AN-6073 查看數據表（PDF） - Fairchild Semiconductor

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AN-6073

Highly Integrated Green-Mode PWM Controller

Fairchild Semiconductor 

AN-6073

Figure 11. FAN6751HL Burst-mode Operation

VO

APPLICATION NOTE

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:

VSLOPE = VSL • D

(4)

where VSL = 0.33V and D = duty cycle.

VFB

1.1

Ids

Vds

Figure 13. Synchronized Slope Compensation

time

Switching Switching Switching Switching

Disabled Disabled Disabled 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

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:

POUT

=

1

2

•

LP

• IPK 2 • fS

•η

(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.

© 2008 Fairchild Semiconductor Corporation

Rev. 1.0.0 • 9/26/08

5

www.fairchildsemi.com

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