DC-DC Converter Isolation for Low Voltage Bus Stability
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Solution Overview
Problem
In hybrid electric vehicles, the high current draw of a conventional starter motor causes significant voltage drops on the low voltage bus, leading to flickering or fading of accessories during engine starts, which is an annoying nuisance due to frequent engine on-off cycles.
Innovation Solution
A system comprising a DC-DC converter coupled to a high voltage battery, a secondary low voltage battery for the starter motor, and an isolation device that decouples the low voltage buses during engine starts, allowing the starter motor to draw current from the secondary battery while maintaining stable voltage to the low voltage load, and recouples the buses post-start to charge the secondary battery.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a conventional starter motor is used to start the ICE, then the starter motor can provide sufficient current to crank the engine, but significant voltage drops occur on the low voltage bus causing accessories to flicker or fade
Solution Approach 1:
The low voltage electrical system is segmented into two separate buses: a first low voltage bus for powering accessories and a second low voltage bus for the starter motor. This segmentation isolates the high current draw of the starter motor from the accessory power supply, preventing voltage drops from affecting accessory operation.
Solution Approach 2:
A DC-DC converter is introduced as an intermediary device between the high voltage battery and the first low voltage bus. This converter provides stable voltage to accessories during engine starting by decoupling them from the starter motor circuit, acting as a buffer that prevents voltage fluctuations from propagating to the accessory load.
2Object-affected harmful factors
If the DC-DC converter is decoupled from the starter motor circuit during engine starts, then stable voltage is maintained to low voltage accessories, but the secondary battery must be capable of providing sufficient current independently
Solution Approach 1:
The system performs preliminary charging of the secondary low voltage battery from the high voltage battery through the DC-DC converter during normal operation. This preliminary energy storage ensures the secondary battery has sufficient charge to independently power the starter motor during engine starting events without causing voltage instability.
3Device complexity
If a single low voltage bus is used to power both accessories and the starter motor, then the system structure is simple, but repeated engine on-off cycles cause annoying flickering of accessories
Solution Approach 1:
The electrical system is divided into separate power domains: one for accessories (first low voltage bus) and one for starting (second low voltage bus). This segmentation eliminates the interaction between accessory loads and starter motor loads, preventing the flickering effect that occurs when they share a common bus, while maintaining manageable system complexity through modular architecture.
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 solution maintains stable voltage to low voltage vehicle accessories during engine starts, preventing interruptions and ensuring continuous operation, while allowing the high voltage battery to charge the secondary battery for future starts, thus addressing voltage sag issues efficiently without additional energy conversion systems.
Implementation Method 1
a DC-DC converter configured for coupling to a high voltage battery, the converter configured to provide an output to a first low voltage bus
Implementation Method 2
an isolation device configured to decouple said first and second low voltage buses
Data Source
AI summary
Systems and methods provide voltage stability for a low voltage bus coupling a DC/DC converter with a low voltage load comprising one or more vehicle accessories. The DC/DC converter can be coupled to a high voltage battery, the low voltage load, and a starter circuit configured for starting an engine. Prior to engaging a starter motor to start the engine, the DC/DC converter can be isolated from the starter circuit so as to be shielded from any large current demand during an engine start event. While starting the engine, the starter motor can draw current from a secondary battery, while the DC/DC converter provides a sufficient and stable voltage to the vehicle low voltage accessories. After completion of the engine start event, the DC/DC converter can be recoupled to the starter circuit, allowing the high voltage battery to charge the secondary battery.