Datasheet

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´
=
(25)
R =
S
» W15m
200mV
I
PEAK
200mV
13A
=
(26)
R =
S
´ »15m 0.22WW
300W
85V 0.92´
2
P =
RS
P
OUT
V ´ h
IN_MIN
2
(27)
´ 5s= 833A s
2
2.5W
0.015 W
I t=
2
(28)
Power Semiconductor Selection (Q1, Q2, D1, D2):
I I =13A³
DM PEAK
(29)
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
-
(30)
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
(31)
UCC28061
www.ti.com .............................................................................................................................................................. SLUS837A – JUNE 2008 – REVISED JULY 2009
The UCC28061 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 25 .
A standard 15-m Ω metal-film current-sense resistor is used for current sensing, as shown in Equation 26 . The
estimated power loss of the current sense resistor (P
RS
) is less than 0.25 W during normal operation, as shown
in Equation 27 .
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 28 .
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 29 :
The MOSFET RMS current calculation (Q1, Q2) is shown in Equation 30 :
To meet the power requirements of the design, IRFB11N50A 500-V MOSFETs were chosen for Q1 and Q2.
The boost diode RMS current (D1, D2) is shown in Equation 31 :
To meet the power requirements of the design, MURS306T3 600-V diodes from On Semiconductor were chosen
for the design for D1 and D2.
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