Vicor Corp. Tel: 800-735-6200, 978-470-2900 Fax: 978-475-6715

ENMods System: FARM3 and MiniHAM

Rev. 2.9

Page 9 of 13

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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

APPLICATION NOTE (CONT.)

N

EMI GND

SR

L

+

BOK

ST

EN

–

Micro-

controller

15 Vdc

27 k

Ω

+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

Ω

Figure 11 — Enable (EN) function

Figure 14 — Ride-through time vs. operating power

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:

Where:

C = capacitance

V = voltage across the capacitor

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

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

is:

1

2

(2)

Where:

P = operating power

Δt = discharge interval

Rearranging Equation 2 to solve for the required capacitance:

C = 2P

Δt / (V

1

2