IW1691-00 datasheet(8/24 Pages) DIALOG

Part #	IW1691-00

Description	Digital PWM Current-Mode Controller for Quasi-Resonant Operation

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Manufacturer	DIALOG [Dialog Semiconductor]

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iW1691

Digital PWM Current-Mode Controller for Quasi-Resonant Operation

Rev. 2.0

iW1691

Page 8

FebRuaRy 3, 2012

_

()

g

ON

g peak ON

M

vt t

i

t

L

×

=

(9.2)

This current represents a stored energy of:

2

_

()

2

M

g

g peak ON

L

E

i

t

=

×

(9.3)

When Q

1 turns off, ig(t) in LM forces a reversal of polarities on

all windings. Ignoring the communication-time caused by the

leakage inductance L

K at the instant of turn-off, the primary

current transfers to the secondary at a peak amplitude of:

_

()

(

)

P

d

g peak ON

S

N

i t

i

t

N

=

×

(9.4)

Assuming the secondary winding is master and the auxiliary

winding is slave.

VAUX

0V

VAUX = -VIN x

NAUX

NP

VAUX = VO x

NAUX

NS

Figure 9.3 : Auxiliary Voltage Waveforms

The auxiliary voltage is given by:

()

AUX

O

S

N

V

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. 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; for example, at the knee of

the auxiliary waveform (see Figure 9.3), then ΔV will also be

small. With the iW1691, ΔV can be ignored.

The real-time waveform analyzer in the iW1691 reads the

auxiliary waveform information cycle by cycle. The part then

generates a feedback voltage V

FB. The VFB signal precisely

represents the output voltage 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 a heavy load. If this is in the normal range, the device

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

the pulse width (T

ON), and off time (TOFF) in order to meet the

output voltage regulation requirements. During this mode

the PWM switching frequency is between 30 kHz and 130

kHz, depending on the line and load conditions.

If less than 0.2 V is detected on V

SENSE it is assumed that the

auxiliary winding of the transformer is either open or shorted

and the iW1691 shuts down.

9.5 Valley Mode Switching

In order to reduce switching losses in the MOSFET and

EMI, the iW1691 employs valley mode switching when

I

OUT is above 50%. In valley mode switching, the MOSFET

switch is turned on at the point where the resonant voltage

across the drain and source of the MOSFET is at its lowest

point (see Figure 9.4). By switching at the lowest V

DS, the

switching loss will be minimized.

Gate

VDS

Figure 9.4 : Valley Mode Switching

Turning on at the lowest V

DS generates lowest dV/dt, thus

valley mode switching can also reduce EMI. To limit the

switching frequency range, the iW1691 can skips valleys

(seen in the first cycle in Figure 9.4) when the switching

frequency becomes too high.

iW1691 provides valley mode switching during constant

output current operation. So, the EMI and switching losses

are still minimized during CC mode. This feature is superior

to other quasi-resonant technologies which only support

valley mode switching during constant voltage operation.

This is beneficial to applications, such as chargers, where

the power supply mainly operates in CC mode.

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