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PRELIMINARY

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

FARM, Filter/Autoranging Rectifier Module

Rev. 4.7

Page 5 of 12

The Filtered, Autoranging Rectifier Module (FARM) provides an

effective solution for the AC front end of a power supply built

with Vicor DC-DC converters. This high performance power

system building block satisfies a broad spectrum of requirements

and agency standards.

In addition to providing transient/surge immunity and EMI

filtering, the FARM contains all of the power switching and

control circuitry necessary for autoranging rectification, inrush

current limiting, and overvoltage protection. This module also

provides converter enable and status functions for orderly power

up/down control or sequencing. To complete the AC front end

configuration, the user only needs to add hold-up capacitors, and

a few discrete components.

Functional Description

Power-Up Sequence. (see Fig. 8):

Upon application of input power, the output bus capacitors

begin to charge. The thermistor limits the charge current,

and the exponential time constant is determined by the

hold-up capacitor value and the thermistor cold resistance.

The slope (dv/dt) of the capacitor voltage versus time

approaches zero as the capacitors become charged to the

peak of the AC line voltage.

The switch that bypasses the inrush limiting PTC (positive

temperature coefficient) thermistor is open when power is

applied, as is the switch that engages the strap for voltage

doubling. (see Fig. 7). In addition, the converter modules

are disabled via the Enable (EN) line, and Bus-OK (BOK)

is high.

If the bus voltage is less than 200V as the slope nears

zero, the voltage doubler is activated, and the bus voltage

climbs exponentially to twice the peak line voltage.

If the bus voltage is greater than 200V, the doubler is

not activated.

If the bus voltage is greater than 235V as the slope

approaches zero, the inrush limiting thermistor is

bypassed. Below 235V, it is not bypassed.

The converters are enabled 50 milliseconds after the

thermistor bypass switch is closed.

Bus-OK is asserted after an additional 50 millisecond

delay to allow the converter outputs to settle within

specification.

Power-Down Sequence. (see Fig. 8) When input power is

turned off or fails, the following sequence occurs as the bus

voltage decays:

Bus-OK is deasserted when the bus voltage falls below

210Vdc.

The converters are disabled when the bus voltage falls

below 190Vdc. If power is reapplied after the converters

are disabled, the entire power-up sequence is repeated. If

a momentary power interruption occurs and power is

reestablished before the bus reaches the disable threshold,

the power-up sequence is not repeated, i.e., the power

conversion system “rides through” the momentary

interruption.

Application Note

Microcontroller

N

L

PTC

Thermistor

+Out

–Out

Strap

EN

BOK

Strap

400

300

200

100

0

90–132V

AC Line

Output

Bus

(Vdc)

Strap

PTC

Thermistor

Bypass

Converter

Enable

Bus OK

50ms

50ms

Power

Up

Power

Down

4.1

1.1

2.1

3.1

5.1

2.2

1.2

Figure 7—Functional block diagram: autoranging rectifier

Figure 8—Timing diagram: power up/down sequence

1.1

2.1

3.1

4.1

5.1

1.2

2.2