SLUAB06 January   2025 AMC131M02 , AMC3330 , ISO7731

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Topologies for Insulation Monitoring in ESS
    1. 2.1 Single Switch Topology
      1. 2.1.1 Test Steps
    2. 2.2 Dual Switch Floating Sampling Topology
      1. 2.2.1 Test Steps
    3. 2.3 Dual Switch Sampling in High Voltage Side Topology
    4. 2.4 AC Injection Topology
  6. 3Comparison of Designs
    1. 3.1 Injection Resistance Influence on Accuracy
  7. 4Capacitor Influence on Sampling Time
    1. 4.1 How to Choose Y Capacitor
    2. 4.2 Sampling Time
  8. 5Key Devices
    1. 5.1 BQ79731-Q1
    2. 5.2 UCC33421-Q1
    3. 5.3 AMC131M02
    4. 5.4 AMC3330
    5. 5.5 ISO773x
  9. 6Topos Simulation Result and Conclusion
  10. 7Summary
  11. 8References

4.2 Sampling Time

Suppose that the power supply Vu charges the capacitor C through the resistor R, V0 is the initial voltage value on the capacitor, Vu is the voltage value of the capacitor after it is fully charged, and Vt is the voltage value on the capacitor at any time t, then the following calculation formula can be obtained:

If the initial voltage on the capacitor is 0, the formula can be simplified to:

From the above formula, it can be seen that because the exponential value can only be infinitely close to 0, but never equal to 0, it takes an infinite amount of time for the capacitor to be fully charged.

While t = RC，Vt = 0.63Vu；while t = 2RC，Vt = 0.86Vu；while t = 3RC，Vt = 0.95Vu；

While t = 4RC，Vt = 0.98Vu；while t = 5RC，Vt = 0.99Vu；

It can be seen that when it reaches 5t, it is basically close to the final value.

Therefore, for the 1500V insulation detection system, the Y capacitor can be selected as 200nF, so it takes 4.5s to detect a voltage of 0.995u.

Based on analysis before, concluded in Table 4-1.

Table 4-1 Comparison of Three Designs
DesignsDesign 1Design 2Design 3
DeviceDeviceDevice
SwitchReed relay × 1Reed relay × 1Reed relay × 1
SSR × 2SSR× 2SSR × 1
Insulation monitoringUIR BQ79731-Q1× 1AMC131M02 × 1UIR BQ79731-Q1
HV sense-AMC0311 × 1-
Insulation samplingAMC3330 × 1AMC3330 × 1AMC3330 × 1
ISOISO7731 × 1ISO7731 × 1
SupplyUCC33421 × 2UCC33421 × 2UCC33421 × 1
Cost summaryUS$10.21US$9.72US$7.8
ShuntYesYesYes
Accuracy (worst case)0.82%0.82%5.9%
HV supplyNeedNeedNeed
Sampling time=R × 2CY=R × 2CY=RCY
  1. The cost is almost entirely from TI devices and >1ku quantity.

Considering cost and accuracy, using double arms and putting control in high voltage can be the better choice for insulation monitoring in energy storage system.