FSQ0565RS Fairchild Semiconductor, FSQ0565RS Datasheet - Page 16

• Manufacturer Part Number: FSQ0565RS
• Description: A Quasi-Resonant Converter (QRC) generally shows lower EMI and higher power conversion efficiency than a conventional hard-switched converter with a fixed switching frequency
• Manufacturer: Fairchild Semiconductor

FSQ0565RS/RQ Rev. 1.0.3
© 2008 Fairchild Semiconductor Corporation
7. Switching Frequency Limit: To minimize switching
loss and Electromagnetic Interference (EMI), the
MOSFET turns on when the drain voltage reaches its
minimum value in quasi-resonant operation. However,
this causes switching frequency to increases at light load
conditions. As the load decreases or input voltage
increases, the peak drain current diminishes and the
switching frequency increases. This results in severe
switching losses at light-load condition, as well as
intermittent switching and audible noise. These problems
create limitations for the quasi-resonant converter
topology in a wide range of applications.
Figure 35. QRC Operation with Limited Frequency
I
DS
t
B
=15
t
I
B
DS
I
DS
=15
t
B
μ
=15
I
DS
s
μ
t
s
s
max
t
μ
s
t
max
s
B
=21
=15
t
t
t
s
s
=21
s
V
V
μ
μ
DS
DS
s
s
μ
s
V
V
DS
DS
I
DS
I
DS
I
DS
t
I
W
DS
=6
FSQ0565 Rev. 00
μ
s
D
A
B
C
16
To overcome these problems, FSQ-series employs a
frequency-limit function, as shown in Figures 35 and 36.
Once the SenseFET is turned on, the next turn-on is
prohibited during the blanking time (t
blanking time, the controller finds the valley within the
detection time window (t
shown in Figures 35 and Figure 36 (Cases A, B, and C).
If no valley is found during t
forced to turn on at the end of t
the devices have a minimum switching frequency of
48kHz and a maximum switching frequency of 67kHz.
8. AVS (Alternating Valley Switching): Due to the
quasi-resonant operation with limited frequency, the
switching frequency varies depending on input voltage,
load transition, and so on. At high input voltage, the
switching on time is relatively small compared to low
input voltage. The input voltage variance is small and the
switching frequency modulation width becomes small. To
improve the EMI performance, AVS is enabled when
input voltage is high and the switching on time is small.
Internally, quasi-resonant operation is divided into two
categories; one is first-valley switching and the other is
second-valley switching after blanking time. In AVS, two
successive occurrences of first-valley switching and the
other two successive occurrences of second-valley
switching is alternatively selected to maximize frequency
modulation. As depicted in Figure 36, the switching
frequency hops when the input voltage is high. The
internal timing diagram of AVS is described in Figure 37.
67kHz
59kHz
53kHz
48kHz
f
s
frequency
Constant
Figure 36. Switching Frequency Range
FSQ0565 Rev.00
CCM
D
Assume the resonant period is 2 us
C
Variable frequency within limited range
W
AVS trigger point
) and turns on the MOSFET, as
B
W
DCM
, the internal SenseFET is
W
(Case D). Therefore,
AVS region
A
B
www.fairchildsemi.com
). After the
17
19
21
V
15
1
1
μ
1
μ
μ
I N
1
s
μ
s
s
s