FARM™

Rev 6.9

vicorpower.com

Page 7 of 11

06/2017

800 927.9474

FARMxxxx

+IN

PC (GATE IN)

PR

–IN

N

EMI GND

N/C

L

+

EN

–

150k

Micro-

controller

Vicor DC-DC

Converter

FARM

BOK

ST

Not used with VI-260/VI-J60

Figure 10 — Enable (EN) function

Bus-OK (BOK) Pin: (See Figure 11) The Bus-OK pin is intended

to provide early-warning power fail information and is also

referenced to the negative output pin.

CAUTION: There is no input-to-output isolation in the FARM. It is

necessary to monitor Bus OK via an optoisolator if it is to be used on the

secondary (output) side of the converters. A line-isolation transformer

should be used when performing scope measurements. Scope probes

should never be applied simultaneously to the input and output as this

will damage the module.

Filter: (See Figure 12) An integral input filter consists of a

common mode choke and Y rated capacitors (line-ground)

plus two X rated capacitors (line-line). This filter configuration

provides common mode and differential mode insertion

loss in the frequency range between 100kHz and 30MHz as

illustrated in Figure 6.

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 13). 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 shut down.

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:

Application Note (Cont.)

+IN

PC

PR

–IN

N

EMI GND

N/C

L

+

BOK

ST

EN

–

Vicor DC-DC

Converter

Micro-

controller

Secondary

referenced

Figure 11 — Bus OK (BOK) isolated power status indicator

4.7nF

L1

4.7nF

N

EMI GND

N/C

L

0.47µF

0.099µF

330µH

+

BOK

ST

EN

–

CM

Figure 12 — Internal filter

(1)

Where:

ε = stored energy

C = capacitance

V = voltage across the capacitor

(2)

ε = Pt = C(V

1

2

–V

2

2

) / 2

Where:

P = operating power

t = discharge interval

V

t

V

t

(3)

C = 2Pt / (V

1

2

–V

2

2

)