Cirrus Logic AN356 User Manual
R1
R2
R
cs
8 1
Boost Diode
Boost
FET
CS1501
CS1601
GDZCD
IFB
GND
CSIAC
VDD
L
B
6
3
57
4
V
DD
STBY
2
V
rect
V
link
33pF
AN356
Application Note
CS1501 & CS1601:
DRIVING THE ZCD PIN FROM THE MOSFET DRAIN
1. Introduction
The CS1601 is designed with ZCD (zero-current detection), providing the controller with the capability to turn on the MOSF ET
when the current through the boost inductor is close to zero. This can also be described as valley/zero crossings switching. This
is implemented by adding a sense winding on the PFC choke to detect the current.
If the addition of an auxiliary winding on the boost inductor is a problem due to design or manufacturing constraints, an alternative
method for implementing this function is presented below.
2. Standard ZCD Configuration
The standard configuration for the CS1501 & CS1601 is an auxiliary winding is added to the PFC boost inductor to provide zerocurrent detection (ZCD) information.
Figure 1. ZCD Using Auxiliary Winding on PFC Boost Inductor
An auxiliary winding is added to the PFC boost inductor to provide zero-current detection (ZCD) information. The ZCD comparator looks for the zero crossing on the auxiliary winding and switches when th e auxilia ry voltage is be low zero. Switching in t he
valley of the oscillation minimizes the switching losses and reduces EMI noise.
For a full description of ZCD operation, see the CS1501 & CS1601 data sheets.
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Copyright Cirrus Logic, Inc. 2011
(All Rights Reserved)
APR ‘11
AN356REV1
3. Alternative Configuration
R1
R2
R
cs
8 1
Boost Diode
Boost
FET
CS1501
CS1601
GDZCD
IFB
GND
CSIAC
VDD
L
B
6
3
57
4
V
DD
STBY
2
V
rect
V
link
33pF
47pF
500 V
Low Voltage
Schottky
R1
V
drainVzcd
–()
I
zcd
-------------------------------------
455kΩ==
[Eq.1]
R2
V
drain
V
zcd
–()
R1
-------------------------------------
V
zcd
⋅ 4.79kΩ==
[Eq.2]
It is possible to implement ZCD with a resistor divider from the drain of the PFC MOSFET. See Figure 2.
AN356
Figure 2. ZCD Alternative Implementation
At lighter loads, the resistor divider circuit may not pull the ZCD pin to its trigger threshold (low) as the ringing voltage itself drifts
away from zero volts. This can cause the drive to turn on while there is still a voltage across the drain to source of the Boost
MOSFET. Using a 47 pF capacitor in series with resistor R1, the issue of DC drift on the drain w aveform is remove d. The value
of the capacitor is minimized to prevent additional capacitive switching losses.
3.1 Resistor Configuration
The maximum current that can be drawn by the ZCD pin is I
on the MOSFET drain = 460 V.
Select R1 to be a standard value of 475 kΩ.
Select R2 to be a standard value of 4.75 kΩ.
Due to internal comparator design, the maximum external capacitor is 33 pF, as shown. This acts as a low-pass filter for high-
frequency switching noise. Values above this should not be used.
The addition of a Schottky diode is required to prevent negative voltages on the ZCD pin from exceeding -0.5 V. See Fig 3.
= 1mA. Set a maximum voltage of 5 V for V
zcd
, with the voltage
zcd
2 AN356REV1
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