Cirrus Logic AN361 - Manual (Page 3)

AN361
AN361REV2 3
3. Film Capacitor Used in CS1601
The value of the link capacitor used in CS1601 is usually small, and in those cases the electrolytic capacitor can be
replaced by the film capacitor. Compared to electrolytic capacitors, film capacitors include the following advantages:
- Smaller tolerance (The tolerance of electrolytic capacitors is about 10% to 20%; film capacitor tolerance can
reach 5%.)
- High RMS current capabilities
- Overvoltage capability of up to twice the rated voltage
- High peak current capability
- No electrolyte (acid) inside
‐
Longer lifetime
The equivalent series resistance (ESR) of electrolytic capacitors is sufficiently high to generate heat if excessive
AC/ripple current is applied. This may decrease the lifetime of the component. Film capacitors, and especially poly-
propylene film type capacitors, exhibit smaller ESR values, which virtually eliminates internal heating, even at very
high currents. This increases the lifetime of the component.
4. Design Example
The Cirrus Logic CDB1601-120W evaluation board, which is used for this example, demonstrates an excellent PFC
boost design. The specified input voltage of the CDB1601-120W ranges from 108VAC to 300VAC. The V
link
voltage
is 460V. For this application note, the input voltage range is modified to be from 90VAC to 265VAC and the V
link
voltage is changed to 400V.
Target specifications:
- Input AC voltage range (V
line
) = 90-265VAC
- Output link voltage (V
link
) = 400V
- Output power (P
o
) = 40W
- Minimum line frequency (f
line(min)
) = 45Hz
Step 1) System Stability
Determine the link capacitor value using Equation 1.
Consequently, for a 40-watt output power converter, the minimum link capacitor is:
Step 2) Link Voltage Ripple
Calculate the peak-to-peak ripple on the link voltage
using the link capacitance in Step 1 and Equation 2.
C
link ratio  
0.25 0.5 ~  
460
V
link
-------------
 
 
2
 0.25
460
400
----------
2
0.33 F watt  =  = = [Eq. 9]
C
link
0.33 40  13.2 F = = [Eq. 10]
V
link rip  
P
o
2  f
line min  
V
link
C
out rip  
  
-----------------------------------------------------------------------------------
40
2  45 400 13.2   
------------------------------------------------ 27 V = = = [Eq. 11]