IW3602 datasheet(11/19 Pages) DIALOG

11 / 19 page

iW3602

AC/DC Digital Power Controller for

Dimmable LED Drivers

Rev. 1.0

iW3602

Datasheet

11 of 19

VCC

VCC(ST)

ENABLE

Start-up

Sequencing

VIN

Figure 9.7 : Start-up Sequencing Diagram

9.6 Understanding Primary Feedback

Figure 9.8 illustrates a simplified flyback converter. When

the switch Q1 conducts during tON(t), the current ig(t) is

directly drawn from rectified vg(t). The energy Eg(t) is stored

in the magnetizing inductance LM. The rectifying diode D1 is

reversely biased and the load current IO is supplied by the

secondary capacitor CO. When Q1 turns off, D1 conducts and

the stored energy Eg(t) is delivered to the output.

vin(t)

TS(t)

VAUX

N:1

VAUX

vg(t)

ig(t)

–

iin(t)

id(t)

Figure 9.8 : Simplified Flyback Converter

In order to tightly regulate the output voltage, the information

about the output voltage and load current must be accurately

sensed. In the DCM flyback converter, this information can

be read via the auxiliary winding or the primary magnetizing

inductance (LM). During the Q1 on-time, the load current

is supplied from the output filter capacitor CO. The voltage

across LM is vg(t), assuming the voltage dropped across Q1

is zero. The current in Q1 ramps up linearly at a rate of:

()

di t

v t

(9.6)

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

()

g peak

vt t

(9.7)

This current represents a stored energy of:

()

g peak

(9.8)

When Q1 turns off, ig(t) in LM forces a reversal of polarities on

all windings. Ignoring the communication-time caused by the

leakage inductance LK at the instant of turn-off, the primary

current transfers to the secondary at a peak amplitude of:

()

g peak

i t

(9.9)

Assuming the secondary winding is master and the auxiliary

winding is slave.

VAUX

VAUX = -VIN x

NAUX

VAUX = VO x

NAUX

Figure 9.9 : Auxiliary Voltage Waveforms

The auxiliary voltage is given by:

()

AUX

+∆

(9.10)

and reflects the output voltage as shown in Figure 9.9.

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 is a

fixed ΔV. If the voltage can be read when the secondary

current is small, for example, at the knee of the auxiliary

waveform (see Figure 9.9), then ΔV is also small. With the

iW3602, ΔV can be ignored.

The real-time waveform analyzer in the iW3602 reads the

auxiliary waveform information cycle by cycle. The part then

generates a feedback voltage VFB. The VFB signal precisely

IW3602 датащи(PDF) 11 Page - Dialog Semiconductor

IW3602 датащи(HTML) 11 Page - Dialog Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

датащи скачать

ссылки URL