Cirrus Logic AN359 User Manual
C
IN
+
Q
B
PFC
CONTROLLER
L
B
R
NTC
R
NTC
CM
Chok e
DM
Chok e
Y-C AP
Y-CAP
FUSE
C1
C2
C3
L1
C4
L2
C5
C
F
AC
X-CAP
X-CAP
X-C AP
V
DC_IN
AN359
Application Note
EMI Filter Modifications when
Replacing L6562 with CS1501
1. Introduction
The Cirrus Logic CS1501 PFC controller is designed to match industry standard critical conduction mode (CRM)
power factor correction (PFC) controllers, with minimal change to conventional designs. This note summarizes the
changes required in the EMI filter circuit of a 120W power supply (PSU), when a ST Microelectronics L6562 PFC is
replaced with a Cirrus Logic CS1501 PFC controller.
2. Overview
When replacing the L6562 PFC controller, care must be taken to ensure the EMI signature is not compromised. Different PFC controllers operate at different frequencies and in different operating modes, requiring EMI filters with
different impedance profiles. An example of maintaining EMI signature when converting from the L6562 to the
CS1501 is described here. For the purpose of simplicity, we will consider only the PFC portion of the PSU circuit.
Cirrus Logic Application Note AN349 gives complete instructions on how to replace the L652.
3. Filter Structure
Figure 1 illustrates the components in an EMI filter for a PFC boost using ST Microelectronics L6562.
http://www.cirrus.com
Figure 1. EMI Filter for a 120 W PSU with an L6562 Analog PFC Controller
Copyright Cirrus Logic, Inc. 2011
(All Rights Reserved)
JUL’11
AN359REV1
AN359
dl V dtLB=
Table 1 lists the circuit components that influence the EMI filter’s impedance profile.
Description Qty. Designator
Capacitor 4700pF
Capacitor 0.1µF
Inductor 1mH
Capacitor 0.1µF
10 mH
Capacitor 1µF
Resistor 5
Capacitor 4700pF
Capacitor 10nF
2
1
1
1
1
1
1
1
1
Table 1. EMI Filter Components
4. Test Results
Initial testing of the circuit demonstrated an increase in the measured EMI when the ST Microelectronics L6562 was
replaced with the Cirrus Logic CS1501. This increased EMI response indicates that the filter has not been designed
to match the CS1501. The use of two NTC resistors, to reduce inrush current, and the bridge rectifier increase the
impedance at the V
CS1501 operates mainly in discontinuous conduction mode (DCM), which requires a lower inductance for the boost
inductor L
bridge and R
. This lower inductance leads to higher switching currents, , being pulled through the
B
resistors. This increases the switching noise at V
NTC
The CS1501 senses the rectified AC voltage (V
inates the filtering use of the capacitor C
in erratic switching (jitter) on the boost FET Q
node. The L6562 controller operates in critical conduction mode (CRM). The digital
DC_IN
.
DC_IN
) as part of its control loop. The bandwidth required for this elim-
DC_IN
. The noise on V
F
, increasing the EMI energy of the PFC.
B
and on the sense voltage into the controller results
DC_IN
C1, C2
C3
L1
C4
L2
C5
R
NTC
C
IN
C
F
Figure 2. EMI Measurement Before EMI Filter Changes
2 AN359REV1
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