ENMODS™

Rev 3.3

vicorpower.com

Page 7 of 13

06/2017

800 927.9474

EN1xxx

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 Figure 1a).

Functional Description (FARM3, see Figures 9 & 10)

Power-Up Sequence

1.1 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.

2.1 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.

3.1 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.

4.1 The converters are enabled ~150 milliseconds after the

thermistor bypass switch is closed.

5.1 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:

1.2 Bus-OK is deasserted when the bus voltage falls below

2.2 The converters are disabled when the bus voltage falls

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 – 132V

AC Line

Output

Bus

Strap

PTC

Thermistor

Bypass

Converter

Enable

Bus OK

~150ms

Power

Up

Power

Down

Timing Diagram, Power Up/Down Sequence

4.1

1.1

2.1

3.1

5.1

2.2

1.2

~150ms

Figure 9 — Functional block diagram: FARM3 module

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