Application information
L6562AT
7.4
Operating with no auxiliary winding on the boost inductor
To generate the synchronization signal on the ZCD pin, the typical approach requires the
connection between the pin and an auxiliary winding of the boost inductor through a limiting
resistor. When the device is supplied by the cascaded DC-DC converter, it is necessary to
introduce a supplementary winding to the PFC choke just to operate the ZCD pin.
Another solution could be implemented by simply connecting the ZCD pin to the drain of the
power MOSFET through an R-C network as shown in figure 3: in this way the high-
frequency edges experienced by the drain will be transferred to the ZCD pin, hence arming
and triggering the ZCD comparator.
Also in this case the resistance value must be properly chosen to limit the current
sourced/sunk by the ZCD pin. In typical applications with output voltages around 400 V,
recommended values for these components are 22 pF (or 33 pF) for CZCD and 330 kΩ for
RZCD. With these values proper operation is guaranteed even with few volts difference
between the regulated output voltage and the peak input voltage
Figure 20. ZCD pin synchronization without auxiliary winding
ZCD
C
ZCD
R
ZCD
5
L6562AT
15/25
STMICROELECTRONICS [ ST ]
