SLUAB09 October   2025 AMC0386-Q1 , TPS61170 , TPS61170-Q1 , TPSI2140-Q1

 

  1.   1
  2.   Abstract
  3. 1Introduction
    1. 1.1 Background
    2. 1.2 System Requirements
    3. 1.3 Typical Challenges
      1. 1.3.1 Influence of Y-Capacitors
      2. 1.3.2 High Potential Testing
      3. 1.3.3 Wide AC Voltage Range
  4. 2Insulation Monitoring Architectures
    1. 2.1 Basic Architecture
    2. 2.2 Dual-Switch Architecture
    3. 2.3 Active Single-Switch Architecture
    4. 2.4 Architecture Comparison
  5. 3Key Components
    1. 3.1 Solid-State Relay
    2. 3.2 Voltage Sensor
    3. 3.3 DC Power Supply
  6. 4Summary
  7. 5Reference
  8.   Trademarks

1.3.1 Influence of Y-Capacitors

The AC side of the onboard charger (OBC) typically incorporates a two‑stage EMI filter, and a Y‑type capacitor as a key element of that filter. In a V2L scenario, an external load is also equipped with an individual EMI filter on the load‑side, adding further Y‑capacitance to the circuit. Because the value of the Y‑capacitor of the load is not known in advance, the combined capacitance can rise to as much as 100nF, which can significantly complicate the insulation-monitoring design.

The Y‑capacitor influences the measurement in two ways, the capacitor introduces an error during transient periods and distorts the steady‑state reading. Influence of Y-Capacitors illustrates a typical AC‑voltage‑sensing circuit. The waveform at the top shows the true AC voltage. In the lower panel, the green trace represents the voltage that is sensed if the Y‑capacitor is ignored; whereas, the red trace shows the voltage that is actually sensed when the effect of the Y‑capacitor is considered.

Influence of Y-Capacitors illustrates two distinct error mechanisms, introduced by the Y‑capacitor, in the AC‑voltage‑sensing loop.

  • Transient‑state error (Figure 1-2): At the moment the measurement begins (t = 0.025s), the capacitor must charge through the sensing resistor. During this charging interval, the measured voltage deviates from the true value; only after the voltage has settled does the reading become accurate.
  • Steady‑state error (Figure 1-3): Since the Y‑capacitor presents a frequency‑dependent impedance, the sensed waveform is phase‑shifted with regard to the actual AC voltage. The sensed voltage leads the true voltage, and the phase lead grows with increasing capacitance.

Influence of Y-Capacitors

TPSI2140 AMC0381 TPS61170 Transient‑State Error Settling Time Figure 1-2 Transient‑State Error Settling Time
TPSI2140 AMC0381 TPS61170 Steady‑State Error Phase DelayFigure 1-3 Steady‑State Error Phase Delay

The time constant (τ = R × C) governs the severity of both effects. A larger τ lengthens the charge‑and‑discharge periods, prolonging the settling time and amplifying the phase deviation. For a given Y‑capacitance, reducing the resistance in the measurement network of the IMD (for example, lowering R) shortens τ, and therefore, improves measurement accuracy.