NCP1606BDR2G【PDF 13/22 页】IC POWER FACTOR CONTROLLER 8SOIC

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Sheet目录45

9页 10页 11页 12页 [13页]14页 15页 16页 17页

NCP1606

http://onsemi.com

out the bulk voltage ripple, then this on time is truly

constant over the ac line cycle.

Note that the maximum on time of the controller occurs

when V

CONTROL

is at its maximum. Therefore, the Ct

capacitor must be sized to ensure that the required on time

can be delivered at full power and the lowest input voltage

condition. The maximum on time is given by:

ON(max)

Ct @ V

CTMAX

CHARGE

(eq. 4)

Combining this equation with equation 1, gives:

Ct w

2 @ P

OUT

@ L @ I

CHARGE

h @ Vac

RMS

@ V

CTMAX

(eq. 5)

where V

CTMAX

= 2.9 V (min)

CHARGE

= 297 mA (max)

OFF TIME SEQUENCE

While the on time is constant across the ac cycle, the off

time in CRM operation varies with the instantaneous input

voltage. The NCP1606 determines the correct off time by

sensing the inductor voltage. When the inductor current

drops to zero, the drain voltage (Vd in Figure 23) is

essentially floating and naturally begins to drop. If the

switch is turned on at this moment, then CRM operation

will be achieved. To measure this high voltage directly on

the inductor is generally not economical or practical.

Rather, a smaller winding is taken off of the boost inductor.

This winding, called the zero current detector (ZCD)

winding, gives a scaled version of the inductor output and

is more useful to the controller.

Figure 28. Voltage Waveforms for Zero Current

Detection

DRIVE

Winding

Drain

0.6 V

OUT

5.7 V

2.1 V

1.6 V

ZCD

Figure 28 gives typical operating waveforms with the

ZCD winding. When the drive is on, a negative voltage

appears on the ZCD winding. And when the drive is off, a

positive voltage appears. When the inductor current drops

to zero, then the ZCD voltage falls and starts to ring around

zero volts. The NCP1606 detects this falling edge and starts

the next driver on time. To ensure that a ZCD event has

truly occurred, the NCP1606s logic (Figure 29) waits for

the ZCD pin voltage to rise above V

ZCDH

(2.1 V typical)

and then fall below V

ZCDL

(1.6 V typical). In this way,

CRM operation is easily achieved.

Figure 29. Implementation of the ZCD Winding

ZCD

200 mV

2.1 v

VCL(POS)

Clamp

Shutdown

Demag

VCL(NEG)

Active

Clamp

1.6 V

Reset

Dominant

Latch

DRIVE

SENSE

ZCD

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