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PRELIMINARY

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

ENMods System: FARM3 and MiniHAM

Rev. 1.3

Page 7 of 12

The ENMods system 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.

The ENMods 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 (Fig 1A).

Functional Description (FARM3, see Figures 9 & 10)

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 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. 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

SR

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 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