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FAN6751HLMY Arkusz danych(PDF) 5 Page - Fairchild Semiconductor

Numer części FAN6751HLMY
Szczegółowy opis  Highly Integrated Green-Mode PWM Controller
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Producent  FAIRCHILD [Fairchild Semiconductor]
Strona internetowa  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

FAN6751HLMY Arkusz danych(HTML) 5 Page - Fairchild Semiconductor

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AN-6073
APPLICATION NOTE
© 2008 Fairchild Semiconductor Corporation
www.fairchildsemi.com
Rev. 1.0.0 • 9/26/08
5
Figure 11. FAN6751HL Burst-mode Operation
Switching
Disabled
Switching
Disabled
Vds
Ids
VFB
VO
time
1.1
Switching
Disabled
Switching
Disabled
Figure 12. FAN6751MR Burst-Mode Operation
Built-in Slope Compensation
A flyback converter can be operated in either discontinuous
current mode (DCM) or continuous current mode (CCM).
There are many advantages when operating the converter in
CCM. With the same output power, a converter in CCM
exhibits a smaller peak inductor current than in DCM.
Therefore, a small-sized transformer and a low-rated
MOSFET can be applied. On the secondary side of the
transformer, the RMS output current of DCM can be twice
that of CCM. Larger wire gauge and output capacitors with
larger ripple current ratings are required. DCM operation
also results in a higher output voltage spike. A large LC
filter is added. Therefore, a flyback converter in CCM
achieves better performance with lower component cost.
Despite the above advantages of CCM operation, there is
one concern—stability. In CCM operation, the output power
is proportional to the average inductor current, while the
peak current remains controlled. This causes sub-harmonic
oscillation when the PWM duty cycle exceeds 50%. Adding
slope compensation (reducing the current-loop gain) is an
effective way to prevent oscillation. The FAN6751
introduces a synchronized positive-going ramp (VSLOPE) in
every switching cycle to stabilize the current loop.
Therefore, the FAN6751 can be used to design a cost-
effective, highly efficient, compact flyback power supply
operating in CCM without additional external components.
The positive ramp added is:
D
V
V
SL
SLOPE
=
(4)
where VSL = 0.33V and D = duty cycle.
Figure 13.
Synchronized Slope Compensation
Over-Power Compensation
The maximum output power of a flyback converter can
generally be designed by the current-sense resistor RS. When
the load increases, the peak inductor current increases
accordingly. As the current-sense signal of the SENSE pin
exceeds the internal limit VSENSE, 0.83V typically, as
VIN=1V, FAN6751 stops the PWM pulse immediately. The
output power of a flyback power supply in DCM is
calculated as follows:
η
=
S
PK
P
OUT
f
I
L
P
2
2
1
(5)
where:
Lp is the transformer primary-side inductance;
IPK is the peak inductor current;
fS is the PWM frequency; and
η is the conversion efficiency.


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