EN1M11 View Datasheet(PDF) - Vicor
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EN1M11 Datasheet PDF : 12 Pages
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Application Note (continued)
PRELIMINARY
The power fail warning time (∆t) is defined as the interval
between BOK and converter shutdown (EN) as illustrated in
Figure 13. The Bus-OK and Enable thresholds are 205V and
185V, respectively. A simplified relationship between
hold-up time, operating power, and bus capacitance is
obtained by inserting these constants in equation (3):
C = 2P∆t / (2052 – 1852)
C = 2P∆t / (7,800)
It should be noted that the series combination (C1, C2,
see Figure 1a) requires each capacitor to be twice the
calculated value, but the required voltage rating of each
capacitor is reduced to 200V.
Allowable ripple voltage on the bus (or ripple current in the
capacitors) may define the capacitance requirement.
Consideration should be given to converter ripple rejection
and resulting output ripple voltage. The ripple rejection (R) of
Vicor converters is specified as a function of the input/output
voltage ratio:
R = 30 + 20log(Vin / Vout)
(4)
For example, a converter whose output is 15V and nominal
input is 300V will provide 56dB ripple rejection, i.e., 10V p-p
of input ripple will produce 15mV p-p of output ripple
(see Figure 17). Equation 3 is again used to determine the
required capacitance. In this case, V1 and V2 are the
instantaneous values of bus voltage at the peaks and valleys
(see Figure 13) of the ripple, respectively. The capacitors
must holdup the bus voltage for the time interval (∆t) between
peaks of the rectified line as given by:
∆t = (π – θ) / 2πf
(5)
Where: f = line frequency
θ = rectifier conduction angle
The approximate conduction angle is given by:
( ) θ = Cos-1 V2/V1
(6)
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 = total output power
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 14, 15, and
16, respectively.
Example
In this example, the output required from the DC-DC converter
at the point of load is 12Vdc at 320W. Therefore, the output
power from the ENMods would be 375W (assuming a converter
efficiency of 85%). The desired hold-up time is 9ms over an
input range of 90 to 264Vac.
Determining Required Capacitance for Power Fail
Warning. Figure 14 is used to determine capacitance for a
given power fail warning time and power level, and shows that
the total bus capacitance should be at least 820µF. Since two
capacitors are used in series, each capacitor should be at least
1,640µF. Note that warning time is not dependent on line
voltage. A hold-up capacitor calculator is available on the
Vicor website, at vicorpower.com.
Determining Ride-through Time. Figure 15 illustrates ride-
through time as a function of line voltage and output power,
and shows that at a nominal line of 90Vac, ride-through would
be 68ms. Ride-through time is a function of line voltage.
Determining Ripple Voltage on the Hold-up Capacitors.
Figure 16 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 12V p-p.
Determining the Ripple on the Output of the
DC-DC Converter. Figure 17 is used to determine the ripple
rejection of the DC-DC converter and indicates a ripple
rejection of approximately 60 dB for a 12V output. Since 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 120 Hz) will be 12mV p-p.
Note that 2nd Generation converters have greater ripple
rejection then either VI-200s or VI-J00s.
A variety of hold-up capacitor assemblies (HUBs) are
available. Please visit the Vicor website @ vicorpower.com.
For more information about designing an autoranging
AC input power supply using the ENMods and Vicor DC-DC
converter modules, contact Vicor Applications Engineering at
the nearest Vicor Technical Support Center, or send E-mail to
apps@vicorpower.com.
•••
Vicor Corp. Tel: 800-735-6200, 978-470-2900 Fax: 978-475-6715
ENMods System: FARM3 and MiniHAM
Set your site on VICOR at www.vicorpower.com
Rev. 1.3
Page 10 of 12
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