TIDUEZ8 Design guide

TIDUEZ8C december 2022 – june 2023

1.1 Insulation Monitoring

Various techniques coexist in the insulation monitoring market. The two most popular methods are AC current injection and an 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 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 design in automotive hybrid electric vehicles (HEV) and EV systems, where isolation is also a critical parameter.

As stated on the safety standards, 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. The 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.

R_stP and R_stN are ±0.1% high-resistance divider branches that are switched between DC+ and PE and DC– and PE respectively. R_inAMC 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, I_iso, is proportional to the bus voltage, the isolation resistances, and the resistive branch switched in.

Under normal conditions, where the isolation barrier is unbroken and the insulation resistance, R_isoN, between DC– and PE is in the order of MΩ, there is only a very small current flow over the switched-in resistor divider, leading to a small input signal at the AMC3330.

In case of deterioration of the isolation barrier, I_iso increases which leads to a higher input signal at the AMC3330. The same behavior is true, depending on R_isoP, for the case where SN is closed and SP is opened.

Figure 1-5 Isolation Voltage on Positive Side Switch – Isolation Current

To calculate the exact values for R_isoN and R_isoP the equivalent circuits shown in Figure 1-5 and Figure 1-6 are used.

When SP is closed while SN is opened, the leakage current creates a voltage in the resistive branch – here referenced as the isolation voltage V_P. According to Kirchhoff's voltage law, Equation 21 can be derived.

Equation 1.

I_{iso} \times R_{isoN} - V_{DC} + V_{P} = 0

Solving for V_P Equation 2 is the result.

Equation 2.

V_{p} = V_{DC} - I_{iso} \times R_{isoN}

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

Equation 3.

I_{iso} = \frac{V_{P}}{R_{isoP} | | (R_{stP} + R_{inAMC})}

Substituting Equation 2 and Equation 3:

Equation 4.

V_{P} \times (1 + \frac{R_{isoN}}{R_{isoP} | | (R_{stp} + R_{inAMC})}) - V_{DC} = 0

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

Equation 5.

SP closed \to V_{P} = \frac{V_{DC}}{(1 + \frac{R_{isoN}}{R_{isoP} | | (R_{stp} + R_{inAMC})})}

V_P can be calculated from the measurement of V_inP with Equation 6.

Equation 6.

V_{P} = V_{inP} \times \frac{R_{stP} + R_{inAMC}}{R_{inAMC}}

Similar equations are found for the reverse state where SP is opened and SN is closed.

Figure 1-6 Isolation Voltage on Negative Switch – Isolation Current

In this case the sign of V_DC is changed since for this equivalent circuit the polarity of the DC connection changes, which also leads to a negative isolation current I_iso.

Equation 7.

I_{iso} \times R_{isoP} + V_{DC} + V_{N} = 0

;

Equation 8.

V_{N} = - V_{DC} - I_{iso} \times R_{isoP}

Equation 9.

I_{iso} = \frac{V_{N}}{R_{isoN} / / (R_{stN} + R_{inAMC})}

Equation 10.

V_{N} \times (1 + \frac{R_{isoP}}{R_{isoN} | | (R_{stN} + R_{inAMC})}) + V_{DC} = 0

Equation 11.

SN c l o s e d \to V_{N} = \frac{- V_{DC}}{(1 + \frac{R_{isoP}}{R_{isoN} | | (R_{stN} + R_{inAMC})})}

Equation 12.

V_{N} = V_{inN} \times \frac{R_{stP} + R_{inAMC}}{R_{inAMC}}

From Equation 7 through Equation 11, the isolation resistances between the DC lines and PE can be computed as with the assumption R_stP = R_stN = R_st:

Equation 13.

R_{isoP} = \frac{- (R_{inAMC} + R_{st}) \times (V_{DC} + V_{N} - V_{P})}{V_{N}}

Equation 14.

R_{isoN} = \frac{(R_{inAMC} + R_{st}) \times (V_{DC} + V_{N} - V_{P})}{V_{P}}

The polarity of the isolation voltage observed at R_inAMC for the negative case, is the opposite of when the resistive branch is switched in for the positive case. AMC3330 is a good fit here, because of the bipolar input voltage range.