FSCM0565R datasheet(17/24 Pages) ONSEMI

17 / 24 page

www.onsemi.com

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

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 (tB). After the

blanking time, the controller finds the valley within the

detection time window (tW) and turns on the MOSFET, as

shown in Figures 35 and Figure 36 (Cases A, B, and C).

If no valley is found during tW, the internal SenseFET is

forced to turn on at the end of tW (Case D). Therefore,

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.

Figure 36. Switching Frequency Range

max=21μs

max=21

μs

tB=15μs

IDS

tW=6μs

tB=15μs

FSQ0565 Rev. 00

VDS

53kHz

67kHz

59kHz

Constant

frequency

VIN

Assume the resonant period is 2 us

AVS trigger point

48kHz

AVS region

CCM

DCM

Variable frequency within limited range

FSQ0565 Rev.00

FSCM0565R Datasheet(PDF) 17 Page - ON Semiconductor

FSCM0565R Datasheet(HTML) 17 Page - ON Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL