Application Note, continued
80
75
70
65
60
55
50
45
40
25
15
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50
Output Voltage
Figure 16—Converter ripple rejection vs. output voltage
Another consideration in hold-up capacitor selection is their
ripple current rating. The capacitors’ rating must be higher than
the maximum operating ripple current. The approximate
operating ripple current (rms) is given by:
I rms = 2P/Vac
(7)
Where: P = operating power level
Vac = operating line voltage
Calculated values of bus capacitance for various hold-up time,
ride-through time, and ripple voltage requirements are given as
a function of operating power level in Figures 13, 14, and 15,
respectively.
Determining Ripple Voltage on the Hold-up Capacitors.
Fig. 15 is used to determine ripple voltage as a function of
operating power and bus capacitance, and shows that the ripple
voltage across the hold-up capacitors will be 12Vac.
Determining the Ripple on the Output of the
DC-DC Converter. Fig. 16 is used to determine the ripple
rejection of the DC-DC converter and indicates a ripple
rejection of approximately 60 dB for a 12 Volt output. If the
ripple on the bus voltage is 12Vac and the ripple rejection of
the converter is 60 dB, the output ripple of the converter due to
ripple on its input (primarily 120Hz) will be 12 mV p-p.
Note that 2nd Generation converters have greater ripple
rejection then either VI-200s or VI-J00s.
For more information about designing an autoranging AC
input power supply using the ARM and Vicor DC-DC
converter modules, contact Vicor Applications Engineering at
the nearest Vicor Technical Support Center (see back cover),
or send an E-mail to apps@vicr.com.
•••
Example
In this example, the output required at the point of load is 12Vdc
at 320 Watts. Therefore, the output power from the ARM would
be 375 Watts (assuming a converter efficiency of 85%). The
desired hold-up time is 9 ms over an input range of 90 to 264Vac.
Determining Required Hold-up Capacitance. Fig. 13 is used
to determine hold-up capacitance for a given hold-up time and
power level, and shows that the total bus capacitance must be
at least 820µF. Since two capacitors are used in series, each
capacitor must be at least 1,640µF. Note that hold-up time is
not dependent on line voltage.
Determining Ride-through Time. Figure 14 illustrates ride-
through time as a function of line voltage and output power,
and shows that at a nominal line of 115Vac, ride-through would
be 68 ms. Ride-through time is a function of line voltage.
Vicor Corp. Tel: 800-735-6200, 978-470-2900 Fax: 978-475-6715
ARM, Autoranging Rectifier Module
Set your site on VICOR at www.vicorpower.com
Rev. 2.4
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