EN1C11_15 Datasheet, PDF (11/13 Pages) Vicor Corporation – Component Power Front-end System for EN Compliance

APPLICATION NOTE (CONT.)
The power fail warning time (Δt) is defined as the interval
between BOK and converter shutdown (EN) as illustrated in
Figure 15. The Bus-OK and Enable thresholds are 205 V and
190 V, respectively. A simplified relationship between
power fail warning time, operating power, and bus capacitance
is obtained by inserting these constants in Equation (3):
C = 2PΔt / (205² – 190²)
C = 2PΔt / (5,925)
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 200 V.
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.
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 15) of the ripple, respectively. The capacitors
must hold up the bus voltage for the time interval (Δt)
between peaks of the rectified line as given by:
Δt = (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 13, 14, and
16, respectively.
EN1xxx
Example
In this example, the output required from the DC-DC
converter at the point of load is 12 Vdc at 320 W. Therefore,
the output power from the ENMods would be 375 W
(assuming a converter efficiency of 85%). The desired
hold-up time is 9 ms over an input range of 90 to 264 Vac.
Determining Required Capacitance for Power Fail
Warning. Figure 13 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 820 μF. Since
two capacitors are used in series, each capacitor should be at
least 1,640 μF. Note that warning time is not dependent on
line voltage. A hold-up capacitor calculator is available on the
Vicor website, at www.vicorpower.com/powerbench/product-
calculators.
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 90 Vac, ride-
through would be 68 ms. 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 12 Vp-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 12 V output. Since the
ripple on the bus voltage is 12 Vac 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 12 m Vp-p.
A variety of hold-up capacitor assemblies (HUBs) are
available. Please visit the Vicor website at
www.vicorpower.com/powerbench/product-calculators.
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
[e-mail].
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ENMODS™
Page 11 of 13
Rev 3.1
06/2015
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