iW1690

Low-Power Off-line Digital PWM Controller

iW1690

+

D1

Q1

N:1

+

–

Figure 9.3.1 Simplified Flyback Converter

In order to tightly regulate the output voltage, the information

about the output voltage and load current needs to be

accurately sensed. In the DCM flyback converter, this

information can be read via the auxiliary winding or the

primary magnetizing inductance (L

the load current is supplied from the output filter capacitor C

The voltage across L

across Q

rate of:

()

()

g

g

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

i

t

L

×

=

(9.2)

This current represents a stored energy of:

2

_

()

2

M

g

g peak

L

E

i

t

=

×

(9.3)

When Q

polarities on all windings. Ignoring the communication-time

caused by the leakage inductance L

t

amplitude of:

_

()

()

P

d

g peak

S

N

i t

i

t

N

=

×

(9.4)

Assuming the secondary winding is master, the auxiliary

winding is slave.

0V

Figure 9.3.2 Auxiliary Voltage Waveforms

The auxiliary voltage is given by:

(

)

AUX

AUX

O

S

N

V

VV

N

=

+∆

(9.5)

and reflects the output voltage as shown in Figure 9.3.2.

The voltage at the load differs from the secondary voltage by

a diode drop and IR losses. The diode drop is a function of

current, as are 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 iW1690, ΔV can be ignored.

The real-time waveform analyzer in the iW1690 reads

this information cycle by cycle. The part then generates a

feedback voltage V

the output voltage under most conditions and is used to

regulate the output voltage.

9.4 Constant Voltage Operation

After soft-start has been completed, the digital control block

measures the output conditions. It determines output power

levels and adjusts the control system to a light load or a

heavy load. If this is in the normal range, the device operates

in the Constant Voltage(CV) mode, and changes the pulse

width (Ton), and off time (Toff) in order to meet the output

voltage regulation requirements. During this mode the PWM

switching frequency is either 45 kHz or 65kHz, depending on

which product option is being used.

If no voltage is detected on V

auxiliary winding of the transformer is either open or shorted

and the iW1690 shuts down.