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

3.3.2 Select Current-limiting Resistor R1

The output state of an ISOM8118-Q1 opto-emulator depends on the primary-side input current. To limit the amount of current flowing into the AN pin, one resistor R1 must be connected in series at the input. The selected current-limiting resistor should meet two criteria: One is to ensure that the GPIO port of the MCU can receive a low level after the voltage at the AC discharge port reaches the judgment threshold VTH, and the other is that the 220VRMS AC voltage will not burn out the ISOM8118-Q1 or the resistor R1 itself after the relay closes normally.

For judgment accuracy, R1 needs to satisfy:

Equation 3. R 1 < V T H - V F I F m i n

The calculation with substituted values yields R1<34.8kΩ. When a maximum voltage is input, the current flowing into the AN equals 220 2 V 34.8 k Ω = 8.9 m A , which is less than the max. IF(ON) of 20mA and meets safety requirements.

The heat generation from the resistor should also be taken into account, and the power dissipation across the resistor R1 is calculated with the equation below:

Equation 4. P R 1 L O S S = ( V A C - V F ) 2 R 1

The calculation with substituted values yields PR1LOSS=1.375W. As a single 1206-package resistor is rated for a power dissipation of 0.25W, at least 6 × 200kΩ 1206-package resistors in parallel are needed for long-term stable operation, allowing for some safety margin.