DC Link Capacitor Discharge Switching to Prevent Resistor Overheating
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Solution Overview
Problem
Existing devices for discharging intermediate circuit capacitors in vehicles require complex monitoring and control systems to prevent damage during disconnection from a high-voltage battery, which increases design and manufacturing complexity.
Innovation Solution
A discharge device that controls a switching element without monitoring the disconnection status, using a fixed number of switching operations and waiting periods to safely discharge the capacitor, ensuring the load resistor does not overheat even if the battery remains connected.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a disconnecting device is activated to separate the high-voltage battery from the intermediate circuit capacitor during discharge, then electrical safety is improved, but the power converter cannot verify whether disconnection was successful, risking capacitor destruction if disconnection fails
Solution Approach 1:
The discharge device is activated in advance as a protective measure when the disconnecting device is commanded to open. The control device activates the discharge device before knowing whether disconnection succeeded, ensuring that if disconnection fails, the capacitor will still be safely discharged through the load resistor. This preliminary action eliminates the need for complex monitoring systems to verify disconnection status.
Solution Approach 2:
The control device monitors its own control signals to the disconnecting device and automatically activates the discharge device based on this self-monitoring. When the control device sends an opening command to the disconnecting device, it simultaneously or subsequently activates the discharge device, creating a self-contained safety mechanism without requiring external monitoring systems.
2Reliability
If the discharge device is activated without verification of battery disconnection, then capacitor protection is ensured, but the load resistor may overheat and be destroyed due to continuous current from the connected battery
Solution Approach 1:
The control device activates the discharge device in periodic switching operations rather than continuously. It activates the discharge device for a specified number of switching operations, then deactivates it for an idle time period, and repeats this cycle. This periodic action allows the load resistor to cool down between discharge cycles, preventing overheating and destruction while still ensuring capacitor discharge safety.
Solution Approach 2:
The discharge device operation is made dynamic through multiple activation cycles with varying parameters. The control device performs the discharge operation in repeated cycles, each with a specified number of switching operations followed by idle time. This dynamic approach adapts to the thermal state of the load resistor, activating discharge only when safe to do so, thereby preventing overheating while maintaining capacitor protection.
3Reliability
If multiple switching operations are performed to discharge the capacitor, then discharge completeness is improved, but the load resistor temperature increases, potentially causing destruction
Solution Approach 1:
The discharge operation is divided into periodic cycles with multiple switching operations followed by idle time periods. Each cycle performs a specified number of switching operations to discharge the capacitor, then pauses to allow the load resistor to cool. This periodic structure enables complete discharge over multiple cycles while preventing thermal destruction by interspersing cooling periods.
Solution Approach 2:
The discharge process maintains continuity of useful action by repeating the switching operations across multiple activation cycles. Rather than performing all discharge operations in a single continuous sequence, the process is divided into repeated cycles that continue until the capacitor is fully discharged, while interspersing idle periods to manage thermal load on the resistor.
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 allows for a simple and effective discharge strategy that avoids capacitor damage without complex monitoring, using a microcontroller, by structuring the power converter to manage load resistor heating and cooling effectively.
Implementation Method 1
the charge stored in the DC link capacitor must be converted into heat
Data Source
Figure 1~2
AI summary
Device (1) for discharging a DC link capacitor (7), comprising a power converter (2) for supplying electrical power to an electric machine (6) driving a vehicle, which has the DC link capacitor (7) and a discharge device (8) connected in parallel to the DC link capacitor (7) with a series connection of a controllable switching element (9) and a load resistor (10), and a high-voltage battery (3) supplying the power converter (2) on the input side, wherein the power converter (2) has a control device (13) which is configured to control the switching element (9) depending on a signal state at an input (14) of the control device (13) to perform a fixed predetermined number of switching operations (18a-18e) in which the switching element (9) conducts, with a fixed predetermined duration (19) and a fixed predetermined waiting time (20) between two successive switching operations (18a-18e),wherein the load resistor (10) and its heat transfer to the power converter (2) are arranged such that the load resistor (10) is not damaged when a maximum continuous voltage of the high-voltage battery (3) is applied during a respective switching operation (18a-18e) and during cooling during the waiting period (20).