IW3614 datasheet(11/18 Pages) DIALOG

11 / 18 page

Rev. 0.7

iW3614

Page 11

PReliminaRy

iW3614

AC/DC Digital Power Controller for

High Power Factor Dimmable LED Drivers

9.6 Understanding Primary Feedback

Figure 9.8 illustrates a simplified flyback converter. When the

switch Q

1 conducts during tON(t), the current ig(t) is directly

drawn from rectified sinusoid v

g(t). The energy Eg(t) is stored

in the magnetizing inductance L

M. The rectifying diode D1

is reverse biased and the load current I

O is supplied by the

secondary capacitor C

O. When Q1 turns off, D1 conducts and

the stored energy E

g(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 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

M). During the Q1 on-time,

the load current is supplied from the output filter capacitor C

The voltage across L

M is vg(t), assuming the voltage dropped

across Q

1 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 has ramped up to:

()

g peak

vt t

(9.7)

This current represents a stored energy of:

()

g peak

(9.8)

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:

()

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 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 9.9), then ΔV will also be

small. With the iW3614, ΔV can be ignored.

The real-time waveform analyzer in the iW3614 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.

9.7 Valley Mode Switching

In order to reduce switching losses in the MOSFET and EMI,

the iW3614 employs valley mode switching during constant

output current operation. 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 9.10). By switching at the lowest

DS, the switching loss will be minimized.

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