NCP1607BDR2G. datasheet(13/23 Pages) ONSEMI

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NCP1607

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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 VCONTROL 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) +

@ V

CTMAX

CHARGE

(eq. 5)

Combining this equation with equation 1, gives:

@ P

OUT @ L @ ICHARGE

h @ Vac 2 @ V

CTMAX

(eq. 6)

where VCTMAX = 2.9 V (min)

ICHARGE = 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 NCP1607 determines the correct off time by

sensing the inductor voltage. When the inductor current

drops to zero, the drain voltage (“Vdrain” 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

DRV

Winding

Drain

VOUT

VCL(POS)

VZCDH

VZCDL

ZCD

VCL(NEG)

VZCD(on)

VZCD(off)

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 NCP1607 detects this falling edge and starts

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

truly occurred, the NCP1607’s logic (Figure 29) waits for

the ZCD pin voltage to rise above VZCDH (2.1 V typical)

and then fall below VZCDL (1.6 V typical). In this way,

CRM operation is easily achieved.

Figure 29. Implementation of the ZCD Winding

ZCD

−

VCL(POS)

Clamp

Shutdown

Demag

VCL(NEG)

Active

Clamp

−

Reset

Dominant

Latch

DRIVE

RSENSE

RZCD

VDD

Vin

NZCD

VZCDH

VZCDL

VSDL

NCP1607BDR2G. Datasheet(PDF) 13 Page - ON Semiconductor

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