FSQ0465RS Fairchild Semiconductor, FSQ0465RS Datasheet - Page 16

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FSQ0465RS

Manufacturer Part Number
FSQ0465RS
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
Datasheet
FSQ0465RS/RB Rev. 1.0.1
© 2008 Fairchild Semiconductor Corporation
Figure 33. QRC Operation with Limited Frequency
One-shot
GateX2
FSQ0465 Rev. 00
V
AVS
V
gate
DS
I
DS
t
B
GateX2: Counting V
=15
t
I
B
triggering
DS
I
DS
=15
t
fixed
B
μ
=15
I
DS
s
μ
t
s
s
max
t
μ
t
s
t
B
max
s
B
=21
=15
1st or 2nd is depend on GateX2
t
t
t
s
s
=21
s
V
V
μ
μ
DS
DS
gate
s
s
μ
s
every 2 pulses independent on other signals .
V
V
DS
DS
I
V
DS
gate
fixed
Figure 35. Alternating Valley Switching (AVS)
I
DS
I
continued 2 pulses
DS
1st valley switching
t
I
W
DS
=6
FSQ0465 Rev. 00
μ
de-triggering
s
t
B
D
A
B
C
fixed
triggering
V
gate
continued another 2 pulses
t
16
B
2nd valley switching
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 34, the switching
frequency hops when the input voltage is high. The
internal timing diagram of AVS is described in Figure 35.
67kHz
59kHz
53kHz
48kHz
f
s
fixed
frequency
Constant
Figure 34. Switching Frequency Range
FSQ0465 Rev.00
CCM
Modulation frequency is approximately 17kHz.
1st valley- 2nd valley frequency modulation.
D
t
B
1st or 2nd is dependent on GateX2
Assume the resonant period is 2 s
C
Variable frequency within limited range
fixed
1st valley switching
AVS trigger point
t
B
B
DCM
V
gate
fixed
continued 2 pulses
AVS region
μ
t
A
B
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17
19
21
V
15
1
1
μ
1
μ
μ
I N
1
s
μ
s
s
s

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