Pre-Charging Circuit for EV Battery Relay Protection
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Solution Overview
Problem
High-voltage systems in electric vehicles face power losses and reduced service life due to switching operations, with challenges in precise voltage measurement between the battery and intermediate circuit capacitor, leading to unsatisfactory charging relay performance.
Innovation Solution
A pre-charging circuit charges the intermediate circuit capacitance to match the battery voltage, allowing for voltage-free switching and precise control of the main relay connection, using a MOSFET circuit for charging current regulation and enabling independent decision between precharging and fast charging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional battery switch or vehicle relay is used for connecting the battery to the load circuit, then the switching operation can be performed, but power losses occur and the service life decreases significantly due to arcing and burn-off at high system voltages
Solution Approach 1:
The patent applies preliminary action by pre-charging the intermediate circuit capacitor before closing the main relay. The pre-charging circuit charges the capacitor to match the battery voltage in advance, so that when the relay closes, there is no voltage difference and thus no arcing or inrush current. This preliminary voltage matching eliminates the harmful effects during the main switching operation.
2Reliability
If a pre-charging circuit is implemented to prevent inrush current peaks, then the service life of the relay is extended, but the charging time increases and the device complexity increases
Solution Approach 1:
The patent changes the parameter of charging current by using a MOSFET circuit to regulate the pre-charging current. This allows precise control over the charging rate, enabling the system to charge the intermediate circuit capacitor efficiently while preventing excessive current peaks. The parameter change in current regulation achieves both protection of the relay and management of charging time.
3Measurement precision
If the voltage difference between battery and intermediate circuit capacitor is measured precisely, then accurate switching control can be achieved, but the measurement precision is insufficient due to microcontroller input tolerances
Solution Approach 1:
The patent introduces an intermediary approach by using a voltage divider circuit with high-precision resistors to scale down the voltage difference before measurement. This intermediary circuit transforms the high-voltage measurement problem into a low-voltage measurement that the microcontroller can handle accurately, compensating for the microcontroller's inherent input tolerances.
4Measurement precision
If additional charging circuits or resistance networks are added to reduce voltage difference, then the voltage matching improves, but the device complexity and space requirements increase
Solution Approach 1:
The patent applies universality by designing the pre-charging circuit with dual functionality: it serves both as a voltage-matching circuit to prevent relay arcing and as a controlled charging circuit for the intermediate circuit capacitor. The same circuit components perform multiple functions, avoiding the need for separate dedicated circuits for each function.
Solution Approach 2:
The patent merges the pre-charging function with the existing power conversion circuitry. The pre-charging circuit is integrated into the overall power management system, sharing components and control logic with the main charging circuit, thereby reducing overall device complexity and space requirements.
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 extends the service life of switching relays by preventing inrush current peaks, ensuring accurate voltage matching, and allowing for efficient charging without time limits, resulting in a more compact and reliable charging system.
Implementation Method 1
using a parasitic intermediate circuit capacitance, which is contacted between the output of the AC/DC converter and the battery to be charged. This intermediate circuit capacity is previously charged from the battery to the current battery voltage (Vbat) via a pre-charging circuit in the converter.
Implementation Method 2
The charging current itself is determined by a MOSFET circuit, which means precise charging current regulation (charging current power limitation across the MOSFET).
Data Source
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AI summary
Pre-charging circuit for protecting a load relay for connecting an energy storage device to a load circuit, arranged in a charging circuit with an intermediate circuit capacitor, wherein an intermediate circuit capacitor is provided between the DC voltage output and the battery and a measuring device is provided which detects the differential voltage Udiff between the voltage Vlink at the intermediate circuit capacitor and the voltage Vbat at the energy storage device and as soon as the differential voltage Udiff exceeds a predetermined setpoint Usoll, a charging process of the intermediate circuit capacitor from the energy storage device takes place until the differential voltage Udiff falls below a predetermined setpoint Usoll and then the load circuit is closed by activating the load relay in order to establish a connection with the energy storage device.