus voltage adapted to regulate the actual bus voltage to a target voltage less than the high voltage. A second controller distributes the target current into a plurality of allocated current commands for respective converters according to respective states of charge of the battery units connected to the converters.
1. Electric vehicle apparatus comprising:
a battery pack comprising series-connected battery units providing a main voltage;
a plurality of DC/DC converters, each having an input coupled to a respective battery unit;
a low-voltage bus, wherein the DC/DC converters have respective outputs coupled in parallel to the low-voltage bus;
a first controller receiving an actual bus voltage, wherein the first controller generates a target current in response to the actual bus voltage adapted to regulate the actual bus voltage to a target voltage; and
a second controller distributing the target current into a plurality of allocated current commands for respective DC/DC converters according to respective states of charge of the battery units connected to the DC/DC converters.
2. The apparatus of claim 1 wherein the first controller is comprised of a proportional-integral-derivative (PID) controller responding to an error between the actual bus voltage and the target voltage.
3. The apparatus of claim 2 wherein the PID controller includes integral control with anti-windup.
4. The apparatus of claim 2 wherein the PID controller includes a plurality of controller gains, wherein the controller gains are adjusted according to a magnitude of the error.
5. The apparatus of claim 1 wherein the first controller is comprised of a feedforward controller responsive to an actual bus current.
6. The apparatus of claim 5 wherein the feedforward controller determines the target current according to a conversion efficiency of the DC/DC converters.
7. The apparatus of claim 6 wherein the conversion efficiency is adaptively determined.
8. The apparatus of claim 6 wherein the conversion efficiency is determined using a lookup table.
9. The apparatus of claim 6 wherein the target current is determined according to a formula:
I
in
=
v
des
·
I
out
v
in
·
η
where I in is the target current to be drawn by the DC/DC converters in combination, odes is the desired target voltage, I out is the actual bus current, v in is an average of the voltages supplied by the battery units, and η is the conversion efficiency.
10. The apparatus of claim 1 wherein the first controller limits a rate of change of each respective allocated current command.
11. The apparatus of claim 1 wherein the second controller is distributed among a plurality of converter modules and a main module, wherein the main module further comprises the first controller.
12. A method of converting a main voltage from series-connected battery units to a target voltage for a bus via parallel power converters each powered by a respective battery unit, comprising the steps of:
measuring a bus voltage;
determining an aggregate current that regulates the bus voltage to the target;
allocating the aggregate current among the converters according to respective states of charge of the battery units.
13. The method of claim 12 wherein the determining step is comprised of:
minimizing an error between the bus voltage and the target using a proportional-integral-derivative (PID) controller.
14. The method of claim 13 wherein the PID controller includes integral control with anti-windup.
15. The method of claim 13 wherein the PID controller includes a plurality of controller gains, and wherein the determining step further comprises adjusting the controller gains according to a magnitude of the error.
16. The method of claim 12 wherein the determining step is comprised of using a feedforward controller to determine the aggregate current according to a measured bus current and a conversion efficiency of the power converters.
17. The method of claim 16 wherein the conversion efficiency is adaptively determined.
18. The method of claim 16 wherein the conversion efficiency is determined using a lookup table.
19. The method of claim 16 wherein the aggregate current is determined according to a formula:
I
in
=
v
des
·
I
out
v
in
·
η
wherein I in the aggregate current to be drawn by the power converters in combination, v des is the target, I out is the bus current, v in is an average of the voltages supplied by the battery units, and η is the conversion efficiency.
20. The method of claim 12 wherein a rate of change of each respective allocated current is limited.