Datasheet

´ 5s= 833A s
2
2.5W
0.015 W
I t=
2
(30)
Power Semiconductor Selection (Q1, Q2, D1, D2):
I I =13A³
DM PEAK
(31)
I =
DS
» 2.3A
4 VÖ
IN_MIN
2
9 Vp ´
OUT
I
PEAK
2
1
6
- =
4 85VÖ ´
2
9 390Vp ´
13A
2
1
6
-
(32)
I =
D
» 1.4A
4 VÖ
IN_MIN
2
9 Vp ´
OUT
I
PEAK
2
=
4 85VÖ ´
2
9 390Vp ´
13A
2
(33)
Brownout Protection
R =
A
» W3M
Hysteresis
7 Am
=
21V
7 Am
(34)
R =
B
» W47k
1.4V R´
A
V 0.75 1.4V´ Ö -
IN_MIN
2
=
1.4V ´ W3M
85V 0.75 1.4V´ Ö -2
(35)
UCC28060
SLUS767E – MAY 2007 – REVISED NOVEMBER 2008 .................................................................................................................................................... www.ti.com
The most critical parameter in selecting a current-sense resistor is the surge rating. The resistor needs to
withstand a short-circuit current larger than the current required to open the fuse (F1). I
2
t (ampere squared
seconds) is a measure of thermal energy resulting from current flow required to melt the fuse, where I
2
t is equal
to RMS current squared times the duration of the current flow in seconds. A 4-A fuse with an I
2
t of 14 A
2
s was
chosen to protect the design from a short-circuit condition. To ensure the current-sense resistors have a high
enough surge protection, a 15-M Ω , 500-mW, metal-strip resistor was chosen for the design. The resistor has a
2.5-W surge rating for 5 seconds. This result translates into 833 A
2
s and has a high enough I
2
t rating to survive a
short-circuit before the fuse opens, as described in Equation 30 .
The selection of Q1, Q2, D1, and D2 are based on the power requirements of the design. Application note
SLUU138 , UCC38050 100-W Critical Conduction Power Factor Corrected (PFC) Pre-Regulator, explains how to
select power semiconductor components for transition-mode PFC pre-regulators.
The MOSFET maximum-pulsed drain current (Q1, Q2) is shown in Equation 31 :
The MOSFET RMS current calculation (Q1, Q2) is shown in Equation 32 :
To meet the power requirements of the design, IRFB11N50N 500-V MOSFETs from International Rectifier were
chosen for Q1 and Q2.
The boost diode RMS current (D1, D2) is shown in Equation 33 :
To meet the power requirements of the design, MURS306T3 600-V diodes from On Semiconductor were chosen
for the design for D1 and D2.
Resistor R
A
and R
B
are selected to activate brownout protection at 75% of the specified minimum operated input
voltage. Resistor R
A
programs the brownout hysteresis comparator, which was selected to provide 21 V of
hysteresis. R
A
and R
B
are shown in Equation 34 and Equation 35 .
In this design example, brownout becomes active when the input drops below 64 V
RMS
and deactivates when the
input reaches 79 V
RMS
.
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