SLUAB09 October 2025 AMC0386-Q1 , TPS61170 , TPS61170-Q1 , TPSI2140-Q1
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.
Figure 1-2 Transient?State Error
Settling Time
Figure 1-3 Steady?State Error Phase
DelayThe 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.