Relay welding may occur when a mechanical relay experiences high inrush current and voltage, leading to arcing that can cause the relay contacts to melt and stick to one another. It is crucial to effectively detect relay welding to maintain the safety and reliability of any system. Welding is a common issue in AC and DC applications in many sectors using high voltage, including electric vehicle (EV).
The EV sector implements high voltage contactors to both charge and power the vehicle, so the arcing experienced by the circuits can cause the mechanical relays to weld. Contrary to mechanical relays, a Solid-State Relay's (SSR) lack of moving parts makes it a good option for diagnosing weld detection. This application brief discusses the implementation of SSRs to detect said welding using an EV BMS as an example. Details on weld detection in EV chargers can be found in L1 and L2 EV Charger Electric Vehicle Service Equipment Design Considerations.
In an EV’s architecture, isolated switches can be used along with voltage dividers to detect welded and stuck open relays. Figure 1 shows an MCU level implementation schematic of the TPSI2140-Q1, an automotive isolated switch, that can control high voltage loads via low voltage signals from an MCU whilst providing isolation between the two.
In order to detect welding in general, voltage must be measured after the mechanical relay. This way, the physical integrity of the relay can be determined based on the voltage value being measured. The switches can then be opened or closed in different combinations via the MCU to measure the voltage at the nodes on the ends of the mechanical relays and determine whether or not there is a relay weld or open in the circuit.
Figure 2 shows the implementation of the TPSI2140-Q1 to diagnose weld-shut and stuck-open relays in the main contactors of an EV. After the switches are closed, a voltage difference between HV+ and HV- will be created briefly in order to be able to measure the voltage. These voltages can then be compared to the expected values for when the mechanical relays are functional and faults can be detected. If the measurements taken at the nodes determine that there is welding or an open detected, the MCU may intervene to protect the system.
The isolated switches are to be closed in unique combinations in order to create different loops which measure different nodes with node B as reference ground. The truth tables regarding the diagnosis of relay functionality can be seen in Table 1, along with voltage divider formulas for each diagnosis.
Diagnosis | SW1 | SW2 | SSR1 | SSR2 | Expected Voltage Value |
---|---|---|---|---|---|
SW1 Checking Weld | - | - | Closed | Closed | |
SW1 Checking Open | Closed | - | Closed | Closed | |
SW2 Checking Weld | Closed | - | Closed | Closed | |
SW2 Checking Open | Closed | Closed | Closed | Closed |
Table 2 shows some example outcomes when a relay’s physical integrity is compromised, the arrows show how a weld or open effects the state of a mechanical relay:
Diagnosis | SW1 | SW2 | SSR1 | SSR2 | Expected Voltage Value | Voltage Value When Physical Integrity Compromised |
---|---|---|---|---|---|---|
SW1 Weld Detected | Open ➜ Closed | - | Closed | Closed | ||
SW1 Open Detected | Closed ➜ Open | - | Closed | Closed | ||
SW2 Weld Detected | Closed | Open ➜ Closed | Closed | Closed | ||
SW2 Open Detected | Closed | Closed ➜ Open | Closed | Closed |
As covered in this application brief, the implementation of SSRs to measure and compare voltages is an effective solution to detect welding. Welding is a common issue in both high voltage AC and DC applications and it is crucial to effectively detect relay welding to maintain the safety and reliability of any system. The TPSI2140-Q1 is used in this application brief because our isolated switches have high switching cycles, quick switching speeds, and high reliability. For more information on isolation and SSRs, see the How to Achieve Higher-reliability isolation and a Smaller Solution Size with Solid-state Relays Technical Article.
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