AN-9737

APPLICATION NOTE

© 2011 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.0 • 4/13/11

2

Figure 2 shows typical waveforms of the simplified circuit

of a flyback converter with CRM. When the MOSFET (Q1)

turns on, the primary current in primary side linearly

increases and is clamped at a certain internal level because

the FL6961 doesn’t have cycle-by-cycle current limit like a

conventional current mode control IC (such as FAN7527B).

Its peak level is determined by the primary magnetizing

inductance value and the fixed on-time. Instead of the cycle-

by-cycle primary current limit, the FL6961 has an over-

current protection (OCP) function. If the current sensing

signal is larger than internal detection level, the FL6961

doesn’t get output signal for operating the MOSFET (Q1).

Figure 2. Key Waveforms of Flyback Converter on

CRM

The FL6961 has a constant on-time across the whole range.

The input average current always follows the peak input

current, as shown in the equation:

ON

PK

MOSFET

AVG

t

I

I

2

1

)

(

=

(1)

This is also proportional to the instantaneous input voltage.

This means the input current shape is always the same as the

input voltage shape. The reverse diode voltage is linearly

increased and is equal to:

P

S

IN

O

DIODE

PK

N

N

V

V

V

+

=

)

(

(2)

During the MOSFET off-time, which is also the diode on-

time; the input current instantly drops to zero, the diode in

the secondary side conducts, and the diode current linearly

decreases. The peak current of the secondary side is the

same as the multiplication of the primary peak current and

the secondary side is:

off

PK

S

P

DIODE

AVG

t

I

N

N

I

2

1

)

(

=

(3)

Since the diode forward-voltage drop decreases as current

decreases, the output voltage reflects the primary winding

and adds additional voltage due to overshoot made by

resonance between the leakage inductance on primary-side

winding and parasitic capacitance on the MOSFET (Q1). As

a result, a superimposed voltage occurs on the MOSFET

during off-time as:

OS

R

IN

off

MOSFET

V

V

V

V

+

+

=

)

(

(4)

overshoot term.

between the primary and secondary side of the transformer

and the output voltage, calculated as:

O

S

P

R

V

N

N

V =

(5)

Figure 3 shows the ideal waveforms of the primary-side

current at MOSFET (Q1) and the secondary-side current at

the diode. The input peak and average current on the

primary

side

follows

input

voltage

instantaneously.

Normally, secondary-side current on the diode is larger than

the primary side because of the turns ratio.

Figure 3. Ideal Waveforms

time

time

time