FARM™

Rev 6.9

vicorpower.com

Page 6 of 11

06/2017

800 927.9474

FARMxxxx

Figure 9 — Offline Power Supply Configuration

Application Note (Cont.)

Off-Line Power Supply Configuration

The FARM maintains the DC output bus voltage between 250 and

with Vicor 300V input converters. The FARM automatically

switches to the proper bridge or doubler mode depending on

the input voltage, eliminating the possibility of damage due to

improper line connection. The FARM1xxx is rated at 500W in

1000W for the low and high input ranges respectively. Either of

these modules can serve as the AC front end for any number and

combination of compatible converters as long as the maximum

power rating is not exceeded.

Strap (ST) Pin: In addition to input and output power pin

connections, it is necessary to connect the Strap pin to the center

junction of the series hold-up capacitors (C1, C2, see Figure 9) for

proper (autoranging) operation. Metal oxide varistors, V1 and

V2 provide capacitor protection. The bleeder resistors (R1, R2,

see Figure 9) discharge the hold-up capacitors when power is

switched off. Capacitors C7 and C8 are recommended if the

hold-up capacitors are located more than 3 inches (75mm)

from the FARM output pins.

Enable (EN) Pin: (See Figure 10) The Enable pin must be

connected to the PC or Gate-In pin of all converter modules to

disable the converters during power up. Otherwise, the converters

would attempt to start while the hold-up capacitors were being

charged through an unbypassed current limiting thermistor,

preventing the bus voltage from reaching the thermistor bypass

threshold, thus disabling the power supply. The Enable output

(the drain of an N channel MOSFET) is internally pulled up to 15V

through a 150kΩ resistor.

A signal diode should be placed close to and in series with the PC

or (Gate-In) pin of each converter to eliminate the possibility of

control interference between converters. The Enable pin switches

to the high state (15V) with respect to the negative output power

pin to turn on the converters after the power up inrush is over.

The Enable function also provides input overvoltage protection

for the converters by turning off the converters if the DC bus

this condition occurs, placing the thermistor in series with the

input voltage, which reduces the bus voltage to a safe level while

limiting input current in case the varistors conduct. The thermistor

bypass switch also opens if a fault or overload reduces the bus

CAUTION: There is no input to output isolation in the FARM, hence the

–OUT of the FARM and thus the –In of the downstream DC-DC

converter(s) are at a high potential. If it is necessary to provide an external

enable/ disable function by controlling the DC-DC converter’s PC pin

(referenced to the –IN) of the converter an opto-isolator or isolated relay

should be employed.

+IN

PC (GATE IN)

PR

–IN

+IN

PC (GATE IN)

PR

–IN

Vicor DC-DC

Converter

R1

R2

C1

C2

V1

V2

D2

D1

C3

C5

C4

C6

F1

F2

Vicor DC-DC

Converter

To additional modules

C7**

C8**

FARM

Filter/Autoranging

Rectifier Module

N

EMI GND

N/C

L

+

BOK

ST

EN

–

Z1

C9

L

PE

N

R3

D4

C11

C10

D3

R4

F3

*

Vicor

Part

Description

Part Number

C1,2

Hold-up capacitors

C3-C6

4,700pF (Y2 type)

01000

C7,8**

Film Cap., 0.61µF

34610

C9

0.47µF

03047

C10,C11 0.001µF

D1, 2

Diode

00670

D3, 4

1N5817

26108

F1, F2

Use recommended fusing for specific

DC-DC Converters

R1, 2

150k , 0.5W

R3, 4***

250

V1,2

30234-220

Z1

MOV 270

30076

Sizing PCB traces:

All traces shown in bold carry significant

current and should be sized accordingly.

± IN

FARM2-xxx

± IN

* See Agency Approvals on FARM data sheet.

**

Required if C1 & C2 are located more than

3 in (75mm) from output of the FARM.

***Not used with VI-260/VI-J60

Not used with VI-260/VI-J60