ENMODS™

Rev 3.3

vicorpower.com

Page 9 of 13

06/2017

800 927.9474

EN1xxx

Operating Power (W)

0

5

10

15

20

25

30

35

40

500

250

*

2,200

µF

1,600

µF

1,300

µF

*

1,100

µF

820

µF

680

µF

Figure 13 — Power fail warning time vs. operating power and

total bus capacitance, series combination of

C1, C2 (see Figure 1a)

Operating Power (W)

0

10

20

30

40

50

60

70

80

90

100

90 Vac

115 Vac

500

250

N

EMI GND

SR

L

+

BOK

ST

EN

–

Micro-

controller

15 Vdc

27 k

+5 Vdc

Secondary

referenced

FARM3

Figure 12 — Bus OK (BOK) isolated power status indicator

+In

PC (Gate In)

PR (Gate Out)

–In

N

EMI GND

SR

L

+

B OK

ST

EN

–

150 k

15 Vdc

Vicor DC-DC

Converter

Micro-

controller

FARM3

250

Figure 11 — Enable (EN) function

Figure 14 — Ride-through time vs. operating power

Application Note (Cont.)

Hold-up Capacitors

Hold-up capacitor values should be determined according to

output bus voltage ripple, power fail hold-up time, and

ride-through time (see Figure 15). Many applications require the

power supply to maintain output regulation during a momentary

power failure of specified duration, i.e., the converters must hold-

up or ride through such an event while maintaining undisturbed

output voltage regulation. Similarly, many of these same systems

require notification of an impending power failure in order to

allow time to perform an orderly shutdown.

The energy stored on a capacitor which has been charged

to voltage V is:

Energy is given up by the capacitors as they are discharged by the

converters. The energy expended (the power-time product) is:

Rearranging Equation 2 to solve for the required capacitance:

(1)

Where:

ε = stored energy

C = capacitance

V = voltage across the capacitor

(3)

C = 2Pt / (V

1

2

–V

2

2

)

(2)

ε = Pt = C(V

1

2

–V

2

2

) / 2

Where:

P = operating power

t = discharge interval

V

t

V

t