iW1700

Zero Power No-Load Off-Line Digital PWM Controller

iW1700

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

t

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 iW1700, ΔV can be ignored.

The real-time waveform analyzer in the iW1700 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 according to a light

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

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

the pulse width (T

output voltage regulation requirements.

If no voltage is detected on V

auxiliary winding of the transformer is either open or shorted

and the iW1700 shuts down.

9.5 Constant Current Operation

The constant current (CC) mode is useful in battery charging

applications. During this mode of operation the iW1700 will

regulate the output current at a constant level regardless of

the output voltage, while avoiding continuous conduction

mode.

To achieve this regulation the iW1700 senses the load

current indirectly through the primary current. The primary

current is detected by the I

the BJT emitter to ground.