Distributed DC/DC Converter Control for EV Battery Balancing
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Solution Overview
Problem
Existing electric vehicle power systems face challenges in maintaining stable low-voltage bus voltage and balancing battery cell states of charge due to high communication bandwidth requirements and increased costs associated with dedicated communication buses, particularly when using multiple DC/DC converters connected in parallel.
Innovation Solution
A distributed/cascaded control approach is implemented, where an outer controller provides slow, coarse control of total current and distribution, with local controllers adjusting current draw based on feedback and feedforward control variables, allowing for voltage regulation within existing communication bandwidth limitations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a bank of DC/DC converters is used with inputs connected to different battery cells to balance electrical load, then battery cell state of charge balancing is improved, but communication bandwidth requirements increase and system complexity increases
Solution Approach 1:
The control system is segmented into a central controller that determines total current and an allocator that distributes current to individual converters. This segmentation allows each converter to operate independently with its own state of charge information while sharing a common control objective, reducing the need for high-bandwidth communication between all system components.
Solution Approach 2:
The control architecture transitions from a flat communication structure to a hierarchical structure with two levels: central controller level for overall current management and allocator level for individual converter distribution. This dimensional change in control organization reduces communication bandwidth requirements by separating fast local control from slower centralized coordination.
2Device complexity
If a single DC/DC converter is used driven directly across the full high voltage of the battery pack, then device complexity is reduced, but component cost increases due to high voltage requirements
Solution Approach 1:
The single high-voltage converter is segmented into multiple lower-voltage converters, each handling a subset of battery cells. This segmentation allows the use of lower-voltage, lower-cost components while achieving the same overall power conversion function through parallel operation of multiple units.
Solution Approach 2:
Multiple lower-voltage DC/DC converters are merged in parallel to achieve the cumulative power conversion capability of a single high-voltage converter. The combined output of all converters provides the total required current while each individual converter uses more economical lower-voltage components.
3Power
If each DC/DC converter is controlled to increase voltage from 4V to 14V covering one cell, then voltage conversion is achieved, but the common output voltage may not remain constant causing voltage instability
Solution Approach 1:
The central controller implements feedback control by continuously monitoring the combined output of all converters and adjusting the total current command to maintain the desired output voltage. This outer-loop feedback ensures that variations in individual converter outputs are compensated to maintain overall voltage stability.
Solution Approach 2:
The control system adds a centralized coordination dimension to the individual converter control. While each converter independently manages its own voltage conversion, the central controller adds an overall system-level voltage regulation layer that coordinates all converters to maintain stable common output voltage.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach maintains stable low-voltage bus voltage and balances battery cell states of charge without the need for high communication bandwidth, reducing costs and ensuring efficient power distribution among battery units.
Implementation Method 1
a DC/DC converter has been used to down convert the high voltage to an appropriate lower voltage to drive a low voltage power bus
Data Source
AI summary
An electrified vehicle has a battery pack comprising series-connected battery units providing a main voltage. To supply a lower voltage bus equally from all battery units, a plurality of DC/DC converters are powered by respective units and have their outputs in parallel. A central module has an outer loop controller generating a target current to regulate the bus voltage to a predetermined voltage and has an allocator distributing the target current into a plurality of allocated current commands according to respective states of charge of the battery units. A plurality of local controllers each adjusts a current of a respective DC/DC converter. Each local controller receives a respective allocated current command as a respective feedforward control variable. Each local controller uses an error between the bus voltage and the predetermined voltage to be integrated as a respective feedback control variable only when the error is above a threshold.