ISL6740IV-T Datasheet, PDF (25/28 Pages) Intersil Corporation – Flexible Double Ended Voltage and Current Mode PWM Controllers

ISL6740, ISL6741
225
180
135
90
45
0
-45
-90
-135
100
1•10³
1•10⁴
FREQUENCY (Hz)
1•10⁵
FIGURE 25B. PREDICTED LOOP PHASE MARGIN
The actual loop gain and phase margin measured on the
ISL6740EVAL2Z demonstration board appear in Figures 26A and
26B:
as the FET on resistance, copper losses, and inductor resistance.
The phase discrepancy above 60kHz is not particularly relevant
to the loop performance since it occurs well above the cross over
frequency. The predicted behavior indicates a much gentler drop
off of phase than was observed in the measured performance.
The discrepancy was not investigated.
Performance
The major performance criteria for the converter are efficiency
and load regulation. These quantities are detailed in Figures 27
and 28.
95
93
91
89
50
40
30
20
10
0
-10
-20
-30
-40
-50
0.1k
1k
10k
FREQUENCY (Hz)
FIGURE 26A. MEASURED LOOP GAIN
100k
225
180
135
90
45
0
-45
-90
-135
0.1k
1k
10k
FREQUENCY (Hz)
100k
FIGURE 26B. MEASURE LOOP PHASE MARGIN
The only major discrepancies between the predicted behavior
and the measured results are the Q of the L-C filter and the phase
behavior above 60kHz. The actual Q appears to be significantly
less than predicted resulting in less gain peaking and a less rapid
phase shift near the resonant frequency. This is most likely the
result of neglecting other losses in the converter’s output, such
87
85
2 3 4 5 6 7 8 9 10
LOAD CURRENT (A)
FIGURE 27. EFFICIENCY vs LOAD V_IN = 48V_t
12.015
12.010
12.005
12.000
11.995
0 1 2 3 4 5 6 7 8 9 10
LOAD CURRENT (A)
FIGURE 28. LOAD REGULATION AT V_IN = 48V
The efficiency, although very good, could be further improved
using a controlled SR method instead of using a self-driven
method with an auxiliary schottky diode. The schottky diode
conducts when the main switching FETs are off. Its forward
voltage drop is considerably larger than that of the SR FETs and
causes a measurable reduction in efficiency. The effect becomes
more significant as the input voltage is increased due to the
reduction of duty cycle (and consequent increase in the OFF
time).
25
FN9111.6
December 2, 2011