iW1707

Low-Power Off-Line Digital Green-Mode PWM Controller

iW1707

depletion NFET or BJT turns on, and the V

begins to charge up again towards the start-up threshold to

initiate a new soft-start process.

In applications where the active start-up is not needed,

the start-up resistor can be directly connected to the V

CC

pin without using the active start-up device, and the ASU

pin can be left unconnected. Refer to Figure 3.3 for the

application circuit.

9.3 Understanding Primary Feedback

Figure 9.2 illustrates a simplified flyback converter. When

the switch Q1 conducts during t

directly drawn from rectified sinusoid v

is stored in the magnetizing inductance L

diode D1 is reverse biased and the load current I

supplied by the secondary capacitor C

D1 conducts and the stored energy E

output.

+

D1

Q1

N:1

+

–

Figure 9.2: Simplified Flyback Converter

In order to tightly regulate the output voltage, the

information about the output voltage and load current need

to be accurately sensed. In the DCM flyback converter,

this information can be read via the auxiliary winding or

the primary magnetizing inductance (L

During the Q1

on-time, the load current is supplied from the output filter

capacitor C

voltage dropped across Q1 is zero. The current in Q1 ramps

up linearly at a rate of:

( )

( )

gg

M

di t

v t

dt

L

=

(9.1)

At the end of on-time, the current has ramped up to:

( )

( )

_

g

ON

g peak

M

vt t

it

L

×

=

(9.2)

This current represents a stored energy of:

_

2

M

g

g peak

L

E

it

=

×

(9.3)

When Q1, turns off at t

polarities on all windings. Ignoring the communication-time

caused by the leakage inductance L

turn-off t

a peak amplitude of:

( )

( )

_

P

d

g peak

S

N

i t

i

t

N

=

×

(9.4)

Assuming the secondary winding is master, and the auxiliary

winding is slave,

0V

1

2

Figure 9.3: Auxiliary Voltage Waveforms

The auxiliary voltage is given by:

(

)

V

AUX

AUX

O

S

N

VV

N

=

+∆

(9.5)

and reflects the output voltage as shown in Figure 9.3.

The voltage at the load differs from the secondary voltage

by a diode drop and IR losses. Thus, if the secondary

voltage is always read at a constant secondary current, the

difference between the output voltage and the secondary

voltage will be a fixed ΔV. Furthermore, if the voltage can

be read when the secondary current is small, ΔV will also be

small. With the iW1707, ΔV can be ignored.

The real-time waveform analyzer in the iW1707 reads this

information cycle-by-cycle. The part then generates a

feedback voltage V