TIDUEZ8 May   2021


  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
    1. 1.1 Insulation Monitoring
    2. 1.2 Isolation Capacitance
    3. 1.3 IEC 61557-8 Standard for Industrial Low-Voltage Distribution Systems
    4. 1.4 Key System Specifications
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Highlighted Products
      1. 2.2.1 TPSI2140
      2. 2.2.2 AMC3330
      3. 2.2.3 TPS7A24
      4. 2.2.4 REF2033
      5. 2.2.5 TLV6001
    3. 2.3 Design Considerations
      1. 2.3.1 Resistive Bridge
      2. 2.3.2 Isolated Analog Signal Chain
        1. Differential to Single-Ended Conversion
        2. High-Voltage Measurement
        3. Signal Chain Error Analysis
      3. 2.3.3 PCB Layout Recommendations
  8. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
      1. 3.1.1 Connectors
      2. 3.1.2 Default Jumper Configuration
      3. 3.1.3 Prerequisites
    2. 3.2 Software Requirements
    3. 3.3 Test Setup
    4. 3.4 Test Results
  9. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Documentation Support
    3. 4.3 Support Resources
    4. 4.4 Trademarks
  10. 5About the Authors

Insulation Monitoring

Various techniques coexist in the insulation monitoring market. The two most popular methods are AC current injection and electric bridge switch.

The AC current injection method is based on generating a square wave signal that is injected into the RC circuit between the high-voltage (HV) lines and Protective Earth (PE) through an RC filter or transformer. Impedance is computed based on charge and discharge of the capacitor. The main drawback of the AC current method is the difficulty on achieving a reliable and accurate design, as well as the need for a bulky transformer to isolate the injection circuit from the HV lines. The AC current method has the advantage that there is no influence of the isolation capacitance. See Section 1.2 for more information.

The electric bridge switch method is proposed in safety standards, such as the IEC 61851-23. The electric bridge switch method is where a known resistive branch is switched across the isolation barrier. In normal operation, no current flows through the resistor bridge because there is no path to PE. This indicates that the system is safe without any isolation breakages.

Figure 1-4 Insulation Monitoring Analog Front End (Simplified)

The design of electric bridge DC insulation monitoring is straightforward and accurate. No bulky transformers are needed, and only small amounts of power are dissipated across the isolation barrier during normal operation.

Apart from industrial low-voltage distribution systems, this is also the most popular solution in automotive hybrid electric vehicles (HEV) and EV systems, where isolation is also a critical parameter.

As stated on the safety standards, designers should limit the operating time of this resistive branch to less than ten seconds. This is due to the safety of the system being compromised while the circuit is in operation.

Figure 1-4 is an example of isolation breakdown measurement using this reference design. Negative side switch (SN) and positive side switch (SP) are implemented with the new TPSI2140 seamless relay isolated switches used to temporarily break the isolation barrier with the known resistive divider path.

RstP and RstN are ±0.1% high-resistance divider branches that are switched between DC+ and PE and DC– and PE respectively. RinAMC is the voltage sensing resistor that serves as the scaled-down voltage input to the AMC3330 reinforced isolated amplifier.

During measurement time, the two resistor branches are switched in at different times. Figure 1-5 shows the equivalent circuit when the SP is on while SN is kept off. The current across the isolation barrier, Iiso(A), is proportional to the bus voltage, the isolation resistances, and the resistive branch switched in.

Figure 1-5 Isolation Voltage on Positive Side Switch - Proper Isolation

Under normal conditions, where the isolation barrier is unbroken and there is high-resistance path to PE (RisoN is in the order of MΩ), no current flows through the switched path as there is no closed path to PE.

In case of deterioration of the isolation barrier, leakage currents can flow to PE through the isolation barrier.

Figure 1-6 Isolation Voltage on Positive Side Switch - Isolation Current

When the resistive branch is switched in, the leakage current creates a voltage in the resistive branch – here referenced as the isolation voltage. The value of the isolation voltage is given by the DC bus voltage minus the voltage drop at the isolation barrier due to the isolation current.

Equation 1. Iiso×RisoN-DC+Vp=0
Equation 2. Vp=DC-Iiso×RisoN

The leakage current across the isolation barrier is given by Equation 3:

Equation 3. Iiso=VpRisoP//RstP+RinAMC

Substituting Equation 2 and Equation 3:

Equation 4. Vp×1+RisoNRisoP//Rstp+RinAMC-DC=0

Hence, the value of the isolation voltage that its served scaled down to the AMC3330 ±1-V range through RinAMC is given by Equation 5.

Equation 5. Sp(closed) Vp=DC1+RisoNRisoP//Rstp+RinAMC

Equivalent circuit and behavior are found for the negative side switching.

Figure 1-7 Isolation Voltage on Negative Switch - Proper Isolation

Figure 1-8 Isolation Voltage on Negative Switch - Isolation Current
Equation 6. Iiso×RisoP+DC+Vn=0;      Vn=-DC-Iiso×RisoP;
Equation 7. Iiso=VnRisoN//RstN+RinAMC
Equation 8. Vn×1+RisoPRisoN//RstP+RinAMC+DC=0
Equation 9. Sn(closed) Vn=-DC1+RisoPRisoN//RstN+RinAMC

From Equation 6 through Equation 9, the isolation resistances between the DC lines and PE can be computed as:

Equation 10. RisoN=RstDC-Vn-VpVp; RisoP=RstDC-Vn-VpVn

The polarity of the isolation voltage observed at RinAMC for the negative case, is the opposite of when the resistive branch is switched in for the positive case. The AMC3330 is really convenient for this application as it offers a bipolar input (VINP – VINN) in the range of –1 V to 1 V.