Datasheet

Output Capacitor Selection
C ³
OUT
= » m147 F
V (V )-
OUT OUT_MIN
2 2
2
P
OUT
h
1
f
LINE
(390V) (240V)-
2 2
2
300W
0.92
1
47Hz
(21)
C =200 Fm
OUT
(22)
2
1 2 300
14
4 0 92 390 4 47 200
OUT
RIPPLE
OUT LINE OUT
P
W
V V
V f C . V Hz Fh p p m
´
´
= = »
´ ´ ´ ´ ´ ´ ´
(23)
I =
COUT_100Hz
= =0.591A
P
OUT
V ´ h Ö
OUT
´ 2
300W
390V 0.92´ ´ Ö2
(24)
I =
COUT_HF
- (I )
COUT_100 Hz
2
P 2
OUT
Ö2
2 ´ ´h V
IN_MIN
4 VÖ
IN_MIN
2
9 Vp
OUT
2
(25)
I =
COUT_HF
- (0.591A)
2
300W 2´ Ö2
2 ´ ´0.92 85V
4 85VÖ ´2
9 390Vp ´
2
» 0.966A
(26)
Selecting an R
S
for Peak Current Limiting
I =
PEAK
» 13A
2P (1.2)
OUT
Ö2
h ´ V
IN_MIN
2 300W 1.2´ Ö ´
2
0.92 85V´
=
(27)
R =
S
» W15m
200mV
I
PEAK
200mV
13A
=
(28)
R =
S
´ »15m 0.22WW
300W
85V 0.92´
2
P =
RS
P
OUT
V ´ h
IN_MIN
2
(29)
UCC28060
www.ti.com .................................................................................................................................................... SLUS767E – MAY 2007 – REVISED NOVEMBER 2008
The output capacitor (C
OUT
) is selected based on holdup requirements as shown in Equation 21 .
Two 100- µ F capacitors were used in parallel for the output capacitor:
For this size capacitor, the output voltage ripple (V
RIPPLE
) is approximately 11 V, as shown in Equation 23 :
In addition to hold-up requirements, a capacitor must be selected so that it can withstand the low-frequency RMS
current (I
COUT_100 Hz
) and the high-frequency RMS current (I
COUT_HF
); see Equation 24 to Equation 26 .
High-voltage electrolytic capacitors generally have both a low- and a high-frequency RMS current rating on the
product data sheets.
The UCC28060 peak limit comparator senses the total input current and is used to protect the MOSFETs during
inrush and over-load conditions. For reliability, the peak current limit (I
PEAK
) threshold in this design is set for
120% of the nominal inrush current that is observed during power-up, as shown in Equation 27 .
A standard 15-m Ω metal-film current-sense resistor is used for current sensing, as shown in Equation 28 . The
estimated power loss of the current sense resistor (P
RS
) is less than 0.25 W during normal operation, as shown
in Equation 29 .
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