Power Relay Diagnostics Using Pre-Charge Resistor
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Solution Overview
Problem
Existing electrical systems for electric or hybrid vehicles face challenges in diagnosing the functionality of power relays, particularly due to the risk of thermal overload and damage from switch-on current when connecting or disconnecting energy sources, and previous diagnostic options are no longer viable.
Innovation Solution
An electrical system with a galvanically isolated DC/DC converter and a diagnostic device equipped with switchable voltage sensors, which systematically tests the power relays by actuating switches and sensors in a prescribed sequence to ensure the functionality of all elements, including power relays, without requiring the DC link capacitor to be charged or reversed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a pre-charge circuit with a resistor is used to limit switch-on current, then thermal overload and damage to components are prevented, but the complexity of the electrical system increases due to additional components and control mechanisms
Solution Approach 1:
A diagnostic resistor is introduced as an intermediary component in the pre-charge circuit. This resistor serves dual purposes: it limits the switch-on current to prevent thermal overload (protective function) and enables voltage measurement for relay functionality diagnosis (diagnostic function). By using this intermediary element, the system achieves both protection and diagnostics without requiring separate complex measurement systems.
Solution Approach 2:
The pre-charge resistor is designed to serve multiple functions simultaneously: current limiting during switch-on, voltage division for measurement purposes, and diagnostic testing of power relay functionality. This multi-functionality reduces the need for separate dedicated components for each function, thereby managing system complexity while maintaining reliability.
2Measurement precision
If diagnostic tests are performed by charging the DC link capacitor, then power relay functionality can be tested, but the risk of thermal overload and component damage increases due to high switch-on current
Solution Approach 1:
The diagnostic procedure is designed to test power relay functionality before the DC link capacitor is fully charged or before high current conditions occur. By performing the diagnosis in a preliminary state with controlled current levels, the system can assess relay functionality without exposing components to dangerous thermal overload conditions that would occur during full power operation.
Solution Approach 2:
The diagnostic resistor acts as a mediator that enables voltage measurement for relay diagnosis while limiting current flow. By measuring the voltage across this resistor rather than directly sourcing high current through the relay, the system achieves diagnostic capability without subjecting components to harmful high current levels.
3Measurement precision
If multiple switches and sensors are used to systematically test power relays, then diagnostic accuracy is improved, but the device complexity and cost increase
Solution Approach 1:
The existing pre-charge resistor is repurposed to serve as both a current-limiting component and a measurement element for voltage sensing. This eliminates the need for separate dedicated measurement resistors or sensors in the pre-charge path, reducing component count while maintaining diagnostic accuracy. The control unit leverages the voltage already present across this resistor for diagnostic purposes.
Solution Approach 2:
The pre-charge circuit components, particularly the diagnostic resistor, provide their own measurement signal without requiring additional active sensing components. The voltage drop across the resistor during normal operation or diagnostic mode inherently provides the information needed for relay functionality assessment, making the system self-diagnostic to some extent.
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 allows for fast and effective diagnosis of power relays, reducing the risk of thermal overload and damage, while ensuring functional safety by testing the power relays and other critical components without the need for charging the DC link capacitor, thereby enhancing the reliability of the electrical system.
Implementation Method 1
The switch-on current resulting from the low inner resistance of the source (usually below 1 ohm) and from the large potential difference can cause thermal overload, damage and even destruction of individual components in the traction network
Implementation Method 2
the switch-on current resulting from the low inner resistance of the source (usually below 1 ohm) and from the large potential difference can cause thermal overload
Implementation Method 3
The electrical system has a diagnostic device with switchable voltage sensors
Data Source
AI summary
An electrical system comprises at least one high-voltage battery, at least one DC link capacitor and at least two power relays, whereby one power relay is arranged between a positive connector of the high-voltage battery and the DC link capacitor, while the other power relay is arranged between a negative connector of the high-voltage battery and the DC link capacitor, whereby the electrical system has a galvanically isolated DC/DC converter that is connected to a voltage source in the electrical system, whereby the DC/DC converter is configured such that it can transmit electric energy to a high-voltage side with the DC link capacitor in order to pre-charge the DC link capacitor, whereby the electrical system has a diagnostic device to test the power relays, whereby the diagnostic device has switchable voltage sensors.