“This application note describes the ZCD (Zero Current Detect) pin driven from the drain of the MOSFET. This document describes the standard ZCD configuration and its alternative and resistor configuration.
This application note describes the ZCD (Zero Current Detect) pin driven from the drain of the MOSFET. This document describes the standard ZCD configuration and its alternative and resistor configuration.
The CS1601 uses a ZCD (Zero Current Detect) design, which enables the controller to turn on the MOSFET when the current through the boost Inductor approaches zero. This can also be described as valley/zero crossing switching. This is achieved by adding a sense winding to the PFC choke to sense the current.
If adding an auxiliary winding to the boost inductor is an issue due to design or manufacturing constraints, an alternative method to achieve this is described below.
Standard ZCD configuration
Standard on the CS1501 and CS1601 is the addition of an auxiliary winding to the PFC boost inductor to provide zero current detection (ZCD) information.
An auxiliary winding is added to the PFC boost inductor to provide zero current detection (ZCD) information. The ZCD comparator looks for a zero crossing on the auxiliary winding and switches when the auxiliary voltage is below zero. Switching in the valley of oscillation minimizes switching losses and reduces EMI noise.
ZCD can be achieved from the drain of the PFC MOSFET through a resistive divider. See Figure 2.
At lighter loads, the resistor divider circuit may not pull the ZCD pin to its trigger threshold (low) because the ringing voltage itself will deviate from zero volts. This can cause the driver to turn on while the voltage still exists between the drain to source of the Boost MOSFET. Using a 47 pF capacitor in series with resistor R1 eliminates the problem of DC drift on the drain waveform. The value of the capacitor is minimized to prevent additional capacitive switching losses.