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

ENMods System: FARM3 and MiniHAM

Rev. 2.9

Page 7 of 13

Set your site on VICOR at vicorpower.com

The ENMod component power front-end system for

EN compliance provides an effective solution for an AC

front end of a power supply enabled with Vicor DC-DC

converters. The ENMod system’s basic building blocks are

the MiniHAM passive harmonic attenuation module, the

FARM3 autoranging AC-DC front-end module (Figure 9) and

a discrete EMI filter.

The ENMod system provides transient/surge immunity,

harmonic current attenuation and EMI filtering, in addition to

all of the power switching and control circuitry necessary for

autoranging rectification, inrush current limiting, and

overvoltage protection. Converter enable and status functions

for orderly power up/down control or sequencing are also

provided. To complete the AC front-end configuration, the user

only needs to add hold-up capacitors, a simple EMI filter, and

a few discrete components (see Fig. 1a).

Power-Up Sequence.

Upon application of input power, the hold-up 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. 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 200 V 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 200 V, the

doubler is not activated.

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

approaches zero, the inrush limiting thermistor is

bypassed. Below 235 V, it is not bypassed.

The converters are enabled ~150 milliseconds after the

thermistor bypass switch is closed.

Bus-OK is asserted after an additional ~150 millisecond

delay to allow the converter outputs to settle within

specification.

Power-Down Sequence.

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

210 Vdc.

The converters are disabled when the bus voltage falls

below 190 Vdc. 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

SR

EMI

Filter

EMI GND

400

300

200

100

0

90–132 V

AC Line

Output

Bus

(Vdc)

Strap

PTC

Thermistor

Bypass

Converter

Enable

Bus OK

~150 ms

Power

Up

Power

Down

4.1

1.1

2.1

3.1

5.1

2.2

1.2

~150 ms

Figure 9 — Functional block diagram: FARM3 module

Figure 10 — Timing diagram: power-up/down sequence

1.1

2.1

3.1

4.1

5.1

1.2

2.2