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L6562A

Transition-mode PFC controller

STMicroelectronics 

Application information

L6562A

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 400V,

recommended values for these components are 22pF (or 33pF) for CZCD and 330k 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

RZCD CZCD

ZCD

5

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