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Q0465R

Green-Mode Fairchild Power Switch (FPS™) for Quasi-Resonant Operation - Low EMI and High Efficiency

Fairchild Semiconductor 

tsmax=21μs

IDS

IDS

tB=15μs

VDS

ts

IDS

tB=15μs VDS

ts

A

IDS

B

IDS

IDS

VDS

C

tB=15μs

ts

IDS

VDS

tB=15μs

IDS

D

tW=6μs

tsmax=21μs

FSQ0465 Rev. 00

Figure 33. QRC Operation with Limited Frequency

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.

fs

67kHz

59kHz

53kHz

48kHz

Assume the resonant period is 2μ s

AVS trigger point

Constant

frequency

CCM

Variable frequency within limited range

DCM

AVS region

1

15μs

1

17μs

1

19 μs

1

21μs

D

C

B

FSQ0465 Rev.00

A

VIN

Figure 34. Switching Frequency Range

Vgate

GateX2

Vgate continued 2 pulses

1st valley switching

Vgate continued another 2 pulses

2nd valley switching

Vgate continued 2 pulses

1st valley switching

One-shot

fixed

fixed

fixed

AVS triggering 1st or 2nd is depend on GateX2 de-triggering triggering

fixed

fixed

fixed

1st or 2nd is dependent on GateX2

VDS

tB

tB

tB

tB

tB

tB

GateX2: Counting Vgate every 2 pulses independent on other signals .

FSQ0465 Rev. 00

1st valley- 2nd valley frequency modulation.

Modulation frequency is approximately 17kHz.

Figure 35. Alternating Valley Switching (AVS)

© 2008 Fairchild Semiconductor Corporation

FSQ0465RS/RB Rev. 1.0.1

16

www.fairchildsemi.com

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