NCP1608BOOSTGEVB
http://onsemi.com
10
Zero
Crossing
Distortion
V
out
(10V/div, ac coupled)
I
in
(500mA/div)
Vin (100V/div)
Figure 13. Full Load Input Current (V
in
= 230 Vac 50 Hz, I
out
= 250 mA)
The zero crossing distortion increases the THD and
decreases the PF of the pre-converter. To meet
IEC61000-3-2 requirements, this is generally not an issue as
the NCP1608 reduces input current distortion with sufficient
margin. If improved THD or PF is required, then zero
crossing distortion must be reduced. To reduce the zero
crossing distortion, the on time is increased as the
instantaneous input voltage is decreasing to zero. This
increases the time for the inductor current to build up and
reduces the instantaneous input voltage at which the
distortion begins.
This method is implemented by connecting a resistor from
V
in
to Ct as shown in Figure 14. The resistor current (I
CTUP
)
is proportional to the instantaneous line voltage and is
summed with I
charge
to increase the charging current of Ct.
I
CTUP
is maximum at the peak of V
in
and is approximately
zero at the zero crossing.
Figure 14. .Add R
CTUP
to Modulate the On Time and Reduce Zero Crossing Distortion
+
AC Line
Ct
+
−
PWM
Ct
L
t
on
Ct
(offset)
V
Control
V
DD
I
charge
DRV
V
in
C
in
R
CTUP
I
CTUP
+
V
in
R
CTUP
The increased charging current at the peak of V
in
enables
the increased sizing of the Ct capacitor without reducing the
control range at Vac
HL
or low output power. The larger value
of the Ct capacitor increases the on time near the zero
crossing and reduces the zero crossing distortion as shown
in Figure 15. This reduces the frequency variation over the
ac line cycle. The tradeoff is that the standby power
dissipation is increased by R
CTUP
. The designer must
balance the desired THD and PF performance with the
standby power dissipation requirements.
