AN-6083

APPLICATION NOTE

© 2008 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.2 • 3/11/09

4

Figure 10.Feedback Circuit

The maximum sourcing current of the FB pin is 1.04mA.

The phototransistor must be capable of sinking this current

to pull FB level down at no load. The value of the biasing

2

OUT

D

Z

B

VV

V

K

mA

R

−−

×≥

(6)

where:

regulator; and

K is the current transfer rate (CTR) of the opto-coupler.

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 to 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 the CCM. Larger wire gauge and output capacitors

with larger ripple-current ratings are required. DCM

operation also results in higher output-voltage spikes. A

large LC filter has to be added. A flyback converter in CCM

achieves better performance with a 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 FAN6791

every switching cycle to stabilize the current loop.

Therefore, the FAN6791 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:

SLOPE

SLOPE

VV

D

= Δ×

(7)

Figure 11.Synchronized Slope Compensation

Constant Power Control

To limit the output power of the converter constantly, a power-

limit function is included. Sensing the converter input voltage

through the VRMS pin, the power limit function generates a

relative peak-current-limit threshold voltage for constant power

control, as shown in Figure 12.

Figure 12. Constant Power Compensation

Leading Edge Blanking (LEB)

A voltage signal proportional to the MOSFET current

MOSFET is turned on, a spike is induced by the diode

reverse recovery and by the output capacitances of the

MOSFET and diode, inevitably appearing on the sensed

signal. Inside the FAN6791, a leading-edge blanking time

of about 270ns is introduced to avoid premature termination

of MOSFET by the spike. Only a small-value RC filter (e.g.

FB