ISL6741_14 Datasheet, PDF (19/28 Pages) Intersil Corporation – Flexible Double Ended Voltage and Current Mode PWM Controllers

ISL6740, ISL6741
15
V (L1:1)
I (L1)
10
5
0.986 0.988 0.990 0.992 0.994 0.996 0.998 1.000
TIME (ms)
FIGURE 9. SECONDARY WINDING VOLTAGE AND INDUCTOR
CURRENT DURING CURRENT LIMIT OPERATION
Figures 8 and 9 show the behavior of the inductor ripple under
steady state and overcurrent conditions. In this example, the
peak current limit is set at 11A. The peak current limit causes
the duty cycle to decrease resulting in a reduction of the average
current through the inductor. The implication is that the converter
can not supply the same output current in current limit that it can
supply under steady state conditions. The peak current limit
setpoint must take this behavior into consideration. A 3.32Ω
current sense resistor was selected for the rectified secondary of
current transformer T2, corresponding to a peak current limit
setpoint of 16.5A.
The short circuit protection involves setting a voltage between 0
and 2V on the SCSET pin. The applied voltage divided by 2 is the
percent of maximum duty cycle that corresponds to a short
circuit when the peak current limit is active. A divider from RTC to
ground provides an easy method to achieve this. The divider
between RTC and GND formed by R13 and R15 determines the
percent of maximum duty cycle that corresponds to a short
circuit. The divider ratio formed by R13 and R15 is:
---------^R-----¹---⁵-----------
R13 + R15
=
------------¹---^.--²---⁷----^k-------------
1.27k + 17.4k
=
0.068
(EQ. 24)
Therefore, the duty cycle that corresponds to a short circuit is
6.8% of D max (97.9%), or ~6.6%.
Performance
The major performance criteria for the converter are efficiency,
and to a lesser extent, load regulation. Efficiency, load regulation
and line regulation performance are demonstrated in the
following figures.
100
95
90
85
80
75
70
0 123456789
LOAD CURRENT (A)
FIGURE 10. EFFICIENCY vs LOAD V_IN = 48V_t
12.5
12.25
12.00
11.75
11.50
11.25
11
0
1
2
3
4
5
6
7
8
9
LOAD CURRENT (A)
FIGURE 11. LOAD REGULATION AT V_IN = 48V
14.0
13.5
13.0
12.5
12.0
11.5
11.0
45 46 47 48 49 50 51 52 53 54
INPUT VOLTAGE (V)
FIGURE 12. LINE REGULATION AT I_OUT = 1A
As expected, the output voltage varies considerably with line and
load when compared to an equivalent converter with closed loop
feedback. However, for applications where tight regulation is not
required, such as those application that use downstream DC/DC
converters, this design approach is viable.
19
FN9111.6
December 2, 2011