UM2027
AC
voltage monitoring and zero-voltage
synchronisation
DocID029048 Rev 1
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The next equation gives the proportional coefficient between V
AC
and V
AC_IM
.
For example, with R
U
= 2 MΩ, V
AC_RMS_Max
= 264 V, and V
AC_IM_Max
= 4 V, the following
resistor values and the K parameter are determined. For high accuracy, a 1% resistor
tolerance is recommended.
on the I
CL
board schematic, the resistor bridges are defined with the following
values:
Zero AC line voltage detection
The TRIAC phase-control needs to be synchronized with the AC line voltage. The zero AC
line voltage crossing detection uses the AC line voltage measurement. Indeed, the zero AC
line voltage occurs when the line voltage (V
L
) and the neutral voltage (V
N
) are equal. In this
case, a comparator (U7), connected to the pin 17 of the MCU, compares the V
L_IM
and
V
N_IM
voltage. As soon as V
L_IM
is lower than V
N_IM
the output comparator switches to the
low level, as shown in
Figure 18: "AC line voltage measurement principle"
. This figure
shows that the typical delay of the ZVS signal versus the real VAC zero is 36 µs for a 230
V 50 Hz grid voltage.
