AN3213 datasheet(32/64 Pages) STMICROELECTRONICS

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ST7570 demonstration board description

AN3213

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Doc ID 17429 Rev 1

Figure 22.

Montecarlo simulation of the transmission line coupling response loaded

with the EN50065-1 LISN impedance

7.1.4

Zero crossing coupling

The zero crossing coupling circuit is aimed at providing an analog signal to the ZC_IN_A pin

synchronous with the mains network voltage. This signal must be centered on VSS and

limited to ±5 V peak (References 1).

The zero crossing circuit is realized through a non-isolated resistive coupling to both neutral

and phase lines, as represented in Figure 23.

The two Zener diodes clamp the mains voltage at ±4.7 V, keeping the same slope as the

mains voltage around the zero crossing. The four 100 k

Ω series resistors limit the Zener

current to 1.5 mA rms. Having two resistors per line helps prevent short-circuits in the case

of resistor degradation.

Figure 23.

Non-isolated zero crossing coupling circuit

This circuit introduces a delay of nearly 30 µs between the mains zero crossing and the

ZC_IN_A input pin, as represented in Figure 24. Figure 25 shows an oscilloscope infinite

persistence snapshot showing an overall jitter lower than 10 µs.

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