Datasheet

Inductor Selection
D =
PEAK_LOW_LINE
V V- Ö
OUT IN_MIN
2
V
OUT
» 0.69
=
390V 85V- Ö2
390V
(7)
L1=L2=
h ´ V xD
IN_MIN PEAK_LOW_LINE
2
P f
OUT MIN
´
» m340 H
=
0.92(85V) 0.69
2
300W 45kHz´
(8)
I =
LPEAK
P Ö
OUT
2
V ´ h
IN_MIN
=
300W Ö
2
85V 0.92´
» 5.4A
(9)
I =
LRMS
I
LPEAK
Ö
6
=
5.4A
Ö
6
» 2.2A
(10)
V V- Ö
OUT IN_MAX
2
2V
» 8
=
390V 265V- Ö
2
2V
N
P
N
S
=
(11)
ZCD Resistor Selection (R
ZA
, R
ZB
)
V N
OUT S
N 3mA´
P
» W16.3k
=
390V
8 3mA´
R =R ³
ZA ZB
(12)
R =R =2
ZA ZB
0kW
(13)
UCC28061
SLUS837A – JUNE 2008 – REVISED JULY 2009 .............................................................................................................................................................. www.ti.com
The boost inductor is selected based on the inductor ripple current requirements at the peak of low line.
Selecting the inductor requires calculating the boost converter duty cycle at the peak of low line (D
PEAK_LOW_LINE
),
as shown in Equation 7 .
The minimum switching frequency of the converter (f
MIN
) under low line conditions occurs at the peak of low line
and is set between 25 kHz and 50 kHz to avoid audible noise. For this design example, f
MIN
was set to 45 kHz:
The inductor for this design would have a peak current (I
LPEAK
) of 5.4 A, as shown in Equation 9 , and an RMS
current (I
LRMS
) of 2.2 A, as shown in Equation 10 .
This converter uses constant on-time (T
ON
) and zero-current switching (ZCS) to set up the converter timing.
Auxiliary windings off of L1 and L2 detect when the inductor currents are 0. Selecting the turns ratio in
Equation 11 ensures that there is at least 2 V at the peak of high line to reset the ZCD comparator after every
switching cycle.
The turns-ratio of each auxiliary winding is:
The minimum value of the ZCD resistors is selected based on the internal zener clamp maximum current rating
of 3 mA, as shown in Equation 12 .
In this design the ZCD resistors were set to 20 k Ω , as shown in Equation 13 .
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