SNLA490 March   2025 ISOM8110-Q1 , ISOM8111-Q1 , ISOM8112-Q1 , ISOM8113-Q1 , ISOM8115-Q1 , ISOM8116-Q1 , ISOM8117-Q1 , ISOM8118-Q1

 

  1.   1
  2.   Summary
  3. 1Relay Contact Sticking in OBCs
    1. 1.1 Relays Used in OBCs
    2. 1.2 Causes and Hazards of Relay Contact Sticking
  4. 2About Opto-emulators
    1. 2.1 Introduction to ISOM811x-Q1 and Key Parameters Like CTR
    2. 2.2 Performance Comparison with Optocoupler and Single-Channel Digital Isolator
  5. 3AC Relay Weld Detection in V2L Circuit for OBCs Based on ISOM8118-Q1
    1. 3.1 What Is V2L for OBCs
    2. 3.2 Design Reference for AC Relay Weld Detection Circuit in V2L
    3. 3.3 Design Examples and Selection Guidelines Based on Typical Circuits
      1. 3.3.1 Select Voltage Judgment Threshold VTH and IFmin
      2. 3.3.2 Select Current-limiting Resistor R1
      3. 3.3.3 Select the pull-up resistor R2
  6. 4Summary
  7. 5References

3.2 Design Reference for AC Relay Weld Detection Circuit in V2L

The ISOM811X-Q1 family features DC and AC inputs and different CTR metrics, with the selection reference shown in the following table:

Part Number CTR Input Type
ISOM8110-Q1 100%~155% Single-direction - DC Input
ISOM8111-Q1 150%~230%
ISOM8112-Q1 255%~380%
ISOM8113-Q1 375%~560%
ISOM8115-Q1 100% to 560% Bidirectional - AC Input
ISOM8116-Q1 150%~230%
ISOM8117-Q1 255%~380%
ISOM8118-Q1 375%~560%

The V2L scenario requires adaptation to the bidirectional AC input on the power grid and load sides, hence the need for the ISOM811[5-8]-Q1 family. A higher CTR means that the change in output current IC is more sensitive to the relay weld on the high-voltage input side. The subsequent example uses the ISOM8118-Q1 with the largest CTR value as a reference for calculation, while other options can be selected based on actual application requirements.

In the application reference circuit, the output pull-up resistor R2 is designed to convert a single current change into a voltage change, and the recommended procedure for relay weld detection is as follows:

  1. The MCU sends a command to disconnect the relay;
  2. The OBC’s inverter generates a safe voltage for the human body of 36Vpk-pk which is connected to the chip input through the current-limiting resistor R1;
  3. If there is a current in excess of IFmin generated on the input side of the ISOM8118 at this time, the secondary-side voltage drop will change;
  4. If the MCU's GPIO port detects a falling edge, it could be determined that the relay contacts are in contact and are at risk of sticking.

If not, it will be determined that the relay contacts are disconnected well at this time and a command should be sent to close the relay. In addition, the OBC enters the V2L state.

Typical Reference Circuit for Relay Weld Detection Based on ISOM8118-Q1 Figure 3-1 Typical Reference Circuit for Relay Weld Detection Based on ISOM8118-Q1