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

3.3.1 Select Voltage Judgment Threshold VTH and IFmin

The minimum forward conduction current of the ISOM8118-Q1 at 25°C is 0.5mA, as noted in the data sheet. However, when the temperature rises above 40°C, its CTR will decrease rapidly, and by 125°C the CTR will be much lower than the nominal typical value noted in the sheet. To ensure circuit stability and reliability across wide temperature ranges, choose 1mA as the minimum IF in designs. With the current-limiting resistor R1’s resistance fixed, a higher input voltage leads to a larger input IF. A higher CTR will result in a larger output IC. The ISOM8118-Q1 features the largest CTR value, improving the judgment accuracy. Given that 36V is the maximum safe voltage for the human body, set the voltage judgment threshold VTH to 36V.

Equation 2. V T H = I F m i n × R 1 + V F

VF is the input voltage drop of the emulated LED, with a typical value of 1.2V.